Ullmann's Encyclopedia of Industrial Chemistry || Lubricants and Lubrication

c© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a15 423

Lubricants and Lubrication 1

Lubricants and Lubrication

Thorsten Bartels, Fuchs Europe Schmierstoffe GmbH, Mannheim, Federal Republic of Germany (Chaps. 9and 16)

Wolfgang Bock, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Sections 10.1 and 10.2,Chap. 11)

Jurgen Braun, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chap. 6)

Christian Busch, Fuchs Lubritech GmbH, Mannheim, Federal Republic of Germany (Chap. 15)

Wolfgang Buss, Fuchs Europe Schmierstoffe GmbH, Mannheim, Federal Republic of Germany(Sections 13.1–13.4)

Wilfried Dresel, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chaps. 5 and 14)

Carmen Freiler, Fuchs Europe Schmierstoffe GmbH, Mannheim, Federal Republic of Germany (Chap. 12)

Manfred Harperscheid, Fuchs Europe Schmierstoffe GmbH, Mannheim, Federal Republic of Germany(Chap. 8)

Rolf-Peter Heckler, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chap. 14)

Dietrich Horner, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chap. 12)

Franz Kubicki, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Section 13.1)

Georg Lingg, Fuchs Petrolub AG, Queretaro, Mexico (Section 10.1)

Achim Losch, Fuchs Europe Schmierstoffe GmbH, Mannheim, Federal Republic of Germany (Section 13.1)

Rolf Luther, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chaps. 7 and 19)

Theo Mang, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chaps. 1–4, 12, Sections13.1–13.4, Chaps. 17, 18)

Siegfried Noll, Fuchs Petrolub AG, Mannheim, Federal Republic of Germany (Chap. 16)

Jurgen Omeis, Fuchs Europe Schmierstoffe GmbH, Mannheim, Federal Republic of Germany (Chaps. 6 and8)

1. Introduction . . . . . . . . . . . . . 62. Lubricants in the Tribological

System . . . . . . . . . . . . . . . . . 72.1. Friction . . . . . . . . . . . . . . . . 82.1.1. Types of Friction . . . . . . . . . . . 82.1.2. Friction and Lubrication Condi-

tions . . . . . . . . . . . . . . . . . . . 102.2. Wear . . . . . . . . . . . . . . . . . . 113. Rheology of Lubricants . . . . . . 113.1. Viscosity . . . . . . . . . . . . . . . . 113.2. Special Rheological Effects . . . 153.3. Viscosity Grades . . . . . . . . . . 164. Base Oils . . . . . . . . . . . . . . . 184.1. Historical Review and Outlook . 194.2. Chemical Characterization of

Mineral Base Oils . . . . . . . . . . 194.3. Refining . . . . . . . . . . . . . . . . 204.3.1. Distillation . . . . . . . . . . . . . . . 204.3.2. Deasphalting . . . . . . . . . . . . . 20

4.3.3. Traditional Refining Processes . . 214.3.3.1. Acid Refining . . . . . . . . . . . . . 214.3.3.2. Solvent Extraction . . . . . . . . . . 214.3.4. Solvent Dewaxing . . . . . . . . . . 234.3.5. Finishing . . . . . . . . . . . . . . . . 244.4. Base Oil Manufacturing by Hy-

drogenation and Hydrocracking 244.4.1. Manufacturing Naphthenic Base

Oils by Hydrogenation . . . . . . . 254.4.2. Production of White Oils . . . . . . 254.4.3. Lube Hydrocracking . . . . . . . . 274.4.4. Catalytic Dewaxing . . . . . . . . . 274.4.5. Wax Isomerization . . . . . . . . . . 284.4.6. Hybrid Lube Oil Processing . . . . 284.4.7. All-Hydrogen Route . . . . . . . . . 294.4.8. Gas-to-Liquids Conversion Tech-

nology . . . . . . . . . . . . . . . . . 294.5. Boiling and Evaporation Behav-

ior of Base Oils . . . . . . . . . . . 31

2 Lubricants and Lubrication

5. Synthetic Base Oils . . . . . . . . . 325.1. Synthetic Hydrocarbons . . . . . 325.1.1. Polyalphaolefins . . . . . . . . . . . 335.1.2. Polyinternalolefins . . . . . . . . . . 335.1.3. Polybutenes . . . . . . . . . . . . . . 335.1.4. Alkylated Aromatics . . . . . . . . 355.1.5. Other Hydrocarbons . . . . . . . . . 355.2. Halogenated Hydrocarbons . . . 355.3. Synthetic Esters . . . . . . . . . . . 355.3.1. Esters of Carboxylic Acids . . . . . 355.3.1.1. Dicarboxylic Acid Esters . . . . . . 365.3.1.2. Polyol Esters . . . . . . . . . . . . . 365.3.1.3. Other Carboxylic Esters . . . . . . 365.3.1.4. Complex Esters . . . . . . . . . . . . 375.3.1.5. Fluorinated Carboxylic Acid Esters 375.3.2. Phosphate Esters . . . . . . . . . . . 375.4. Polyalkylene Glycols . . . . . . . . 385.5. Other Polyethers . . . . . . . . . . 395.5.1. Perfluorinated Polyethers . . . . . . 395.5.2. Polyphenyl Ethers . . . . . . . . . . 395.5.3. Polysiloxanes (Silicone Oils) . . . 405.6. Other Synthetic Base Oils . . . . 425.7. Mixtures of Synthetic Lubricants 436. Additives . . . . . . . . . . . . . . . 436.1. Antioxidants . . . . . . . . . . . . . 436.1.1. Mechanism of Oxidation and An-

tioxidants . . . . . . . . . . . . . . . 436.1.2. Compounds . . . . . . . . . . . . . . 446.2. Viscosity Modifiers . . . . . . . . . 466.2.1. VI Improvement Mechanisms . . . 466.2.2. Structure and Chemistry of Viscos-

ity Modifiers . . . . . . . . . . . . . 466.3. Pour Point Depressants . . . . . . 466.4. Detergents and Dispersants . . . 486.4.1. Metal-Containing Compounds

(Detergents) . . . . . . . . . . . . . . 486.4.2. Ashless Dispersants (AD) . . . . . 496.5. Antifoam Agents . . . . . . . . . . 496.6. Demulsifiers . . . . . . . . . . . . . 506.7. Dyes . . . . . . . . . . . . . . . . . . 516.8. Antiwear and Extreme Pressure

Additives . . . . . . . . . . . . . . . 516.9. Friction Modifiers . . . . . . . . . 546.10. Corrosion Inhibitors . . . . . . . . 546.10.1. Antirust Additives . . . . . . . . . . 546.10.2. Metal Passivators . . . . . . . . . . . 557. Lubricants in the Environment . 567.1. Current Situation . . . . . . . . . . 567.1.1. Economic Consequences and Sub-

stitution Potential . . . . . . . . . . 577.1.2. Agriculture, Economy, and Politics 587.2. Biodegradable Base Oils for Lu-

bricants . . . . . . . . . . . . . . . . 587.2.1. Synthetic Esters . . . . . . . . . . . 59

7.2.2. Polyalkylene Glycols . . . . . . . . 597.2.3. Polyalphaolefins . . . . . . . . . . . 597.2.4. Relevant Properties of Biodegrad-

able Base Oils . . . . . . . . . . . . . 597.3. Additives . . . . . . . . . . . . . . . 607.4. Products (Examples) . . . . . . . . 608. Lubricants for Internal Combus-

tion Engines . . . . . . . . . . . . . 628.1. Four-Stroke Engine Oils . . . . . 628.1.1. General Overview . . . . . . . . . . 628.1.1.1. Fundamental Principles . . . . . . . 628.1.1.2. Performance Specifications . . . . 638.1.1.3. Formulation of Engine Oils . . . . 638.1.1.4. Additives . . . . . . . . . . . . . . . . 648.1.2. Characterization and Testing . . . 658.1.2.1. Physical and Chemical Testing . . 658.1.2.2. Engine Testing . . . . . . . . . . . . 658.1.2.3. Passenger Car Engine Oils . . . . . 668.1.2.4. Engine Oil for Commercial Vehi-

cles . . . . . . . . . . . . . . . . . . . 668.1.3. Classification by Specification . . 688.1.3.1. MIL Specifications . . . . . . . . . 688.1.3.2. API and ILSAC Classification . . 688.1.3.3. ACEA Specifications . . . . . . . . 688.1.3.4. Manufacturers’ Approvals . . . . . 698.1.3.5. Future Trends . . . . . . . . . . . . . 698.2. Two-Stroke Oils . . . . . . . . . . . 718.2.1. Application and Characteristics . . 718.2.2. Classification . . . . . . . . . . . . . 728.2.2.1. API Service Groups . . . . . . . . . 728.2.2.2. JASO Classification . . . . . . . . . 738.2.2.3. ISO Classification . . . . . . . . . . 738.2.3. Oils for Two-Stroke Outboard En-

gines . . . . . . . . . . . . . . . . . . 748.2.4. Environmentally Friendly Two-

Stroke Oils . . . . . . . . . . . . . . . 748.3. Tractor Oils . . . . . . . . . . . . . 758.4. Gas Engine Oils . . . . . . . . . . . 758.5. Marine Diesel Engine Oils . . . . 768.5.1. Low-Speed Crosshead Engines . . 768.5.2. Medium-Speed Engines . . . . . . 768.5.3. Lubricants . . . . . . . . . . . . . . . 779. Gear Lubrication Oils . . . . . . . 779.1. Introduction . . . . . . . . . . . . . 779.2. Requirements of Gear Lubrica-

tion Oils . . . . . . . . . . . . . . . . 789.3. Tribology of Gears . . . . . . . . . 789.3.1. Friction Conditions of Gear Types 789.3.2. Specific Gear and Transmission

Failure . . . . . . . . . . . . . . . . . 829.4. Gear Lubrication Oils for Motor

Vehicles . . . . . . . . . . . . . . . . 839.4.1. Lubricants forGearDrives inCom-

mercial Vehicles . . . . . . . . . . . 84


9.4.2. Lubricants for Gear Drives in Pas-senger Cars . . . . . . . . . . . . . . 85

9.4.3. Lubricants for Automatic Trans-missions and CVTs . . . . . . . . . 86

9.5. Multipurpose Lubricants in Ve-hicle Gears . . . . . . . . . . . . . . 90

9.6. Gear Lubricants for IndustrialGears . . . . . . . . . . . . . . . . . . 91

10. Compressor Oils . . . . . . . . . . 9310.1. Gas Compressor . . . . . . . . . . 9310.1.1. Displacement Compressors . . . . 9310.1.2. Dynamic Compressors . . . . . . . 9510.1.3. Preparation of Compressed Air . . 9510.1.4. Oils for Compression of Other

Gases . . . . . . . . . . . . . . . . . . 9510.1.5. Characteristics of Gas Compressor

Oils . . . . . . . . . . . . . . . . . . . 9510.1.6. Standards and Specifications of

Compressor Oils . . . . . . . . . . . 9610.2. Refrigerator Oils . . . . . . . . . . 9610.2.1. Introduction . . . . . . . . . . . . . . 9610.2.2. Minimum Requirements . . . . . . 9810.2.3. Classification . . . . . . . . . . . . . 9910.2.4. Viscosity Selection . . . . . . . . . 10411. Turbine Oils . . . . . . . . . . . . . 10611.1. Demands on Turbine Oils –

Characteristics . . . . . . . . . . . 10611.2. Formulation . . . . . . . . . . . . . 10711.3. Specifications . . . . . . . . . . . . 10811.4. Turbine Oil Circuits . . . . . . . . 10811.5. Monitoring and Maintenance of

Turbine Oils . . . . . . . . . . . . . 10811.6. Life of (Steam) Turbine Oils . . . 10911.7. Gas Turbine Oils – Application

and Requirements . . . . . . . . . 10911.8. Fire-Resistant, Water-Free Flu-

ids for Power Station Applica-tions . . . . . . . . . . . . . . . . . . 112

11.9. Lubricants for Water Turbinesand Hydroelectric Plants . . . . . 112

12. Metalworking Fluids . . . . . . . 11312.1. Mechanism of Action . . . . . . . 11312.2. Water-Miscible Cutting Fluids . 11412.2.1. Composition . . . . . . . . . . . . . 11412.2.2. Corrosion Protection and Corro-

sion Test Methods . . . . . . . . . . 11412.2.3. Concentration of Water-Mixed

Cutting Fluids . . . . . . . . . . . . . 11412.2.4. Stability of Coolants . . . . . . . . . 11512.2.5. Foaming Properties . . . . . . . . . 11512.2.6. Preservation of Coolants with Bio-

cides . . . . . . . . . . . . . . . . . . 11512.3. Neat Cutting Fluids . . . . . . . . 11612.3.1. Specifications . . . . . . . . . . . . . 116

12.3.2. Composition . . . . . . . . . . . . . 11612.4. Application . . . . . . . . . . . . . . 11712.4.1. Machining with Geometrically De-

fined Cutting Edges . . . . . . . . . 11712.4.2. Machining with Geometric Non-

Defined Cutting Edges . . . . . . . 11812.5. Storage . . . . . . . . . . . . . . . . 12012.6. Environmental Aspects . . . . . . 12012.7. New Trends in Coolant Technol-

ogy . . . . . . . . . . . . . . . . . . . 12013. Forming Lubricants . . . . . . . . 12213.1. Sheet Metal Working Lubricants 12213.1.1. Deep Drawing . . . . . . . . . . . . 12213.1.2. Stretch Drawing and a Combina-

tion of Stretch and Deep Drawing 12413.1.3. Shear Cutting . . . . . . . . . . . . . 12413.1.4. Choice of Lubricants . . . . . . . . 12513.1.5. Sheet Metal Forming in Automo-

bile Manufacturing . . . . . . . . . 12613.2. Lubricants for Wire, Tube, and

Profile Drawing . . . . . . . . . . . 12613.2.1. Wire Drawing . . . . . . . . . . . . . 12613.2.2. Profile Drawing . . . . . . . . . . . . 12813.2.3. Tube Drawing . . . . . . . . . . . . . 12813.2.4. Hydroforming . . . . . . . . . . . . . 12913.3. Lubricants for Rolling . . . . . . 13013.3.1. Rolling Steel Sheet . . . . . . . . . 13113.3.2. Rolling Aluminum Sheet . . . . . . 13313.3.3. Rolling of Other Materials . . . . . 13413.4. Lubricants for Solid Metal

Forming . . . . . . . . . . . . . . . . 13414. Lubricating Greases . . . . . . . . 13714.1. Introduction . . . . . . . . . . . . . 13714.2. Components of Greases . . . . . . 13814.2.1. Thickeners . . . . . . . . . . . . . . . 13814.2.1.1. Simple Soaps . . . . . . . . . . . . . 13814.2.1.2. Complex Soaps . . . . . . . . . . . . 14014.2.2. Other Ionic Organic Thickeners . 14214.2.3. Nonionic Organic Thickeners . . . 14214.2.4. Inorganic Thickeners . . . . . . . . 14314.2.5. Miscellaneous Thickeners . . . . . 14314.2.6. Temporarily Thickened Fluids . . 14314.3. Base Oils . . . . . . . . . . . . . . . 14314.3.1. Mineral Oils . . . . . . . . . . . . . . 14414.3.2. Synthetic Base Oils . . . . . . . . . 14414.4. Grease Structure . . . . . . . . . . 14414.5. Additives . . . . . . . . . . . . . . . 14514.6. Manufacture of Greases . . . . . 14614.6.1. Metal Soap-Based Greases . . . . . 14614.6.2. Oligourea Greases . . . . . . . . . . 14714.6.3. Gel Greases . . . . . . . . . . . . . . 14714.7. Grease Rheology . . . . . . . . . . 14814.8. Performance . . . . . . . . . . . . . 14914.8.1. Test Methods . . . . . . . . . . . . . 149


14.8.2. Analytical Methods . . . . . . . . . 15014.9. Applications . . . . . . . . . . . . . 15014.9.1. Roller Bearings . . . . . . . . . . . . 15014.9.2. Cars, Trucks, Construction Vehi-

cles . . . . . . . . . . . . . . . . . . . 15114.9.3. Steel Mills . . . . . . . . . . . . . . . 15314.9.4. Mining . . . . . . . . . . . . . . . . . 15314.9.5. Railroad, Railway . . . . . . . . . . 15314.9.6. Gears . . . . . . . . . . . . . . . . . . 15414.9.7. Food-Grade Applications . . . . . . 15414.9.8. Textile Machines . . . . . . . . . . . 15414.9.9. Applicationswith PolymericMate-

rials . . . . . . . . . . . . . . . . . . . 15414.10. Ecology and the Environment . 15414.11. Grease Tribology . . . . . . . . . . 15515. Solid Lubricants . . . . . . . . . . 15515.1. Classification . . . . . . . . . . . . . 15615.1.1. Class 1: Structural Lubricants . . . 15615.1.2. Class 2: Mechanical Lubricants . . 15615.1.3. Class 3: Soaps . . . . . . . . . . . . 15815.1.4. Class 4: Chemically Active Lubri-

cants . . . . . . . . . . . . . . . . . . 15815.2. Characteristics . . . . . . . . . . . 15815.2.1. Crystal Structures of Lamellar

Solid Lubricants . . . . . . . . . . . 15815.2.2. Heat Stability . . . . . . . . . . . . . 15915.2.3. Thermal Conductivity . . . . . . . . 15915.2.4. Adsorbed Films . . . . . . . . . . . 15915.2.5. Chemical Stability . . . . . . . . . . 15915.2.6. Particle Size . . . . . . . . . . . . . . 15915.3. Products Containing Solid Lu-

bricants . . . . . . . . . . . . . . . . 16015.3.1. Powders . . . . . . . . . . . . . . . . 16015.3.2. Dispersions and Suspensions . . . 16015.3.3. Greases and Grease Pastes . . . . . 16015.3.4. Pastes . . . . . . . . . . . . . . . . . . 16115.3.5. Dry-Film Lubricants . . . . . . . . 16215.4. Industrial Uses of Products Con-

taining Solid Lubricants . . . . . 16315.4.1. Screw Lubrication . . . . . . . . . . 16315.4.2. Roller-Bearing Lubrication . . . . 16315.4.3. SlideBearing, SlideGuideway, and

Slide Surface Lubrication . . . . . 16415.4.4. Chain Lubrication . . . . . . . . . . 16415.4.5. Plastic and Elastomer Lubrication 16516. Testing and Analysis . . . . . . . . 16516.1. Base Oil Categories and Evalu-

ation of Various Petroleum BaseOils . . . . . . . . . . . . . . . . . . . 165

16.2. Laboratory Methods for TestingLubricants . . . . . . . . . . . . . . 165

16.2.1. Density . . . . . . . . . . . . . . . . . 16516.2.2. Viscosity . . . . . . . . . . . . . . . . 16616.2.3. Refractive Index . . . . . . . . . . . 16716.2.4. Structural Analyses . . . . . . . . . 16716.2.5. Flash Point . . . . . . . . . . . . . . 16716.2.6. Surface Phenomena . . . . . . . . . 16716.2.7. Cloud Point, Pour Point . . . . . . 16716.2.8. Aniline Point . . . . . . . . . . . . . 16816.2.9. Water Content . . . . . . . . . . . . . 16816.2.10. Ash Content . . . . . . . . . . . . . . 16816.2.11. Acidity, Alkalinity . . . . . . . . . . 16816.2.12. Aging Tests . . . . . . . . . . . . . . 16816.2.13. Hydrolytic Stability . . . . . . . . . 16916.2.14. Corrosion Tests . . . . . . . . . . . . 16916.2.15. Oil Compatibility of Seals and In-

sulating Materials . . . . . . . . . . 16916.2.16. Evaporation Loss . . . . . . . . . . . 16916.2.17. Analysis of Grease . . . . . . . . . . 16916.3. Mechanical –Dynamic Testing

Methods for Lubricants . . . . . 16916.3.1. Tribological System Categories

within Lubricant Tests . . . . . . . 16916.3.2. Standardized and Nonstandardized

Test Methods for Lubricants . . . . 16916.3.3. Common Mechanical –Dynamic

Testers . . . . . . . . . . . . . . . . . 17017. Economic Aspects . . . . . . . . . 17518. Disposal of Used

Lubricating Oils . . . . . . . . . . 17718.1. Possible Uses of Waste Oil . . . . 17718.2. Legislative Influences on Waste

Oil Collection and Recondition-ing . . . . . . . . . . . . . . . . . . . . 177

18.3. Re-refining . . . . . . . . . . . . . . 17819. Toxicology and Occupational

Health . . . . . . . . . . . . . . . . . 18219.1. Safety Aspects of Handling Lu-

bricants (Working Materials) . . 18219.1.1. Polycyclic Aromatic Hydrocar-

bons (PAK, PAH, PCA) . . . . . . 18219.1.2. Nitrosamines in Cutting Fluids . . 18319.2. Skin Problems Caused by Lubri-

cants . . . . . . . . . . . . . . . . . . 18320. References . . . . . . . . . . . . . . 184


The entire topic was coordinated by TheoMang and Wilfried Dresl

Abbreviations

AAS atomic absorption spectrometryACEA Association des Constructeurs Eu-

ropeens d’Automobiles (Associationof European Car Manufacturers)

AD ashless dispersantAENOR Association Espanola de Normal-

izacion y Certification (Spanish As-sociation for Normalization and Cer-tification)

AFNOR Association Francaise de Normation(French Association for Normaliza-tion)

AGMA American Gear Manufacturers Asso-ciation

ASTME American Society of Tool and Manu-facturing Engineers (USA)

ATF automatic transmission fluidATIEL Association Technique de l’Industrie

Europeenne des Lubrifications (Tech-nical Association of the European Lu-bricant Industry)

AW antiwearAWT Almen – Wieland testCAFE Californian Act for Fuel EmissionCCEL China Certification Committee for

Environmental Labelling of productsCCMC Committee of Common Market Au-

tomobile Constructors (EU)CCS cold cracking simulatorCEC Coordinating European Council for

the development of performance testsfor lubricants and engine oils

CETOP Comite Europeen des TransmissionsOleohydrauliques et Pneumatiques(European Oil Hydraulic and Pneu-matic Committee)

CFMS closed-field magnetron sputteringCONCAWE The Oil Companies’ Organization

for the Conservation of Clean Air andWater in Europe

CRC Coordinating Research Council (EU)CVT constantly variable transmissionDD detergent and dispersantDGMK Deutsche Wissenschaftliche

Gesellschaft fur Erdol, Erdgas undKohle (German Scientific Society forMineral Oil, Natual Gas and Coal)

DKA Deutscher Koordinationsausschuß imCEC

DLC diamond-like amorphous carbonDSC differential scanning calorimetryEBT electron beam texturingECP Environmental Choice Program

(Canada)EHD elastohydrodynamicEHEDG European Hygienic Equipment De-

sign GroupELGI European Lubricating Grease Insti-

tuteEP extreme pressureFCC fluid catalytic crackerFM friction modifierFTMS Federal Test Methods Standardiza-

tion (USA)FVA Forschungsvereinigung Antrieb-

stechnik (German Research Asso-ciation for Drive Technology)

FZG Forschungsstelle fur Zahnrader undGetriebebau (German Research Cen-ter for Toothed Wheel and GearingEngineering)

GfT Gesellschaft fur Tribologie (GermanSociety for Tribologie)

GOST Gossudarstwenny Obschtschesso-jusny Standart (former Soviet Union,Governmental Union Standard)

HC hydrocrackedHD heavy dutyHLB hydrophilic-lipophilic balanceHPDSC high-pressure differential scanning

calorimetryHRC Rockwell C hardnessHTHS high-temperature high shearHVI high viscosity indexIBAD ion-beam-assisted depositionICP inductively coupled plasma atomic

emissionIFP Institut Francais du Petrol (French In-

stitute of Petroleum)ILMA Independent Lubricant Manufactur-

ers Association (USA)ILSAC International Lubricant Standardiza-

tion and Approval CommitteeIP Institute of Petroleum (UK)ISO VG ISO viscosity gradeJEA Japanese Environmental AssociationJIS Japanese Industrial StandardLDF long drain field testLVFA low-velocity friction apparatus


LVI low viscosity indexMIL Military Standard (USA)MLDW Mobil Lube DewaxingMQL minimum quantity lubricationMRV mini rotary viscosimeterMSDW Mobil Selective DewaxingN – D – M (refractive index) n-density-molec-

ular massNLGI National Lubricating Grease Institute

(USA)OEM original equipment manufacturerPCMO passenger car motor oilPEP passive extreme pressurePPD pour point depressantRAL UZ Reichsausschuß fur Lieferbedingun-

gen und Gutersicherung, Umweltze-ichen (German Imperial Committeefor Quality Control and Labelling,Environmental Symbol)

RNT radionuclide techniqueRVT Reichert Verschleiß Test (Reichert

wear test)SCDSC sealed capsule differential scanning

calorimetrySHPD super-high performance dieselSRE standard reference elastomerSRV Schwing –Reibverschleiß Gerat

(translatorisches Oszillations-Prufgerat) (translatory oscillation ap-paratus)

STLE Society of Tribologists and Lubrica-tion Engineers

STOU Super Tractor Oil UniversalSUS Saybolt universal secondsTAN total acid numberTBN total base numberTDA thermal deasphaltingTEI Thailand Environmental InstituteTEWL transepidermal water lossTG thermogravimetryTGL TechnischeNormen,Gutervorschriften

und Lieferbedingungen (formerGDR, Technical Standards, QualitySpecifications and Terms of Deliv-ery)

TOST turbine oil oxidation stability testTRIP transformation induced plasticsTSSI temporary shear stability indexUEIL Union Europeenne des Independents

en Lubrifiants (EuropeanUnion of In-dependent Lubricant Manufacturers)

UHPD ultra-high performance diesel

UHVI ultra-high viscosity indexUNITI Bundesverband Mittelstandischer

Mineralolunternehmen (GermanFed-eral Association of Middle-class OilCompanies)

UTTO universal tractor transmission oilVAMIL Regeling Willekeurig Afschrijving

Milieu-Investeringen (Dutch regula-tion of discretion deductions for in-vestments into the environment)

VCI vapor-phase (volatile) corrosion in-hibitor

VDA Verband der Automobilindustrie(German Automobile Industry As-sociation)

VDMA Verband Deutscher Maschinen- undAnlagenbau (German EngineeringFederation)

VDS Volvo drain specificationVGB Technische Vereinigung der

Großkraftwerkbetreiber (GermanTechnicalAssociation ofLargePowerPlant Operators)

VGO vacuum gas oilVHVI very high viscosity indexVI viscosity indexVIE viscoisty index extendedVII viscosity index improverVOC volatile organic compoundsVTC viscosity-temperature coefficientWGK Wassergefahrdungsklasse (Water

Hazard Class, Water Pollution Class)XHVI extra-high viscosity indexXRF X-ray fluorescence spectrometyZAF zinc- and ash-free

1. Introduction

The most important function of lubricants isthe reduction of friction and wear. Scientific re-search has shown that 0.4% of gross domesticproduct could be saved in terms of energy inWestern industrialized countries if current tribo-logical knowledge, i.e., the science of friction,wear, and lubrication, was just applied to lubri-cated processes.

Apart from important applications in internalcombustion engines, vehicle and industrial gear-boxes, compressors, turbines, or hydraulic sys-tems, there are a vast number of other applica-tions which mostly require specifically tailored


lubricants. Between 5000 and 10 000 differentlubricant formulations are necessary to satisfymore than 90% of all lubricant applications.

Mineral oil components continue to formthe quantitatively most important foundationof lubricants. Petrochemical components andincreasingly derivatives of natural, harvestableraw materials from the oleo-chemical indus-try are finding increasing acceptance becauseof their environmental compatibility and sometechnical advantages

Lubricants today are classified into twomajorgroups: automotive lubricants and industrial lu-bricants. Industrial lubricants can be sub-dividedinto industrial oils and industrial specialties; i.e.,greases, metalworking lubricants, and solid lu-bricant films.

Process oils are often included in lubricantstatistics. These are oils which are included asrawmaterials in processes, but above all as plas-ticizers for the rubber industry. Process oil’s onlylink with lubricants is that they are mineral oilproducts resulting from the refining of base oilsbut they often distort lubricant consumption fig-ures. Theywill not be covered in this book.How-ever, to provide a degree of comparison, theyhave been included in the following lubricantstatistics.

Apart from the most common lube oils, themany thousands of lubricant applications neces-sitate a diverse number of systems which is sel-dom equaled in other product groups.

The group next to oils are emulsions, whichas oil-in-water emulsions are central to water-miscible cutting fluids (Chap. 12) and rollingemulsions. In these cases, the lubricant manu-facturer normally supplies a concentrate whichis mixed with water locally to form an emulsion.The concentration of these emulsionswithwaterare generally between 1 and 10%. The annualconsumption of such emulsions in industrializedcountries is about the same as all other lubricantstogether. From this point of view, the volumetricproportion of these products (as concentrates) issignificantly under-rated in lubricant statistics.

The next group of lubricant systems arewater-in-oil emulsions (invert emulsions). Theirmost important application is in metal forming.These products are supplied ready-to-use or asdilutable concentrates. Fire resistant HFB fluidsare designed as water-in-oil emulsions too.

In some special cases, oil-in-oil emulsions aredeveloped as lubricants and these are primarilyused in the field of metalworking.

Water-based solutions in the form of non-dispersed systems are sometimes used in chip-forming metalworking operations.

Greases (Chap. 14) are complex systems con-sisting of base oils and thickeners based onsoaps or other organic or inorganic substances.They are available in semiliquid form (semifluidgreases) through to solid blocks (block greases).Special equipment is required for their produc-tion (grease-makingplants).Agroupof productsclosely related to greases are pastes.

Solid lubricant suspensions normally containsolid lubricants in stable suspension in a fluidsuch as water or oil. These products are oftenused in forging and extrusion and other metal-working processes. Solid lubricant films can alsobe applied as suspensions in a carrier fluidwhichevaporates before the lubricant has to function.

Solid lubricant powders can be applied di-rectly to specially-prepared surfaces.

In the case of dry-film lubricants (Chap. 15),solid lubricants are dispersed in resin matrices.Dry-film lubricants are formed when the solvent(principally water or hydrocarbons) evaporates.

Molten salts or glass powder are used for hotforming processes such as extrusion. These arenormally supplied as dry powders and developlubricity when they melt on the hot surface ofthe metal.

Polymer films are used when special surfaceprotection is required in addition to lubricity(e.g., the pressing of stainless steel panels). To-gether with greases, these products are also usedto some extent in the construction industry.

2. Lubricants in the TribologicalSystem

Tribology (derived from theGreek tribein, or tri-bos meaning rubbing) is the science of friction,wear, and lubrication. Although the use of lubri-cants is as old as mankind, scientific focus on lu-bricants and lubrication technology is relativelynew. The term tribology was first introduced in1966 and has been used globally to describe thisfar-reaching field of activity since 1985. Eventhough efforts had beenmade since the 16th cen-tury to describe the whole phenomenon of fric-


Figure 1. Sliding and rolling

tion scientifically (Leonardo da Vinci, Amon-tons, Coulomb), the work always concentratedon single aspects and lubricants were not evenconsidered. Some research work performed upto the early 1970s totally ignored the chemicalprocesses which take place in lubricated frictionprocesses.

The tribological system (commonly referredto as the tribosystem) consists of four elements:the twocontacting partners, the interface bet-ween the twoand themedium in the interface andthe environment [52]. In lubricated bearings thelubricant is located in this gap. In plain bearings,the material pair are the shaft and the bearingshells; in combustion engines they are the pis-ton rings and the cylinder wall or the camshaftlobes and the tappets and in metalworking, thetool and the workpiece.

The variables are the type of movement, theforces involved, temperature, speed, and dura-tion of the stress. Tribometric parameters – suchas friction, wear, and temperature data – can begathered from the stress area. Tribological stress

is the result of numerous criteria of surface andcontact geometry, surface loading, or lubricantthickness. Tribological processes can occur inthe contact area between two friction partners –which can be physical, physicochemical (e.g.,adsorption, desorption), or chemical in nature(tribochemistry).

2.1. Friction

2.1.1. Types of Friction

Friction is the mechanical force which resistsmovement (dynamic or kinetic friction) or hin-ders movement (static friction) between slidingor rolling surfaces. These types of friction arealso called external friction.

Internal friction results from the friction bet-ween lubricant molecules; this is described asviscosity (Chap. 3).

The causes of external friction are, above all,themicroscopic contact points between two slid-


Figure 2. ‘Walzreibung’, mixing of rolling and sliding motions.a) Rolling in metal forming; v1, initial speed of the sheet metal; v2, final speed of the sheet metal; v3, speed of the roller; vr,speed difference in the roll gap (sliding part); N, neutral point (non-slip point, pure rolling)b) Engagement of gear teeth, 1, 2, 4. high sliding/rolling ratio; 3. pitch circle (pure rolling, no slip)

ing surfaces; these cause adhesion, material de-formation, and grooving. Energywhich is lost asfriction can be measured as heat and/or mechan-ical vibration. Lubricants should reduce or avoidthemicro-contactwhich causes external friction.

Sliding friction occurs in a pure sliding mo-tion with no rolling and no spin (Fig. 1).

The coefficient of friction µ is defined as thedimensionless ratio of the friction force F andthe normal forceN . The proportionality betweennormal force and frictional force is often given

in dry and boundary friction conditions but notin fluid-film lubrication.

Rolling friction is the friction generated byrolling contact. In roller bearings, rolling fric-tion mainly occurs between the rolling elementsand the raceways, whereas sliding friction oc-curs between the rolling elements and the cage.The main cause of friction in roller bearings issliding in the contact zones between the rollingelements and the raceways. It is also influencedby the geometry of the contacting surfaces and


the deformation of the contacting elements. Inaddition, sliding also occurs between the cagepockets and the rolling elements.

If rolling motion and sliding motion com-bine to any significant extent, as for gear toothmeshing, special terminology has been created.The word ‘Walzreibung’ which is derived from‘Walzen’ (rolling, e.g., steel rolling) is usedin Germany. Situations in which a high slid-ing/rolling ratio occur require totally differ-ent lubrication than does pure sliding. Figure 2shows this ‘rolling friction’ during rolling andduring gear meshing.

Static Friction. The static coefficient of fric-tion is defined as the coefficient of friction cor-responding to the maximum force that must beovercome to initiate macroscopic motion bet-ween two bodies (ASTM).

Kinetic Friction. Different from static fric-tion, kinetic friction occurs under conditions ofrelative motion. ASTM defines the kinetic coef-ficient of friction as the coefficient under con-ditions of macroscopic relative motion of twobodies. The kinetic coefficient of friction (dy-namic coefficient of friction) is usually some-what smaller than the static coefficient of fric-tion.

Stick – slip is a special formof frictionwhichoften results from very slow sliding movementswhen the friction partners are connected to asystem which can vibrate. The process is influ-enced by the dependence of the coefficient ofsliding friction on speed. This generally occurswhen the static coefficient of friction (f stat) islarger than the dynamic coefficient of friction(f dyn). Stick – slip is normally encountered withmachine tools which operate with slow feeds.Stick – slip can cause chatter marks on compo-nents.

2.1.2. Friction and Lubrication Conditions

In tribological systems, different forms of con-tact can exist between contacting partners.

Solid friction (dry friction) occurs whentwo solids have direct contact with each other

without a separating layer. If conventional ma-terials are involved, the coefficients of frictionand wear rates are high. Lubrication technologyattempts to eliminate this condition.

Boundary Friction. The contacting sur-faces are covered with a molecular layer ofa substance whose specific properties can sig-nificantly influence the friction and wear char-acteristics. Boundary friction layers are of greatimportance in practical applications when thick,long-lasting lubricant films to separate two sur-faces are technically impossible. Boundary lu-bricating films are created from surface-activesubstances and their chemical reaction products.Adsorption, chemisorption, and tribochemicalreactions also play significant roles.

Fluid Friction. In fluid friction, both sur-faces are fully separated by a fluid lubricantfilm (full-film lubrication). This film is eitherformed hydrostatically or more commonly, hy-drodynamically (Fig. 3). Liquid or fluid frictionis caused by the frictional resistance, because ofthe rheological properties of fluids.

Figure 3. Hydrostatic lubrication as a form of fluid friction

If both surfaces are separated by a gas film,this is known as gas lubrication.

Mixed friction occurs when boundary fric-tion combines with fluid friction. Machine ele-ments which are normally hydrodynamically lu-bricated experiencemixed frictionwhen startingand stopping.


Solid lubricant friction occurs when solidlubricants are used (Chap. 15).

Stribeck Diagram. The friction or lubrica-tion conditions between boundary and fluid fric-tion are graphically illustrated by use of Stribeckdiagrams (Fig. 4) [53]. These are based on thestarting-up of a plain bearing whose shaft andbearing shells are, when stationary, separatedonly by amolecular lubricant layer. As the speedof revolution of the shaft increases (peripheralspeed) a thicker hydrodynamic lubricant filmis created what initially causes sporadic mixedfriction but which, nevertheless, significantly re-duces the coefficient of friction. As the speedcontinues to increase, a full, uninterrupted film isformed over the entire bearing faces; this sharplyreduces the coefficient of friction. As speed in-creases, internal friction in the lubricating filmadds to external friction. The curve passes amin-imum coefficient of friction value and then in-creases, solely as a result of internal friction. Thelubricant film thickness shown in Figure 4 de-pends on the friction and lubrication conditionsincluding the surface roughness R.

Hydrodynamic Lubrication. In hydrody-namic lubrication the lubricant is pulled into theconical converging clearance by the rotation ofthe shaft. The created dynamic pressure carriesthe shaft.

On the basis of the Navier – Stokes theory offluid mechanics, Reynolds created the basic for-mula for hydrodynamic lubrication in 1886. Theapplication of the Reynolds’ formula led to the-oretical calculations on plain bearings. The onlylubricant value was viscosity.

Elastohydrodynamic Lubrication (EHDRegime). Hydrodynamic calculation on lubri-cant films was extended to include the elasticdeformation of contact faces (Hertzian contacts,Hertz’s equations of elastic deformation) andthe influence of pressure on viscosity (Chap. 3).This enables application of these elastohydrody-namic calculations to contact geometries otherthan that of plain bearings – e.g., those of rollerbearings and gear teeth.

Figure 5 shows the elastic deformation of theball and raceway of a ball bearing.

2.2. Wear (→Abrasion and Erosion)

Wear is created by the processes of abrasion,adhesion, erosion, tribochemical reactions, andmetal fatigue which are important to lubricationtechnology.

Types of Wear. There are several criteriawhich could be used to classify the differenttypes of wear. For example, according to thetypes of (kinematic) friction which lead to wear(sliding wear, rolling wear, fretting wear), ac-cording to wear mechanisms (adhesive wear,abrasive wear, tribochemical wear) or accordingto the shape of the wear particle.

There is also highly specialized wear termi-nology for different lubricant applications; thisis oriented to the geometry of the various bear-ing faces (e.g., clearance and crater wear in thefield of chip-forming tools).

The Wear Process. Wear can be measuredgravimetrically, volumetrically or in terms ofarea over a period of time or against increas-ing load. Uniformly decreasing wear which sta-bilizes at a very low level can be described asrunning-in wear. This can be controlled by thetribochemical reactions of the additives in thelubricants. Wear can occur at relatively constantspeed and ultimately lead to the functional fail-ure of the bearing. Wear at an increasing wearrate can lead to progressive wear. In time, thematerial damage caused by wear will also leadto the failure of a component.

Analysis of machine element failure as a re-sult of wear and lubrication can be performed byuse of practically oriented tests. Determinationof the effect of different greases on the projectedfailure of roller bearings has become increas-ingly important.

3. Rheology of Lubricants

Consistency, flow properties, or viscosity in thecase of oils, are key parameters to create lubrica-tion efficiency and the application of lubricants.

3.1. Viscosity (→Rheometry)

The measure of internal friction in a fluid isviscosity. Viscosity and its dimensions are best


Figure 4. Stribeck graph according to H. Czichos and K.-H. Habig [53]a) Boundary friction (h→ 0 ); b) Mixed film friction (h≈R); c) Elastohydrodynamic lubrication (h>R); d) Hydrodynamiclubrication (h>>R)

explained with a model of parallel layers offluid which could be viewed molecularly. If thispacket of fluid layers is sheared (τ ), the individ-ual fluid layers are displaced in the direction ofthe shearing force. The upper layers move morerapidly than the lower layers because molecularforces act to resistmovement between the layers.These forces create resistance to shearing andthis resistance is given the term dynamic viscos-ity. The difference in velocity between two givenfluid layers, related to their linear displacement,is referred to as shear rate S. This velocity gra-dient is proportional to the shear stress (τ ). Theproportionality constant η is called dynamic vis-cosity and has the unit Pa · s.

τ = η · S

The laboratory determination of viscosity inrun-out or capillary tubes is influenced by theweight of the fluid. The relationship between dy-namic viscosity and specific gravity is referred

to as kinematic viscosity ν. The following unitanalysis applies:

ν =η

�

[Pa·s

kg·m−3

]or

[N·s·m3

m2·kg

]or

[kg·m·s−2·s·m3

m2·kg

]or

[m2

s

][

mm2

s

]or (Centistoke) (3.1)

Fluids which display the above proportionalityconstant between shear stress and shear rate arereferred to as Newtonian fluids, i.e., the viscos-ity of Newtonian fluids is independent of shearrate (→Fluid Mechanics). Deviations from thisNewtonianbehavior are sometimes referred to asstructural viscosity. Those viscosities are namedas apparent viscosities.

Influence of Temperature on Viscosity(V – T Behavior). The viscosity of all oilsused for lubrication purposes drops significantly


Figure 5. Improvement of hydrodynamic lubrication clearance between two rollers by Hertzian deformation (elastohydrody-namic contact, EHD contact), pressure distribution in the Hertzian contact

when their temperature increases. In linear sys-tems, this V –T behavior is hyperbolic and thepractical differentiation necessary in practice isdifficult to replicate and the interpolation bet-ween two measured viscosities is also problem-atic. For these reasons, V –T behavior has beenallocated to a functionwhich results in a straight-line graph if suitable coordinates are selected (Ubbelohde –Walter equation).

loglog (ν + C) = K − m·logT (3.2)

In this double-logarithmic formula, C and Kare constants, T is temperature in Kelvin and mis the V –T line slope. Figure 6 show the lin-ear and Ubbelohde –Walter double-logarithmicV –T curves of three oils with significantly dif-feringV –T lines (naphthenic oil, paraffinicHC-II / Group III oil / Chap. 4) and a natural veg-etable (rapeseed) oil.

The constant C for mineral oils in the V –Tequation is between 0.6 and 0.9.

The constant C only plays a minor role in theviscosity calculation, larger differences are onlyapparent at very low temperatures.

TheVogel – Cameron equation is used for therapid, computer-based calculation of dynamicviscosity. The Vogel – Cameron viscosity – tem-perature equation is:

η = A·exp(

B

T + C

)(3.3)

where A, B, and C are constants and T is againtemperature in Kelvin.

Them value in the Ubbelohde –Walter equa-tion which represents the double-logarithmicV –T graph slope is sometimes used to charac-terize the V –T behavior of oils. Them value forlubricant base oils is between 4.5 and 1.1 [54].The smaller values apply to oils which are lessaffected by temperature.


Figure 6. V –T behavior of various oilsA) Linear; B) Double-logarithmica) Paraffinic base oil; b) Naphthenic base oil; c) Rapeseed oil

Figure 7. Graphical illustration of viscosity index (VI)

m =loglog (ν1 + 0.8) − loglog (ν2 + 0.8)

logT2 − logT1(3.4.)

The viscosity – temperature constant (VTC) wasintroduced to better differentiate V –T behaviorwhen the influence of temperature is low.

V TC =ν40 − ν100

ν40(3.5)

The evaluation ofV –T behavior at low temper-atures according to Ubbelohde –Walter lines orother straight-lineV –T graphs often leads to in-accuracies. These dependencies do not apply to


base oils which can suffer thickening caused bycrystallization of some components (e.g., paraf-fins) at low temperatures or those whose poly-mer molecules simulate viscosity effects at lowtemperatures.

Viscosity Index. The viscosity index VI isfar more important for the description ofviscosity – temperature behavior than the mvalue or the viscosity – temperature constant.This value was first introduced in the USAin 1928. It was based on the then greatest(VI = 100) and the smallest (VI = 0) temperaturedependence of US base oils.

VI is graphically illustrated in Fig-ure 7. Values over 100 are calculated as VIE.

Viscosity index is also defined in the mostinternational standards.

Table 1 shows various V –T characteristicsfor a number of oils.

Viscosity – Pressure Dependency. The sig-nificance of viscosity – pressure dependency(V – p behavior) was, and still is, underestimatedfor numerous lubrication applications. V – p be-havior has become a part of the calculation ofelastohydrodynamic lubricant films.

V – p behavior can be described by the for-mula:

ηp = η1·eα(p−p1) (3.6)

where ηp is the dynamic viscosity at a pressurep, η0 is the dynamic viscosity at 1 bar and α isthe viscosity – pressure coefficient:

α =1ηp

·(

dηp

dp

)T

(3.7)

V – p dependence is defined by the chemi-cal structure of the substances with the stericgeometry of the molecules being of particularsignificance.

Figure 8 and 9 show viscosity – pressure re-lationship for a number of oils with differentchemical structures.

α can increase significantly with falling tem-perature which has an exponential effect on vis-cosity.

Viscosity – Shear Rate Relationship. Theviscosity of Newtonian fluids is a constant (pro-portionality factor) between the shear force τ

and the shear rate S and not dependent on shearrate. Lubricants which display a dependence onshear rate are known as non-Newtonian or fluidswith structural viscosity.

Oils containing polymers with specific ad-ditives or thickeners and mineral oils at lowtemperatures (long-chain paraffin effects) dis-play such structure-viscous behavior. At normalapplication temperatures, most major lubricantbase oils such as hydrocarbon oils (mineral oilraffinates or synthetic hydrocarbons), syntheticesters and natural fatty oils can withstand veryhigh shearing forces (e.g., 109 s−1) as foundin highly loaded machine elements (e.g., gear-boxes) and are independent of shear rate.

Engine oils containing polymer V –T im-provers (Chap. 8) or polymer ash-free disper-sants display structural – viscosity effects at lowand high temperatures.

While the reduction in viscosity caused by theshearing of structure-viscous fluids is reversible,polymer-based oils can suffer a permanent re-duction in viscosity. In these cases, the shear-ing forces lead to a mechanical change or re-duction in the size of the polymer molecules sothat their desired effects are minimized. Theseeffects have been observed, in particular, withmultigrade engine oils and high-VI hydraulicoils.

3.2. Special Rheological Effects

Apart from the above-described effects, lubri-cants are subject to further rheological phenom-ena and in particular, colloidal systems consist-ing of solid or fluid dispersions (solid disper-sions or emulsions).

Even small mechanical loading such as vig-orous stirring can cause a system to change com-pletely, e.g., pasty systems can break down intolow-viscosity systems (Fig. 10). If continuousmechanical load causes apparent viscosity to fallover time and the original viscosity is restoredafter a certain rest period, such fluids are calledthixotropic. This effect is used for sheet forminglubricants (Chap. 13).

Time-based viscosity changes can also becaused by the separation of colloidal particlessuch as paraffins from mineral base oils, or ad-ditives during the storage of lubricants. This un-desirable effect can occur during the cooling


Figure 8. Viscosity – pressure relationship of various oils1) Aromatic oil; 2) Naphthenic oil; 3) Paraffinic oil; 4) Biodegradable polyol ester

Table 1. Various V – T characteristics for several oils [54]

Kinematic viscosity mm2/s Viscosity index m Constant Viscositytemperatureconstant

40 ◦C 100 ◦C (VI) (VTC)

Naphthenic spindle oil 30 4.24 40 4.05 0.847Paraffinic spindle oil 30 5.23 105 3.68 0.819Medium solvent extract 120 8.0 −50 4.51 0.939Medium polyalkylene glycol 120 20.9 200 2.53 0.826Medium silicone oil 120 50.0 424 1.14 0.583Multigrade motor oil (SAE 10W-30) 70 11.1 165 2.82 0.841Ester oil 30 5.81 140 3.40 0.806

phase following production or by long-term stor-age at low temperatures. This separation processcan also be independent of temperature and becaused by the solubility status of additives in thebase oils.

Greases. The rheological description ofgreases is also complex because of their com-plex system structure (fluids, soap thickeners,solid thickeners, additives, Chap. 14). Themath-ematical description of apparent viscosity hasnot gained general acceptance in practice.

At lower shear loads, greases behave simi-larly to structure-viscous substances but at highshear loads, like Newtonian base oils.

3.3. Viscosity Grades

To simplify the classification of lubricants ac-cording to their application, viscosity gradeswere introduced which are now internationallyaccepted. ISO viscosity grades apply to indus-trial lubricants while Society of Automitive En-gineers (SAE) classifications apply to automo-tive engine and gear oils (see Fig. 11).

ISO Viscosity Grades. 18 viscosity gradesare laid down in the ISO standard 3448. Over therange from 2 to 2500mm2/s, these are the inter-national standard number series E6 rounded towhole numbers when the 6 numerals correspondto one power of ten (the first and fourth power of10 are reduced). The viscosity grades were also


Figure 9. Increase of the viscosity – pressure coefficient with falling temperature according to H. Holland [55]

Figure 10. Various lubricant structures (a, b, c) with high sensitivity against shear stress

adopted into or added to national standards suchas ASTM or DIN.

Viscosity grades are not used for all industriallubricants. Particularly oils for chip-forming andchip-less metalworking processes are not classi-fied in this way.

Apart from the viscosity grades, ISO 3448defines tolerances as well as median viscosities.

Other Viscosity Grades.Engine Oils. Todefine theviscosity of engine

oils, two or three viscosity thresholds were se-lected to define flow properties at low tempera-tures and to define a minimum viscosity at hightemperatures. Maximum viscosity at low tem-perature should ensure the rapid oil circulationto all lubrication points and permit a sufficiently

higher cranking speed for starting and the mini-mum viscosity at 100 ◦C should ensure that ad-equate lubrication of the bearings occurs at hightemperatures. The classification system was in-troduced by the SAE together with the Ameri-can Society for Testing and Materials (ASTM)and is used throughout the world. Low temper-ature viscosity is measured as dynamic viscos-ity with a specially constructed rotational vis-cosimeter (cold cranking simulator) at low shearrates (Chaps. 8 and 11).

Automotive Gear Oils. Specific SAE viscos-ity grades havebeen created for automotive gear-box, axle and differential oils. Compared withengine oils, the low temperature behavior ofthese oils is more heavily weighted.


Figure 11. Comparison of viscosity grades for various applications

Industrial Gear Oils. AGMA (AmericanGear Manufacturers Association) defines nineviscosity ranges for industrial gear oils.

Viscosity Grades for Base Oils. Mineralbase oils are traditionally classified accordingto Saybolt Universal Seconds (SUS). A 150 Nbase oil shows a viscosity of 150 SUS at 100 ◦F.

4. Base Oils

In terms of volume, base oils are themost impor-tant components of lubricants, they account formore than 95% of lubricant formulations. Thereare lubricant families (e.g., some hydraulic andcompressor oils) in which chemical additives


only account for 1% while the remaining 99%are base oils. On the other hand, other lubricants(e.g., somemetalworking fluids, greases, or gearlubricants) can contain up to 30% additives.

4.1. Historical Review and Outlook

Although the most important requirement ofbase oils in the 1950s was the correct viscos-ity and the absence of acidic components, baseoils in the 1960swere downgraded to solvents orcarriers for additives in the euphoria surround-ing chemical additives. In the 1970s, there wasa realization that some synthetic fluids with uni-form basic chemical structures offered perfor-mance superior to that of mineral base oils. Atthat time, the considerably higher price of theseproducts hindered their market acceptance. Inthe 1980s however, lower-price, quasi-synthetichydrocracked oils were introduced in WesternEurope which closely matched the properties ofsynthetic hydrocarbons (Shell, BP, Fuchs). In the1990s, base oil developmentswere influencedbythe ever-increasing demands on lubricant per-formance, and by environmental and health andsafety criteria. This led to chemically pure oilssuch as hydrocrackedproducts, polyalphaolefinsand esters gaining acceptance. Natural fatty oils,particularly their oleochemical derivatives, haveexperienced a renaissance because of their tech-nical characteristics but, above all, because oftheir rapid biodegradability.

The trend towards ever-greater performanceand even better environmental compatibilitywillcontinue in the first decade of the new millen-nium. The significantly higher price of the newlubricants, which will be increasingly charac-terized by their base oils and less so by theirchemical additives, will probably be acceptedby users who will benefit from long product lifeand lower overall system costs. In Western Eu-rope in 1998 [56], about 10 wt% of base oilswere synthetic products (including Group 3 hy-drocracked oils (Section 16.1). These, however,represented about 40% of base oil value. In Ger-many, furthermore, ca. 5% of lubricant base oilswere rapidly biodegradable (natural and syn-thetic) esters in 1999. It is safe to assume thatin Western Europe at least, synthetic and quasi-synthetic oils will in terms of value, have over-

taken traditional mineral base oils by the year2005.

4.2. Chemical Characterization ofMineral Base Oils

Crude oil generally consists of many thousandsof single components and these are reflected inthe processing of each fraction. Thus, mineraloil fractions have always been characterized bydefining their technical properties or to identifyand quantitatively determine groups of compo-nentswith similar chemical character. Advancedphysicochemicalmethods are, however, increas-ingly being used in routine testing.

Rough Chemical Characterization.Viscosity –Gravity Constant (VGC). This

value enables only rough chemical characteriza-tion of oils. Values near 0.800 indicate paraffiniccharacter whereas values near to 1.000 point to amajority of aromatic structure (ASTM 2501-91)

Aniline Point. The aniline point is also usedto characterize the hydrocarbon structure ofmin-eral oils (see Section 16.2.8).

Carbon Distribution. The most importantmeans of analysis for characterization of min-eral oil hydrocarbons is the determination ofcarbon in terms of its three categories of chemi-cal bond – aromatic (CA), naphthenic (CN), andparaffinic (CP).

N –D–M analysis uses physicochemicaldata which are easy to obtain. These includerefractive index, density, and molecular mass.Molecular mass can be determined by measur-ing the viscosity at different temperatures (e.g.,ASTM D 2502). Carbon distribution is givenin %CA, %CN and %CP (100% in total).N –D–M analysis also determines the averagetotal number of rings per molecule (RT) and thebreakdown into aromatic and naphthenic rings(RN) per molecule (RN =RT −RA).

Brandes [57] created a method of deter-mining carbon distribution according to spe-cific bands in the infrared spectrum (see Sec-tion 16.2.4). Exact determination of aromaticcarbon content can be performed by NMR(ASTM 5292).


Hydrocarbon Composition. To determinethe hydrocarbon composition of the base oil, thecomponenets are first separated by chromatog-raphy and the fractions are then subjected to ad-vanced analytical procedures.

To differentiate mineral oils in the boilingrange from 200 to 550 ◦C, high ionizing voltagemass spectrometry (ASTM D 2786-91) is usedfor saturated fractions and ASTM 3239-91 foraromatic fractions. Saturated fractions are sepa-rated into alkanes (0-ring), 1-ring, 2-ring, 3-ring,4-ring and 5-ring naphthenes. The aromatic frac-tions are subdivided into seven classes: monaro-matics, diaromatics, triaromatics, tetraaromat-ics, pentaaromatics, thiophenoaromatics, andunidentified aromatics.

Polycyclic Aromatics in Base Oils. Poly-cyclic aromatics (PAH, polycyclic aromatic hy-drocarbons, or, in general, PCA, polycyclic aro-matics) formed by the combustion of gasolinecan accumulate in engine oils. They can alsoaccumulate in quenching oils after long periodsof heavy-duty use.

In traditional solvent refining processes, PAHlargely remain in the extract. Nonsolvent ex-tracted distillates contain PAH in line with theirboiling point.

The carcinogenic characteristics of non-severely treated distillates in the petroleumindustry was established by the InternationalAgency for Research on Cancer (IARC) in 1983[58]. This led to considerable limitations in themanufacture and application of naphthenic baseoils and the ending of the use of aromatic ex-tracts as lube base oils.

PAH can be determined by HPLC with an-thracenes or other aromatics as markers or byGC–MS after appropriate sample preparation.

The IP 346method, adopted into national leg-islation in several countries, does not analyzePAH directly but an extract is obtained in di-methyl sulfoxide (DMSO), in which PAH accu-mulate. As a rule, the extract largely containsnaphthenes or mono-aromatics. According to IP346, DMSO extracts contain only 0.1% PAH.After numerous skin-painting tests on mice, thecarcinogenicity of petroleum products corre-sponds to the percentage of DMSO extract.

4.3. Refining (→Oil Refining)

Lubricant refineries are divided into integratedand nonintegrated plants (Fig. 12). Integrated re-fineries are linked to primary crude oil refineriesand are fed with vacuum distillate by pipeline.Nonintegrated refineries purchase vacuum dis-tillate on the open market or buy atmosphericresidues and perform their own vacuum distilla-tion. Occasionally they perform vacuum distil-lation on crude oil.

4.3.1. Distillation

By way of fractional distillation, products areremoved from crude oil which approximatelymeet the viscosity grades ultimately required.Often only four or five cuts suffice to fulfil lu-bricant requirements. The viscosity of the pri-mary vacuum distillate is not identical to that ofthe finished base oils formed in hydrocrackingprocesses.

After the corresponding separation of thelighter components from the crude oil by atmo-spheric distillation, the lubricant components arein the atmospheric residue. Figure 13 shows theyields of the various cuts in conventional lube oilrefiningwith the correspondingboiling ranges ofa typical lube crude. The atmospheric residue isnow subjected to vacuum distillation to removethe components required for lubricants.

4.3.2. Deasphalting (→Oil Refining,Chap. 3.6.5.1.)

The vacuum residue still contains highly viscoushydrocarbons which can supply valuable com-ponents for lube base oils. These highly viscousbase oils, commonly known as brightstocks, areseparated from the asphaltenes by extraction.Brightstocks are produced in lube oil refinerieswhen the use of the asphaltene byproduct (hardasphalt) is worthwhile. The quality of the hardasphalt for the manufacture of high quality bi-tumen depends on the crude oil. Deasphaltingis usually carried out with propane as solvent.Brightstocks can be manufactured with viscosi-ties of more than 45mm2/s at 100 ◦C.

Figure 14 shows a flow sheet for the manu-facture of suitable feeds for lube oil refining bydistillation and propane deasphalting.


Figure 12. Integrated (1) and not integrated (2) lube refineryA) Atmospheric distillation; B) Vacuum distillation; C), D) Processing of vacuum distillates for non-lube productiona) Fractionating vacuum distillation; b), c) Lube refining processes

4.3.3. Traditional Refining Processes

The vacuum distillates still contain compo-nents which can detrimentally affect aging,viscosity – temperature behavior and flowingcharacteristics, and components which are haz-ardous to health. To eliminate these compoundsseveral refining methods were developed, ofwhich solvent refining has become the most ac-cepted method over the past few decades. Newplants increasingly use hydrotreatment.

4.3.3.1. Acid Refining

Acid refining has become less popular becausethe acid sludgewaste produced is difficult to dis-pose of and this method has been replaced bysolvent extraction. Acid refining is still used tosome extent for the re-refining of used lubricat-ing oils (Chap. 18) and for the production of verylight-colored technical or pharmaceutical whiteoils and petroleum sulfonates as byproducts.

When the distillates are treated with con-centrated sulfuric acid or fuming sulfuric acid(oleum), substances which accelerate oil agingare removed.Oleum treatment (wet refining) notonly readily removes reactive oil componentssuch as olefins but also reduces the aromatic con-tent, which in turn increases the viscosity indexof the product. Reactions of oleum with satu-rated paraffinic structures lead to refining losses.Acid-refined oils require complex neutralizationand absorption follow-up treatment to remove alltraces of acid and undesirable byproducts.

4.3.3.2. Solvent Extraction

Solvent extraction (Fig. 15) creates base oilswhich are known as solvent raffinates or sol-vent neutrals (SN). Extraction processes usingsolvents create both a base oil and, after evap-oration of the solvent, an aromatic-rich extract.The selectivity of the extraction media for aro-matics is an important selection parameter.


Figure 13. Yield of the various cuts in conventional lube oil refining of a typical lube crude

Figure 14. Flow sheet for the manufacture of suitable feeds for lube oil refining by distillation and propane deasphalting


Figure 15. Solvent extraction

In particular the selectivity towards poly-cyclic aromatics has attracted attention becauseof the carcinogenicity of these compounds.Common extractants are furfural, N-methyl-2-pyrrolidone (NMP), and phenol. Sulfur dioxideshould also be mentioned because of its his-torical importance as B. Edeleanu introducedextraction to petroleum technology in 1912. In1999, sulfur dioxide was only used in very fewrefineries (one in Germany until 1999) for therefining of naphthenic distillates.

In recent years several furfural extractionplants have been converted into NMP plants,and even phenol plants have taken a back seat.NMP is a nontoxic solvent and can be used ina low solvent-to-oil ratio with high selectivity.This generates significant energy savings. NMPin new plants results in physically smaller unitsand thus lower capital expenditure.

Depending on the crude, the extract part ofparaffinic oils can be 30 to 50%. As the standardrequirement of solvent neutral oils is a viscos-ity index of at least 95, the extraction severity ismatched to this demand.

A higher proportion of aromatic and naph-thenic hydrocarbons in the distillates requiresgreater extraction severity and thus a largerquantity of extract. The percentage share ofthe extract is a major economic factor inconventional lube refining. Aromatic extractshave a very high viscosity – pressure coefficient

(Chap. 3) and in the past were often used inmetal-forming operations, such as cold extru-sion.

A 1-point increase in viscosity index as a re-sult of greater extraction severity creates, on av-erage, 1% more extract. Some refineries pro-duce so-called semiraffinates with VIs of bet-ween 70 and 80. It is cheaper to adjust theVI later with VI improvers than to produce15 or 20% more extract. These semiraffinates,however, have disadvantages resulting from thelower extraction severity – e.g., lower oxidationstability, possibly higher amounts of sulfur orhigher amounts of polycyclic aromatics.

Solvent extraction is generally only eco-nomic to a minimum VI of 50 of the vacuumgas oil [59].

4.3.4. Solvent Dewaxing (→Waxes,Chap. 4.2.3.1.)

In traditional refining processes, solvent extrac-tion is followed by solvent dewaxing. Long-chain, highmelting point paraffins (waxes) crys-tallize at low temperature and thus negatively af-fect the cold flowproperties of lube oil distillatesand lead to a high pour point. Their removal has,therefore, been an important consideration sincethe beginning of crude oil refining.

Dewaxing by crystallization of paraffins atlow temperatures and separation by filtration are


the principal processes in traditional refining.Compared with catalytic dewaxing with hydro-gen, urea dewaxing to separate n-paraffins is ofrelatively minor importance in lube oil refining.Crystallization methods involve mixing the sol-vent with the oil; this improves filtration, as aresult of dilution, and promotes the growth oflarge crystal formations.

Solvents used for dewaxing are ketones andchlorinated hydrocarbons.Ketone dewaxing (di-methyl ketone or methyl ethyl ketone, MEK) isnormally used for pour points down to −12 ◦C.For lower pour points, the Di –Me (dichloro-ethane – dichloromethane mixtures) method isused. This also enables the manufacture of hardand soft waxes. These waxes are only valuableproducts if their quality and processing is goodand they can then be sold, e.g., as candle wax orfor coatings. They are often used as cracker feedin fluid catalytic crackers (FCC) and the crackeryield is then assessed by use of special formulas.

Figure 16 illustrates the conventional solventdewaxing process.

4.3.5. Finishing

A finishing stage often follows extraction anddewaxing. In the past, methods which employedabsorbents were often used but these days pre-dominantly hydrofinishing processes are used.Finishing should improve the color of the prod-uct and remove surface-active substances whichcan negatively affect the air-release or demulsi-fying properties of a lubricating oil.

In general, the hydrofinishing process is re-ferred to as mild hydrotreating and focuses onimproving color, odor, and ultraviolet stability.Ferrofining (BP) has achieved major economicimportance as a finishing process. The processparameters do not generally lead to desulfur-ization. More severe processes with pressuresin excess of 100 bar can bring about significantdesulfurization and some dearomatization.

Additional finishingwith absorbents (bleach-ing clays, bauxite) is sometimes used for themanufacture of refrigerator, transformer, or tur-bine oils. The subsequent filtration process infilter presses or other filtering equipment repre-sents additional complexity and the disposal ofthe filter residue is an increasing problem.

Lube Crudes. The finished base oils have ahigh sulfur content, especially after conventionalsolvent refining. Hydrogenating processes al-most fully eliminate sulfur but desulfurizationconsumes hydrogen.

The vacuum gase oils (VGO) are the directfeed for lube refining. TheirVI , wax content, andsulfur content is especially important. A highVI leads to low extraction losses in solvent ex-traction and low hydrogen consumption duringhydrogenation. High wax content increases pro-duction costs.

Table 2 shows theVI andwaxcontent ofVGOfrom various lube crudes.

Table 2. VI and wax content of vacuum gas oil (VGO) from variouscrudes

Crude VI after solventdewaxing

Wax in VGO (wt%)

Pennsylvania 100 0Ordovician 85 13Brent 65 19Arab light 60 9Arab heavy 55 9Iranian light 55 16Lagomedio 45 10Urals 40 11Iranian heavy 40 16ANS 15 8

4.4. Base Oil Manufacturing byHydrogenation and Hydrocracking(→Oil Refining, Chap. 3.3., →Oil Refining,Chap. 3.5.)

Hydrogenation and hydrocracking in the manu-facture of lubricant base oils significantly in-fluence the chemical structures of mineral oilmolecules. Unstable molecules are chemicallystabilized by the removal of the heteroatoms(sulfur, oxygen, nitrogen) and severe hydrogen-ation can convert aromatics into saturated naph-thenic or paraffinic structures. In addition to thehydrogenation process, hydrocracking breaksdown or cracks larger molecules into smallerones. Larger molecular structures can re-formfrom small fragments. The principal process cri-teria are temperature, pressure, the catalyst andspace velocity. If special conditions are met, afocal point of the process is the isomerizationof paraffinic structures. Beside the saturation of


Figure 16. Conventional solvent dewaxing process

aromatics, opening of the naphthenes rings canoccur.

Lubricant base oils can be much more eas-ily tailored using these processes than is pos-sible with simple solvent refining separation.The future of lube base oil production thus lieswith hydrogenation and hydrocracking. An ad-ditional advantage of advanced hydrocracking isthe lower dependence on the quality of the crudeoil.

In addition to hydrocracking and hydroisom-erization, the following typical chemical pro-cesses occur during hydrogenation (Table 3).

While the processes mentioned are generallyreplacing solvent extraction for the manufac-ture of base oils, there are hydrogenation pro-cesses which either crack long-chain paraffinswith specific catalysts into light products and re-move them from the base oil or convert them intoisoparaffins with good low-temperature charac-teristics without a significant loss in yield. If thepreviously mentioned hydrogenation processesare combined with this type of catalytic hydro-genating dewaxing, this is called the ‘all-hydro-gen route’.

4.4.1. Manufacturing Naphthenic Base Oilsby Hydrogenation

Only about 10% of the petroleum base oilsused in lubricants are naphthenic. Before 1980,

naphthenic oils were significantly more impor-tant (USA 25%). In 1983, a publication by theIARC defined a number of mineral oil productsas carcinogenic, among them naphthenic oils.Although the IARC publication and subsequentlegislation and classification requirements led tothe disappearance of this type of oil from impor-tant markets, naphthenic base oils were still re-quired for some lubricant applications (greases,metalworking oils, and refrigerator oils) becauseof their outstanding solubility and these are nowmanufactured with severe hydrogenation whichproduces noncarcinogenic base oils. Figure 17shows a flowchart for the manufacture of naph-thenic base oils. Severely hydrotreated Group Cdistillates are themost accepted naphthenic baseoils in western markets.

4.4.2. Production of White Oils

White oils – light-colored, odorless, low-aromatic or aromatic-freemineral oil raffinates –are used in a number of applications as medicaland pharmaceutical white oils or technical whiteoils in, above all, the food and beverage indus-tries as food-grade lubricants. BASF Germanywas the first to develop white oil manufactur-ing from the high-pressure hydrogenation tech-nology used to liquefy coal. Up to that point,white oils were manufactured exclusively byacid treatment.


Table 3. Typical chemical processes during hydrogenation

Desulfurization MercaptansR – SH → RH+H2SDisulfidesR – S – S –R → RH+H2SSulfidesR – S –R → RH+H2SThiophene

Denitrogenation

Polyaromatic saturationViscosity Index: (a) −60; (b) 20Pour point: (a) +50 ◦C; (b) +20 ◦C

Naphthenic ring openingViscosity Index: (a) 20; (b) ∼130Pour point: (a) +20 ◦C ; (b) −10 ◦C

IsomerizationViscosity Index:(a) 125; (b) 125Pour point: (a) +20 ◦C; (b) −40 ◦C

Figure 17.Manufacture of naphthenic base oils


The feeds for hydrogenated white oil man-ufacturing are generally naphthenic or paraf-finic solvent-treated vacuum distillates or un-treated vacuum distillates. In the BASF pro-cess hydrogenation takes place in two stages:the first at 300 – 380 ◦C and 80 – 200 bar with aNiMo catalyst and the second at 200 – 300 ◦Cand 100 – 200 bar with an Ni catalyst. The firststage produces the technical qualities (accordingto theU.S. Food andDrugAdministration)whilethe second stage produces the medical grades(e.g., German Pharmacopeia). Figure 18 showsa flow chart for white oil manufacture [59].

4.4.3. Lube Hydrocracking

The principal elements of the lube hydrocrack-ing process are the cracking of low-VI compo-nents and the saturation of aromatics.

Hydrocracking can be performed with theprincipal objective of producing lubricating oilsand to convert vacuum gas oil into high-gradefuels. Fuel hydrocracker residues are excellentfeeds for the manufacture of lubricant base oilsand hydrocrackers used to make olefin feedsfor steam crackers can supply a premium qual-ity feed for lube base oils. The severity hasthe most important effect on the quality of thebase oil produced. High severity (e.g., 80%light products) generates a high-VI and low-evaporation oil. While hydrocracking refineriesfor base oils with normal VIs and normal Noackevaporation (high viscosity index, HVI oils,Group II oils, Section 16.1) at moderate sever-ity are being built in the USA and the Far East,based mainly on Chevron Technology, West-ern European hydrocracking plants were de-signed for low-evaporation base oils (Shell, PetitCouronne, France, 1972; Union Fuchs, Wessel-ing, Germany, 1986 and BP, Lavera, France).These produce Group III oils, both from waxesand from vacuum gas oils with hydrocrackerresidues.

Hydrocracked base oils differ from solvent-extracted oils by their extremely low aromaticcontent and their chemical purity, i.e., onlytraces of heteroatoms such as sulfur, nitrogen,or oxygen are present. Furthermore, Group IIIoils which were manufactured at high severityor from waxes have Noack evaporation charac-teristics which are about 50% lower than equiv-

iscous, solvent-extracted vacuum distillates. Ifcatalytic dewaxing is performed, even lowerpour points can be achieved than with SN oils.

Since 1995, more than half of all new baseoil manufacturing plants have been built or areplanned using hydrocracking technology.

The increasing number of fuel hydrocrack-ers, which are a response to increasing demandsfor low-sulfur oils (diesel fuels with < 50 ppmsulfur), represents a growing source of hydroc-racked feeds. Fuel hydrocracker residue can beprocessed into high-quality, hydrocracked baseoils by distillation and subsequent dewaxing(wax isomerization) at acceptable cost.

During the hydrocracking process, especiallyif vacuum gas oils are severely treated to cre-ate very high viscosity index (VHVI) oils, poly-cyclic aromatics can be formed along with sat-urated structures (aromatics to naphthenes orparaffins and isoparaffins) under some processconditions. These have to be removed by subse-quent high-pressure hydrogenation or by extrac-tion if hybrid methods are used. This subsequenthydrogenation can also be manipulated so thatit also produces a significant increase in VI .

4.4.4. Catalytic Dewaxing

The most complex stage of traditional base oilrefining is solvent dewaxing. In addition to thehigh capital expenditure and, above all, operat-ing cost, the solvent-related limits to the achiev-able pour point are a disadvantage.

Thus, various methods have been developedwhich can removeunbranched and less branchedparaffins, and someother petroleumcomponentsby catalytic and hydrogenation reactions or con-vert them into components which improve thelow-temperature characteristics of the base oil.

The first technologies were based on the cat-alytic cracking. In 1979, Mobil introduced theMobil lube dewaxing process (MLDW). Thedevelopment of catalysts since then has ledto strong hydroisomerization activity processes(Mobil selective dewaxing, MSDW).

While the MLDW process mainly involvesthe cracking of long-chain paraffins and the pro-duction of larger quantities of light byproductsand poor base oil yields, MSDW processes cre-ate high-yield, nonwaxy isoparaffins from thewaxy parts of the feed.


Figure 18. Flow chart for white oil manufacture (BASF)

At the beginning of the 1990s, Chevron suc-cessfully introduced its isodewaxing process. In1999 it was the most important technology forcatalytic dewaxing [59]. The process combinesrelatively high yields with low pour points andhigh VIs. The method uses an intermediate-poresilicoaluminumphosphate molecular sieve (e.g.,Pt on SAPO-11) as catalyst. In terms of processeconomy, high base oil yields but also thefor-mation of mainly high-value C5+ liquids as by-products are of significance.

Dewaxing catalysts must possess the rightbalance between hydrogenation activity andacid activity. Increasing hydrogenating proper-ties usually leads to a reduction in paraffin iso-merization and thus a rise of the pour point.Higher acid activity increases cracking and thusyield losses. Sulfur and nitrogen in the feed playimportant roles. Nitrogen is detrimental to acidactivity and sulfur is catalyst poison.

Isodewaxing in principle enables manufac-ture of lubricant base oils with pour points below− 45 ◦C, however, dewaxing to very low pourpoints (depending on process conditions and cat-alysts) leads to VI losses.

A major success of this new dewaxing pro-cess are the molecular sieve catalysts – zeo-lites with exactly defined mesh sizes. Althoughhighly branched paraffins or polycyclic naph-thenes or aromatics are not affected by the ze-olites, long-chain less or unbranched moleculesare isomerized by the catalysts [60–62].

Figure 19 shows the flow chart for cat-alytic dewaxing (hydrodewaxing) including ahydrofinishing reactor.

4.4.5. Wax Isomerization

VHVI oils have been manufactured from waxcuts from solvent extraction processes since theearly 1970s. The conversion of long-chain nor-mal paraffins or less branched paraffinic compo-nents into isomerized products with good low-temperature characteristics and high VIs suc-ceeded, with suitable catalysts, in isomerizinghydrocracking processes. Because the input feedis already aromatic- and naphthene-free, the cat-alysts can be fully optimized to the conversion ofthe paraffinic material. At relatively high yieldlosses, the process creates high-quality but com-paratively expensive hydrocracked oils (ShellXHVI, Exxon Exxyn, Mobil MWI-2 catalyst).

4.4.6. Hybrid Lube Oil Processing

The combination of traditional solvent refin-ing with severe hydrotreating and hydrocrack-ing processes is known as hybrid processing. Tocreate higher VIs (> 105) and to reduce sulfurcontent, hydrotreating can follow solvent extrac-tion (e.g., in the manufacture of turbine oils).

The combination of amild furfural extractionwith hydrocracking can produce hydrocrackedoils by low-hydrocracking conversion in smallreactors. The low hydrogen consumption addsto the economy of such processes. The introduc-tion of a hydrocracking stage into a conventionalsolvent refinery offers the attractive possibilitiesof de-bottlenecking if the corresponding dewax-ing capacity is available.

Catalytic dewaxing can, on the other hand,follow solvent refining. This presupposes that


Figure 19. Flow chart for catalytic dewaxing (hydrodewaxing) including hydrofinishing

the catalytic dewaxing catalysts are not poisonedby the sulfur and nitrogen components in thesolvent raffinate. In the Mobil processes, thisis more likely with MLDW catalysts than withMSDW catalysts.

Several other high-paraffin componentsshould be considered as future base oil feeds.These could include high-wax (> 70%) naturaldistillates and Fischer – Tropsch waxes or syn-thetic fluids from natural gas.

The quality of low-quality lube feeds (VGO)can be improved by hydrogenation before sol-vent extraction (Hy-starting or Hystart).

4.4.7. All-Hydrogen Route

The production of base oils by hydrocrackingand catalytic, solvent-free dewaxing is calledthe all-hydrogen route. Figure 20 shows a flowsheet for such a refinery used to manufactureVHVI oils. Depending on the severity of the hy-drocracker, Group II HVI or Group III VHVIoils can be produced (HC-I and HC-II oils, Ta-ble 4). Leading examples for HC-I oils are theChevron refinery in Richmond, California andthe Conoco – Pennzoil refinery in Lake Charles(Exel Paralubes) [63]. Some HC-I refinerieshave been prepared to produce HC-II oils by theall-hydrogen route by increasing the severity ofthe hydrocracker. Another example of a hydro-cracker refinery is the Petro-Canadas plant [64].

The use of a fuel hydrocracker for the pro-duction of base oils using the all-hydrogen routewas first realized by the SK Corporation (Ulsan,Korea) in 1995. By recycling the hydrocrackerbottom and the special integration of the fuel hy-drocracker in the lube oil process, SK also devel-oped a specific method (UCO process, Fig. 21)[65].

4.4.8. Gas-to-Liquids ConversionTechnology

As a result of efforts to increase the valueof natural gas in logistically favorable loca-tions, the chemical liquefaction of natural gas(also the chemical reaction route)was developed(on the basis of the Fischer – Tropsch process;→Waxes, Chap. 5.). This process creates high-quality liquid products and paraffin wax. High-quality UHVI oils can then be obtained fromnatural gas by part oxidation, polymerization,and isomerization [66], [67].

The base oil market could undergo dra-matic changes if Fischer – Tropschwaxes,whichare part of gas-to-liquids technology, becomemore generally available. Shell has used suchwaxes from its Bintulu plant in Malaysia for itsXHVI’s. New technologies for smaller, efficientplants have been developed by Entech and Syn-troleum in the last years.


Figure 20. All-hydrogen route in Chevron’s Richmond (California, USA) refinery

Table 4. API/ATIEL classification of base oils.

Group Sulfur (wt%) Saturates (wt%) Viscosity index

I > 0.03 and/or < 90 80 – 120I > 0.03 and/or < 90 80 – 120II ≤ 0.03 and ≥ 90 80 – 120III 0.03 and 90 > 120IV All polyalphaolefins (PAOs)V All base oils not included in Groups I – IV

Figure 21. VHVI base oil production based on fuel hydrocracker residue (SK Corporation, Korea)


4.5. Boiling and Evaporation Behaviorof Base Oils

The target of base oil distillation is the viscositydesired at 40 ◦C and 100 ◦C. The same distilla-tion cut (same boiling point distribution) withdifferent chemical structures leads to differentviscosities. A highly naphthenic cut produces ahigher viscosity than a paraffinic cut. In otherwords, equiviscous cuts of different chemicalstructures have different boiling-point distribu-tions.

Figure 22.Distillation curves of three paraffinic solvent re-fined cutsA) Light cut (low boiling); B) Medium light cut; C) Heavycut (high boiling); D) Blend of A and C, equiviscous withB

In practice, ISO viscosity grades or other re-quired viscosities are created by blending dif-ferent cuts. If the boiling points of both cuts aretoo far apart, the flash point drops significantlyand evaporation increases. Figure 22 shows the

distillation curves of three paraffinic solvent re-fined cuts. One line is a blend of cut A withcut C which creates an equiviscous oil with cutB. The share with a boiling range up to 380 ◦Cincreases from 10% (equiviscous cut) to 20%(equiviscous blend).

Apart from the use of a variety of laboratorydistillation apparatus to determine the boilingpoint distribution of a base oil, gas chromato-graphic methods of determining boiling pointhave gained popularity over the last few years.Both nonpolar packed and capillary gas chro-matographic columns can be used. The injectionport temperature is between 360 and 390 ◦C de-pending on the length of the column. The initialcolumn temperature can be as low as − 50 ◦C,the final column temperature is between 360 and390 ◦C and the programming rate is usually be-low 10 ◦C/min.

In recent years, the evaporation characteris-tics of lubricants have become increasingly im-portant quality criteria. The reasons for this arethe emissions created when a lubricant evapo-rates, and the accompanying change to the lu-bricant’s composition. The topic of evaporationbehavior has become increasingly important inline with the trend towards lower viscosity oilsfor most applications (energy-saving oils).

As a simple laboratory method, Noack evap-oration (1 h at 250 ◦C) has become the estab-lished method of characterizing evaporation be-havior in lubricant specifications with the evap-oration losses being given in wt%. Gas chro-matographicmethods are also used but these canproduce somewhat deviating results.

This led to a simulated Noack evaporationmethod using gas chromatography. In the USA,a gas chromatographic process was developed todetermine engine oil volatility (ASTM D5480).In this case, evaporation takes place at 371 ◦C(700 ◦F) and the evaporating components aremeasured to ca. n-C22. Although this process iseasily reproducible, the values obtained are notcomparable with those from the Noack method.

Figure 23 shows typical evaporation lossesfor well-cut paraffinic solvates; values for VHVIoils (HC-II oils, API Group III oils and typicalester base oils (Chap. 5) are shown for compar-ison.


Figure 23. Typical evaporation losses (Noack evaporation) of various lube base oils (250 ◦C, 1 h)

5. Synthetic Base Oils

Classes of synthetic lubricants according toGunderson and Hart [68] and Shubkin et al.[69] are listed in Table 5.

Table 5. Classes of synthetic lubricant

Gunderson and Hart (1962)[68]

Shubkin et al (1993) [69]

Alkylated aromaticsChlorofluorocarbon polymers Chlorotrifluoroethylene

CycloaliphaticsDialkylcarbonates

Dibasic acid esters EstersFluoro estersNeopentyl polyol esters

PerfluoroalkylpolyethersPhosphate esters Phosphate esters

PhosphazenesPolyalphaolefinsPolybutenes

Polyalkylene glycols Polyalkylene glycolsPolyphenyl ethers

SilahydrocarbonsSilicate estersSiloxanes Siloxanes

Although many of the synthetic base oilsavailable today had been developed decades ago,their use on a large technical scale has increasedonly slowly because of their considerably highercost [70], [71] (Table 6).

Table 6.Relative costs of synthetic base oils (relative to 1 formineraloils in 1999)

Type Relative cost

Alkylated aromatics 2−3Polybutenes 3−5Dibasic acid esters 7−15Polyalkylene glycols 7−20Polyalphaolefins 7−25Neopentyl polyol esters 10−20Siloxanes 40−200Perfluoroalkylpolyethers 400−800

However, the consumption of polyal-phaolefins, the most common synthetic lubri-cating oils, has increased enormously since the1980s [72–74].

Synthetic base oils usually are prepared by re-action of a few defined chemical compounds – inmany cases based on petroleum also – and tai-lored to their application by the right choice ofreaction conditions.

Synthetic base oils have been classified ac-cording to both the production process and theirchemical composition [71], [75]. From a chemi-cal point of view the latter method is preferable.

5.1. Synthetic Hydrocarbons

Synthetic hydrocarbons were developed simul-taneously in Germany and the United States. InGermany low-temperature performance and the


need to overcome the general shortage of petro-leum base stocks was the driving force [76]. Allsynthetic hydrocarbons – and the other econom-ically important synthetic lubricating oils – canbe synthesized starting with ethylene.

5.1.1. Polyalphaolefins

The term polyalphaolefin (PAO) is derived fromthe source of this class of base oil, usually α-decene or a mixture of α-olefins containing,in general, a minimum of six and a maximumof twelve carbon atoms. Linear α-olefins werefirst used for the synthesis of lubricating oilby Montgomery, Gilbert, and Kline [77].Polyalphaolefins have been described in detailin [74].

Oligomerization of α-olefins is gener-ally carried out by boron trifluoride-basedFriedel – Crafts oligomerization with alcoholsas co-catalysts. Possible mechanisms have beendescribed in detail in [78]. The boron trifluo-ride method has become predominant for theproduction of grades with lower viscosities,2 – 10mm2/s. For higher viscosities, 40 and100mm2/s, other catalysts must be used, andfor the production of grades with viscosities of10 – 25mm2/s, C-12 and C-14 olefins can beused; these yield mainly dimers [79]. In the nextstep the unsaturated olefins obtained are hydro-genated, using, e.g., nickel on kieselguhr or pal-ladium on alumina as catalysts. In a third and fi-nal step the saturated oligomers are distilled. Theoligomerization of α-decene or α-olefin mix-tures always results in complex mixtures of iso-mers with more branching than expected. Be-cause the rearrangements are intramolecular, themolecular masses of the products can be keptwithin a narrow range. A typical saturated α-decene trimer for example looks like a three-pointed star [80] (Fig. 24).

Polyalphaolefins therefore have several ad-vantages – narrow boiling ranges, very low pourpoints, VI > 135 for all grades with kinematicviscosity > 4mm2/s at 100 ◦C. Their volatilityis lower than that of all possible and equivis-cous mineral oil grades and they contain onlysmall quantities of unsaturated and polycyclicaromatic compounds andonly traces of nitrogen,sulfur, or other impurities. Although in someoxidation tests without additives some mineral

oils seem superior to polyalphaolefins, this isbecause of the presence of natural antioxidantsin the mineral oils that have survived the re-fining process. The response of the syntheticproducts to antioxidants and their extreme pres-sure/antiwear synergists is better [81–83] thanthat of mineral oils. Their low polarity, on theother hand, leads to poorer solvency for very po-lar additives and can cause problems with seals.Therefore, PAOs tend to be used in combinationwith smaller amounts of (dicarboxylic acid) es-ters or solvent-refined mineral oils.

Polyalphaolefins have been used traditionallyin aerospace and lifetime applications, but todaythey are used in a wider variety of applicationsand have gained additional importance becauseof the increasing need for them in automotivelubricants [84].

5.1.2. Polyinternalolefins

Polyinternalolefins (PIO) are rather similar topolyalphaolefins. Both kinds of hydrocarbonare prepared by the oligomerization of lin-ear olefins. The difference is that polyinter-nalolefins aremade from cracked paraffinic basestocks. Plants for the manufacture of polyinter-nalolefins and polyalphaolefins therefore looksimilar (Fig. 25). Internal olefins are more dif-ficult to oligomerize and the resulting productshave VIs 10 to 20 units lower than the VIs ofequiviscous polyalphaolefins [85].

Mixtures of polyalphaolefins with polyeth-ylenes with kinematic viscosities from 100 to2000mm2/s at 100 ◦C are available commer-cially.

While the cationic polymerization of ethyl-ene with aluminium chloride yields oils withVIs up to 120 and molecular masses from 400to 2000, the polymerization of propylene givesoils with lower VIs and poor thermal stability.Both kinds of lubricating oil have lost impor-tance, but copolymers of both base materials aresaid to have the potential for gaining importanceagain.

5.1.3. Polybutenes (→Polyolefins, Chap. 4.)

Polybutenes (PBs) as synthetic base oils havebeen described in detail in [86]. They consist


Figure 24.Molecular structures of typical polyalphaolefin oligomers

Figure 25. Schematic of a polyinternalolefin (polyalphaolefin) plant


mainly of isobutene and are, therefore, often alsoknown as polyisobutenes (PIBs).

Polybutenes are produced by Lewis-acid cat-alyzed polymerization of the raw C4 stream thatbeside isobutene contains the other two butenesand the butanes (→Butenes). The process yieldsa copolymer with a backbone built mainly fromisobutene units. The higher the molecular mass,the lower the content of other butenes, that is,the lower the molecular mass the more complexthe structure. Polybutenes contain one doublebond at the end of the carbon chain. Therefore,polybutenes are less resistant to oxidation thanpolyalphaolefins, polyinternalolefins, and alky-lated aromatics – above 200 ◦C they begin todepolymerize and form gaseous products.

Polybutenes with molecular masses fromabout 300 to 6000 are important asVI improvers.They are used as components in two-stroke oils,gear and hydraulic oils, metal working lubri-cants, greases, compressor lubricants, and wire-rope protectives [87]. One of the options for fu-ture polybutene applications is the increased useas synergistic components in binary or ternarybase oil systems [88].

5.1.4. Alkylated Aromatics

Alkylnaphthalenes as lubricants have been avail-able in Germany since ca. 1930 [89], [90].Low-cost dialkylbenzenes are produced as by-products in the manufacture of linear andbranched monoalkylbenzenes that themselvesare feedstocks for themanufacture of detergents.The Friedel – Crafts alkylation of benzene witholefins tends to yield polyalkylated products,because of the reactivity of the primary prod-uct. With excess benzene, a suitable olefin, e.g.,propene, and the right choice of a catalyst thereaction can be controlled and specialized alkyl-benzenes can be synthesized.

Lower-cost dialkylbenzenes are used in awide variety of industrial areas, e.g., in trans-former oils, where high resistance to gas evolu-tion is important. Tailored grades, especially thelinear products, although having poorer proper-ties than polyalphaolefins, still are an option be-cause of their excellent solvency, their suitabil-ity for low temperatures, and their compatibilitywith elastomers [91]. The most widespread useof these products is, however as refrigerator oils.

5.1.5. Other Hydrocarbons

Cyclic saturated or alicyclic hydrocarbons(naphthenes) occur naturally as components ofmineral oils. Naphthenes do not yet have compa-rable synthetic analogs available on a technicalscale.

5.2. Halogenated Hydrocarbons

Chlorinated hydrocarbons are very stable andnot combustible. They were once widely usedas insulating oils, heat-transfer fluids, and hy-draulic fluids, but this use has been discontin-ued because of the environmental problems theycause.

Commercial liquid chlorofluorocarbons,all oligomers of chlorotrifluoroethylene(→Fluorine Compounds, Organic, Chap. 4.7.),contain between two and twelvemonomer units.They are suitable for use as lubricantsmainly be-cause of their extraordinary chemical stability,including against oxygen.

Their low corrosivity, the good low-tempera-ture characteristics of the low-viscosity grades,and their lubricating properties are also advan-tageous. Disadvantages are their high volatilityand their not ideal V –T dependence. Their den-sity naturally is relatively high (between 1.7 and2 g/mL at 40 ◦C). Chlorofluorocarbons are usedas lubricants in oxygen compressors, in pumpsfor mineral acids, halogens, and oxygen, and inmills ormixers for strong oxidizing agents. Theyare also suitable lubricants for turbine pumps inrocket engines and as base oils for non-flamma-ble hydraulic fluids [92].

5.3. Synthetic Esters

5.3.1. Esters of Carboxylic Acids (→Esters,Organic)

Synthetic ester lubricants for jet engines weredeveloped inGermany duringWorldWar II [93].


Carboxylic groups, because of their strongdipole moments, reduce the volatility and in-crease the flash point of lubricating oils, and atthe same time positively affect thermal stability,(the bonds of the COOgroup are thermallymorestable than the C –C bond), solvency, lubricity,and biodegradability. On the other hand, how-ever, they negatively affect the hydrolytic stabil-ity of a lubricant and the reactivity with metalsor alloys that contain copper or lead [94].

5.3.1.1. Dicarboxylic Acid Esters [95]

Two types of ester molecule or their combina-tions have been found to be most suited as lubri-cant components:

1) Esters of branched primary alcohols withstraight-chain dicarboxylic acids, and

Di(2-ethylhexyl)adipate

2) Esters of straight-chain primary alcoholswith branched dicarboxylic acids

Diundecyl(2,2,4-trimethyl)adipate

The alcohols that are needed for the pro-duction of dicarboxylic acid ester oils can beobtained synthetically by the hydroformylationof olefins with carbon monoxide and hydrogen(→Oxo Synthesis). The dicarboxylic acids canbe obtained either by oxidation of vegetable oils,e.g., castor oil, as for azelaic and sebacic acid[96], or by cleavage of corresponding alicyclichydrocarbons with oxygen, as for adipic acid[97].

Esters of straight-chain dicarboxylic acidshave better V –T characteristics than mineraloils, and higher VI values. The values decrease

with increasing branching, but branching im-proves the low-temperature properties. Esterswith little branching have the best properties,especially esters with methyl groups adjacent tothe carboxyl group.

Dicarboxylic acid ester oils can improve theV –T characteristics of automotive engine oilswithout having a negative effect on the low-tem-perature viscosity or the Newtonian flow behav-ior. Such oils are suitable for diesel engines also.Esters have also gained importance as lubricityadditives in greases and in polyalphaolefins asadditives for the improvement of the compati-bility with elastomers.

5.3.1.2. Polyol Esters

Since the 1940s neopentyl glycol ((2,2-di-methyl)-1,3-propandiol), trimethylolethane and-propane (1,1,1-tris(hydroxymethyl)ethane and-propane), and pentaerythritol (2,2-bis(hy-droxymethyl)-1,3-propanediol) have been usedas the alcoholic components of esters [98]. Be-side excellent thermal andoxidation stability, theesters have good V –T and lubricating proper-ties and excellent viscosity characteristics at lowtemperatures.

The mechanism of the polymerization ofpolyol esters during aging has been studied [99]and benzotriazole derivatives have been pro-posed as multifunctional additives [100]. Sincethe 1960s the polyol esters have gained impor-tance as lubricants for high-temperature appli-cations, for example in aircraft engines that aremeant for speeds beyond Mach 2, and they havebecome even more important as biodegradableesters.

5.3.1.3. Other Carboxylic Esters

Esters of some aromatic polycarboxylic acids(1), mainly phthalic acid (2), trimellitic acid,and pyromellitic acid (3), with monofunctionalalcohols are also used as base oils. The latterare suited as high temperature lubricants. Thetime dependence of the low-temperature viscos-ity of some grades is rather pronounced. There-fore their use in low temperature applications isproblematic.


1

2

3

Monoesters of fatty acids and monofunc-tional alcohols are used in metal working. Thesulfurized esters are excellent extreme pressureadditives for all kinds of lubricant [101].

Dimeric acid esters, formed by dimerizationof esters of oleic or tallow fatty acid, are used asengine oil and thickener components.

5.3.1.4. Complex Esters

Esters that contain straight or branched diols orpolyalkylene glycols and straight or brancheddicarboxylic acids as well as (mono)carboxyl-ic acids and monofunctional alcohols have at-tracted interest. Usually first the diol is esteri-fied with the dicarboxylic acid, then, dependingon the desired product, the intermediate prod-uct is reacted with either a carboxylic acid or amonoalcohol. Two types of complex ester havebecome important:

A)

ROH · · · HOOCrCOOH · · · HORrROH· · · HOOCrCOOH · · · HOR

B)

RCOOH · · · HORrROH · · · HOOCrCOOH· · · HORrROH · · · HOOCR

The same scheme is valid when neopentanederived polyols [102] or glycerol [103] are in-volved.

Complex esters have higher molecularmasses and higher viscosities than the commonesters. Esters of type B are superior to type Awith regard to flash point, pour point, and low-temperature viscosity. In type A polyalkyleneglycols lead to lower pour points than aliphaticglycols.

High molecular mass complex esters of thetype:

R · · · (HOrOH · · · HOOCrCOOH)n · · · R

have also been named polymer esters and findusemainly in applications wheremineral oil andsolvent resistance is required.

5.3.1.5. Fluorinated Carboxylic Acid Esters

Esters of fluorinated acids are readily hydro-lyzed and the free acids have a strong oxidiz-ing effect, therefore, only esters of nonfluori-nated carboxylic acids with fluorinated alcoholscan be used as lubricants. The thermal stabilityand the oxidation stability of fluorinated estersis better than that of the nonfluorinated analogs.Some types of rubber shrink in their presenceand some antioxidants have a negative effecton their stability. Fluorinated esters were in thefirst place intended for high-temperature appli-cations [104]. It can be assumed that their usewill be limited because of competition from thefluorinated polyethers.

5.3.2. Phosphate Esters (→PhosphorusCompounds, Organic, Chap. 10.) [105], [106]

Tertiary esters of phosphoric acid with alcoholsor phenols were first prepared 150 years ago butwere not introduced as antiwear additives be-fore 1930. They have also gained importance asplasticizers, fire-resistant hydraulic fluids, com-pressor oils, and synthetic lubricants. They areusually divided into triaryl, trialkyl, and alky-laryl phosphates and are made by reaction ofphosphoryl chloride with phenols or alcohols.Their properties range from low-viscosity fluids


to high-melting solids. With increasing molecu-lar mass trialkyl phosphates change from water-soluble to insoluble liquids. The triaryl phos-phates have higher viscosities and are insolublein water. An aryl side-chain reduces the meltingpoint. The properties of alkylaryl phosphates arebetween those of alkyl and aryl phosphates.

3ROH+POCl3 → OP(OR)3 + 3HCl

The hydrolytic stability varies between goodand poor.Alkylaryl phosphates aremore suscep-tible to hydrolysis than trialkyl or triaryl phos-phates. Phosphoric acid esters are less stablethan carboxylic acid esters, but more stable thansilicic or boric acid esters.

The thermal stability of triaryl phosphates isbetter than that of the alkyl compounds. Withaminic antioxidants and rust inhibitors, triarylphosphates can be used up to 175 ◦C, but alkyl-diaryl phosphates only up to 120 ◦C. Branchingof the alkyl radicals reduces the thermal stabil-ity, the effect becomes stronger with decreasinglength. In general phosphoric acid esters are notcorrosive. The high spontaneous ignition tem-peratures of up to 600 ◦C underline the good fireresistance of some products. Trialkyl and alky-laryl products have pour points down to− 65 ◦C,when they contain VI improvers.

The lubricating properties of the phosphatesare excellent, particularly on steel. They can beblended with almost all lubricants and additives.Their dissolving power is, on the other hand, re-sponsible for their incompatibility with rubbers,varnishes, and plastics. Nylon, epoxy and phe-nol – formaldehyde resins are stable.

5.4. Polyalkylene Glycols(→Polyoxyalkylenes) [108–110]

The first polyalkylene glycols suitable as lubri-cants were developed during World War II. Thefirst patent was probably published in 1947 byRoberts and Fife [107].

Polyalkylene glycols are prepared by the re-action of epoxides, usually ethylene and propyl-ene oxide, with compounds that contain activehydrogen, usually alcohols or water, in the pres-ence of a basic catalyst, e.g., sodium or potas-sium hydroxide. Polymers with statistically dis-tributed alkylene groups are made by use of a

mixture of alkylene oxides. Separate additionleads to block copolymers. Because ethyleneoxide is more reactive than propylene oxide,random copolymers tend to have the propyleneoxide units at the chain ends.

Terpolymers with, for example, tetra-hydrofuran have also been prepared. Pure tetra-hydrofuran polymers can be obtained by poly-merization of tetrahydrofuran in the presenceof Friedel – Crafts catalysts. They are colorlessoily or waxy substances of very low toxicity.

Themiscibility of polyoxyalkyleneswithwa-ter increases with the number of ethylene oxideunits. In solution the water-soluble grades arepractically nonflammable. The solubility in wa-ter decreases with increasing molecular massand number of ether bonds. Solubility in hy-drocarbons increases with the molecular mass.Polyalkylene glycols in general are soluble inaromatic hydrocarbons.

The molecular mass and viscosity ofpolyalkylene glycols can be significantly influ-enced during production and be adjusted withinnarrow limits. The possibility of engineeringproducts in this way distinguishes them frommany other lubricants.

Low-molecular-mass polyalkylene glycolscontainingmore than 50%propylene oxide havepour points that go down to− 65 ◦C. The lateralmethyl groups are responsible for the disruptionof crystallization. On the other hand pure high-molecular-mass polyethylene glycols are wax-like solids with pour points near + 4 ◦C.

Kinematic viscosities range from 8 to100 000mm2/s at 40 ◦C. The VI valuesof polyalkylene glycols usually lie around200. High-molecular-mass polyethylene gly-cols have VI values of up to 400.

Prolonged heating of polyalkylene glycols toabove 150 ◦C leads to depolymerization. Tracesof alkali or alkaline-earth metals promote thedegradation. The decomposition can be pre-vented by addition of aminic antioxidants, to theextent that the oils can be used as heat-transferfluids up to 250 ◦C.

The polar nature of polyalkylene glycolsgives the products strong affinity for metals, andso the lubricating film remains intact even athigh surface pressure, a property useful in lubri-cants and metal cutting fluids. Because swellingof elastomers decreases with increasing viscos-ity, polyalkylene glycols can be used with natu-


ral and with synthetic rubbers in hydraulic oilsand brake fluids. Because of their low toxic-ity polyalkylene glycols find application in thefood, pharmaceutical, tobacco, and cosmeticsindustries. Products with high ethylene oxidecontent are up to 80% biodegradable. High vis-cosity water-soluble products are shear-stableliquid thickeners. Hydrophilic and hydrophobicfractions in block copolymers give them surfac-tant properties.

5.5. Other Polyethers

Alkylated aryl ethers, like alkyl ethers, havelower viscosities, lower VI values, lower pourpoints, and lower boiling points than the corre-sponding alkanes. Asymmetric substitution alsoreduces the pour point, but has an adverse effecton the V –T relationship. Although such prod-ucts have low viscosities, some have found useas components of lubricants used in the life sci-ences.

5.5.1. Perfluorinated Polyethers (→FluorineCompounds, Organic, Chap. 6.1.) [111–113]

Photochemical polymerization of tetra-fluoroethylene in the presence of oxygen, fol-lowed by fluorination with elemental fluorineleads to products of type A (Eq. 5.1) in the caseof tetrafluoroethylene. With hexafluoropropyl-ene products of type B (Eq. 5.2) result. Anionicpolymerization of hexafluoropropylene epoxideleads to products of type C (Eq. 5.3), and Lewisacid-catalyzed polymerization of 2,2,3,3-tetra-fluorooxetane leads to products of type D (Eq.5.4).

(5.1)

(5.2)

(5.3)

(5.4)

The density of the perfluorinated polyethers (PF-PEs) is nearly twice that of hydrocarbons. Theyare immiscible with most of the other base oilsand non-flammable under nearly all practicalconditions. The more common types A and Bhave a good to very good V –T and V – p depen-dence, and low pour points [114]. The viscosityof linear PFPEs changes less with temperatureand pressure than that of the non-linear variety;in air they are stable up to 400 ◦C. PFPEs are re-markably inert chemically and have an excellenthydrolytic stability.

The shear stability of PFPEs is better than thatof other polymeric lubricants, but in the presenceof steel and under boundary lubrication condi-tions PFPEs do not perform very well [115].

Perfluoropolyalkyl ethers have all the prop-erties required by modern spacecraft: as lubri-cants and hydraulic fluids they resist to ther-mal and oxidative attack above 260 ◦C, possessgood low-temperature flow characteristics, andare fire-resistant. They can also be used as powertransmission and inert fluids and in transformersand generators as dielectrics with outstandingproperties.

5.5.2. Polyphenyl Ethers

Polyphenyl ethers are formed by reaction of phe-nols and aryl halides [116]. Abbreviated formu-las are in use. They contain the substitution po-sition and the number of phenyl rings (φ) andether bonds, for instance ppp5P4E stands forφ –O –φ –O –φ –O –φ –O –φ (p – p – p).


(5.5)

The aromatic groups increase the stability, butnegatively affect the V –T dependence of thesepolyethers. Alkyl groups lower the high melt-ing points. para Derivatives have lower volatili-ties; for ortho products the volatilities are higher.Spontaneous ignition occurs between 550 and600 ◦C and alkyl substitution reduces this by ca.50 ◦C. With the usual elastomers swelling oc-curs.

The oxidation stability of polyphenyl ethersis only slightly lower than that of polyphenylsor tetraarylsilanes. Alkyl substituents reduceit. The thermal decomposition temperature is465 ◦C. Short-chain substituents reduce it to be-low 380 ◦C andwith higher alkyl groups it dropsto below 350 ◦C, that is, to temperatures typi-cal for aliphatic hydrocarbons. Trifluoromethylgroups decrease the thermal decomposition tem-perature to below270 ◦C.Coke formation is low.It increases with alkyl substitution, particularlyin the presence of methyl groups.

Polyphenyl ethers are the most radiation-resistant lubricants. At low temperatures radi-ation affects the viscosity more pronouncedlythan at high temperatures. It increases viscosity,acidity, evaporation loss, corrosivity, and cokeformation, but reduces flash and ignition point.

Between 200 and 300 ◦C the lubricatingproperties of polyphenyl ethers are reported tobe comparable with those of mineral and esteroils and better than those of polysiloxanes andaromatic hydrocarbons. Alkylated polyphenylethers have better properties than the unsub-stituted ethers. Polyphenyl ethers are suitedas high-temperature or radiation-resistant hy-draulic fluids and lubricants.

5.5.3. Polysiloxanes (Silicone Oils)(→Silicones, Chap. 3.)

Silicone oils suitable as lubricants are generallystraight-chain polymers of the dimethylsiloxane

(4) and phenylmethylsiloxane series (5). Theyhave been described in detail [117], [118].

4

5

Methyl silicone oils are simply made fromquartz and methanol in the end (Fig. 26), butcommercial technology employs sophisticatedsteps, e.g., the so-called equilibration – treat-ment of a mixture of siloxanes of different mo-lecular mass with strong acid or basic catalystswith the aim of achieving a narrow Gaussianmolecular mass distribution [119].

Among the unique properties of silicone lu-bricating oils is their immiscibility with manyorganic fluids, the low temperature-dependenceof their physical properties, and their physio-logical inertness. Silicone oils can be producedwith very low pour points and high viscosity byincreasing the asymmetry of the molecules, usu-ally by partial replacement of the dimethylsilylgroups by phenylmethylsilyl groups. The den-sity of silicone oils is in the region of that of wa-ter, those of dimethylsilicone oils are slightly be-low, those of phenylmethyl silicone oils slightlyabove.

The extremely low viscosity – temperaturecoefficients (VTC), < 0.6 for low-viscosity di-methylsilicone oils aremainly due to the extraor-dinary flexibility of their Si –O chains. Low tomedium viscosity silicone oils have Newtonianbehavior up to high shear rates, but with increas-ing viscosity the apparent viscosity decreaseswith increasing shear rate (pseudoplastic behav-ior). Compressibility and viscosity changes athigh pressure depend strongly on the methyl-to-phenyl ratio of the silicone oils and are relativelylarge, again mainly because of the flexibility oftheir Si –O chains.


Figure 26. Preparation of dimethylsilicone oils

The thermal decomposition of silicone oilsbegins at ca. 300 ◦C. In general the decomposi-tion products are not corrosive, but they lowerthe viscosity. Up to 200 ◦C the oxidation sta-bility of silicone oils is superior to that of hy-drocarbons, esters, and polyalkylene glycols. Athigher temperature the formation of siloxyl andsilyl radicals via cross-linking leads to the for-mation of polymer molecules and thus results ingels.

Some iron and cerium compounds are espe-cially suitable as oxidation inhibitors for siliconeoils.At elevated temperatures gel formation nev-ertheless occurs readily in the presence of sele-nium or tellurium. In the presence of chlorineeven explosions are possible. Si –O bonds canalso be broken by hydrolytic attack, but in con-trast with silicic ester oils formation of silica orsilicic acid gel does not occur.

The surface tension and the foaming tendencyof silicone oils are much lower than those of

mineral oils; silicone oils are, therefore, used asdefoamers for the hydrocarbons.

Silicone oils can be found in all kindsof industrial and military installations. Di-methylsilicone oils are used as lubricants forbearings and gears with rolling friction. In slid-ing friction their performance depends on themetal pairs, they are used, for example, as lu-bricants for bronze or brass on aluminum. Sili-cone oils are among the best lubricants for plas-tic bearings, but in precision instrument applica-tions spreading should be prevented by use of anepilamization agent. They are suitable lubricantsfor rubber parts, also, and also serve as switchand transformer oils. Silicone oils with higherphenyl substitution are mainly used for the lu-brication of turbines, ball bearings, and all kindsof instrument, especially at high temperatures.Their radiation stability is also remarkable.

Silicone oils can be used as base oils for allkinds of lubricating greases [120].


5.6. Other Synthetic Base Oils

Of the following group of synthetic fluids thathave been proposed as base oils for lubricantsor that have been developed especially for thatuse, none has gained yet real economic or com-mercial importance in that area during the last30 years, all still wait for their niche applica-tion, even though some have really astonishingproperties.

Acetals have been investigated because oftheir high stability in alkaline media; they are,however, sensitive to acidic hydrolysis.

Adamantane derivatives, mainly the es-ters of 1,3-dihydroxy-5,7-dimethyladamantane,have been reported as being superior to the com-mon dicarboxylic acid esters [122].

Aromatic amines, in particular theirtrialkylsilyl-substituted derivatives have beenconsidered as lubricants because of their goodheat resistance [121].

The performance of dialkylcarbonates, thediesters of carbonic acid, is similar to that ofcommon dicarboxylic acid esters. They havebeen reported to be preferable to common es-ter with regard to lower toxicity and better sealcompatibility [123]. In addition, no acid com-pounds are formed on decomposition.

Heterocyclic Boron, Nitrogen, and Phos-phorus Compounds. Triazine derivatives such

as melamine and cyanuric acid form the nu-clei for fire-resistant and high-temperature lubri-cants [121], [122]. Polycarboranesiloxanes (6)have been reported to be comparable with sili-cone oils, but can be used at temperatures above220 ◦C [124]. The fluorine-containing phospha-zene derivatives (7) that have been developedmainly for the use in fire-resistant high-tempera-ture hydraulic fluids [125] and for use as lubric-ity additives in perfluorinated polyethers [126],[127].

6

7

Polyalkylene sulfides (polythioether oils)are polyalkylene glycols in which all C –O –Cbonds are replaced byC – S –C bonds; they havegood oxidation stability, and higher viscositiesand lower pour points than the correspondinghydrocarbons [128].

Polyphenyl sulfides (polyphenyl thio-ethers, C-ethers) are polyphenyl ethers inwhich all C –O –C bonds are replaced byC – S –C bonds. This gives them lower pourpoints and better boundary lubrication proper-ties, but reduced oxidative and thermal stability[129], [130].

Silahydrocarbons. Compared with their hy-drocarbon analogs, silahydrocarbons, mainlytetraalkylsilanes have lower pour points, lowervolatilities, higher VI values up to 155, and su-perior thermal stability [121,131,132]. They re-spond to antioxidants and antiwear additives inthe same way as PAOs [133].


Silicate esters, (ortho)silicic acid esters,have been known since the middle of the 19thcentury. Their properties differ strongly fromthose of siloxanes. Despite their good thermalstability and their low pour points their use is re-stricted to hydraulic, heat-transfer, and coolingfluids and to oils for automatic weapons becauseof their poor hydrolytic stability [134]. With thedevelopment of polysilicate clusters the short-comings of the simple esters can, perhaps, beovercome [135].

5.7. Mixtures of Synthetic Lubricants

Today few lubricants contain only one base oil,firstly becausemixing two ormore base oilswithdifferent properties often leads to a lubricantwith the desired performance, and secondly be-cause many of the more polar synthetic base oilsserve as additives in less polar oils, e.g., esters inhydrocarbons and vice versa. In thickened sys-tems even immiscible base fluids like hydrocar-bons and complex esters, perfluorinated ethers,or polyalkylene glycols can be combined.

6. Additives

Base fluids –mineral oil and also synthetic prod-ucts – generally cannot satisfy the requirementsof high performance lubricantswithout using thebenefit of modern additive technology.

Additives can be classified into types that

1) influence the physical, e.g., V –T character-istics, demulsibility, low temperature proper-ties, etc., and chemical properties, e.g., oxi-dation stability, of the base fluids

2) affect primarily the metal surfaces modify-ing their physicochemical properties, e.g.,reduction of friction, increase of EP behav-ior, wear protection, corrosion inhibition,etc.

Additives are used at contentrations from afew ppm (antifoam agents) up to 20wt%or even

more. They can assist each other (synergism) orthey can lead to antagonistic effects. Some addi-tives are multifunctional products that decreasethe possibility of additives interfering with eachother.

Of course, there are some properties that can-not be influenced by additives, e.g., volatility,air release properties, thermal stability, thermalconductivity, compressibility, or boiling point..

Although well balanced and optimized ad-ditive systems can improve the performance oflubricants enormously, the formulation of highperformance lubricants requires also excellenthigh quality base fluids.

6.1. Antioxidants

The function of a lubricant is mostly limitedby the aging of the lubricant base stock. Typ-ical characteristics of aged lubricants are discol-oration and a characteristic burnt odor. In ad-vanced stages the viscosity will begin to risesignificantly and acidic oxidation products areformed, which in turn may induce corrosion andlubricant problems. This aging process can bedelayed tremendously by the use of antioxidants.

6.1.1. Mechanism of Oxidation andAntioxidants

The aging of lubricants can be differentiated intotwo processes: the oxidation process by reactionof the lubricant molecules with oxygen and thethermal decomposition (cracking) at high tem-peratures. In practice the oxidative aging of thelubricant is the dominating process which in-fluences significantly the lifetime of the lubri-cant.Causedby steadily increasedpower densityand reduced lubricant volumes (higher load : oilratio) as well as extended service life in thelast years, the thermal stress on the lubricantmolecules grows constantly.

The oxidation of hydrocarbons can be de-scribed by the well-known free radical mech-anism via alkyl and peroxy radicals [136]. Themain reaction steps are shown in Figure 27.


Figure 27.Mechanism of autoxidation

The difference in reactivity of the miscella-neous radicals formed during autoxidation ex-plains why linear and saturated hydrocarbonsexhibit a much higher oxidation stability com-pared to branched, aromatic and unsaturated hy-drocarbons.

The typical oxidation products that will beformed by these oxidation processes includealkyl hydroperoxides, dialkyl peroxides, alco-hols, aldehydes, ketones, carboxylic acids, andesters. By polycondensation processes high mo-lecular mass oxidation products are formed.These products are responsible for the typicalviscosity increase of aged oil. Further polycon-densation and polymerization of these oxidationproducts lead finally to oil-insoluble polymersthat can be observed as sludge and varnish-likedeposits.

Because of the acidic character of most of theoxidation products the danger of corrosion is in-creased. Also the attack of alkyl peroxy radicalson the metal surface may be responsible for cor-rosive wear. Furthermore such dissolved metalscan form salts which also precipitate as sludge.

Nonhydrocarbon-based lubricants may be-have totally different. Thus the aging ofpolyalkylene glycols will lead to a decrease ofthe viscosity caused by the decomposition of thepolymeric structure.

Normally refined mineral base stocks con-tain traces of nitrogen-, sulfur- and oxygen-con-taining heterocycles as well as mercaptans, thio-ethers, and disulfides that may act as so-called‘natural antioxidants’ or as pro-oxidants thatwillaccelerate the oxidation of the lubricant. Usuallythe antioxidants are dominating thus providinga relatively good oxidation stability of the baseoil. On the other hand hydrocracked base stocks(API group II and III) and synthetic lubricantslike PAOs (API group IV) require a well bal-anced antioxidant combination in order to ex-ploit the advantages of that kind of lubricants[137–139].

Antioxidants can be classified in primary an-tioxidants (radical scavengers) and secondaryantioxidants (peroxide decomposers).

Radical scavengers compete successfullywith the lubricant molecules in the reactionwith reactive radicals of the propagation process.They react preferably with the radical oxidationproducts forming resonance-stabilized radicalsthat are so unreactive that theywill stop the prop-agation of the autoxidation. Peroxide decom-posers convert hydroperoxides into non-radicalproducts thus also preventing the chain propa-gation reaction.

In finding the optimum synergistic antioxi-dant andmetal deactivator combination themax-imum delay of oil oxidation is achieved.

6.1.2. Compounds

Phenolic Derivatives (→Antioxidants,Chap. 4.1.). Sterically hindered mono-, di-and polynuclear phenol derivatives belong tothe most effective antioxidants acting as rad-ical scavengers and are used in many appli-cations. Typically the phenols are substitutedin the 2 and 6 positions with tertiary alkylgroups. The most common substituent is thetertiary butyl group. The most simple deriva-tives are 2,6-di-tert-butylphenol (2,6-DTB) (8)and butylated hydroxytoluene, 2,6-di-tert-butyl-4-methylphenol (BHT) (9). The advantage ofpolynuclear phenols like 4,4′-methylenebis(2,6-di-tert-butylphenol) (10) or types with high-mo-lecular-mass substituents in the 4 position is thereduced volatility due to the higher molecularmass that makes these products suitable for hightemperature applications.


8

9

10

Aromatic Amines (→Antioxidants,Chap. 4.2.). Oil-soluble secondary aromaticamines represent another important class of an-tioxidants that act as radical scavengers. Typicalproducts are a large number of alkylated di-phenylamines (11), N-phenyl-1-naphthylamine(PANA) (12) and the polymeric 2,2,4-trimeth-yldihydroquinoline (TMQ) (13). Because of itspoor solubility in mineral oil the latter is com-monly used in greases and polar lubricants.

11

12

13

Compounds Containing Sulfur and Phos-phorus (→Antioxidants, Chap. 4.3.; Sec-tion 6.8). The most famous representatives ofthis group of additives are the zinc dithio-phosphates which mainly act as radical scav-engers [140]. Because of their multifunctionalproperties (antioxidant, antiwear additive, ex-treme pressure additive, corrosion inhibitor)zinc dithiophosphates are still being used asstandard additives.

In addition, a large number of ashlessdithiophosphoric acid derivatives, the so-calledO,O,S-triesters, exist. These are reaction prod-ucts of dithiophosphoric acid with olefins, cy-clopentadiene, norbornadiene, α-pinene, poly-butene, unsaturated esters like acrylic acid es-ters, malenic acid esters and other chemicalswith activated double bonds [140]. All this addi-tives show antiwear and also antioxidant proper-ties, although their performance as antioxidantis not really as good as that of the metal dithio-phosphates.

Organosulfur compounds are typicalperoxide decomposers. Zinc and methy-lenebis(dialkyldithiocarbamates) have beenfound to be highly efficient [140].

Organophosphorus Compounds. Triaryl-and trialkylphosphites are the most commontypes of this group. They are not only perox-ide decomposers, but they also can limit pho-todegradation. Because of their relatively poorhydrolytic stability their application is restrictedto sterically hindered derivatives.

Other Compounds. Surprisinglyorganocop-per compounds in combination with peroxidedecomposers can act as antioxidants althoughcatalytic quantities of copper ions usually actas pro-oxidants [140]. Overbased phenates andsalicylates of magnesium and calcium behave asantioxidants at higher temperatures. Many othercompounds have been proposed as antioxidantsbut none of these could gain a real relevance.

Synergistic Mixtures. Commonly severaltypes of antioxidation and combinations of themare used. Well known is the synergistic effectof aminic and phenolic antioxidants. The com-bination of radical scavengers and peroxide


decomposers is called heterosynergism. So thecombination of phenolic antioxidantswith phos-phites is known as highly efficient especially inhydrotreated base stock [141]. As metals mayact as catalysts in the oxidation process typ-ical rust inhibitors and metal passivators arealso used as synergistic compounds (see alsoSection 6.10).

6.2. Viscosity Modifiers

6.2.1. VI Improvement Mechanisms

In the simplest case, a desired viscosity indexcan be achieved by mixing fluids with corre-sponding viscosity index improvers (VIIs). Butfor a number of technical and commercial rea-sons, viscosity modifiers are often used.

Viscosity modifiers are chain-like polymericmolecules whose solubility depends on chainlength, structure and chemical composition[142], [143]. As a rule, the base oil solubilityof these polymer chains deteriorates when thetemperature falls and improves with increasingtemperature so that an increase in viscosity in-duced by viscosity modifiers also increases theviscosity index.

The absolute increase in viscosity and the VIdepends on the molecular mass and the concen-tration of viscosity modifiers in the formulation[144]. In practice and depending on the pro-jected application, molecular masses of 10 000to 250 000 are used. Concentrations are usuallybetween 3 and 25 wt%. As a result of their highmolecular mass, viscosity modifiers are alwaysdissolved in a base fluid.

Apart from their thickening effect whichis schematically illustrated in Figure 28A asa function of molecular mass, shear stabilityserves as a second characteristic [145]. Accord-ing to Figure 28B, increasingmolecularmass re-duces shear stability if the polymer concentra-tion remains constant.

The reason for this effect is mechanically- orthermally-induced chain degradation [146]. De-pending on the type and duration of the load, anumber of different molecular sizes are created.The resulting drop in viscosity is described bythe Permanent Shear Stability Index

[KV before shearing] − [KV after shearing][KV before shearing] − [KV of the base oil]

e.g.,11ct − 8ct11ct − 5ct

where KV denotes the kinematic viscosity.If the relaxation time for the polymer chains

is short (10−6 s), the high-molecular moleculesadopt a temporary alignment. This drop in vis-cosity is described by the temporary shear sta-bility index (TSSI).

Both values are of great importance to auto-motive applications, especially to engine, gearand hydraulic oils because the specified charac-teristics of such an oil do not just apply to thefresh oil but should remain throughout the draininterval. As the reduction in molecular mass inpractice is overshadowed by oxidation and othereffects, a series of laboratory tests have been es-tablished to characterize shear stability [147].

6.2.2. Structure and Chemistry of ViscosityModifiers

The most important monomers for viscositymodifiers are listed in Table 7.

It is also possible to sort these polymersaccording to their supramolecular chain struc-ture, independently of the base monomers used(Fig. 29).

dispersing. Dispersing viscosity modifiersare in principle the link to ashless dispersantswhich are discussed in Section 6.4.2.

6.3. Pour Point Depressants

With the exception of polyalkylated naphtha-lenes, pour point depressants (PPD) are closelylinked to a series of viscosity modifiers. Thema-jor difference between pour point depressantsand viscosity modifiers is their application con-centration and the selection of monomer build-ing blocks [148].

Although paraffin crystallization cannot besuppressed, the crystalline lattice and thus themorphology of the paraffin crystals can be sig-nificantly altered by PPDs. While polyacry-lates and ethylene vinyl acetate copolymersare used for crude oils and petroleum, spe-cial polyalkylmethylacrylates are normally usedfor mineral oil-based formulations. Due to co-crystallization of paraffinic components in the


Figure 28. Thickening efficiency and shear stability of VMs

Table 7. Types of polymeric viscosity modifier

VM Type Description Main applications

OCP olefin copolymers engine and hydraulicoils

PAMA polyalkyl (meth)acrylates gear and hydraulic oils

PIB polyisobutene gear oils, raw materialfor ashless dispersants

SIP hydrogenatedstyrene – isoprenecopolymers

engine oils

SBR hydrogenatedstyrene – butadienecopolymers

engine oils


Figure 29. Viscosity modifiers, chain structures, and monomer make-up

base oil and the polymer chain, the crystalmorphology is altered. Instead of the needle-like paraffin crystals which rapidly cause paraf-fin gelation, densely packed, round crystals areformed which hardly affect flowing propertieseven at temperatures below the pour point. Thecorresponding polyalkyl methacrylates-PPDSare in principle, comb-like and containC12 –C24paraffinic side chains [149]. By determining thepolymer solubility and crystallization of paraf-finic components in the base oil, the optimumPPD-mix and concentration can be determinedfor every base oil mixture. However, as a widerange of base oils are used in practice, standardproducts are often used which cover almost theentire spectrum. In such cases, efficiency is con-trolled via PPD concentration.

6.4. Detergents and Dispersants

Detergents and dispersants (DD), often calledheavy-duty (HD) additives have been indispens-able for the development of modern engineoils for gasoline and diesel combustion motors.These lubricants are especially severely stresseddue to the high engine temperatures and the ad-ditional influence of aggressive blow-by gasesof the combustion process. DD additives keepoil-insoluble combustion products in suspensionand also prevent resinous and asphalt-like oxi-dation products from agglomerating into solidparticles. These additives prevent oil thickening,sludge and varnish deposition on metal surfacesand corrosive wear.

The first detergent additives usedweremetal-containing compounds, often with high alkalinereserve.

To meet the dramatically increased require-ments of modern high-performance engine oilsnew ashless dispersants with improved dispers-ing properties have been developed. As theseashless compounds possess also cleansing prop-erties in fact there is no real difference betweendetergents and dispersants. Thus, it seems to bemore appropriate to speak of metal-containingand ashless DD or HD additives [150], [151].

Mechanism of Action. Detergents and dis-persants are generally molecules having a largeoleophilic hydrocarbon ‘tail’ and a polar hy-drophilic head group. The tail section serves asa solubilizer in the base fluid, while the polargroup is attracted to contaminants in the lubri-cant. Solid contaminants are enveloped formingmicelles whereby the nonpolar tails prevent theadhesion of polar soot particles on metal sur-faces as well as the agglomeration into largerparticles (peptidization).

6.4.1. Metal-Containing Compounds(Detergents)

Phenates represent an important class of de-tergents which are synthesized by reaction ofalkylated phenols with elemental sulfur or sul-fur chloride followed by the neutralization withmetal (calcium, magnesium, barium) oxides orhydroxides. Calcium phenates are currently the


most widely used types. Basic calcium phen-ates can be produced by using an excess of themetal base. Beside their good dispersant proper-ties they also possess greater acid neutralizationpotential.

Salicylates are generally prepared by car-boxylation of alkylated phenols with subsequentformation of divalent metal salts. Typically alsothese products are overbased by an excess ofmetal carbonate (calcium and magnesium) toform highly basic detergents that are stabilizedby micelle formation. Salicylates exhibit addi-tional antioxidant properties and have proven ef-fective in diesel engine oil formulations.

Thiophosphonates are produced by the re-action of polybutene (molecular mass from 500to 1000) with phosphorus pentasulfide followedby hydrolysis and formation of metal (calcium,formerly also barium) salts.

The reaction products consist mainlyof thiopyrophosphonates combined withthiophosphonates and phosphonates [152].

Sulfonates are metal salts of long-chainalkylarylsulfonic acids which can be dividedinto the petroleum and synthetic types. Be-side their excellent anticorrosion properties (seeSection 6.10.1) neutral and especially so-calledoverbased sulfonates with colloidally dispersedmetal oxides or hydroxides (Fig. 30) addition-ally exhibit an excellent detergent and neutral-ization potential that makes them very cost-effective multifunctional DD additives for en-gine oils.

Calcium sulfonates are relatively cheap prod-ucts with good general performance. Magne-sium compounds have excellent anticorrosionproperties but tend to formhard ash after thermaldecomposition. Barium sulfonates are hardlyused anymore due to toxicological concerns.

6.4.2. Ashless Dispersants (AD)

Ashless dispersants are generally derived fromhydrocarbon polymers, usually from poly-butenes with molecular masses of 500 to 3000[153].

Polybutenes are thermally coupled withmaleic anhydride (MA) to give alkylsuccinic an-hydrides. In a further reaction step oligomericaminoalkylenes are added to the anhydride toform thermally stable imides. By using suitableraw materials and reaction control, this two-stage, one-pot reaction can produce a variety ofproducts. A sample reaction scheme is shown inFigure 31.

Novel developments are monofunctional andbifunctional polyisobutylene succinic anhy-drides (PIBSAs). Apart from the bifunctionalamines (triethylene tetramines, tetraethylenepentamines), alcohols such as pentaerythritolsare used to improve the base character and theresulting interactions with elastomers.

The dispersion of oxidation, nitration andsoot particles can be controlled by product mix-ture and the purity of the components.

Equally chlorine-free alternatives are low-molecular-mass, dispersant PAMAS. Contraryto hydrocarbon-based oligomers, the dispers-ing units are spread along the chains, wherebythe type of dispersing groups can vary greatly[154]. Current research reports advantages ofthese type of AD with regard to soot dispers-ing and improved fuel efficiency characteristics[155].

6.5. Antifoam Agents

The foaming of lubricants is a very undesirableeffect that can cause enhanced oxidation by theintensive mixing with air, cavitation damage aswell as insufficient oil transport in circulationsystems that can even lead to lack of lubrica-tion. Beside negative constructive influences thefoaming tendency is influenced by the surfacetension of the base oil and especially by the pres-ence of surface-active substances such as deter-gents, corrosion inhibitors and other ionic com-pounds.

Two types of foam can be discriminated:Surface foam can be controlled by antifoam

agents. Effective defoamers are usually not solu-


Figure 30.Model of the structure of overbased sulfonates

ble in the base oil and therefore have to be finelydispersed. The particle size of the dispersed de-foamers should be smaller than 100µm or evensmaller than 10µm [156].

The inner foam refers to finely dispersed airbubbles in the lubricant that can form very sta-ble dispersions. Unfortunately the common de-foamers that control the surface foam tend to sta-bilize the inner foam. Generally the air releaseproperties of lubricants cannot be improved byadditives. Lubricants which need excellent airrelease properties, e.g., turbine oils have to beformulated using specially selected base oils andadditives.

Silicone Defoamers. Liquid silicones, espe-cially linear and cyclic polydimethylsiloxanes,are the most efficient antifoam agents at verylow concentrations of 1 to max. 100mg/kg. Toguarantee a stable dispersion silicones usuallyare predissolved in aromatic solvents.

Compared to other additives silicone de-foamers have the disadvantage of being partic-ularly easily carried out of the lubricant due totheir insolubility and their strong affinity to polarmetal surfaces.

Silicone-free defoamers are being usedmore and more in many applications, espe-cially in metalworking processes. Cutting fluidsand hydraulic fluids have to be silicone-free toguarantee the subsequent application of paintsor lacquers on the workpieces.

The main representatives of silicone-free de-foamers are special poly(ethylene glycols) andmiscellaneous organic copolymers. Also trib-utylphosphate has been proposed as antifoamagent.

6.6. Demulsifiers

Most of the industrial oils in circulation systems(hydraulic, gear, turbine and compressor oils)require good or excellent demulsification prop-erties to separate water from the lubricating sys-tem. Without demulsifiers lubricating oils canform relatively stable water-in-oil emulsions.

In principle, all surface-active substances aresuitable demulsifiers. One of the first knowntypes have been alkaline-earth metal salts of or-ganic sulfonic acids particularly barium and cal-cium dinonylnaphthenesulfonates. Nowadays


Figure 31. Reaction mechanism for the manufacture of ashless dispersants

special poly(ethylene glycols) and other ethoxy-lated substances are used.

Surprisingly the same class of chemical sub-stances is used as emulsifiers. Here the molecu-lar mass, the degree of ethoxylation and the treatrate are very important to guarantee the demul-sifying properties.

6.7. Dyes

For marketing, identification or leak detectionpurposes some lubricants contain dyes [157].Most of these substances are solids which of-ten are suspended in mineral oil or dissolved inaromatic solvents to make their handling easier.

Generally oil-soluble azo dyes are used.Some of these products may be removed in thenear future because they have to be labeled aspotential carcinogenes.

Fluorescent dyes are typically used to detectleaks under UV light when the coloration of lu-bricants is not appreciated.

6.8. Antiwear and Extreme PressureAdditives

When two contacting parts of a machinery startto move and the hydrodynamic lubrication hasnot yet build up or in the case of severe stressand strong forces the lubricating system runs inthe area of mixed friction. In this case antiwear(AW) and extreme pressure (EP) additives arenecessary in any metalworking fluid, engine oil,hydraulic fluid or lubricating grease to preventwelding of the moving parts respectively to re-duce wear.

Function of AW/EP Additives. Because oftheir polar structure AW/EP additives form


layers on the metal surface by adsorption orchemisorption that guarantees their immediateavailability in the case of mixed friction condi-tions. When the hydrodynamic lubricating filmis not yet or no longer valid, temperature willincrease and the AW and EP additives can re-act with the metal surface forming tribochem-ical reaction layers (iron phosphides, sulfides,sulfates, oxides and carbides – depending on thechemistry of the additive) that will prevent directcontact between the sliding metals.

AW additives are mainly designed to reducewear when the running system is exposed tomoderate stress whereas EP additives are muchmore reactive and are used when the stress of thesystem is very high in order to prevent the weld-ing of the moving parts that otherwise wouldlead to severe damage.

Phosphorus Compounds. Under conditionof medium stress organic phosphorus com-pounds work excellent as antiwear additives.

Most of these additives are neutral and acidicphosphoric acid esters, their metal or amine saltsor amides. As the acidic form of these com-pounds is the most reactive one, the reactivitydecreases with the degree of neutralization.

Tricresylphosphate (TCP) is the most impor-tant neutral phosphoric acid triester. For toxico-logical reasons TCP should be free of o-cresol.

Amine salts of mixtures of mono- and dialkylphosphoric acid partial esters are highly effi-cient FZG boosters. Some types exhibit addi-tional anticorrosion properties.

Ethoxylated mono- and dialkylphosphoricacids are even more polar due to their hy-drophilic structure what makes them more ef-ficient.

Also phosphites could gain some significa-tion. Because of the inherent hydrolytic instabil-ity of this chemical group in practice only steri-cally hindered derivatives like triarylphosphitesand long chain trialkylphosphites are used forsome applications.

Compounds Containing Sulfur and Phos-phorus. The most important additives of thisgroup are the zinc dialkyldithiophosphates(ZnDTP) [159]. They are synthesized by re-action of primary and secondary alcohols(C3 –C12) or alkylated phenols with phos-phorus(V)sulfide followed by neutralization of

the resulting dialkyldithiophosphoric acids withzinc oxide. Beside the neutral species also basicZnDTPs can be obtainedwhen the neutralizationis done with an excess of zinc oxide.

Zinc dialkyldithiophosphate/zinc dialryldithiophosphate

ZnDTPs based on isopropanol or n-buta-nol are solids, whereas mixtures of short andlong chain alcohols are liquid. The thermal andhydrolytic stability of ZnDTPs and thus theirreactivity (AW/EP-performance) can be influ-enced by the structure of the alkyl groups. Sothe thermal stability increases with the chainlength of the alkyl groups and their structurein the sequence secondary, primary and aro-matic. By carefully directed alcohol composi-tion the specific requirements of different appli-cations can selectively be adjusted. Beside ex-cellent antiwear and extreme pressure propertiesZnDTPs are also efficient antioxidants and evenmetal passivators. This multifunctional proper-ties makes them the widest spread cost effectiveadditive group that is used nowadays in hugequantities in engine oils, shock absorber oils andhydraulic fluids.

Beside ZnDTPs also ammonium, antimony,and molybdenum dialkyldithiophosphates areknown. The latter are effective antiwear addi-tives with remarkable friction reducing proper-ties and excellent antioxidant behavior.

Ashless dialkyldithiophosphoric acid-O,O,S-triesters have higher hydrolytic stability thanthe corresponding acids, but poorer antioxidantproperties. Similar to the ZnDTPs the reactiv-ity can be influenced by variation of the organicsubstituents.

Triphenylphosphorothionate (TPPT) is dis-tinguished by its extraordinary thermal stabilitywhat makes it suited for high temperature lubri-cation.


Compounds Containing Sulfur and Nitro-gen.

Zinc-bis(diamyldithiocarbamate) and theashless methylene-bis(di-n-butyldithiocarb-amate) are highly effective EP additives and ex-cellent antioxidants. Beside these main speciesalso antimony and tungsten derivatives areknown. Dithiocarbamates are predominantlyused in lubricating greases and to some extentin gear oil formulations.

Dialkyl-2,5-dimercapto-1,3,4-thiadiazole(DMTD) derivatives (see Section 6.10) are usu-ally classified as metal passivators and sulfurscavengers, but they also exhibit excellent EPproperties. The use of thiadiazoles and dithio-carbamates is limited by their relatively highprice.

Sulfur Compounds. From the early days oflubrication until now elementary sulfur has beendissolved directly in mineral oil (up to 1.5%) toimprove the EP properties of metal working flu-ids. Oil-soluble organic sulfur compounds, theso-called sulfur carriers, of the general formulaR – Sx –R offer improved solubility and exactcontrol about the reactivity of the sulfur.

Inactive and active sulfur carriers can be dif-ferentiated. The inactive types with predomi-nantly disulfide bridges (x = 2) possess relativelystable C – S bonds which will react at elevatedtemperatures only. The active forms with x bet-ween 3 and 5 (so-called pentasulfides) are muchmore reactive as the sulfur of the relatively la-bile polysulfidebridges can easily bemade avail-able even at low temperatures.Moreover numer-ous sulfur carriers with specific distribution ofthe different polysulfide bridges (x = 1 – 5) areused to cover the whole field of application withits varying stress requirements. The mechanismof action begins with physical adsorption fol-lowed by chemisorption and finally cleavage ofthe sulfur – carbon bond and reaction of sulfurwith the metal surface (Fig. 32). Generally thisreaction takes place at temperatures over 600 ◦C[158].

Active sulfur carriers are excellent EP addi-tives. Because of the high reactivity with nonfer-

rous metals active sulfur carriers cannot be usedin the machining of nonferrous metals. Inactivesulfur carriers need higher temperatures to setthe sulfur free. Therefore they are much morecompatible with nonferrous metals and show tosome extent antiwear properties.

The polarity and thus the affinity to themetal surface is determined by the organic sub-stituents.

Sulfurized hydrocarbons are obtained by di-rect sulfurization of olefins (e.g., diisobutene,terpenes) with elementary sulfur in the presenceof H2S, their reaction with dichlorodisulfide oroxidation of mercaptans to disulfides. The firstmethod will lead to products with high sulfurcontents up to 45%. The other will form inac-tive sulfur carriers that are predominantly used ingear oils and mild metalworking fluids (cuttingfluids). Active pentasulfides are used for severemachining processes, e.g., broaching.

Sulfurized fatty oils and fatty acid esters areproduced by sulfurization of the unsaturated rawmaterials with sulfur (dark colored products) orwith sulfur in the presence of H2S to obtain lightcolored products that are used predominantly inmodern lubricants instead of the formerly usedsperm oil products. The combination of sulfurwith the friction reducing properties of the fattyraw materials leads to excellent EP additiveswith high load carrying capacity in the four balltestingmachine. Sulfurized fatty acidmethyl es-terswill lead to productswith lowviscosities thatare used, e.g., for deep-hole drilling.

PEP Additives. Overbased sulfonates, espe-cially calcium and sodium salts, can be used ashighly efficient boosters in combinationwith ac-tive sulfur carriers to formulate metal workingfluids with extremely high load carrying behav-ior. These overbased products are called PEPs(passive EP).

Chlorine Compounds. Dispite their excel-lent AW/EP properties, chlorine compounds arebeing increasingly replaced by other additivesfor environmental and toxicological reasons.

Solid Lubricating Compounds (seeChap. 15). Mainly finely ground powders ofgraphite and molybdenum disulfide and theirdispersions are used as solid additives. Solidlubricants possess excellent emergency running


Figure 32.Mechanism of sulfur carriers under extreme pressure conditions

properties when the oil supply is breaking down.Also for extreme high temperature applicationssolid lubricating compounds are used due totheir high thermal stability. Other compoundsthat are used especially in lubricating greasesare polytetrafluoroethylene, calcium hydroxideand zinc sulfide.

6.9. Friction Modifiers

In the case of liquid (hydrodynamic) lubricationfriction can be reduced only by the use of baseoils with lower friction coefficients and lowerviscosity respectively high viscosity indices. Inthe area of low slide velocities, moderately in-creased loads and low viscosities at higher tem-peratures the liquid lubrication can easily pro-ceed to mixed friction conditions. In this caseso-called friction modifiers have to be used toprevent stick – slip oscillations and noises by re-ducing frictional forces. They work at temper-atures where AW and EP additives are not yetreactive. Therefore, friction modifiers (FM) canbe regarded asmild AWor EP additives workingat moderate temperatures and loads in the areaof beginning mixed friction.

Friction modifiers can be classified into dif-ferent groups regarding their function [160]:mechanically working FMs (solid lubricat-ing compounds, e.g., molybdenum disulfide,graphite, PTFE, polyamide, polyimide, fluori-nated graphite), adsorption layers forming FMs(e.g., long-chain carboxylic acids, fatty acid es-ters, ethers, alcohols, amines, amides, imides),tribochemical reaction layers forming FMs (sat-urated fatty acids, phosphoric and thiophos-phoric acid esters, xanthates, sulfurized fattyacids), friction polymer-forming FMs (ethoxy-lated dicarboxylic acid monoesters, dialkyl ph-thalates, methacrylates, unsaturated fatty acids,

sulfurized olefins) and organometallic com-pounds (molybdenum compounds like mo-lybdenum dithiophosphates, molybdenum di-thiocarbamates, their synergistic combinationwith ZnDTPs, copper containing organic com-pounds).

Typical applications of friction modifiers aremodern fuel economy oils, slide way oils, au-tomatic transmission fluids (ATF) that containso-called anti-squawk additives, and lubricantsfor limited-slip axles that contain so-called an-tichatter additives.

6.10. Corrosion Inhibitors

Anticorrosion additives are molecules with longalkyl chains and polar groups that can be ad-sorbed on the metal surface forming denselypacked, hydrophobic layers. The adsorptionmechanism can base on a physical or chemi-cal interaction of the polar anticorrosion additivewith the metal surface [161].

Because of this high surface activity, anticor-rosion additives compete with other polar addi-tives like antiwear and extreme pressure addi-tives for the metal surface and can therefore re-duce their efficiency. Thus a FZG damage loadstage of > 12 can be reduced down to a dam-age load stage of 8 if highly efficient and polaranticorrosion additives are added.

Corrosion inhibitors can be divided into twomain groups: antirust additives for the protec-tion of ferrous metals and metal passivators fornonferrous metals.

6.10.1. Antirust Additives

Sulfonates. Petroleum sulfonates (ma-hogany sulfonates) are byproducts at the pro-


duction of white oils by treatment with oleum.The resulting acid tar contains long-chainalkylarylsulfonic acids that can be neutralizedwith lyes. Sodium sulfonates with low molec-ular masses (below approx. 450) are typicallyused as low-priced emulsifiers and detergentswith additional anticorrosion properties in wa-ter based metal working fluids, engine oils andrust preventatives. Sulfonates with higher mo-lecular masses are highly efficient corrosioninhibitors especially when based on divalentcations like calcium, magnesium, and barium.The importance of the barium compounds de-creasing constantly due to toxicological andecotoxicological concerns.

Nowadays synthetic alkylbenzene sulfonatesare used preferably in spite of a higher pricelevel due to their higher and more constant qual-ity. They are reaction products of specificallydesigned monoalkylbenzenesulfonic acids (typ-ically C24 alkyl groups) and dialkylbenzenesul-fonic acids (typically twoC12 alkyl groups) withalkaline and alkaline-earth metal hydroxides.

A special group of synthetic sulfonates arethe dinonylnaphthenesulfonates of which theneutral calcium and barium salts are distin-guished by additional demulsifying propertiesand a good compatibility with EP additives.

Beside the neutral or only slightly basic sul-fonates overbased sulfonates with high alkalinereserve (TBN 100 to 400mg KOH/g) play animportant role especially in the formulation ofengine oils. There they exhibit detergent proper-ties and can neutralize acidic oxidation products.

Carboxylic Acid Derivatives. Lanolin(wool fat) and salts of the lanolin fatty acidmostly in combination with sulfonates havelong been known as corrosion inhibitors in rustpreventatives.Oxidized paraffinswith their highcontent of hydroxy- and oxocarboxylic acids arestill used for that purpose.

Zinc naphthenates are especially used in lu-bricating greases.

Alkylated succinic acids, their partial estersand half amides are known as highly efficient,

not emulsifying antirust additives even at verylow treat rates of 0.01 to 0.05%. Thereforethese additives are used preferably in turbine oilsand hydraulic fluids. 4-Nonylphenoxyacetic acidand derivatives have a similar performance.

Another widespread group are amides andimides of saturated and unsaturated fatty acids.The best known product of this type is N-acylsarcosine that shows a strong synergisticeffect with imidazoline derivatives. Addition-ally these additives have good water-displacingproperties.

Amine Neutralized Alkylphosphoric AcidPartial Esters. Some special amine salts ofmono- or dialkylphosphoric acid partial estersexhibit excellent anticorrosion properties in ad-dition to their highly efficient antiwear proper-ties [162]. Because of the well-known antag-onism of anticorrosion and antiwear additivesthis behavior makes them one of themostly usedcomponents in ashless industrial oils.

Vapor phase corrosion inhibitors (VCIs)for closed systems are substances with highaffinity to metal surfaces and relatively high va-por pressure to guarantee their availability onparts that are not steadily in direct contact withthe corrosion inhibited lubricant. The mostlyused product group for this application areamines. Morpholine, dicyclohexylamine, anddiethanolamine have proved to be highly effi-cient for that purpose. Because of toxicologicalconcerns, that refer mainly to the nitrosamineforming potential of secondary amines, theseproducts are being partly substituted by tertiaryamines like diethanolmethylamine and similarproducts.

Another group of oil soluble VCIs are lowmolecular carboxylic acids (n-C8 to n-C10).

6.10.2. Metal Passivators

Metal passivators can be classified into threegroups: film forming compounds, complexforming chelating agents, and sulfur scavengers.


Figure 33. Function of benzotriazole as film forming agent

The fundamental function of the film form-ing types consists in the building of passivatingprotective layers on the nonferrousmetal surfacethus preventing the solubilization of metal ionsthat would work as pro-oxidants. The complexforming agents are able to build oil-soluble com-plexes with significantly reduced catalytic activ-ity regarding the influence of nonferrous metalions on the oxidative aging process of lubricants.Sulfur scavengers are even able to scavenge cor-rosive sulfur by integrating it into their molecu-lar structure. Mechanism and function of thesethree types of metal passivator are shown in Fig-ures 33, 34, and 35.

Figure 34. Function of N,N ′-disalicylidene-propyl-enediamine as chelating agent

Metal passivators in combination with an-tioxidants show strong synergistic effects as theyprevent the formation of copper ions respec-tively suppress their behavior as pro-oxidants.Thus these additives are used in nearly everyformulation of modern lubricants.

The mostly used metal passivators arebenzotriazole and tolyltriazole as well as theiralkylated liquid derivatives with improved sol-ubility. Typical treat rates of these film formingpassivators are between 0.005 and 0.03%.

Furthermore 2-mercaptobenzothiazole (re-duced importance) and especially 2,5-dimercapto-1,3,4-thiadiazole derivatives areused as highly efficient film forming pas-sivators. The later can also act as sulfurscavenger by building in sulfur into thealkyl – sulfur – thiadiazol bonds (see Fig. 35).

N,N ′-disalicylidenealkylendiamines (where‘alkylen’ is ethylene or propylene) belong tothe group of chelating agents. Beside their EPand antiwear properties zinc dialkyldithiophos-phates and dialkyldithiocarbamates have somemetal-passivating properties (see Section 6.8).

7. Lubricants in the Environment

Lubricants and functional fluids are omnipresentdue to their widespread use and they thus pol-lute the environment in small, widely-spreadamounts and rarely in large, localized quantities.

Normally biodegradability of at least 60%according to OECD 301 or 80% according toCEC L-33-A-93 is considered as main objectivecriteria for ‘Bio-Lubricants’. But the other cri-teria will get more and more importance in thenear future.

7.1. Current Situation

Every year, about 40 – 50% of the approxi-mately 5× 106 t of used lubricants in Europeend up polluting the environment. This is eithertechnically desired (total-loss lubrication) or fol-lows leaks, or other problems.

The environmental damage caused by min-eral oil-based lubricants is largely caused by the


Figure 35. Function of dialkyl-2,5-dimercapto-1,3,4-thiadiazole as sulfur scavenger

approximately 40% of lubricants which are notproperly disposed of. This figures include total-loss applications, the residual oil in about 90mil-lion oil cans and 20× 106 oil filters, spillagesduring topping-up, leaks, drips from separatedoil-line and hydraulic couplings, accident lossesand all manner of emission losses (CONCAWEReport no. 5/96).

Even though mineral oil products can be rel-atively rapidly biodegraded by the microorgan-isms present in nature, these natural degradingsystems are exhausted by the volume of thelosses.

In the past, mineral oil-based products wereused almost exclusively. Leakages and othercauses allowed a part of these lubricants to pol-lute the environment. The problem has spurredsociety and politics into action, reflected by di-verse legislation and recommendations as wellas the issuing of environmental awards to prod-ucts which cause less damage. The use of en-vironmentally friendly materials and substanceswill play an increasingly important part in thescope and procedures of the EU Eco Audit.

Technically, and disregarding developmentand overall costs, more than 90% of all lu-bricants could be made rapidly biodegradable.However, this change will only take place if theecological benefits of using renewable raw ma-terials is made clear and best of all, quantified inmonetary terms.

7.1.1. Economic Consequences andSubstitution Potential

In Germany the selection of rapidly biodegrad-able lubricants, especially those classified as‘not water pollutant’ reduce the expense of oil

spillages or disposal as well as the cost of com-plying with health and safety regulations to aminimum. Compared to mineral oil-based prod-ucts, the selection of low-evaporation ester andvegetable oil-based lubricants can allow lowerviscosities to be used. This enables faster ma-chining and cuts drag-out losses. This, in turn,increases economy, increases productivity andgenerates product optimization.

The vast majority of rapidly biodegradablelubricants currently on the market are based onsaturated or unsaturated ester oils. The acids re-quired for esterification are mostly from veg-etable oils. Compared to equivalent mineral oils,ester oils are significantly more expensive. Thisis amajor barrier to thewider acceptance of suchoils. But the overall cost of a lubricant resultsfrom the interaction between the following char-acteristics (in alphabetical order):

– Aging and temperature stability– Disposal costs– Machine compatibility (multifunctional oil)– Machine investments– Maintenance measures– Oil change intervals– Operating costs– Reconditioning possibilities– Reduction in storage costs– Reduction in emission reduction measures– Reduction in the cost of preventative healthand safety measures

– Risk minimization and lower clean-up costsafter spillages

– Simpler plant license procurement

Completely newmix of economic aspects canresult from every new application. In particular,the use of fully saturated synthetic esters offers anumber of opportunities. Pure vegetable oil for-


mulations are now only used for lower demands,for example total-loss chain saw or mold releaseoils and some types of greases.

7.1.2. Agriculture, Economy, and Politics

The world market price of vegetable oils islargely influenced by the price of soybeans.Since half of the world’s soy production comesfrom the USA, vegetable oil prices are heavilydependent on the size of US soy harvest. On theGerman market, the price of rapeseed (in Ham-burg, Rostock or Mannheim) is particularly sig-nificant.

Increasing sales of environmentally harmlesslubricants could have a positive effect on pricesand this, in turn, would make such lubricantseven more interesting.

Until now, in Europe rapeseed and sunfloweroil as lubricant base oils have been in the fore-front because of the following reasons:

– They are abundantly available– Their thermal oxidation stability are accept-able in some applications

– Their flowing properties are better than thoseof other vegetable oils

The disadvantages of the abovementionedvegetable oils are:

– The oxidation, hydrolytic and thermal stabil-ity of vegetable oils is not adequate to allowtheir use in circulating systems and

– High-performance additives for vegetableoils have not undergone sufficient devel-opment, especially those which are eco-toxicologically safe and ecologically harm-less.

Apart from pure vegetable oils, oleo-chemical esters can also be used because theirthermal-oxidation stability compared to rape-seed oil opens up a wider field of applications.However, these products are considerably moreexpensive, a factorwhich in turn, limits their use.

In 1999, nevertheless, 20 000 ha of rapeseedwere cultivated in Germany only for applicationin lubricants. This corresponds to a volume ofnearly 20 000 t/a from which an approximatelysimilar quantity of lubricating or hydraulic oilsare manufactured. It can be assumed that about20 000 to 25 000 t of oleochemical esters are

used for biodegradable lubricants per year so thatbiodegradable lubricants account for just over4% of all lubricants (Germany 1999).

In Germany as most important market forbiodegradable lubricants today a number of po-litical initiatives were started in recent years, ofwhich some were included in two comprehen-sive reportswhichwere submitted to theGermanparliament.– Resolution dated June 16, 1994: Measuresto promote the use of rapidly biodegradablelubricants and hydraulic fluids

– Resolution dated November 28, 1995: Newsfrom the Federal Ministry for Food, Agri-culture and Forestry: ‘Report on the useof rapidly biodegradable lubricants and hy-draulic fluids and measures taken by theGovernment’ (New Edition 1999)

– Regulation for introduction of environmen-tal friendly lubricants to market (June 2000):This program supports the substitution ofmineral based lubricants by ester based prod-ucts.The use of environmentally friendly fluids is

mandatory in the public sector.Research and Development on ecologically

harmless fluids and the clarification of unan-swered questions is sponsored through projects.

7.2. Biodegradable Base Oils forLubricants

The most important biodegradable base oils aredifferent types of ester oil, e.g.:– Vegetable oil from harvestable raw materi-als, for example rapeseed or sunflower oil

– Semisaturated, transesterified ester oilswith natural fatty acids, for exampletrimethyolepropane trioleate

– Fully saturated, synthetic esters based onchemical modified vegetable oils or mineraloil, for example, diisotridecyl adipateOther in principle biodegradable base oils

mostly are not considered as ‘environmentalfriendly lubricants’ because of their petrochem-ical origin:– Poly(alkylene glycols) (PAG)– Low viscosity polyalphaolefins (PAO 2)– Some special types of synthetic hydrocarbonup to a viscosity of 6mm2/s at 100 ◦C


Natural Fats and Oils (Triglycerides). Theworldwide market for natural oils amounts to100× 106 t/a. The dominant crops are soybean(Northern America), palm (SE Asia, SouthernAmerica) and rapeseed oil (Europe).

In Germany, nearly 900 000 t vegetable oilsare used only for industrial applications – withincreasing numbers in the last years. The mainpart today is transesterified to biodiesel (rape-seed methyl ester).

Apart from the abovementioned environmen-tal aspects, the use of natural oils also has anagri-political benefit of enabling the agriculturalindustry to shift its production to technical rawmaterials. The lubricants industry has investedsignificant sums of money in developing andmarketing biodegradable lubricants.

Natural fatty oils such as castor oil, palm oil,rapeseed oil, soybean oil, sunflower oil, lard, de-gras and sperm oil, i.e., triglycerides of more orless unsaturated fatty acids, have been used in lu-bricants for years. These base oils are biodegrad-able and, compared to mineral oils, have excel-lent tribological properties (low friction coeffi-cient, good wear protection). Their range of useis limited by lower stability against thermal ox-idative and hydrolytic stress and partly inferiorcold flow properties. These limits can be im-proved gradually either with additives, or withthe selection, cultivation or genetically modifi-cation of new types of plants. With new typesof ‘High oleic sunflower oils’ (HOSO) withan amount of oleic acid of more than 90% itseems possible to formulate oils for higher per-formance levels.

7.2.1. Synthetic Esters

The collective name ‘synthetic esters’ covers abroad range of chemicals with different qualitiesand prices. For the development of environmen-tally acceptable lubricants, esters have to be se-lected which fulfil the ecological requirementsand have more favorable properties than naturalfatty oils.

These properties are mainly thermal-oxidative resistance, better low temperature be-havior and better resistance to hydrolysis.

At present, polyol esters such as trimethylol-propane esters (TMP esters) or glycerol trioleatedominate. The basis of these aremainly alcohols

frompetrochemical and oleochemical origin andfatty acids derived from natural oils.

In regard to hydrolytic stability, ‘normal’polyol esters differ only slightly from rapeseedoil; the difference in oxidation resistance ismuchgreater. Both characteristics are significantly im-proved with complex esters of polyols and sat-urated fatty acids. Normally an improvementin hydrolytic stability worsens the base fluid’sbiodegradability.

Themost important chemical reactions to im-prove the properties of esters are transesteri-fication, (selective) hydrogenation, ozonolysis,dimerization.

7.2.2. Polyalkylene Glycols (see Section 5.4)

Polyethylene glycols (PEG) are biodegradable.They are not miscible with mineral oils or es-ters, but are water-soluble which, at the presenttime, is considered to be a disadvantage. If PEGis released through a leak or an accident, it willmigrate quickly in the ground or in the water.

The chemical industry is attempting to de-velop biodegradable, non-water-soluble PAGs.These could be alternatives to esters.

7.2.3. Polyalphaolefins (see Section 5.1.1)

Low viscous polyalphaolefins (PAO 2) arebiodegradable. These base stocks have only lim-ited application in the formulation of lubricants.The new types of biodegradable, higher viscos-ity synthetic hydrocarbons may have a greaterinfluence in the near future.

7.2.4. Relevant Properties of BiodegradableBase Oils

Evaporation Loss. The evaporation loss ofesters and vegetable oils is very low. The lowerevaporation loss of ester oils has great advan-tages for example in regard to emissions frommachine tools in metalworking and for the emis-sions, especially the particle emissions, of inter-nal combustion engines.


Viscosity –Temperature Behavior. Usingan ester instead of a mineral oil improves theviscosity – temperature behavior of a lubricant.Thehigher viscosity index (VI) of an ester resultsin a wider temperature range in application butwith the recommended working viscosity. Alsothe higher VI can lead to polymer-free multi-grade lubricants with improved shear stability.

Boundary Lubrication. Most vegetableoils, synthetic esters and glycols display ex-cellent lubricity in boundary lubrication condi-tions. Because of their high degree of polaritythese lubricants are superior to mineral oil basedlubricants. This has been proven by a series oftests. Experimental investigations on twin disktest rigs showed that the friction coefficient ofvegetable oils, synthetic esters and glycols ishalf that of mineral oils:

Figure 36 compares feiction coefficients of amineral oil and a TMP ester of the same viscos-ity grade. As can be seen, the friction coefficientfor this ester is half that of a mineral oil.

7.3. Additives (see also Chap. 6)

Extreme Pressure/Antiwear Additives.Most vegetable oils and synthetic esters used inlubricants display have a high degree of polarity.This characteristic results in better lubricity thanmineral oils in boundary conditions. Sulfurizedfatty materials are environmentally friendly EPadditives. Sulfur as an additive in esters providesanAWeffect. Sulfur carriers with 15% total sul-fur and 5% active sulfur have proven effective inrapidly biodegradable esters and vegetable oils.

Corrosion Inhibitors. Vegetable oils andsynthetic esters show a high polarity. This alsoapplies to corrosion inhibitors and can result ina competitive reaction on the metal surface.

Special calcium sulfonates, succinic acidderivatives or ashless sulfonates can be used toprovide corrosion prevention.

Antioxidants. Special phenolic and aminicmaterials are suitable antioxidants for the for-mulation of biodegradable lubricants.

7.4. Products (Examples)

Because of their physical properties, refined,chemically unchanged vegetable oils can beused as alternatives to petroleum products in alarge number of lubricants. Compared to min-eral oils, their performance in most applicationsis limited by their relatively poor aging resis-tance. The reasons for this are the unsaturatedhydrocarbon chains in the natural fatty acids andthe easy hydrolysis of the ester compounds.

Vegetable oils, at present, are primarily usedfor total-loss applications and products withrelatively low technical specifications. At facevalue, using rapeseed oil as a basis for engineoils does not appear very promising – the presentoperating conditions of engine oils cannot be sat-isfied by chemically unmodified vegetable oils.

Over many years the development in the lu-bricants market had the main aim to formulateproducts for ‘long oil drain intervals’ or for life-time. This was not possible with vegetable oil.But today the opposite development can be rec-ognized in some applications – the substitutionof circulating oils by total-loss lubricants:

– Example 1: Minimum Quantity Lubricationin metalworking

– Example 2: Die-casting and forming insteadof cutting and grinding

– Example 3: Oil-refreshing systems for lubri-cation of four-stroke engines

These ideas are not primarily intended for useof chemically unchanged vegetable oils in newapplications. The main idea behind all examplesis the lower overall impact of lubricants and theirdegradation products on the environment by useof total-loss lubricants.

For four-stroke engines, a novel lubricationconcept for diesel engines was developed whichcounteracts the relatively poor aging resistanceof vegetable oils by continuous refreshing (Plan-totronic system). This has permitted the use ofrapeseed and sunflower oils for engine lubrica-tion. In this lubrication system, used oil is grad-ually burned along with the fuel itself withoutnegatively influencing emission values.

The concept can be seen as a way of continu-ously refreshing a vegetable-based engine oil oras a way of performing a continuous oil change.Combustion in the engine was only made pos-sible by the use of low-additive vegetable oils –


Figure 36. Friction coefficients of two base fluids as function of slip

Figure 37. Scheme of lubrication points in an engine

burning conventional oils seems highly con-tentious considering the additives commonlyused in such oils. The use of vegetable oils forengine lubrication depends on the progressiveburning of used engine oil with the fuel. Theadvantage of the oil refreshing method is that

chemically unmodified, low additivated veg-etable oils can be used and that no waste oilhas to be disposed of. Another advantage is theautomated oil-change, which helps to reduce themaintenance costs especially for stationary en-gines (power plants).


8. Lubricants for InternalCombustion Engines

8.1. Four-Stroke Engine Oils

Technically and commercially, engine oils arethe number 1 among lubricants and functionalfluids; on the global lubricants market, they ac-count for more than 60%.

While demand in Europe is more or less stag-nating and will decline slightly in the foresee-able future in spite of increasing vehicle regis-trations, there is still considerable growth poten-tial in Southeast Asia and third world thresholdcountries (China, India, Korea etc.) [201].

8.1.1. General Overview

Historically the development of engine oils overthe last 50 years has been focussed on the spec-ifications issued by the international automo-bile industry. Starting with the first specifica-tions published by the US Army (MIL specs),today there are three internationally recognizedsets of minimum requirements. In Europe, theseare ACEA, in the USAAPI, and ILSAC in Asia.Details of these specifications can be found inSection 8.1.3.

In principle, all specifications reflect the suc-cessive adaptation of oil qualities to develop-ments in engine design. Back in the 1950s,monogrades dominated the engine oil market.The viscosity of these oils was matched to theprevailing ambient temperature and thereforehad to be changed between summer and winter.The 1960s saw the development of mineral oil-based multigrades, i.e., combined summer andwinter oils, initially high-viscosity types (SAE20W-50) and later the present standard 15W-40. Base oil distillation and refining processescaught up with the new viscosity and perfor-mance requirements (see Chap. 4). This led tonew qualities with comparatively low evapora-tion losses and optimized cold flowing proper-ties. Semisynthetic, and since the 1920s, fullysynthetic oils have dominated the premium-quality market, especially in the passenger carsector. Since the 1990s, a niche for environmen-tally friendly, biodegradable products has beencreated in the German-speaking market [202].

8.1.1.1. Fundamental Principles

Engine oils have to fulfil a wide range of func-tions in engines. The purely tribological taskconsists of guaranteeing the functional reliabil-ity of all friction points in all operating condi-tions (Fig. 37). In addition, engine oils have toperformanumber of other functions. This beginswith the sealing the cylinder and ends with thetransport of sludge, soot, and abraded particlesto the oil filter.

Starting with the tribological functions, thethree classic sections of the Stribeck graph aresatisfied, from hydrodynamic full lubrication tothe elastohydrodynamic area in bearings to theboundary friction conditions at TDC and BDC[203]. All friction pairings and a whole seriesof parameters are covered. The temperaturesencountered range from ambient in the Arctic(− 40 ◦C) to sump temperatures of 100 ◦C topeak values of over 300 ◦C under the pistoncrown.

During the combustion process, the engineoil helps to seal the piston and cylinder. At thesame time, it should burn off at the cylinder wallwithout leaving any residue. The engine oil dis-sipates heat from the piston and thus cools it. Theblow-by gases formed when fuel is burned andtheir reaction byproducts have to be neutralizedand held in suspension. The same applies to thesoot and sludge particles formed by incompletecombustion. The oil also transports dirt and anyabraded particles to the oil filter and ensures itsfilterability. In addition, any water formed dur-ing the combustion process should be emulsifiedand even when higher water concentrations arepresent and when the phases separate as temper-ature falls, the oil should protect against corro-sion.

Engine oils should reduce friction and wearduring extreme, low-temperature start-ups aswell as when the lubricating film is subject tohigh temperature and pressure in bearings andaround the piston rings. Whereas the oil shouldstill flow well and be pumpable without aera-tion at low temperatures (down to − 40 ◦C) toavoid metal-to-metal contact during cold start-ups, the lubricating film must perform satisfac-torily in bearings and hydraulic tappets [204]. Atlow temperatures, additives must not precipitateand the oil must not gel. At the upper end of the


temperature scale, the oilmust offer far-reachingresistance to thermal andmechanical aging. Andfinally, the stability of the lubricating film shouldnot be diminished by fuel dilution of up to 10%.

The viscosity of an engine oil is an indica-tor of how readily a load-carrying film can beformed at all lubrication points in an engine. Theadequate and rapid circulation of the oil at lowtemperatures [205]must be achieved at cold startcranking speeds. On the other hand, viscositymust not fall too much at high temperatures sothat adequate lubricating film stability is givenat high thermal loads.

Corresponding threshold values were deter-mined by the SAE and the ASTM as shown inTable 8.

According to this list, all viscosity grades canbe described by their minimum kinematic vis-cosity at 100 ◦C. Additional dynamic viscositythresholds apply to winter grades, which displaythe letterW. These values are determined in coldcranking simulators (CCS) or in minirotary vis-cometers (MRV). The dynamic viscosity valuegiven by the CCS is a measure of flow prop-erties [206] at low temperatures whereby thehigh shearing rate can mask paraffin crystalliza-tion. In the MRV, a so-called threshold viscosityof max. 60 000mPa · s has been determined for10 ◦C lower temperatures to ensure that the oilpump does not draw air.

Figure 38. Comparison of monogrades and multigrades

The high-temperature high-shear viscosity isan additional criterion for evaluating lube filmstability at high shear rates and high tempera-tures in summer grades. Modern engine oils aremultigrade oils whose low temperature charac-teristics are indicated by theWand the high tem-

perature viscosity by the number following theW. TheV –T behavior ofmultigrade oils is sche-matically sketched in Fig. 38.

8.1.1.2. Performance Specifications

The engine oil market is shifting away fromconventional products to semisynthetic and syn-thetic formulations. Along with their higherprice, the economic and ecological demands onthese oil have also risen (see Chap. 4).

At present, the oil change intervals for carsare between 10 000 and 20 000 km and 30 000and 60 000 km for trucks. In the future, thesefigures are expected to double. A retrospectiveview of specific oil performance shows that thisdevelopment has been in progress for the last 50years [207]. Figure 39 shows that oil consump-tion per energy unit has fallen eight times duringthis period.

As the established CEC engine tests do notallow comprehensive testing of all required oilproperties, additionally in-house methods arenow being used. A series of automobile manu-facturer original manufacturer (OEM) specific,long-term trials,whichhavenowbeen takenoverfor developing oils, are part of these engine tests.In total, these tests represent an enormous tech-nical advance and financial expense because theengine tests themselves have been supplementedby radionuclide techniques [208]. The advan-tages of this technology lie in the on-line mon-itoring of wear in defined running conditions,from running-in to full-throttle operation as wellas the selective examination of critical enginecomponents.

8.1.1.3. Formulation of Engine Oils

Engine oils are complex mixtures of base oilsand additives. Compared to other groups of lu-bricants, the base oils [209] play an importantrole (see Chaps. 4 and 5). Mixtures of base oilsare selected which have the necessary viscosityand performance to correspond to a rough classi-fication. The final products are then marketed asconventional, mineral oil-based, unconventionalsemisynthetic (hydrocracked) and synthetic en-gine oils. Precise international nomenclature di-vides base oils into five groups:


Table 8. Engine oil viscosity classification (SAE J 300 revised 4/1997).

SAE viscosity grade Low temperaturecranking viscosity(mPa · s) at temp.in ◦C (CCS)

Low temperaturepumping viscosity(mPa · s) at temp.in ◦C (MRV)

Low shear-rate kinematic viscosity at100 ◦C (mm2/s)

High shear-rateviscosity at 150 ◦Cand 106 s−1 (mPa · s)

Maximum Maximum Minimum Maximum Minimum0W 3250 at − 30 60 000 at − 40 3.8 – –5W 3500 at − 25 60 000 at − 35 3.8 – –10W 3500 at − 20 60 000 at − 30 4.1 – –15W 3500 at − 15 60 000 at − 25 5.6 – –20W 4500 at − 10 60 000 at − 20 5.6 – –25W 6000 at − 5 60 000 at − 15 9.3 – –20 – – 5.6 < 9.3 2.630 – – 9.3 < 12.5 2.940 – – 12.5 < 16.3 2.9∗

40 – – 12.5 < 16.3 3.7∗∗

50 – – 16.3 < 21.9 3.760 – – 21.9 < 26.1 3.7

∗ For 0W, 5W, 10W. ∗∗ For 15W, 20W, 25W and Monogrades.

Figure 39. The history of oil stress

– Group 1: SN mineral oils with saturates< 80, 80<VI< 120, S> 0.1

– Group 2: HC oils with saturates > 80,80<VI< 120, S< 0.1

– Group 3: HC oils with saturates > 80;VI> 120, S< 0.1

– Group 4: Polyalphaolefins– Group 5: Esters and others

8.1.1.4. Additives

Depending on the base oil used and the requiredengine performance, engine oils can contain upto 30 different additives whose percentage con-tent can range from 5 to 25% in total. As a

rule, performance additives make up the largestgroup.

Performance Additives. The followingspecies of chemical components are summarisedunder the general term of performance additives(Table 9).

Table 9. Performance additives

Antioxidants phenols and aminesAntiwear agents metal dithiophosphates, carbamatesDetergents Ca and Mg sulfonates, phenolates,

salicylatesDispersants polyisobutene and ethylene-propene

oligomers with nitrogen and/or oxygencontaining functional groups

Friction modifiers MoS compounds, alcohols, esters, fattyacid amides, etc.

Antimisting agents silicones and acrylates


These additive groups are described in de-tail in Chapter 6. Particularly in the case of en-gine oils, the substance categories listed gener-ally perform more than one function. ZnDTPs,for example, are primary antiwear additives butalso have a secondary antioxidant character re-sulting from a specific decomposition mecha-nism [210]. Furthermore, complex formulationsof a number of individual components typicallydisplay synergistic as well as antagonistic inter-actions which have to be matched to the appli-cation considered. The composition of the baseoil components has an additional effect on thesespecific interactions. Considerable experienceand know how is thus necessary to create an op-timum formulation.

Viscosity Modifiers and Pour-Point De-pressants (see Section 6.2). Viscosity modifierscan be divided into two groups, the nonpolar,nondispersing and the polar, dispersing group.The first group are only really needed to set theviscosity of multigrade oils. In an absolute con-centration of 0.2 to 1.0%, viscosity modifierscan generate a viscosity increase of between 50and 200% depending on chemical structure andbase oil solubility. Due to special modification,dispersing viscosity improvers are often ash-less dispersants with additional thickening ef-fects. Furthermore, viscosity improvers andpourpoint depressants have an effect on low temper-ature behavior of a formulation (pour point, coldcranking simulator, mini rotary viscometer) andare essential parts of the high-temperature high-shear viscosity. In the USA, additional demandsare made on low-temperature stability (gelationindex)which cannot be achievedwithout the vis-cosity improvers and pour point depressants be-ing matched to the base oil.

8.1.2. Characterization and Testing

8.1.2.1. Physical and Chemical Testing

The physicochemical properties of an engine oilcan be determined in the laboratory with stan-dard test methods (Section 16.3). This charac-terizationmainly focuses on rheological test val-ues and the previously shown SAE classificationsystem.

Various viscosity tests are used to deter-mine exact low- and high-temperature viscosi-ties [211]. The viscosity thus determined is acharacteristic of the engine oil at a definedengine state. At low temperatures (− 10 to− 40 ◦C), a mini rotary viscometer with a lowshear gradient is used to determine the apparentviscosity and thus the oil’s flowability in the areaof the oil pump. In addition, maximum viscos-ity as the threshold of viscosity is determined infive graduated steps. The dynamic CCS viscos-ity, which is determined at − 5 to − 30 ◦C witha high shear gradient, is also an apparent viscos-ity which represents the tribological conditionsat the crankshaft during cold starts. The maxi-mum values laid down in SAE J 300 guaranteereliable oil circulation during the start-up phase.The rheological characteristics at higher thermalloads which occur during full-throttle operationare described by the dynamic viscosity at 150 ◦Cand a shear rate of 10−6 s−1 or high-temperaturehigh-shear (HTHS). The corresponding thresh-old values also guarantee an adequate lube filmeven in these conditions.

Apart from the rheological characteristics,the Noack evaporation test (see Section 16.3) isused to test the volatility of base oils and addi-tives aswell as foaming tendency and air release.The compatibility of high-additive oils with sealmaterials is tested on standard reference elas-tomers in static swelling and subsequent elonga-tion tests [212]. The viscosity loss resulting frommechanical load is described in Section 6.2.

8.1.2.2. Engine Testing

Because realistic engine oil tests cannot be per-formed only over year-long field trials, a numberof international committees have created meth-ods of testing engine oils in defined test enginesoperated in reproducible and practically relevantconditions.

In Europe, the Coordinating European Coun-cil for the Development of Performance Testsfor Lubricants and Fuels (CEC) is responsi-ble for testing, approval and standardization[213]. Performance requirements are set-up inthe form of Association des Constructeurs Eu-ropeen d’Automobiles (ACEA) limits which aredecided together with the additive and lubricantindustries. In the USA, this task is performed by


the automobile industry and the API. This insti-tution lays down test procedures and limits. TheAsian ILSAC has largely adopted the Americanspecifications for automobiles.

In principle, the test procedures detailed inSection 8.1.3 focus on the following general per-formance criteria:

– oxidation [214] and thermal stability– dispersion of soot and sludge particles– protection against wear [215] and corrosion– foaming and shear stability [216]

8.1.2.3. Passenger Car Engine Oils

Car engines include all gasoline and light dieselengines with direct or indirect injection. Rele-vant passenger car engine tests are listed in Ta-ble 10.

Table 10. Passenger car engine tests

Test engine Test procedure Test criteria

Peugeot XUD 11 CEC L-56-T-95 soot handlingpiston cleanliness

Peugeot TU3 H CEC L-55-T-95 oxidationcleanlinessring sticking

Peugeot TU3 S CEC L-38-A-94 cam and tappet wearSequence II D ASTM STP B15 M P1 bearing corrosionM 111 SL CEC L-53-T-95 black sludge

cam wearSequence IIIE ASTM STR 315 M P2 oxidation

wearcleanliness

Sequence VE sludgepiston cleanlinesscorrosion and wear

BMW M52 valve trainair entrainmentwear

VW T4 oil oxidationTBN depletionpiston cleanliness

M111 FE CEC L-54-T-96 fuel efficiencyVW-DI P-VW 1452 piston cleanliness

ring stickingVW-TD CEC L-46-T-93 piston cleanliness

ring sticking

8.1.2.4. Engine Oil for Commercial Vehicles

Commercial vehicles include trucks, buses, trac-tors, harvesters, construction machines and sta-tionary machinery powered by diesel engines.

Apart from the prechamber diesels engineswhich have largely been superceded in Europe,the engines are usually highly turbocharged di-rect injection motors. Economic and ecologicalaspects alongwith high injection pressures, haveimproved combustion and thus reduce emis-sions. As an initiative of ACEA, oil change in-tervals have been extended up to 100 000 km forlong haulage.

Long life and reliability are the criteria for thecommercial vehicle sector. The HD oils have tomatch these requirements. Relevant heavy-dutyengine tests are listed in Table 11.

Table 11. Heavy-duty engine tests.

Test engine Test procedure Test criteria

Caterpillar 1K/1N piston cleanlinesswearoil consumption

Cummins M11 valve train wearsludge

Mack T8 soot handlingMack T9 cylinder wearGM 6.2 Liter valve train wearOM 364LA CEC L-42-T-99 piston cleanliness

cylinder wearsludgeoil consumption

OM 602A CEC L-51-A-98 wearcleanlinessoxidationoil consumption

OM 441LA CEC L-52-T-97 piston cleanlinesscylinder wearturbocharger deposits

HD oils are allocated to the following cate-gories with increasing performance:

– heavy duty (HD)– severe heavy duty (SHPD), and– extreme heavy duty (XHD)

Independent of the viscosity grade and thebase oils used, classic HD oils have a high re-serve alkalinity and thus a higher content ofalkaline-earth salts and organic acids [217].Alsoregarding ashless dispersants, the oils are de-signed for at soot dispersing. Special viscosityimprovers are used generally to avoid additionaldeposits.

Oils for vehicle fleets pose a particular chal-lenge. As opposed to special products, theseshould simultaneously satisfy as many car andtruck demands as possible. Possible pistoncleanliness provided by high concentrations of


Table 12. Engine oil classification according to API SAE J 183

Gasoline engines (Service classes)

API-SA Regular engine oils possibly containing pour point improvers and/or foam inhibitors.API-SB Low-additive engine oils low-power gasoline engines. Include additives to combat aging,

corrosion and wear. Issued in 1930.API-SC Engine oils for average operating conditions. Contain additives against coking, black sludge,

aging, corrosion and wear. Fulfil the specifications issued by US automobile manufacturers forvehicles built between 1964 and 1967.

API-SD Gasoline engine oils for more difficult operating conditions than API-SC. Fulfil the specificationsissued by US automobile manufacturers for vehicles built between 1968 and 1971.

API-SE Gasoline engine oils for very severe demands and highly-stressed operating conditions (stop andgo traffic). Fulfil the specifications issued by US automobile manufacturers for vehicles builtbetween 1971 and 1979. Covers API-SD; corresponds approximately to Ford M2C-9001-AA,GM 6136 M and MIL-L 46 152 A.

API-SF Gasoline engine oils for very severe demands and highly-stressed operating conditions (stop andgo traffic) and some trucks. Fulfil the specifications issued by US automobile manufacturers forvehicles built between 1980 and 1987. Surpasses API-SE with regard to oxidation stability, wearprotection and sludge transportation. Corresponds to Ford SSM-2C-9011 A (M2C-153-B), GM6048-M and MIL-L 46 152 B

API-SG Engine oils for the severest of conditions. Include special oxidation stability and sludge formationtests. Fulfil the specifications issued by US automobile manufacturers for vehicles built between1987 and 1993. Specifications similar to MIL-L 46 152 D

API-SH Specification for engines oils built after 1993. API-SH must be tested according to the CMA’sCode of Practice. API-SH largely corresponds to API-SG with additional demands regardingHTHS, evaporation losses (ASTM and Noack tests), filterability, foaming and flash point.Furthermore, API-SH corresponds to ILSAC GF-1 without the Fuel Economy test but with thedifference that 15W-X multigrade oils are also permissible.

API-SJ Supercedes API-SH. Greater demands regarding evaporation losses. Valid since 10/96.

Diesel engines (Commercial classes)

API-CA Engine oils for low-power gasoline and normally aspirated diesel engines run on low-sulfur fuels.Corresponds to MIL-L 2 104 A Suitable for engines built into the 1950s.

API-CB Engine oils for low-to-medium power gasoline and normally aspirated diesel engines run onlow-sulfur fuels. Corresponds to DEF 2101 D and MIL-L 2104 A Suppl. 1 (S1). Suitable forengines built from 1949 on. Offer protection against high-temperature deposits and bearingcorrosion.

API-CC Gasoline and diesel engine oils for average to difficult operating conditions. Corresponds toMIL-L 2 104 C Offer protection against black sludge, corrosion and high-temperature deposits.For engines built after 1961.

API-CD Engine oils for heavy-duty, normally aspirated and turbocharged diesel engines. Covers MIL-L 45199 B (S3) and corresponds to MIL-L 2 104 C Satisfies the requirements of Caterpillar Series 3.

API-CD II Corresponds to API-CD. Additionally fulfils the requirements of US 2-stroke diesel engines.Increased protection against wear and deposits.

API-CE Engine oils for heavy-duty and high-speed diesel engines with or without turbocharging subject tofluctuating loads. Greater protection against oil thickening and wear. Improved piston cleanliness.In addition to API-CD, Cummins NTC 400 and Mack EO-K/2 specifications must be fulfilled.For US engines built after 1983.

API-CF Replaced API-CD for highly turbocharged diesel engines in 1994. High ash. Suitable for sulfurcontents > 0.5%.

API-CF-2 Only for 2-stroke diesel engines. Replaced API-CD II in 1994.API-CF-4 Engine oil specification for high-speed, 4-stroke diesel engines since 1990. Meets the

requirements of API-CE plus additional demands regarding oil consumption and pistoncleanliness. Lower ash content.

API-CG-4 For heavy-duty truck engines. Complies with EPA’s emission thresholds introduced in1994. Replaced API-CF-4 in June 1994.

API-CH-4 Replaces API-CG-4. Suitable for sulfur contents > 0.5%.All engines (Energy Conserving)(API-EC I) (min. 1.5% less fuel consumption than an SAE 20W-30 reference oil in a 1982, 3.8 liter, Buick

V6 gasoline engine. Sequence VI test)(API-EC II) Same as API-EC I but with minimum 2.7% lower fuel consumptionAPI-EC Replaces API-EC I and II. Only together with API-SJ. Cuts in fuel consumption: 0W-20, 5W-20

> 1.4%, 0W-XX, 5W-XX > 1.1%, 10W-XX, others > 0.5%, Sequence VI A test: In a 1993, 4.6liter Ford V8 engine. Reference oil 5W-30


over-based soaps is sacrificed because gasolineengines are prone to self-ignition if high propor-tions of metal detergents are present. As a re-sult, other components are selected, such as theskillful use of unconventional base oils alongwith detergents, dispersants, VI improvers andantioxidants.

8.1.3. Classification by Specification

8.1.3.1. MIL Specifications

These specifications originate from the USForces which set the minimum requirementsfor their engine oils. These are based on cer-tain physical and chemical data along with somestandardized engine tests. In the past, these clas-sifications were also used in the civilian sectorto define engine oil quality. In the 1980s, thesespecifications have become almost irrelevant forthe West European market.

MIL-L-46152 A to MIL-L-46152 E. Thesemilitary specifications have nowbeen discarded.Engine oils which meet these specifications aresuitable for use in US gasoline and diesel en-gines. MIL-L-46152 E (discarded in 1991) cor-responds to API SG/CC.

MIL-L-2104 C classifies high-additive en-gine oils for US gasoline and normally aspiratedand turbocharged diesel engines.

MIL-L-2104 D coversMIL-L-2104C and re-quires an additional engine test in a highly-charged Detroit two-stroke diesel engine. In ad-dition, Caterpillar TO-2 and Allison C-3 speci-fications are fulfilled.

MIL-L-2104 E similar in content toMIL-L-2104 C The gasoline engine tests have been up-dated and includemore stringent test procedures(Seq. III E / Seq. V E)

8.1.3.2. API and ILSAC Classification

The API together with the ASTM and the SAEhave created a classification in which engine oilsare classified according to the demands madeon them, bearing in mind the varying condi-tions in which they are operated and the differ-ent engine designs in use (Table 12). The testsare standard engine tests. The API has defined aclass for gasoline engines (S = service oils) and

for diesel engines (C = commercial). Diesel en-gines in passenger cars are almost unheard ofin the USA. In addition, a number of fuel econ-omy stages has been determined (EC= energyconserving).

8.1.3.3. ACEA Specifications

As a result of persistent internal differences, theCCMC was disbanded and succeeded by theACEA. CCMC specifications remained valid inthe interim period. The new ACEA classifica-tions (Table 13) came into force on January 1,1996.

Table 13. Engine oil classification according to ACEA – Part I

Gasoline engines

A1-96 Category for low-viscosity engine oilsparticularly low HTHS viscosity(< 3.5mPa · s). This engine oil type wasinitially favored by Ford and Rover.Recommended viscosity grades are xW-30and xW-20.

A2-96 Category for conventional and low-viscosityengine oils (no restriction on the permissibleviscosity grades) with increased demands onthe former CCMC G4. Out-performs API SH.

A3-96 Category for conventional and low-viscosityengine oils (no restriction on the permissibleviscosity grades). Out-performs ACEA A2-96with regard to Noack (evaporation losses),piston cleanliness and oxidation stability.Furthermore, out-performs API SH, CCMCG4 and G5.

Car diesel engines (Light duty diesel engines)

B1-96 Category for low-viscosity engine oils withparticularly low HTHS viscosity (correspondsto A1-96).

B2-96 Category for conventional and low-viscosityengine oils (no restriction on the permissibleviscosity grades) with increased demands onthe former CCMC PD 2.

B3-96 Category for conventional and low-viscosityengine oils (no restriction on the permissibleviscosity grades). Out-performs ACEA B2-96with regard to cam wear, piston cleanlinessand viscosity stability in high soot conditions.

Truck dieselengines

(Heavy-duty diesel engines)

The new ACEA categories for truck diesel engines are largelybased on Mercedes –Benz specification sheets.E1-96 Corresponds in general to the former CCMC

D4.E2-96 Largely based on Mercedes –Benz Sheet

228.1. Mack T8 test is also required.E3-96 Largely based on Mercedes –Benz Sheet

228.3. Mack T8 test is also required.


The ACEA specifications were revised in1996 and replaced by 1998 versions. Tests couldbe performed to 1996 specifications until March1, 1999. The 1998 specifications became validon March 1, 1998. 1996 specifications ceased tobe valid on March 1, 2000. When ACEA speci-fications are quoted, the year must no longer begiven.

Additional foaming tests were introduced forall categories and the elastomer tests were mod-ified (Table 14).

Table 14. Engine oil classification according to ACEA – Part II

Gasoline engines

A1-98 Category for Fuel Economy engine oils withespecially low HTHS viscosity(< 3.5mPa · s). Recommended viscositygrades are xW-30 and xW-20. Compared to a15W-40 reference oil, fuel savings of ≥ 2.5%must be proven in a Mercedes test engine (M111).

A2-98 Category for conventional and Fuel Economyengine oils. Replaces A2-96.

A3-98 Category for conventional and Fuel Economyengine oils with greater requirements thanA2-98. Surpasses ACEA A2-98 with regardto Noack (evaporation losses), pistoncleanliness and oxidation stability. ReplacesA3-96.

Car diesel engines (Light duty diesel engines)

B1-98 Category for Fuel Economy engine oils withespecially low HTHS viscosity (same asA1-98). Recommended viscosity grades arexW-30 and xW-20. Compared to a 15W-40reference oil, fuel savings of ≥ 2.5% must beproven in a Mercedes test engine (M 111).Replaces B1-96.

B2-98 Category for conventional and Fuel Economyengine oils. Replaces B2-96.

B3-98 Category for conventional and Fuel Economyengine oils. Surpasses ACEA B2-98 withregard to cam wear, piston cleanliness andviscosity stability in high soot conditions.Replaces B3-96.

B4-98 New category for direct injection dieselengines (TDI).

Truck dieselengines

(Heavy-duty diesel engines)

E1-98 Corresponds to ACEA E1-96.E2-98 Corresponds to ACEA E2-96.E3-98 Corresponds to ACEA E3-96.E4-98 Largely based on Mercedes –Benz Sheet

228.5. No OM 364 A engine test required butMack T8 and T8E.

(E5xx) (Planned category which should include allrequirements which reduce wear caused bysoot and deposits)

8.1.3.4. Manufacturers’ Approvals

Apart from the specifications listed in Tables 13,14, and 15, a number of manufacturers demandtests on their own engines (Table 15).

8.1.3.5. Future Trends

New generations of engines using optimizedtechnologies advance the concept of tailor-made, special oils.

The continuing optimization of the combus-tion process to increase the efficiency of gasolineengines has led to the development of direct-injection gasoline engines (GDI engines) whichmay offer fuel savings of about 20%. On thediesel side, direct injection with unit pumps orcommon rail technology using pressure of up to3000 bar have become the norm. These designswhich originate in truck engines, offer powerincreases of up to 50% at almost constant fuelconsumption. In the truck area, reducing ex-haust emissions has the highest priority. Thepresent thresholds of Euro 2 and Euro 3 (from2001) will be easily surpassed with special ex-haust recycling and catalytic converter systems.As these generations of engineswill require low-sulfur diesel fuels (10 ppm), new demands willbe made on the lubricants used.

Furthermore, the surface treatment of pis-tons and cylinders has improved to such an ex-tent that topology-specific oil consumption issteadily falling. In addition, oil change intervalshave been constantly increased. All in all, threefactors will characterize engine oils of the fu-ture – fuel efficiency, long oil drain intervals,and low emissions.

Fuel Efficiency. As a result of strictlimits to fuel consumption in the USA(CAFE=Californian Act for Fuel Emissions)and the proven fuel economy effect of low-viscosity engine oils [218], this topic is attract-ing attention in Europe and Asia. As a rule,engine-based savings can reach a theoretical8− 10% [219]. As engine oils cannot totallyeliminate frictional losses, saving potentials of4 and 5% present enormous challenges. How-ever, values of between 3 and 4% are alreadypossible today.


Table 15.Manufacturers’ approvals

VW-Norm Application area

VW 50000 Low-viscosity oils for gasoline and normally-aspirated diesel engines. Only SAE 0W-XX,5W-XX and SAE 10W-XX oils. XX > 40 oils were not included after 10/91.

VW 50101 Multigrade engine oils without low-viscosity characteristics for gasoline and normally-aspirateddiesel engines.

VW 50200 New specification for car gasoline engines with extended oil change intervals.VW 50300 New specification for car gasoline engines with extended service intervals. Surpasses the

requirements of 50200 (HTHS ≥ 2.9 mPa · s).VW 50301 Planned specification for turbocharged car gasoline engines with extended service intervals

(HTHS > 3.5 mPa · s).VW 50500 All-season engine oils for diesel engines with or without turbocharging.VW 50501 All-season engine oils especially for Unit Pump diesel engines.VW 50600 New specification for diesel engines with extended service intervals (HTHS ≥ 2.9 mPa · s).VW 50601 Planned specification for Unit Pump diesel engines with extended service intervals.

MB Sheet Application area

MB 226.0 Monograde engine oils for normally-aspirated diesel engines.MB 227.0 Monograde engine oils for turbocharged diesel engines.MB 227.1 Multigrade engine oils for turbocharged diesel engines.MB 228.1 Multigrade engine oils for turbocharged diesel engines. Oil change intervals up to 30 000 km.MB 228.3 SHPD engine oils for highly turbocharged diesel engines. Extended oil change intervals up to

45 000 km.MB 228.5 UHPD (ultra high performance diesel) engine oils for highly turbocharged diesel engines.

Extended oil change intervals in light-duty engines up to 45 000 km. Up to 160 000 km possible inheavy-duty diesel engines (with service interval displays).

MB 229.1 Engine oils for cars (gasoline and diesel engines). Introduced 05/97. More stringent than ACEAA2-96/A3-96 and B2-96/B3-96.

MB 229.3 Engine oils for cars with extended oil change intervals (30 000 km).

MAN in-housespecification

Application area

MAN 270 Monograde engine oil turbocharged and non-turbocharged diesel engines.MAN 271 Multigrade engine oil turbocharged and non-turbocharged diesel engines.MAN QC13-017/M 3275

SHPD engine oils for all diesel engines with oil change intervals up to 45 000 km.

MAN M3277 UHPD engine oils for all diesel engines with oil change intervals up to 80 000 km.MAN 3271 Oil for engines run on gas (natural gas, propane or butane).

Scania Application area

ACEA E3 Oil change intervals up to 60 000 km.LDF approval ACEA E3 oil with special long drain field-test approval. Oil change intervals up to 120 000 km.VDS Volvo drain specification for extended oil change intervals (50 000 km).VDS-2 Specified for Euro 2 engines (60 000 km).(ACEAE4-98) DAF recommends oils with the following viscosities; 10W-40, 10W-30 and 5W-40. Oil change

intervals up to 100 000 km.Oil Type 1 Normal quality (ACEA E1-96, E2-96)Oil Type 1∗ Type 1 plus corrosion protectionOil Type 2 Higher quality (SHPD, ACEA E3-96)

Reduction in car fuel consumption in urbanconditions are achieved by lowering frictionallosses during cold start-ups which simultane-ously result in less wear and a lowering of vis-cosity in constant throttle conditions. There isa plateau-like optimum for the correlative fuelsavings in engines with HTHS values between2.5 and 2.9mPa · s.

According to OEMs, HTHS values must notbe lower than minimum 2.6mPa · s in all manu-facturer’s approvals and new engine oil develop-ments because of possible wear between criticalmaterial pairings.

Long Drain Intervals. Concerns abouthigher wear and thus shorter life caused by low-


viscosity oils run contrary to the trends of thenew generation of engines which are designedto cope with even longer oil change intervals.As stated in manufacturer’s specifications, oilchange intervals for cars are currently 15 000 kmfor gasoline engines and 30 000 km for dieselengines. At present there are demands to doublethese figures without detracting from the fuelefficiency and wear performance of the oil. Asa result of these diverging requirements, futureengine oil specifications might contain a wholeseries of new OEM-specific engine tests.

The radionuclide technique, as a proven on-line tool, is experiencing a renaissance for ex-amining the effects on wear in various operatingconditions such as during running-in or to deter-mine long-term stability.

Apart from long-term wear, very high de-mands are made on high oxidation stability andlow evaporation losses. This strengthens thetrend towards synthetic and unconventional oilas the basis for such high performance engineoils. A graphic correlation between evaporationtendency (Noack) and oil consumption is givenin Figure 40. A technically realized milestonefor fully synthetic engine oils based on presentsynthetic base oils is an evaporation loss of 5 to6%.

The suitability of extendedoil change intervaloils is tested in thermally stressed test engineswhich run hot and without oil top-up. The typ-ical indicators of aging like viscosity and TBNare measured.

Low Emission. Heavy-duty engine oils al-ready achieve drain intervals of 100 000 km. Be-cause of the ever increasing number of trucks onroads, this is a useful contribution to improvingenvironmental compatibility.

Apart from the CO, HC and SO2 emissionswhich are seen to be caused by the fuel, par-ticulate emissions play a significant role in HDengines. These particulates are a result of incom-plete combustion and are a mixture of fuel- andlubricant-based components. As the oil-basedparticles are largely caused by highly volatileelements in the formulation, evaporation losseshave a direct effect on reducing pollutants (seeFig. 40). Furthermore, it is assumed that sulfurcompounds in diesel fuels will poison the cat-alytic converters of future Euro 3 and Euro 4 en-gines. The call for low sulfur (10 ppm) and most

probably low phosphorus engine oils will notonly lead to a different assessment of additivesbut also to the rejection of solvent neutral oilswhich, as a rule, contain between 0.2 and 1.0%sulfur. Similar to passenger cars unconventionaland synthetic base oils are preferred so that thetrend towards low viscosity, fuel economy oilswill spread definitively to the heavy-duty sector.

8.2. Two-Stroke Oils

8.2.1. Application and Characteristics[220–222]

Two-stroke engines are mostly used when highspecific power, lowweight and low price are keyparameters. These engines are thus often usedin motorcycles, boats (outboard engines), jet-skis, lawn mowers, chain saws and small vans,whereby the vast majority are found in motor-cycles and boats.

Almost all two-stroke engines use total-losslubrication, i.e., the oil is not circulated as in thecase of four-stroke engines but added to the fuel.A large part is burnt in the combustion process,about one quarter is exhausted as unburnt oilmist. Simple engines as found in older mopedsstill use the premix method, whrere a suitabletwo-stroke oil to is added manually the fuel tankat a ratio of about 1 : 20 to 1 : 100. More ad-vanced designs use an automatic oil meteringsystem. These either add a constant amount ofoil to the fuel or add oil according to engine load-ing. Typical oil : fuel ratios are between 1 : 50and 1 : 400.

In the majority of simple two-stroke, the en-gine breathes through classic carburetors. Con-trary to four-stroke engines, the fresh fuel/airmixture in a two-stroke engine scavenges thecylinder after combustion. This simultaneouscharging and emptying causes about 30% of thefresh mixture to be exhausted without burning.

This disadvantage, alongwith the only partialburning of the oil, causes many two-stroke en-gines to generate comparatively high emissions.In highly populated areas with a large number ofsmall motorcycles, such as in many Asian cities,this leads to severe odor, smoke and noise pol-lution.

In recent years, these typical disadvantageshave been countered by some advances in two-


Figure 40. Oil consumption and relative oil generated particulate emissions versus evaporation loss

stroke technology. The development of direct orindirect fuel injection has led to significantly re-duced emissions and improved fuel efficiency.

Today’s engines require correspondinglyhigh quality oils for reliable operation and longlife. The principal criteria for the quality of two-stroke oils are:

– lubricity and antiwear properties– cleaning function (detergent/dispersantproperties)

– avoidance of deposits in the exhaust system– low smoke– spark plug cleanliness and the avoidance ofpreignition

– good fuel miscibility even at low tempera-tures

– corrosion protection– good flowing properties

About 85− 98% of two-stroke oils are baseoils with the rest consisting of various additives.In principle, all common base oils can be used,ranging from brightstocks, solvent neutral typesto fully synthetic polyalphaolefins. Higher qual-ity two-stroke lubricants often contain varioussynthetic esters and this is particularly the casefor biodegradable oils which were specially de-veloped for marine outboards.

The additives in two-stroke oils are usu-ally matched to the requirements of the en-gine. As in the case of four-stroke engine oils,antiwear additives are included in two-stroke

oils. Along with the most commonly used zincdialkyldithiophosphates, nonash-forming typessuch as dithiophosphoric acid esters of alkyl andaryl esters or phosphoric acids are used.

The following DD additives are added tothe oil: Alkali or alkaline-earth salts of sul-fonates and/or phenolic compounds, and poly-butene succinimides.

Furthermore, two-stroke oils contain smallquantities of antioxidants, corrosion inhibitors,defoamers and flow improvers in addition to an-tiwear and DD additives.

Low-smoke two-stroke oils contain a signif-icant amount of fully synthetic fluids, i.e., poly-butenes (about 10 to 50%). Apart from goodlubricity compared to mineral oils, these fluidsoffer much cleaner combustion and significantlyless coking [223], [224].

8.2.2. Classification

The basis for all classification systems are a se-ries of laboratory and functional tests, the latterbeing bench tests performed on the latest gener-ation of two-stroke engines.

8.2.2.1. API Service Groups

The API currently lists three categories (Ta-ble 16) which cover all engines from low-power


Table 16. API groups

API Application Test engine Test criteria

TA mopeds, lawn mowers, electricitygenerators, pumps

Yamaha CE 50 S (50 cm3) piston seizing, exhaust system deposits

TB scooters, small motorcycles Vespa 125 TS (125 cm3) preignition, power loss due tocombustion chamber deposits

TC high-performance motorcycles, chainsaws

Yamaha Y 350 M2 (350 cm3) YamahaCE 50 S

preignition, power loss due tocombustion chamber deposits pistonseizing, ring sticking

lawn mowers to high-performance motorcycles.Engine tests are no longer performed becausethe specified test engines are no longer manu-factured. In future, it is planned to replace theAPI groups with Japanese JASO and global ISOclassifications. There are still a number of oilson the market with API classifications becausethis system was widely accepted in the past.

8.2.2.2. JASO Classification

Japanese Automotive Standards Organization(JASO), to which all major Japanese vehiclemanufacturers belong, classifies two-stroke oilsinto three groups, FA, FB and FC (Table 17).

Table 17. JASO performance categories (Reference oil: JATRE1= 100)

Test criteria JASO FA JASO FB JASO FC

Lubricity > 90 > 95 > 95Detergenteffect

> 80 > 85 > 95

Exhaust smoke > 40 > 45 > 85Exhaustdeposits

> 30 > 45 > 90

Table 18. Engine test criteria for JASO classifications

Test engine Test criteria Test parameters

Honda Dio AF 27 lubricity piston ring wear, ringscuffing, piston seizing

Honda Dio AF 27 detergent effect piston ring sticking as aresult of lacquering,coking, deposits onpiston and incombustion chamber

Suzuki SX 800 R exhaust smoke smoke particlesSuzuki SX 800 R exhaust deposits back pressure in

exhaust system

All three categories use the same test enginesand the corresponding performance category isallocated according to predetermined thresh-olds. The test results are determined in compari-son to an exactly defined, high-performance ref-erence oil (JATRE 1) and published as an Indexrelative to JATRE 1 (Table 18). The key test cri-teria are the lubricity and detergent effect of theoil as well as its tendency to create smoke anddeposits in the exhaust system. The first specifi-cation for a low-smoke oil was created with thelaying-down of JASO FC.

8.2.2.3. ISO Classification

In the mid-1990s, after JATRE 1 oils were testedin European engine tests, it became clear thatJASO FC oils could no longer satisfy the lat-est demands of European two-stroke engines.An series of extended tests which satisfied alldemands were thus developed in Europe. In ad-dition to the testing of smoke, exhaust systemdeposits, lubricity and detergent effect accord-ing to JASO, a new category with a 3-h HondaDio test to quantify improved piston cleanlinessand detergent effect was added. The referenceoil for all tests was JATRE 1. These new guide-lines were created by CEC working parties inwhich European engine and lubricant manufac-turers are represented.

The ISO now classifies two-stroke oils intothree categories, ISO-L-EGB, -EGC and -EGD.A fourth category (-EGE) is currently beingdrafted with strong European representation.

The categories ISO-L-EGB and -EGC mir-ror the requirements of the JASO categories FBand FC while requiring additional proof of pis-ton cleanliness. ISO-L-EGC and -EGD requireproof of low smoke similarly to JASO FC. Ta-ble 19 shows all engine-based evaluation crite-ria.


Table 19. Summary of the ISO categories (Reference oil: JATRE 1= 100)

Test criteria ISO-L-EGB (incl. JASO FB) ISO-L-EGC (incl. JASO FC) ISO-L-EGD

Lubricity > 95 > 95 > 95Smoke > 45 > 85 > 85Exhaust deposits > 45 > 90 > 90Detergent effect > 85 (1-h test) > 95 (1-h test) > 125 (3-h test)∗Piston cleanliness > 85 (1-h test)∗ > 90 (1-h test)∗ > 95 (3-h test)∗∗ New requirements in addition to JASO FC.

8.2.3. Oils for Two-Stroke OutboardEngines

Neither API nor JASO or ISO classificationscontain quality guidelines for outboard engineoils. These are usually oils whose formulationand characteristics have been matched to theengine technologies which have become estab-lished for powering boats. The main differenceto other two-stroke oils lies in their additivechemistry. The additives in these oils are allnonash-forming because these engines all dis-play a marked coking tendency in certain partsof the combustion chamber, such as the ringgrooves. The performance categories of two-stroke oils for outboard engines were primar-ily developed by the American National Ma-rineManufacturers Association (NMMA). Backin 1975, the minimum requirements of suchoils were incorporated into the TCW specifi-cation. In 1988, a far-reaching revision was is-sued with the title TCW 2. During the follow-ing years, problemswere encounteredwith tech-nologically advanced engines which were runon TCW or TCW 2 oils. This initiated a fur-ther tightening-up of minimum requirements,whichwas released as TCW3. In 1997, the stan-dards for oil qualitywere again increased to keeppace with continuing developments in enginetechnology. The new specification, TCW 3-R(R= recertified), now includes laboratory testsand tests on five different engines, three ofwhichare outboards. Table 20 shows the tests whichhave to be performed on a newly developed out-board engine oil to achieve TCW 3-R classifica-tion. The costs generated by testing a TCW 3-Rproduct development have never been so high.The engine tests alone generate costs of about$ 150 000− 200 000.

Table 20. Test criteria for NMMA TCW 3-R

Engine Tests

Test engine: Test criteria:Yamaha CE 50 S lubricity (seizures)Yamaha CE 50 S power loss due to preignitionMercury 15 HP (2 runs) compression losses, ring jamming,

piston cleanlinessOMC 40 HP ring jamming, piston cleanlinessOMC 70 HP ring jamming, piston cleanliness

Laboratory Tests

Low-temperature viscosity limited viscosity at −25 ◦CMiscibility mixing with fuel at −25 ◦CCorrosion protection standard rust tests compared to

reference oilCompatibility stability after mixing with reference

oilFilterability flow rate compared to pure fuel

8.2.4. Environmentally Friendly Two-StrokeOils [225]

The ever increasing severity of environmen-tal legislation is also effecting the develop-ment of two-stroke oils, especially outboardengine oils. Such ecologically optimized oilsoften have regionally differing classificationswhich reflect local environmental legislation andtheir biodegradability depends on varying min-imum requirements. At the international level,the International Council of Marine IndustryAssociations ( ICOMIA) has specified harmo-nized requirements [223]. In 1997, the ICOMIAStandard 27-97 was passed for environmentallyfriendly outboard engine oils. From a technicalpoint of view, products thus labeled must ful-fil at least TCW 3-R as well as offering verylow algae, daphnia and fish toxicity and rapidbiodegradability as defined by ISO and OECDstandards. These oils are based on fully syn-thetic components with the base oils usually be-


ing rapidly biodegradable synthetic esters. Byusing correspondingly high quality esters, theseproducts are the very best two-stroke oils andcan even used for lubricating chain saws. Theuse of ester-based lubricants combines the high-est technical performance with improved envi-ronmental compatibility.

8.3. Tractor Oils

Relatively newer generations of constructionand agricultural machinery make differing de-mands on functional fluids. For reasons of sim-plified servicing but also because of the generalwish to rationalize stocks, universal oils weredeveloped which satisfy the various functionalrequirements of such machines.

These oils should guarantee long machinerylife in all climatic conditions as well as extend-ing service intervals and reducing down-time.

These days, two different oil technologies areusedwhich are characterizedby their applicationarea. They are universal tractor transmission oils(UTTO)and super tractor oils universal (STOU).

The demands made on tractor oils have in-creased sharply in line with advances in vehicletechnology and ease of operation.

Technical advances in tractors required a sig-nificant improvement in operating fluids as theydid in the whole automotive area. Apart frombig improvements in additive technology to copewith greater mechanical demands, the agingstability of the fluids has increased reflectingdramatically higher specific power outputs andlower oil volumes.

All-season use is now a standard requirementfor tractor oils as it is in the automotive area. Theresult of this is that the viscosity grades and thustemperature ranges have been extended fromSAE 15W-30 and 10W-30 to 15W-40, 10W-40and 5W-40.

The performance of such universal oils as hy-draulic fluids corresponds to, at least, HLP andHVLP levels because of the additives includedto guarantee universal use.

The use of these products in vehicle gear-boxes and wet brakes makes much greater de-mands on the fluid. Highly-stressed mechani-cal drive units make great demands on the wearprotection and life of the lubricant. A specialchallenge to oil formulations is the use of these

products in wet brakes which began back in the1960s. Not least because of safety considera-tions, such oils must offer high thermal stabil-ity and balanced and stable friction characteris-tics in brakes. Fine-tuning these oils with specialfriction modifiers is one of the most difficult as-pects of developing such oils.

While UTTO oils can satisfy the above-listedapplications, their use as engine oils requirevastly different additive packages. Apart fromadditives for friction and wear control, low-temperature stabilizers, oxidation, corrosion andfoam inhibitors, engine oils additionally requiresignificant quantities of detergents and disper-sants. As a rule, normally aspirated engines re-quire, at least, an API CE oil and if a tur-bocharger is fitted, the oil should be API CFor CF-4. In many cases, tractor engine manu-facturers issue approvals for oils after they havesuccessfully completed tests in the correspond-ing engines.

The latest generation of tractor oils not onlyinclude products with significantly higher per-formance than in the past but also oils whichoffer better environmental compatibility. Suchproducts often contain rapeseed or sunflowerbase oils or synthetic, rapidly biodegradable es-ters.

Tractor manufacturers now issue their own,in-house fluid specifications which satisfy allspecific requirements of the corresponding ma-chinery. Table 21 lists several manufacturerspecifications.

Table 21.Manufacturer specifications for tractor oils

Manufacturer Oil type Specification

Ford STOU ESN-M2C-159-CFord UTTO ESN-M2C-86-CJohn Deere STOU JDM J 27John Deere UTTO J 20 C, J 20 DMassey Ferguson STOU CMS M1139, CMS

M1144Massey Ferguson UTTO CMS M1135, CMS

M1143

8.4. Gas Engine Oils [226–228]

Gas has long been used to power vehicles andstationary engines. Such engines require a vari-ety of lubricating oils, depending on the type ofengine and the operating conditions.


There is currently no universal, harmonizedspecification for gas engine oils. The large vari-ations in operating conditions between mobileand stationary engines generally require oilswith different additive packages. A general dif-ferentiation is made between high-, medium-,and low-ash types which are recommended bythe manufacturers in line with the designed useof the engine. As a rule, gas engine oils are sub-ject to oxidation and nitration conditions whichcan accelerate the aging of the oil. Gas-poweredcars normally use the same conventional engineoils as are used in gasoline-powered engines.Diesel engines usually need oils with a highertotal base number and with more dispersantsthan gasoline engine oils. Typical qualities forcar engines are API SH and/or ACEAA3 (gaso-line engines) as well as API CG-4 and/or ACEAB3 (diesel engines). Similarly to the automo-tive sector, multigrade oils are used to cope withvariable operating conditions and to guaranteereliable lubrication at low ambient temperatures.As the number of compressed natural gas (CNG)powered cars increases, there is increasing pres-sure to develop oils which are especially formu-lated for these applications. This could mark thebeginning of a future, uniform specification.

Special multigrade oils have already been de-veloped for use in heavy diesel engines, withCNG-powered buses being the major appli-cation. These have been tested and approvedby various engine manufacturers. Examplesof these approvals are Mercedes –Benz Sheet226.9 or MAN M 3271. These oils were testedin bench tests as well as in realistic field trials.

Stationary gas engines canmake significantlymore complex demands on the oil and this hasan effect on their development. While the com-mon ACEA or API bench tests suffice for CNG-powered car engines, laboratory tests on gas en-gine oils are limited to initial oil screening. Thereal development takes place in field trials inwhich engines often have to run for years beforethey are evaluated. While particular attention ispaid to sludge formation, valve train wear andlow temperature flowing in car engines, other ef-fects are important in stationary engines. Partic-ularly important is controlling oil aging causedby oxidation and nitration but also preignitionwhich is caused by high levels of ash-formingadditives. This problem ismost prevalent in two-stroke engines which generally need low-ash

oils. Simply because of their long service lives,gas engine oils should protect against valve seatrecession and spark plug fouling. As regardsrunning on corrosive gases (caused by sulfur orhalogen contamination), special attention mustbe paid to adequate corrosion protection. Thedevelopment and application of gas engine oilsnormally takes place in close cooperation bet-ween the oil and engine manufacturers.

8.5. Marine Diesel Engine Oils [229]

These lubricants are heavily influenced by thetype of fuel used and the design of the enginesthemselves. A number of similar engines arealso used in stationary applications to generateelectricity with conventional fuels or with steampower.

8.5.1. Low-Speed Crosshead Engines

Low speed diesel engines generating up to1000 kW per cylinder at 50 to 120 rpm usethe crosshead principle (large engines withover 900mm bores and 3000mm strokes and20 000 kW outputs). In crosshead designs, thecylinder block and the crankcase are separateunits. Sealing is provided by stuffing boxes andthe liners are lubricated with cylinder oils bymeans of dosing devices. Depending on the boreand stroke of the cylinder, up to 16 dosing de-vices may be fitted.

8.5.2. Medium-Speed Engines

Medium-speed engines run at around 200 to100 rpm. The overall design of these engines isroughly similar to that of vehicle internal com-bustion engines, they do not use the crossheaddesign and the crankshaft is connected to thepistons and thus the cylinders with connectingrods. In such designs, the same oil is used tolubricate the cylinder walls and the crankshaftbearings and forms part of a circulation system.These engines are sometimes called trunk pistonengines.

These days, marine diesel engines are fuelledby the worst and heaviest crude oil fuel cuts.


These include vacuum residues, propane deas-phalting residues, heavy solvent extracts fromlube refining and other byproducts (→HeatingOils, Chap. 2.).

8.5.3. Lubricants

Cylinder oils in crosshead engines are total-lossproducts which lubricate the sliding motion ofthe piston rings in the cylinder liner. To avoidthe deposit of combustion residues, these oilsmust have good dispersant properties. In addi-tion, they must be capable of neutralizing thecorrosive acids which result from the high sul-fur levels in the fuels. To satisfy these require-ments, cylinder oils have a large proportion ofover-based components (up to 30% over-basedcalcium sulfonates or other over-based compo-nents). The alkalinity required for neutralizationleads to TBNs of up to 100. When additives forsuch oils are being selected, special attentionmust be given to the good colloidal solubilityof the additives in the base oil to avoid precip-itation. The large surface areas of cylinders incrosshead engines requires the applied oil to dis-perse rapidly and reliably and this is achieved bygood spreadability.

The stuffing boxes fitted to crosshead enginesensure that the crankcase oils are hardly con-taminated by combustion chamber residues andthese oils thus contain relatively few additives.Additives to combat thermal oxidation are es-sential and the neutralizing capacity can be rel-atively low with a TBN of 5. The contaminationof crankcase oil with cylinder oil leads to in-creased emulsifying capacity which is undesir-able. Water separation is therefore another vitalcharacteristic of crankcase oils.

An increase in dispersant and detergent prop-erties caused by the ingress of cylinder oil canalso lead to wear problems as the effect of typ-ical antiwear and EP additives such as zinc di-alkyldithiophosphate is thereby diminished.

The type of oil used to lubricate trunk pistonengines (combined crankcase and piston lubri-cation) is also largely determined by the sulfurcontent of the fuel. Oils with TBNs of 12 to 40are common and the neutralizing capacity of theoils is very soon exhausted in some cases so thatthe TBN of used oils can be much lower.

During the development of such oils, tests areperformed on one- or three-cylinder Bolnes en-gines but final results are only available fromvery time-consuming on-board trials lasting atleast one year.

Significant restrictions on the sulfur contentof marine diesel fuels will be imposed in thecoming years. This will result in a lowering ofthe high proportion of over-based componentsnecessary for such oils at present.

9. Gear Lubrication Oils

9.1. Introduction

Gear lubrication oils are of particular signifi-cance for the transmission. Apart from the im-portant function of lubricating the sliding rollingcontacts, the oils also fulfil the task of coolingand removing the friction heat generated in thesliding rolling contacts.

In general, the lubricant is a transmissioncomponent which can be relatively simply pro-duced at low cost. However, in order to ensurethe reliable and long-term operation of a trans-mission, the selection of the lubricant, in com-parison to the selection of the other machinecomponents, is of decisive significance duringthe transmission construction.

The following base oils are not only used forthe lubrication of transmissions but also manyother applications:

– mineral oils– synthetic hydrocarbons (polyalphaolefins)– poly(alkylene glycols) (homopolymers)– esters (eco-friendly oils, mainly on syntheticbasis)

– rapeseed oil, castor oil

Apart from the various base oils, the type andquantity of the additive has a significant influ-ence on the function and service life of a trans-mission.

In principle, any type of lubricant, includingengine oils, can be used in any transmission,thus ensuring for the moment the transmission’sfunctionality.

However, the selection of a lubricant whichis not adapted to the respective constructioncomponents and the operating and environmen-tal conditions of a transmission can, given an


early failure of the transmission, lead to maxi-mum consecutive damages up to the breakdownof complete systems. The resulting repair andstandstill times of the system lead to unforeseencosts.

9.2. Requirements of Gear LubricationOils

In many areas of machine construction, thetransmission plays an important role. Figs. 41and 42 give a general survey over the varioustransmission types used today.

Each of these transmission types makes spe-cific requirements on the lubricant which mustbe met to ensure the reliable function of the ma-chines and plants. Thus, the heavily loaded lu-bricants in hypoid gears require high oxidationstability, together with a very good scuffing andscoring and wear load capacity, due to the highload of the tooth contacts. At the same time, theformationof a load-capable and separating lubri-cating film for sufficient lubrication and coolingof the sliding rolling contacts in hypoid gearsrequires a lubricant with an adequately high vis-cosity at operating temperature.

On the other hand, lubricants used in hydro-dynamic gears, such as torque converters, hydro-dynamic clutch couplings or retarders, do notneed to have a good scuffing and scoring loadcapacity. However, they must have a high oxi-dation stability. Due to the viscosity-dependentlosses, lubricants which are used in hydrody-namic gears therefore have a clearly lower vis-cosity at operating temperature in comparison tolubricants used in hypoid gears.

The abovementioned transmission types areused in machine and plant construction withvarying degrees of frequency. Figure 43 presentsthe service lives currently required of transmis-sions according to the most important industriesin the area of machine construction.

The heat development in a transmission hasan influence on the temperature of the oil sumpand the temperature in the oil tank. Heat accel-erates the oil’s aging process and, therefore, cancause a reduction of the oil’s service life. Theheat development and, thus, the oil temperature,is determined by the transmission type, the trans-ferred power, the specific load, as well as by

the switching periods – permanent or intermit-tent operation – and by the environmental con-ditions – use of the transmission in warm or coldclimates, or in a mobile or stationary system.

In transmissions for which long service livesare required, such as paper machines or presses,the lubricant has to be changed according to themechanical and thermal claim, in compliancewith the oil producers’ recommendations. As-suming that the average oil sump temperaturein such transmissions is approx. 90 ◦C, the oilshould be changed in intervals of 2500 operatinghours. An increase of the oil sump temperatureof 10K leads to a 50% reduction of the servicelife, reducing the temperature by the same valuedoubles the oil’s service life.

Today, transmissions for which short ormedium service lives are required, as for exam-ple in passenger cars or mobile hydraulic sys-tems, are mainly filled with life-time oil fillings.The oil sump temperatures in these transmis-sions often rise up to 130 ◦C. Therefore, the useof synthetic lubricants is recommended in theseapplications. In general, the service life of syn-thetic lubricants is three times longer than theservice life of mineral oils. However, the cur-rent prices for synthetic lubricants are also threetimes higher than those for mineral oils.

9.3. Tribology of Gears

There are significant differences between the tri-bology of gear drives [230] and the tribology ofjournal and roller bearings, since the lubricationconditions characterizing the sliding rolling con-tacts in toothed wheels differ from those in jour-nal or roller bearings. As shown in Figure 44, thelubrication conditions in the tooth contacts arethe most difficult.

9.3.1. Friction Conditions of Gear Types

Figure 45 gives a schematic overview of the geartypes currently used [231], [232].

According to its specifically different geo-metric ratios and the operating conditions, eachgear type is limited to a maximum transferableload.

The main key factors are the sliding rollingcontact pressure and the specific load in the toothcontact and the gear’s rotational speed, aswell as


Figure 41. Gears with a constant gear ratio

Figure 42. Gears with a variable gear ratio

Figure 43. Guide values for the service lives required of gears

the currently effective rolling and sliding speedproportions in the tooth contact. The distribu-tion of these factors during a tooth engagementis shown in Figure 46 on a nonprofile correctedspur gear.

The load and surface velocity of the toothflanks are characterized by continuously chang-ing conditions alongside the entire tooth engage-ment distance. The sum of the velocity of the

tooth flank equals the effective hydrodynamicvelocity, vΣ. The difference of the velocity ofthe tooth flank equals the sliding velocity vg. To-gether with the locally effective pressure or so-called sliding rolling contact pressure, this slid-ing speed causes a friction load flowing tangen-tially to the tooth flank and, thus, an increase intemperature in the sliding rolling contact. Onlyin the pitch point contact C, as shown in Fig-


Figure 44. Schematic overview of the narrowing of the gap. (A) hydrodynamic journal bearings, constant sliding speed atgood conformity and good lubrication conditions. (B) Cylindrical roller bearing, relatively high rolling speed at low slidingspeed, poor conformity and medium lubrication conditions. (C) Tooth contact lubrication, poor conformity along the toothengagement length at different sliding velocities in tooth-tip direction and under lubrication conditions inferior to A and B.

ure 46, a pure rollingmovement can be observedwithout sliding speed in the whole tooth engage-ment.

For the toothing shown in Figure 46, a staticloaddistribution alongside the tooth engagementdistance can be assumed.

The tooth engagement is a vibrating systemwhich, according to the geometry and the oper-ational area, shows a dynamic load distributionwhich differs and sometimes clearly exceeds thestatic load distribution. In this vibrating system,the lubricant and, especially its operational vis-cosity have an absorbing effect.

Lubrication Film Generation in ToothContact. A significant key factor for the assess-ment of the lubrication generation and conditionis the minimum lubricant film thickness. A suf-ficient minimum lubricant film thickness is nec-essary to avoid the metal surface contact of thetooth flanks and to lead the generated heat outof the contact zone by means of friction and ab-sorption.With theminimum lubricant film thick-ness determined according to [234], [235], thecurrent lubrication condition in the tooth con-tact can be assessed. The non-stationary slidingrolling contact conditions are the reasons for thedifferences in the lubrication conditions in eachcontact point of the tooth engagement distance.

In the pitch point C of a tooth engagementwhere – as in a roller bearing – a pure roll-offor rollingmovement without slide speed propor-tions is generated, relatively optimal lubricationconditions can be assessed. The lubrication con-ditions during the tooth engagement and disen-gagement, however, are more unfavorable dueto relatively high slide speed proportions.

Another factor with negative impact on thegeneration of a lubrication film in a tooth con-tact becomes effective during a new tooth en-gagement of each tooth when stripping off of thelubrication film by the tooth tip of the engagingtoothed wheel. The lubrication film must be re-generated on this tooth tip with each new toothengagement and breaks with each tooth disen-gagement.

When a gear is characterized by a morelubrication-unfavorable toothing geometry dueto the relatively large slide – speed proportionsalong the tooth engagement length in this typeof gear, for example in a hypoid or cylindricalworm gear, these gears are subject to signifi-cantly larger friction losses and higher slidingrolling contact temperatures than a simple spurgear.

Lubrication Conditions. To avoid toothdamage, a detailed observation and assessmentof the lubrication conditions in the tooth contactsis particularly important.

Depending on the local load, pressure, tem-perature and based on the material values of thetoothed wheels and the lubrication values of thelubricant, a lubricant film thickness flowover thetooth engagement distance of a toothing can becalculated [236]. Today, thismethod is generallyused to calculate the pitch point C. Due to thefast progress made, a more complex observationof the tooth engagement is only a matter of timeand is already partly carried out as a standardmethod by some companies.

The specific film thickness of the lubricantfilm is assessed according to the Eq. (10.1):

λ = hmin/Ra = 2hmin/ (Ra1 + Ra2) (10.1)


Figure 45. Important gear types

where λ is the specific film thickness accordingto [237], hmin is the minimum film thickness ac-cording to [233], and Ra is the arithmetic aver-agemean roughness of the tooth flanks of pinion(Ra1) and wheel (Ra2) according to [238].

According to the evaluation of numerous ob-servations, a differentiation between two rangesis made.

Specific Film Thickness λ > 2. In general, thetooth flank surfaces are separated by a suffi-ciently thick hydro- or elastohydrodynamic lu-bricant film. The viscosity of the applied lubri-cant is the decisive property of the lubricant.Only little wear occurs in the long term. Surfacedamage is not to be expected.


Figure 46. Load and speed distribution in tooth contact according to: A, beginning of tooth engagement; B, beginning ofsingle-tooth engagement; C, pitch point, no sliding velocity; D, end of single-tooth engagement; E, end of tooth engagement;DTE, double tooth engagement; STE, single tooth engagement; FZE, tooth force within tooth engagement line; FZmax, max-imum tooth force within tooth engagement line; vΣ, hydrodynamic effective velocity; vg, sliding velocity; vt, circumferentialvelocity.

Specific Film Thickness λ < 0.7 (Borderline).Numerous industrial gears are operated in thisrange which is also called borderline lubrica-tion. Falling below the critical value leads to anincreased risk of surface damage as well as in-creased wear. In this range, the right lubricantin connection with a suitable lubricant additivesystem is particularly important.

If borderline lubrication is reached in thetooth contacts, the tooth flanks have to be pro-tected against direct metal-to-metal contact inorder to avoid tooth damage.

With sufficiently large film thickness rates(λ > 2), the effect of the lubricant viscosity issufficient. In slowly running gears connectedwith low oil sump temperatures and low circum-ferrential velocities, the lubrication conditionsduring borderline lubrication (λ < 0.7) is gen-erally improved by using polar oil compounds,fatty acid or solid matter particles (molybdenumdisulfide, graphite etc.). Due to the low circum-ferrential speeds, the mentioned compounds canform physical deposits on the surfaces and de-velop a permanent protective layer.

In quickly running gears, in connection withrelatively high oil sump temperatures and highcircumferrential velocities, EP additives im-prove the lubrication effect during borderline lu-brication (λ < 0.7). Depending on the contacttemperature, the EP additives begin a chemi-cal reaction with the surface of the tooth flanks.

This reaction leads to the generation of metallicsoaps which serve as a slide layer, thus prevent-ing metal contact.

9.3.2. Specific Gear and TransmissionFailure

When constructing a gear, it must, first of all,be ensured that all sliding rolling contact points,especially the toothings, are supplied with suf-ficient quantities of lubricant in all operationmodes. If this is the case, significant failures inthe toothings, with the exception of corrosion orthe like, will normally not occur in non-loadedgears. Failures in toothed wheels are always theresult of excessive load, i.e., a too high localload of the respective material – lubricant com-bination.

Toothed wheel damage influenced by the lu-bricant is divided into three categories: Wear,scuffing and scoring, and fatigue (micropittingand pitting) [239].

Wear (→Abrasion and Erosion). Continu-ous wear is predominantly observed at low cir-cumferrential velocities and during borderlinelubrication, see [240].

The wear of a toothing can be significantlyinfluenced by hardening the toothed wheels andselecting a suitable lubricant.


Scuffing and scoring of tooth flanks occurspredominantly at medium to high circumferren-tial velocities and in case-hardened toothings.According to [241], [242] the contacting sur-faces weld together for a short time. The highrelative velocity of the contact surfaces towardseach other breaks this welded joints immedi-ately, thus causing the typical scuffing and scor-ing phenomenon.

Scuffing is a very sudden damage which canalready be caused by one single overload. Sud-den scuffing can lead to the total destruction oftooth flanks.

Freshly produced toothed wheel surfaceswithout run-in are extremely subject to scuff-ing. By adding to the lubricant EP additives, thescuffing load capacity can be improved by a fac-tor of 5 andmore. The calculation of the scuffingload capacity is standardized according to [235],[243].

Fatigue.Micropitting on tooth flanks can be observed

in case-hardened toothings in practically allspeed ranges. Tooth flanks with rough surfacesare especially subject to micropitting. Micropit-ting develop predominantly in the area of nega-tive sliding velocities or in slip areas below thepitch circle as damage due to endurance failureaccording to [244].

A suitable lubricant with a positive influenceon the micropitting must have a sufficiently highviscosity as well as a suitable lubricant additivesystem. When selecting a suitable additive sys-tem, the operational viscosity or temperature ofa gear is of superior significance.

Pitting is an endurance failure which is ob-served on both quenched and tempered and case-hardened toothings for all types of gears and atall circumferrential speed ranges [245].

Pitting is observed predominantly on thetooth flank centers in the height of the pitch cir-cle since this is where the highest load occurs(single-tooth engagement) or the largest stress-cycle amplitudes. Here, the material shows thequickest fatigue.

Recent examinations [235], [246] haveshown that, in principle, an influence of the lu-bricant on the pitting load capacity is possible.

In general, an increase in the lubricant’s vis-cosity has a service-life extending effect. How-ever, it is not always possible and reasonable

to increase the lubricant’s viscosity in a gear(higher splash losses). Currently, lubricants witha lower viscosity are used more and more often.

Suitable lubricant additive systems exist –above all for lubricantswith a lowviscosity (ATFtechnologies) – which have a positive impact onthe pitting load capacity of the toothings [246].

9.4. Gear Lubrication Oils for MotorVehicles

Gear lubrication oils for motor vehicles haveto meet the specific requirements of the geardrives (see Section 9.2). Mostly, the gearboxeswhich serve to change the gear ratio are man-ual transmissions with synchronization. Theycan also be automatic or semiautomatic trans-missions or constantly variable transmissions(CVT). In automatics or CVTs, the lubricant isnot only responsible for the gear drive lubrica-tion but also for the function-related operationof wet clutches, wet brakes as well as torqueconverters and retarders.

Today, it is not possible to meet all working,operating and ambient requirements of the men-tioned transmissions with only one lubricant.Currently, more and more tailor-made individ-ual lubricant solutions are being developed forspecific applications, special transmissions andtransmission family types. Like the developmentof new engine oils, the current development ofgear lubrication oils for motor vehicles has notyet come to an end and is mainly characterizedby several factors:

– The requirements and specifications issuedby the motor, transmission and vehiclemanufacturers intend to considerably ex-tend the oil-change intervals. Today, mostof the manual transmissions and axle gearsin passenger cars are operated with oil-fills for life (250 000 km). With respect toautomatic transmissions in passenger cars,these long oil-change intervals will soonbe achieved. For commercial vehicles, itis planned to achieve transmission ser-vice lives of 350 000 km, 500 000 km up to1 000 000 km without oil-change.

– The use of mineral base oils is decreasing inthe industrialized countries. Today, environ-mentally friendly, synthetic and ester-based


base oils are often used for reasons of envi-ronmental protection. Due to the better ther-mal oxidation stability, ester-based oils areused together with PAO and hydrocrackedbase oils.

Attempts are, furthermore,made to reduce lu-bricant toxicity, especially the chlorine contentof commonly used additives.

Modern vehicles are expected to ensure animproved performance, a reduced noise emis-sion and a reduced fuel consumption. This trendis expressed in the increasing use of lubricantswith a low viscosity together with a reduced fillvolume. Furthermore, multigrade oils are cur-rently used on a large scale to considerably re-duce the fuel consumption.

9.4.1. Lubricants for Gear Drives inCommercial Vehicles

Commercial vehicles are divided into so-calledlight trucks with a maximum load of 3 to 6 tand heavy trucks with a maximum load of morethan 6 t. In 1999, the worldwide annual demandfor gear lubrication oils for commercial vehi-cles amounted to approximately 647 000 t. In thesame year, some 1.84× 106 commercial vehi-cles were produced in Western and Eastern Eu-rope, which corresponds to drive-line lubricantvolume demand of 140 000 t of gear lubricationoils for the commercial vehicles’ gearboxes andaxle gears for the European market. In 1999, thenumber of manufactured light trucks amountedto some 1.505× 106 vehicles.

Because the fill volume per transmission unitis two to six times larger and amounts to 10 to30 L, heavy trucks are farmore interesting for theoil manufacturers. At the same time, the oil de-velopment also confronts a bigger challenge inmeeting the requirements made on the oils usedin the commercial vehicles’ large transmissions.In 1999, the number of manufactured heavytrucks amounted to approximately 335 000 in allEuropean countries.

Tables 22 and 23 list the most important cur-rent (2000) specifications issued by the trans-mission manufacturers and automotive compa-nies, split intomanual transmission and rear axlegears for heavy trucks.With respect to the speci-fications, themodified newSAEclasses are to be

taken into account. Above all, the new classifi-cation SAE J2360 is to be mentioned which willreplace the old MIL-PRF-2105E. In addition tothis, the new SAE ‘Automotive Gear LubricantViscosity Classification’ is also to be taken intoaccount.

Table 22. Specifications for lubrication oils for rear axle gears ofcommercial vehicles

Daimler – Chrysler 235.8 SAE 75W-90, extended drain,250 000 km to 300 000 km

Volvo transmission oilspecification 97312, STD1273,12

SAE 75W-80/85 fortransmissions made by Meritorextended drain, 400 000 km or3 years

MAN 342 SL (synthetic, longdistance)

extended drain, 320 000 km

Scania STO 1:0 (part synthetic,synthetic)

API-GL5+, extended drain,more than 120 000 km

RVI-TDL axle SAE 75W-90, extended drain,350 000 to 400 000 km or 2 to3 years

ZF TE-ML 12 A-C (mineral,part synthetic)


ZF TE-ML 12 D (synthetic) extended drain, 300 000 km

Table 23. Specifications for manual transmission gears of commer-cial vehicles

Daimler – Chrysler 235.11 SAE 75W-80/90, extendeddrain, 250 000 to 300 000 km

Volvo Transmission OilSpecification 97307, STD1273,07

SAE 75W-90, extended drain,400 000 km or 3 years

MAN 341 SL (synthetic, longdistance)

for gears made by ZF and Eaton,extended drain, 320 000 km

MAN M 3343mL/SL (mineral,synthetic, long distance)

total drive-line oils, extendeddrain, 180 000 km

RVI-TDL gear box SAE 75W-90, extended drain,350 000 to 400 000 km or 2 to3 years

ZF TE-ML 02 A–C (mineral,part synthetic)


ZF TE-ML 02 D (synthetic) extended drain, 300 000 km

This trend in the development of lubricantsfor commercial vehicles leads to a significantchange in the current ratio between service filland relubrication on the one hand and the fac-tory fill on the other hand. The proportion of theservice fill and relubrication of currently 75% –with a factory fill proportion of 25% – will bereduced to 20% for the service fill and relubri-cation – with a factory fill proportion of 80%during the next ten years.

All mentioned specifications constitute achallenge for the development of gear lubrica-tion oils for commercial vehicles. Apart from thevery high chemical and physical requirements


included in these specifications concerning theoxidation stability (test at 150 – 160 ◦C), corro-sion, filterability, foam etc. (see Section 9.6) thementioned specifications include a large numberof mechanical – dynamic tests and require veryhigh safety levelswith respect to the scuffing andscoring, pitting and wear resistance of the tooth-ings using different, standardized, nonstandard-ized and company-internal test procedures. Thismakes the development of new oils increasinglycomplicated and expensive. A particular obsta-cle in the development of oils for commercialvehicles is the improvement and optimization ofthe synchronization behavior during upshift ordownshift operations.

Although many commercial vehicles ofAmerican manufacturers still do not use syn-chronized transmissions, almost all commercialvehicles inEurope are equippedwith these trans-mission systems. The SSP 180 test bench hasprovenvery successful in the oil development fora sufficient synchronization behavior, see alsopage 173.

The main problem in the oil development isto adjust the same friction or friction constancyduring a required gear service life of approx-imately 100 000 gearshift operations with dif-ferent material – lubricant pairs without signif-icantly changing the other chemical – physicaland dynamic –mechanical properties.

9.4.2. Lubricants for Gear Drives inPassenger Cars

The development of passenger cars is mainlyinfluenced by the worldwide fierce competitionof numerous producers. For the oil producers,this means to supply approved lubricants whichhave to be available in the same quality world-wide or at least in the production locations forgear-filling purposes.

A passenger car’s sportiness requires a highperformance and a high torque in identical oreven smaller spaces for gearboxes. Due to theeconomy required at the same time, the lubri-cants are expected to optimize the total effi-ciency of the engine and total drive-line withouthaving to be changed during the entire servicelife of more than 250 000 km.

The higher performance causes an increasedheat development in the transmissions which ac-

celerates the aging of the gear lubrication oils.The design of new vehicles leads to the develop-ment of carefully selected bodies, often with alow air resistance. This is often connected withan inferior air circulation and cooling of thetransmissions. Additional capsules facilitate thereduction of driving-related noise to increase thedriving comfort. At the same time, the poor heatdissipation leads to a heat stowing in the cap-sular spaces. Especially these factors increasethe temperature in the vehicle’s transmission andaggravate the operational conditions, thus facil-itating the aging behavior of the oil.

New passenger cars include transmissionswith a higher number of gear stages so that to-day passenger cars using gear drives with sixstages are not unusual. To achieve this, all tech-nical possibilities of dimensioning and materialhave to be exploited. This always has an effect onthe corresponding lubricant when new lighter ormore solidmaterials have to be used in the trans-mission as new friction partners with a frictionand wear behavior so far unknown.

An increasing number of vehicles utilizes au-tomatic transmissions and four-wheel drive sys-tems. The necessary control elements and sensorsystems consisting of electric components suchas measuring units and sensors etc., are installedin the vehicles’ transmissions. The compatibilityof these sensible components with the oil leadsto special requirements made on the lubricantand its development.

In Europe almost 90% of all passenger carsare equipped with synchronized manual trans-mission systems. The variety of the friction ma-terial partners used in the synchronization ofpassenger cars is constantly increasing so thatheremore andmore individual solutions are alsorequired with respect to the lubricant. Anotherrequirement on the gear lubrication oils forman-ual transmission systems, as well as for rear axlegears, is the very high scuffing resistance to en-sure the protection of the spur gears’ which havesignificantly more sliding proportions than theplanetary gears’ toothings normally used in au-tomatic transmission systems.

All mentioned development trends are re-flected in the increasingly stricter specificationsand test procedures issued by leading automo-tive companies.

Several important specifications for lubrica-tion oils for manual transmissions and transaxle


drive system gears for passenger cars are listedbelow:

VW, Seat, Skoda, Audi, TL 726, API GL4 SAE 75W/SAE 80W (lifetime gearoil formanual transmissions and transaxles)Daimler – Chrysler page 235.10Volvo STD 1273,08VW TL 52157 (75W 90, synthetic lifetime)VW TL 52171 (75W, synthetic lifetime)VW TL 52178 (75W, mineral lifetime)Ford factory fill for life M2C 200CBMW 602 00.0 (synthetic lifetime)

9.4.3. Lubricants for AutomaticTransmissions and CVTs

The most important consumers of lubricantsfor automatic transmission and drive systems(ATFs) are off-highway vehicles and machinesas well as commercial vehicles, mainly buses.In addition to this, ATFs are also used to fillpower steering transmission systems in trucks,commercial vehicles and passenger cars.

90%of all passenger cars on theNorthAmer-ican and Asian vehicle market utilize automatictransmission systems. The total estimated ATFvolume worldwide for the year 2001 amountsto approximately 992 000 t. The total volumebreaks down as follows: North America (61%),Asia Pacific (15.5%), Europe (12.5%), LatinAmerica (8.8%) and Middle East (2.2%).

Most manufacturers of automatic transmis-sion systems prescribe fluids for the applicationwhich meet the listed specifications:

Ford Mercon (revised September 1992)Ford MerconVGM Dexron II DGM Dexron II EGM Dexron IIIGM Dexron IIIGAllison C-4Daimler – Chrysler 236.2Daimler – Chrysler Mopar ATF3+, MoparATF 4+Leyland E85

These specifications refer to the Asian, NorthAmerican and Europeanmarkets and to the busi-ness for service fill and relubrication of vehicles.Currently, the factory fill is subject to stricterspecifications:

General Motors, Factory Fill GM 6418-MFord Factory Fill for Life, M2C 202BDaimler – Chrysler Factory Fill for Life, MS9602

European manufacturers of vehicles and, es-pecially, transmission systems have issued par-ticular specifications mainly for off-highwayconstruction machines and commercial vehi-cles:

ZF TE-ML-14, Standard drain, 60 000 kmZF TE-ML-14, Extended drain, 120 000 kmVoith G607, Standard drain, 60 000 kmVoith G1363, Extended drain, 120 000 kmRenk standard drainDaimler – Chrysler 236.8VW TL 52178

Worldwide, there are further specificationsfor off-highway construction machines andcommercial vehicles with importance for the oildevelopment:

Caterpillar TO-4ZF-TE-ML 03ZF-TE-ML 09KomatsuKomatsu –Dresser

ATF Lubricant Requirements for Hydro-dynamic Transmissions. Automatic transmis-sions make use of the versatile possibilities ofoften series-connected hydrodynamic transmis-sions for the torque transfer and conversion.The hydrodynamic clutches facilitate an im-proved and smooth driving and operating be-havior, especially with respect to heavy vehi-cles with a high inertia mass. A hydrodynamictransmission displays a constantly variable op-eration, whereby the resulting torque indepen-dently adapts to the respective load status bychanging the rotational speed.

Apart from the hydrodynamic start-upclutches, hydrodynamic brakes, the so-called re-tarders and intarders are used, especially in com-mercial vehicles. They serve to limit the outputspeed when driving down long slopes in orderto reduce the load on the vehicle’s brakes or togo easy on them.

The fluid machines ensure the uncoupling ofthe drive-line and output torque and, thus, a con-stantly smooth and comfortable force transfer in


the drive shaft. They will, however, only workadequately with slip and are, thus, characterizedby significant fluid losses. Apart from the largerweight, this constitutes another reason for thegenerally higher fuel consumption of vehicleswith automatic transmissions in comparison tovehicles with manual transmissions.

During start-up procedures and permanentoperation as well as during braking procedures,the lubricant as a force transfer medium in thehydrodynamic transmissions is subject to an ex-tremely quick generation of heat and load de-spite installed radiator systems in comparisonto gear drives. Due to the function-related fluidlosses, hydrodynamic clutches, torque convert-ers and brakes always require low-viscosity,mildly addedgear lubricationoilswith a highox-idation resistance. Short-term oil temperaturesof more than 160 ◦C during the operation ofa vehicle are not unusual. Especially for thesefluid machines, a good viscosity – temperaturebehavior, corrosion protection, optimal foamingbehavior and air separation properties are amust.With respect to the viscosity and V –T behaviorlubrication oils for automatic transmissions arevery similar to engine oils.

To facilitate the distinctionbetweenATFs andthe engine oils, ATFs are always dyed with redcolor.

ATF Lubricant Requirements for WetClutches and Brakes. Like in the synchroniza-tion, the friction characteristics are of great im-portance for a comfortable and regular gearshiftbehavior during the transmission’s entire servicelife in wet clutches and brakes as well. A wetclutch or brake consists of disk-plate fin blocks.The clutches and brakes are opened and closedhydraulically. Apart from the desired frictioncharacteristics, the oil predominantly serves toensure the cooling of the friction partners. Of-ten the oil is injected from the interior into thedisk-plate blocks via the rotating shaft.

To adjust a lubricant to a desired friction be-havior or friction characteristic, a test in the ma-chine or, at least, a test of the disk-plates finsused later in the machine is required. Therefore,test benches have been developed especially forthis purpose which enable a more precise ad-justment of the gearshift behavior to the selectedmaterial – lubricant combination.

A tester which demonstrates the differentsliding – friction behavior of oils is the low-velocity friction apparatus (LVFA). Figure 47gives an example of measured friction behaviorfor some lubricants as a function of the slidingspeed.

Tests of original disk-plates can also be car-ried out using an SAENo. 2 tester or aDKA fric-tion test bench. These testers work with givencentrifugal masses which reduce the speed ofthe disk-plate blocks under defined test con-ditions. Static and dynamic friction coefficientand friction losses are determined using a high-resolution measuring technology.

According to the definition, the static frictionis measured during the tests at low differentialrotational speeds (< 2min−1) or towards the endof the gear-shifting operation.

To test the gearshift and friction behavior, es-pecially in case of permanent slip of the fric-tion partners, the transmission manufacturer ZFhas developed another test bench. The so-calledGK test bench or CSTCC (continuously slip-ping torque converter clutch) enables the sim-ulation of almost all real operating conditions.Even slip-controlled clutches of hydrodynamicclutches and torque converters can be simulatedusing the GK test bench. Figure 48 shows pos-sible operating conditions.

Gear Lubrication Oils for CVT Applica-tions. Constantly variable transmissions in mo-tor vehicles enable the operation of a combus-tion engine along certain preferred characteristiccurves in the torque – rotational speed field. Incontrast to all other vehicle transmissions, theCVT enables an ideal alignment of the supplyfield of a combustion engine with the requestfield of the vehicle.

Up until today, mainly three constantly vari-able transmission concepts have proven to besuccessful to meet these requirements.

B-CVT Belt Drives. For passenger cars witha low or medium drive power of up to 150 kW,the belt drive, mostly a ‘Van Doorne’ belt, hasbeen very successful (Fig. 49). The belt em-braces the cone pulleys which have been hydro-statically and axially applied to the drive andoutput shafts. Thus, the radii of the belt drive’scourse as well as the gear ratio can be variedconstantly.


Figure 47. LVFA (low velocity friction apparatus)Determination of the dependence of friction coefficient on sliding velocity to avoid stick – slip behavior

Figure 48. GK test bench. Test runs, variety of operating conditionsA) Opening and closing; B) DKA operation (limited); C) Similar to LVFA;. D) Static operation (friction vibrations)

For the instationary slide – roll contacts bet-ween the belt drive and the cone’s contact sur-face, the lubricant is an extremely important con-struction component. The contact points are of-ten subject to very high contact temperatures andmixed-lubrication conditions. Here, a very goodwear protection of the oil against the extremelyhardened surface of the cone pulleys is required.In addition, the cone’s finely ground contact sur-

faces require from the lubricant a very high pit-ting load capacity. On the one hand, a slip ofthe belt drive due to insufficient contact pres-sure forces should be avoided by means of con-trol engineering. This cannot be avoided totally.Accordingly, the lubricant also has to ensure asufficient scuffing load capacity for these oper-ating conditions.


Figure 49. Adjusting device – principle of a B-CVT (Van Doorne Belt)A) Gear ratio in position LOW; B) Gear ratio in position TOP

Nearly all belt drives of vehicles have atoothed wheel gear stage as well as a hydro-dynamic start-up clutch. In order to increasethe driving comfort and reduce the losses, slip-controlled clutches are used. Thus, the same re-quirements apply for both CVT oils and ATFs.Currently, CVTs are filled with ATF oils whichare slightly modified lubricant variations orhave been individually adjusted to the respec-tive CVT. The viscosity, additives and base oilsare very similar to ATFs, however, the so-calledfriction modifiers are of a greater importance.

T-CVT Traction Drives. The belt drive sys-tem’s mechanics has reached its limits in powertransmission at a driving power of slightly morethan 150 kW, i.e., inmid-range passenger cars orlimousines. Currently, constantly variable trans-mission concepts are tested in this respect whichare based on so-called traction drives (Fig. 50).

The constantly variable adjustment of a trac-tion drive takes place by tipping the transmissionor idle wheels which, axially pressed together,roll on the semi or full-toroids’ courses in aforce-conclusive way. In traction drives as well,the lubricant is to be considered an importantconstruction component and is as significant as

the material, the surface treatment and the hard-ening of the rollers and toroids. Of all transmis-sion types, semi and full-toroid transmissionsstand out due to the highest surface pressure andcircumferential speeds in the slide – roll contactpoints. Surface pressures of 4500N/mm2 andcircumferential speeds of approximately 50m/sin traction drives are not unusual.

In tractiondrives, the frictiondependsheavilyon the oil, the material pair and the slip. Mainlythose lubricants are used which enable a hightransmission performance at the lowest slip pos-sible, thus having a high friction coefficient.

Adequately added, naphthene-based hy-draulic fluids have proven very successfulfor these applications. However, synthetic cy-cloaliphatic oils with a particularly high frictioncoefficient are even more suitable for these pur-poses. When using traction fluids in transmis-sions, e.g., in roller bearings and toothed wheelsconnected in series, the higher friction, however,leads to excessive and undesired overheating.

Another disadvantage of the traction fluidsis a relatively low flash point of 130 to 150 ◦C.Therefore, the very high contact temperatures in


Figure 50. Adjusting device – principle of a T-CVT (traction drive)

the slide – roll contacts cause undesired evapo-ration losses.

Hydrostatic dynamic powershift drives areused in agriculture and in tracked vehicles witha usually very high drive power of more than300 kW. In these transmission systems, plane-tary gear stages branch some of the drive powerinto a closed hydrostatic circulation systemconsisting of a controllable adjustment pump,mostly an axial piston pump, and a hydrostaticconstant engine, mostly with inclined axles. Thebranchingof the power and the control of the out-put speed depending on the volume flow takesplace through the adjustment of the axial pistonpump. These transmissions are always adjustedvariably and under load in the entire adjustmentrange.

The requirements made on the lubricantsused in this respect are limited to the toothings,roller bearings and hydraulic systems. For rea-sons of pumpability, the very good V –T behav-ior of the low-viscosity hydraulic oils used isof great importance. In addition, these transmis-sions are to be protected against wear and cor-rosion using a suitable lubricant. ATFs are alsooften used.

9.5. Multipurpose Lubricants in VehicleGears

Multipurpose lubricants are used in agriculturaland working machines such as tractors, har-vesters, etc.

In these vehicles, the long-term and perfectfunction of the wet, force-conclusive additionalclutches andbrakes is to be ensured.The scuffingload capacity of the balancing hypoid transmis-

sions is to be guaranteed using a suitable lubri-cant. Friction, hydraulic and wear requirementshave to be met. In order to ensure the drivingoperation even at low temperatures, torque con-verters have to work adequately and safely, evenunder conditions of permanent slip of the wetclutches. Therefore, multipurpose oils have al-most always a low viscosity and stand out due toa very good V –T behavior. The presence of wa-ter and dirt has a significant impact on these oils,especially in respect of their foaming behavior.

This is aggravated by the fact that the men-tioned requirements often have to manage withonly one oil circulation system.

Against the background of these require-ments it is easy to understand that major manu-facturers of tractors and agricultural machines,such as Ford, John Deere and Massey Fergu-son, have developed their own lubricant speci-fications for UTTOs and STOUs. The most im-portant of these specifications are listed below:

UTTO SpecificationsMassey Ferguson M1127AMassey Ferguson M1127BMassey Ferguson M1135Massey Ferguson CMS-M1143 LimitsFord ESN-M2C-86CFord ESN-M2C-134DJohn Deere J20CJohn Deere J20DMS 1207

STOU SpecificationsMassey Ferguson CMS-M1139Massey Ferguson CMS-M1144 LimitsFord ESN-M2C-159-CJohn Deere J27


9.6. Gear Lubricants for IndustrialGears

Industrial gear applications differ from vehicletransmissions mainly due to their larger varietyand higher number of combinations of toothingtypes and sizes used (Fig. 45). Most importantare worm gears, planetary gears and helical spurgears with crossed axis. Gears for industrial ap-plication are also applied in a larger variety ofpossible working and ambient conditions. Theystand out due to much higher torque and perfor-mances, connected with clearly larger housingdimensions. At the same time, the gears size alsorequires larger lubricant quantities. The servicelife requirements on industrial gears are clearlyhigher than thosemade on vehicle transmissions(see Fig. 43).

In comparison to vehicle gear lubricants, in-dustrial gear oils meet less requirements at oneand the same time.With respect to industrial ap-plications, the user has mainly to regularly andpunctually change the oils used in accordancewith the instructions of the oil producers. In ad-dition, once-through lubrication is often applied.In this respect, the environmental compatibilityof the lubricants used should especially be takeninto account.

In opposite to vehicle gears, the type of lu-brication in industrial gears can also be verydifferent. Usually, vehicle gears are equippedwith an oil immersion or injection lubricationsystem. According to their working conditions,industrial transmissions can, however, be lubri-cated manually by dropping or pouring, oper-ating in an oil sump, through oil mist or – aswell – through an oil injection system. Oftenlarger oil lubrication systems are employed, e.g.,in printing presses or paper machines, with fillsof several hundreds of liters of lubricant. Fig-ure 51 gives a schematic overview of a lubrica-tion system commonly used today.

During the lubrication with an oil lubricationsystem, the total volume of the oil must not beselected in a too low quantity so that the air in theoil can be released. In this respect, the oil’s airseparation and foaming properties play a veryimportant role since air is a bad lubricant. Thepurity of the lubricant during the operation ofthese systems is another central factor with re-spect to the gear’s service life and, thus, the oil’sfiltration and filterability.

The viscosity-dependent pumpability of theoil, especially at cold temperatures as well asduring the start-up of such systems, is also tobe taken into account precisely. The wrong vis-cosity selection of a lubricant can lead to thestandstill of the entire system. A guideline forthe adequate viscosity selection as a function ofload and speed for spur gears and worm gears inindustrial applications is given in [247].

In comparison to the lubrication oils for ve-hicles transmissions, the worldwide number ofspecifications concerning the properties of thelubricants for industrial applications is quitesmall. Important specifications issued by thegearmanufacturers and end consumers are listedbelow:

David Brown S1.53101 (Jan 1985)U.S. Steel 224DIN 51517 part 1 to part 3Flender sheet A, 02.03.1999, Rev. 0

These specifications cover both simplemechanical – dynamic model test proceduresand common component testers and test stan-dards with roller bearings and toothed wheels.Apart from these specifications usedworldwide,many transmission and system manufacturersare currently issuing their own, more sophis-ticated specifications for their industrial gearswith increasing requirements.

V – T Behavior and Viscosity Index. Ac-cording to the ambient and operating conditions,the required V –T behavior of gear lubricationoils constitutes a very important requirementduring the application. Here, the base oil’s prop-erties according to the viscosity range is of greatimportance. Worldwide, the lubrication oils forindustrial gears are subject to the ISO viscositygrade conversions. In the American region aswell as in a large part of the automotive indus-try, the SAE gear viscosity numbers apply. In theAmerican and, mainly, in the Asian regions, theviscosity ranges for AGMA lubricant are usedmost often.

Corrosion and Rust Protection. Espe-cially corrosion and corrosion protection play avery important role with respect to lubricationoils for industrial gears. Due to the required longservice lives of industrial gears, severe corro-sion can facilitate an unexpectedly quick failure


Figure 51. General layout of a lubrication system for large industrial gears

of bearings, toothed wheels and other impor-tant gear components. This has to be avoidedby using a protecting lubricant combined witha suitable additive. Some industrial applicationsare aggravated by the presence of salt water inthe gear, thus requiring special test methods.

Oxidation Stability. The aging behavior of alubricant is connected with a change in viscosityand with an increase in the acid number. Today,sensitive gears are often tested on a regular ba-sis. Should the acid number change noticeably,the oil producers recommend an oil change.

Flash Point and Pour Point. According tothe application, the specifications almost alwaysdefine applicable flash and pour points.

Demulsibility and Water Separation. Of-ten the water cannot be prevented from pene-trating the lubricant. Once the water proportionsin the oil have exceeded a certain limit, the onlypossibility left is to change the oil or to separateoil and water. In large oil systems, the waterwill sink to the bottom of the oil tank due tothe higher specific weight and can be releasedthrough a drain cock. This, however, requires agood water separation behavior of the lubricant.


Air Separation. Naturally, air is mixed intothe oil in each gear. Since air is also a poor heatconductor, the air should be separated from theoil as quickly as possible in order to minimizethe air’s proportions in the oil.

Paint Compatibility. Because of the im-proved protection and extended service life,the interiors of large industrial gears are alsocoated with paint which can be attacked, soft-ened and stripped off by oils. This appliesespecially for one-component paints whereastwo-components paints are generally resistantagainst polyalkylene glycols.

Seal Compatibility. Like paints, seals oftenhave organic components which can be attackedby lubricants. The risk of an attack rises consid-erably with an increase in the oil temperature.The elastomer compatibility tests of importantspecifications for the combination of lubricantand seal include both static and dynamic tests.

Foaming. Especially contamination causethe development of partly large quantities offoam in oils. This foam development can reachdimensions so that the foam escapes the gearthrough de-aerators, thus resulting in a contami-nation of the environment. In order to avoid this,only those oils should be used which in theirfresh condition stand out due to a very low foam-ing tendency.

Environmental and Skin Compatibility.Today, especially the environmental and skincompatibility is considered very important by agrowing number of gear manufacturers and sys-tem operators due to the effect on health andenvironment. Numerous specifications requirecompatibility tests.

Open gear drives are often found in the ce-ment industry – the so-called milling gears – inrotary kilns in the iron and steel industry, in coal-burning plants or in open-cut mines. These openlarge gears are often lubricated with sprayableadhesion lubricants. Apart from the require-ments made on the load capacity and wear pro-tection, priority is also given to the adhesionproperties, sprayability, pumpability and corro-sion protection. In the mentioned systems, solidso-called compounded lubricants have proven

to be successful as additives to such adhesionlubricants. Today, the health and environmentalimpact which can occur in oil mist and spraylubrication systems as well as the direct lossesdue to oil-containing waste air minimize carefuloil separation processes. Open gears can also belubricated manually or with the help of dosingpumps in regular intervals with high-viscosityoils.

10. Compressor Oils

10.1. Gas Compressor

10.1.1. Displacement Compressors

Reciprocating Piston Compressors. In re-ciprocating piston compressors the oil is alsosubject to enormous stress resulting from thehigh temperatures created when the medium iscompressed (which can cause oxidation and leadto deposits) and in the case of air, the oxygen en-richment. The cleanest possible air or gas shouldbe compressed because contaminants can accel-erate oxidation andwear. In the case of drive unitlubrication, the lubrication of the bearings is ofprimary importance.

For reciprocating piston compressors, nor-mally, lubricants based on mineral oil accordingto DIN 51 506 –VCL, VDL (or PAO- or diester-based lubricants) are used with viscosity gradesof ISOVG68 to ISOVG150. Mobile compres-sors are often lubricated with monograde en-gine oil (SAE 20 – SAE40) [250], [251]. Smallto medium-sized piston compressors are usedfor pressures up to 10 bar.

Rotary Piston Compressors, Single Shaft,Rotary Vane Compressors. The pressurechambers of rotary piston compressors arecooled and lubricated by total-loss systems orby direct oil injection. The lubrication of ro-tary piston compressors is similar to the lubri-cation of the cylinders in reciprocating pistoncompressors insofar as the lubricant is subjectto high outlet temperatures in both cases. In thecase of oil-injected and oil cooled rotary pis-ton compressors, a quantity of oil is continu-ously injected into the compressor chambers.The quantity of the oil is such that the outlet


Table 24. Overview of normally used compressor oils. The viscosity and the quality recommendations of the compressor manufacturers mustbe taken into consideration.

Viscosity classification Type piston compressorsa Screw compressorsoil-injected

Sliding vanecompressorsa,b

Turbo compressors (axialand radial)c

ISO VG 32 MO TDL 32HC-oils TDL 32EPPAO synth. oils

ISO VG 46 MO TDL 32HC-oils TDL 32 EPPAO synth. oilsPOE

ISO VG 68 MO MO MO(SAE20W-20) PAO HC-oils diester

diester PAO HC-oilsPOE

ISO VG 100 MO MO(SAE30) PAO diester

diester HC-oilsISO VG 150 MO(SAE40) PAO

diester

MO=mineral oil; PAO= polyalphaolefin; HC= hydrocracked oil; POE= biologically degradable polyol esters.Diester, polyolester and PAO: for very hard working conditions, increase of service intervals is possible.HC-oils: for medium and hard working conditions.MO: for normal and medium working conditions.Lubricants for roots-compressors: HL, CL, CLP; ISO VG 100-150, DIN 51 524, DIN 51 517.Lubricants for vacuum pumps: ISO VG 68-150.a Total-loss lubrication: HD-monograde motor oils HD 20W-20, HD 30, HD 40.b For oil-injected compressors in mobile equipment (e.g., railways, buses): multigrade motor oils (e.g., 10W40, 15W40). Foroil-injected compressors in stationary units: turbine oils according to DIN 51 515TDL, air compressor oils according toDIN 51 506VCL, VDL. For hard working conditions: monograde motor oils HD20W-20, HD30, MIL 2104 D.c Turbine oils according to DIN 51 515TDL or TDL-EP with extreme pressure additives.

temperature does not exceed 100 – 110 ◦C. Atthe same time, it seals the pistons against thehousing and protects against wear. The coolingof the medium results in an increase in compres-sion performance. Cooling and sealing increasethe volumetric efficiency and thus the overallefficiency of the compressor. The oils used arenormally VCL or VDL according to DIN 51 506with an ISOVG between 68 and 150 or mono-grade SAE20 – SAE40 engine oils .

Screw Compressors. In lube-injected screwcompressors, the oil has a lubricating, sealingand cooling function. The lubricant is injectedinto the pressure chamber between the rotors atabout 3 – 4 bar. It then forms a hydrostatic and ahydrodynamic lubricating film.The oil thereforelubricates the meshing rotors and the plain androller bearings which are part of the geared cou-pling. Furthermore, it seals the gaps between therotor and the housing. It also helps absorb heatand dissipate this via radiators. The temperatureof the compressed air of about 80 ◦C to 100 ◦C

is adjusted by the quantity of oil injected. Down-stream oil separators (normally cartridge filters)remove the oil from the air.

As the viscosity of the oil is of primary im-portance to elastohydrodynamic lubrication andthus for the mechanical stability of the film, itmust be matched to start-up and normal run-ning conditions. As a rule, ISOVG46 lubricantscover most manufacturer’s recommended vis-cosity thresholds of about 10mm2/s at operatingtemperature to about 500mm2/s when starting-up. This range also satisfies most applicationsin central Europe. Higher viscosity ISOVG68oils or synthetic ester-PAG or PAO-based lubri-cants are used in countries with high ambienttemperatures. In recent years, lubricants basedon hydrocracked oils have found increasing ac-ceptance.

Roots Compressors. Recommended lubri-cants include DIN 51 517CL and CLP orHDSAE oils in the viscosity grades ISOVG68and ISOVG100 [250].


10.1.2. Dynamic Compressors

Turbo Compressors. The oils for turbocompressors lubricate bearings, radial shaftseals and possibly gears via a positive-feed cir-cuit. In some cases, the bearings are lubricatedwith grease. Ideally, the same lubricant shouldbe used for the compressor and its drive. Mostoften, DIN 51 515TDL32 and TDL46 turbineoils or TDL-EP grades.

Turbo compressors are principally used forcreating compressed air in mines and industrialmanufacturing plants [249], [250].

10.1.3. Preparation of Compressed Air

The oil injected into oil-cooled screw and rotarypiston compressors is always removed from thecompressed air. The oil which is mixed with thehighly compressed air is removed and collectedin single or multistage downstream oil separa-tors. Before the oil is re-circulated, it is filteredand cooled. Depending on the specific require-ments, the compressed air may then pass a num-ber of subsequent treatment stages such as re-frigerant dryers or absorption dryers. Very lowresidual oil quantities in the air can be achievedby the fitting of a series of in-line oil separators[252–254].

10.1.4. Oils for Compression of Other Gases

Oxygen Compressors. Because of explo-sion hazards when oxygen is compressed, pres-sure chambers lubricants must be mineral oil-free. Water and water-based solutions such asglycerol can be used for cylinder lubrication.Mineral oil-based products may be used forcompressor drives if it does not come into con-tact with the pressure chambers [250].

Acid Gas Compressors. Gases often con-tain acidic components such as SO2 or NOx .If standard compressor oils were used for suchapplications, the lubricating oil would soon be-come over-acidified. To counter this, lubricantsare used for such applications which containhighly alkaline additives. These components canneutralize the acidic components in the gas. Inthese cases, it is recommended that monogradeengine oils (20W-20, 30W, 40W) with high al-kaline reserves (high TBN) are used [250].

Inert Gas Compressors. When inert gasesare compressed, the same rules as for air com-pressors should be used [248], [250].

Hydrocarbon Compressors. Hydrocar-bons such as ethane, propane etc. are easilysoluble in mineral oil. This causes the vis-cosity of the lubricating oil to fall if min-eral oil- based products are used. For thisreason, higher viscosity mineral oils such asISOVG100 and ISOVG150 must be usedin piston compressors whose crankcases aresubject to low inlet pressures (1− 3 bar). Inthe case of screw compressors (high pressure;10− 15 bar), ISOVG68− 100 ester- or poly-glycol-based lubricants with lower hydrocarbonsolubility are recommended [248], [250].

Vacuum Pump Lubrication. Vacuumpumps are compressors whose inlet is connectedto the chamber where the vacuum is created.VDL compressor oils can be used for low vac-uums. Greater vacuums require synthetic oilswith low vapor pressures (mostly synthetic esteroils). The lubricant selection must consider ifthe medium to be extracted is not air, but for ex-ample a refrigerant. In such cases, a compatiblerefrigerator oil can be used [250].

10.1.5. Characteristics of Gas CompressorOils

Compressors whose chambers are lubricatedpose particular safety problems if air or ag-gressive gases contact the lubricant. The selec-tion of the most suitable lubricant depends onthe type of compressor in question, the pres-sures involved, the outlet temperatures and thetype of gas being compressed. Piston com-pressors which generate the highest pressuresare particularly problematic. Turbo compres-sors which only have lubricated bearings andnon-lubricated pressure chambers pose the leastproblems. Rotary and screw compressors withoutlet pressures under 10 bar and correspond-ingly low outlet temperatures are examples ofaverage compressor lubrication application. Ta-ble 24 shows an overview of normally used com-pressor oils.


10.1.6. Standards and Specifications ofCompressor Oils

DIN51 506 describes the classification and re-quirements of lubricating oils which are usedin piston compressors with oil-lubricated pres-sure chambers (also for vacuum pumps). Lu-bricants for screw and oil-injected rotary vaneand screw compressors are not included inDIN 51 506 [251]. Table 25 shows the clas-sification of air compressor oils according toDIN 51 506. Tables 26, 27, and 28 contain theminimum requirements of air compressor oilsaccording to DIN 51 506.

Table 25. Classification of air compressor oils according toDIN 51 506 – Table 1, September 1985

Maximum compressed air temperature, ◦C

Lubricating oilcategory

For compressors onmoving (mobile)equipment for brakes,signals and tippers

For compressors withstorage tanks and pipenetwork systems

VDL up to 220 up to 220VC up to 220 up to 160a

VCLVB up to 140 up to 140

a Rotary multi-vane compressors designed for a once-throughlubrication can be operated at compressor end temperatures ofup to 180 ◦C using lubricating oils doped in the same manner asmotor lubricants or doped compressor oils, provided that therequirements specified for VCL lubricating oils in table 2 arecomplied with.

According to this standard, such lubricantsare pure mineral oils or mineral oils with ad-ditives to increase aging resistance and corro-sion protection. The classification of the lubri-cants depends on the expected outlet temper-atures and the general application. DIN 51 506differentiates between lubricants for mobile ap-plications and stationary applicationswith reser-voirs. Principal differences between the listedgroups: VB/VBL, VC/VCL as well as VDL arethe use of oxidation and corrosion inhibitors, ag-ing stability and residue formation and the qual-ity of the base oils (cuts). The difference betweengroup VB/VBL and group VC/VCL lies in theaging behavior (formation of Conradson cokeafter air-induced aging). Group VDL oils haveto pass a more difficult aging test (formation ofConradson coke after air-induced aging in thepresenceof ferrous oxide).DIN 51 506VDLoilsdisplay the best thermal and oxidation stabilityand form the least residues. The selection cri-

teria in DIN 51 506 were adopted into the 1983ISO/DP 6521 draft [251], [255]. Table 29 de-fines the requirements of air compressor oils forreciprocating piston compressors.

The selection criteria for screw compres-sor and piston compressor oils differ greatly.In oil-flooded rotary vane and screw compres-sors, the injected oil is constantly in contactwith the 80− 100 ◦C hot medium being com-pressed. The compressed medium and the oilare well mixed and the oil has to be sepa-rated by downstream separators and filters. Thisplaces special demands on the lubricant. Onthe whole, these include low foaming, excel-lent air release and good demulsification (sep-aration) of condensed water. In addition ofcourse, the specifications regardingwear protec-tion (FZG≥ 10, DIN 51 354), minimum forma-tion of deposits, good corrosion protection etc.also apply. Most manufacturers of oil-injectedrotary and screw compressors issue their ownlubricant specifications. A draft of ISO/DP 6521for oil-injected screw compressors has existedsince 1983. Table 30 describes the requirementsof air compressor oils for screw compressors.

10.2. Refrigerator Oils

10.2.1. Introduction

The longevity expected of refrigeration com-pressors is closely connected to the high qualitywhich is required of refrigerator oils. The inter-action with other substances which the refriger-ator oil comes into contact with, and especiallythe extremely high and low temperatures, makesvery specific demands on refrigerator oils.

The principal function of a compressor oil isto lubricate the pistons or rotors and to lubricateand seal the valves and, in some cases, the slip-ring seals. Furthermore, the refrigerator oil mustdissipate heat away from hot compressor com-ponents and assist in sealing the compressionchambers and valves. The refrigerator oil servesas a hydraulic control and functional fluid in re-frigeration compressors [256], [257]. It is vitalthat refrigerator oil which reaches and collectsin colder sections of the circuit in the form ofoil vapor or oil mist or as a result of splashingmust be returned to the compressor by mechan-ical means (oil separator) or via the refrigerant


Table 26. The minimum requirements of air compressor oils according to DIN 51 506 – VB, VBL

Grade VB is a pure mineral oil, Grade VB-L contains additives L to increase aging resistance and corrosion protectionLube Oil Group VB and VB-L

Viscosity grade ISOVG 22

ISOVG 32

ISOVG 46

ISOVG 88

ISOVG100

ISOVG150

ISOVG220

ISOVG320

ISOVG460

Kinematic viscosity(DIN 51 561 / 51562 part 1)

min. 19.8 28.8 41.4 61.2 90 135 198 288 414mm2/s at 40 ◦C to to to to to to to to to

max 24.2 35.2 50.6 74.8 110 165 242 352 506mm2/s at 100 ◦C 4.3 5.4 6.6 8.8 11 15 19 23 30Flash point, ◦C (COC) min.(DIN ISO 2592)

175 175 195 195 205 210 225 225 255

Pour point, ◦C, max.(DIN ISO 3016)

−9 −9 −9 −9 −9 −3 0 0 0

Ash, wt% max.(DIN 51 575)

VB: max. 0.02% oxide ash; VB-L: Sulf. ash to be stated by the supplier

Water soluble acids(DIN 51 558 part 1)

neutral

Neutralization number(acid), mg KOH/g max.(DIN 51 558 part 1)

VB: max. 0.15 mg KOH/g; VB-L: to be stated by the supplier

Water,%(DIN ISO 3733)

0.1 max. 0.1 max 0.1 max 0.1 max 0.1 max 0.1 max 0.1 max 0.1 max 0.1 max

Aging characteristics 1.5%CRC max. after air aging

(DIN 51 352 part 1)

2.0 2.0 2.0 2.0 2.0 2.5 2.5 2.5 2.5

% CRC max. after air/Fe2O3aging

(DIN 51352 part 2)

not required

Kinematic viscosity (at 40 ◦C),max. of

20% dist. residue(DIN 51 536 / 51 561 / 51 562part 1)

not required

Distillation residue% CRC max. of 20%distillation residue(DIN 51 356 / 51 551)

not required

Figure 52. Principle of a vapor compression refrigeration cycle


Table 27. The minimum requirements of air compressor oils according to DIN 51 506 – VC, VCL

Grade VC is a pure mineral oil, Grades VC-L and VD-L contain additives L to increase aging resistance and corrosion protectionLube Oil Group VC and VC-L

Viscosity grade ISOVG 32

ISOVG 46

ISOVG 68

ISOVG100

ISOVG150

Kinematic viscosity(DIN 51 561 / 51562 part 1)

min. 28.8 41.4 61.2 90 135mm2/s at 40 ◦C to to to to to

max 35.2 50.6 74.8 110 165mm2/s at 100 ◦C 5.4 6.6 8.8 11 15Flash point, ◦C (COC) min.(DIN ISO 2592)

175 195 195 205 210


−9 −9 −9 −9 −3

Ash, wt% max.(DIN 51 575)

VC: max. 0.02% oxide ash; VC-L: Sulf. ash to be stated by the supplier


neutral

Neutralization number(acid), mg KOH/g max.(DIN 51 558 part 1)

VC: max. 0.15 mg KOH/g; VC-L: to be stated by the supplier

Water,%(DIN ISO 3733)

0.1 max 0.1 max 0.1 max 0.1 max 0.1 max

Aging characteristics 1.5% CRCmax. after air aging

(DIN 51 352 part 1)

1.5 1.5 1.5 1.5 2.0

% CRC max. after air/Fe2O3aging

(DIN 51352 part 2)

not required

Kinematic viscosity (at 40 ◦C),max. of

20% dist. residue(DIN 51 536 / 51 561 / 51 562part 1)

not required

Distillation residue% CRC max. of 20% distillationresidue(DIN 51 356 / 51 551)

0.3 0.3 0.3 0.3 0.75

flow (refrigerant solubility) in all operating con-ditions. Figure 52 shows the principle of a vaporcompression refrigeration cycle.

10.2.2. Minimum Requirements

The basic requirements of refrigerator oils arelaid down in DIN 51 503-1. This standard de-fines the basic requirements of refrigerator oilsaccording to themediumbeing compressed. Theintroduction of new chlorine-free, polar refriger-ants such as HFCR134a (to replace CFCR12),necessitated a revision of DIN 51 503 whichappeared as DIN 51 503-1 in November 1997[258].

Refrigerator oils are classified in alphabeti-cal groups according to the refrigerants beingcompressed:

KAA Refrigerator oils not soluble in ammo-niaKAB Refrigerator oils soluble in ammoniaKC Refrigerator oils for partially- and fully-halogenated fluorinated and chlorinated hy-drocarbons (CFC, HCFC)KD Refrigerator oils for partially- and fully-fluorinated hydrocarbons (FC, HFC)KE Refrigerator oils for hydrocarbon refrig-erants such as propane or isobutane

The various refrigerants available are de-scribed in DIN 8960 and the ASHRAE standard(ANSI/ASHRAE34-1992;AmericanSociety ofHeating,Refrigerating andAirConditioningEn-gineers) [259].

In addition to appearance, density (ISO3675)and viscosity (DIN 51 550), several other prop-erties are defined and determined:


Table 28.Minimum requirements of air compressor oils according to DIN 51 506 – VDL

Lube Oil Group VDL

Viscosity grade ISO VG 32 ISO VG 46 ISO VG 68 ISO VG 100 ISO VG 150Kinematic viscosity

(DIN 51 561 / 51 562-1)cST at 40 ◦C 28.8 to 35.2 41.4 to 50.6 61.2 to 74.8 90 to 110 135 to 165mm2/s at 100 ◦C 5.4 6.6 8.8 11 15Flash point, ◦C (COC), min.

(DIN ISO 2592)175 195 195 205 210


− 9 − 9 − 9 − 9 − 3

Ash, wt%, max.(DIN 51 575)

sulf. ash to be stated by the supplier


neutral

Neutralization number (acid), mgKOH/g, max.

(DIN 51 558 part 1)

to be stated by the supplier

Water, %(DIN ISO 3733)

0.1 max. 0.1 max. 0.1 max. 0.1 max. 0.1 max.

Aging characteristics% CRC max. after air aging(DIN 51 352 part 1)

not required

% CRC max. after air/Fe2O3 aging(DIN 51352 part 2)

2.5 2.5 2.5 3.0 3.0

Distillation residue % CRC max. of20% distillation residue

(DIN 51 356 / 51 551)

0.3 0.3 0.3 0.3 0.6

Kinematic viscosity at 40 ◦C max. of20% distillation residue(DIN 51 536 / 51 561 / 51 562-1)

maximum of five times the value of the new oil

U-tube flowing DIN 51 568Flashpoint DIN-ISO 2592Neutralization number DIN 51 558-3Saponification number DIN 51 559Oxide ash DINEN6245Water content DIN 51 777-1Pour point DIN-ISO 3016Refrigerant miscibility EDIN 51 514Refrigerant compatibility with R 134aASHRAE97/83

Table 31 gives an overview of the importantrefrigerants and suitable refrigerator oils on themarket.

DIN 51503-2 describes criteria for evaluatingused refrigerator oils [259].

10.2.3. Classification

Normally used refrigerator oils are listed in Ta-ble 32.

Dewaxed Naphthenic Refrigerator Oils.Naphthenic mineral oils are still the most signif-

icant group of oils for refrigerators using ammo-nia refrigerants alongwithCFC andHCFC (e.g.,R 22).Naphthenic refrigerator oils generally dis-play very low pour points, good cold flowing aswell as high thermal and chemical stability. Se-lected cuts are normally used [256], [262].

Paraffinic Refrigerator Oils. Paraffinic re-frigerator oils are ideal for use in R 11 and R 12turbo compressors (ISOVG68and100) becauseof their good V –T behavior. These oils are notrecommended for other compressors because oftheir generally inadequate solubility in refriger-ants (miscibility gap, e.g., R 22). The boundariesbetween paraffinic and naphthenic oils are notrigid [256], [257].

Semisynthetic refrigerator oils are mix-tures of highly-stable alkylbenzenes and highly-refined naphthenic mineral oils. The presenceof alkylbenzenes greatly improves the solubilityand thermal stability of the naphthenic compo-nents. The proportion of synthetic componentsis usually between 30 and 60%. Semisyntheticoils are recommended for CFC/HCFC systems,

100 Lubricants and LubricationTable29.T

herequirem

entsof

aircompressoroilsforreciprocatingpiston

compressors–ISO/DP6521

–Draft1983

Mineraloil-basedlubricantsforreciprocatingpiston

compressors

Category

ISO-L-D

AA

ISO-L-D

AB

TestMethod

Viscosity

grades

3246

68100

150

3246

68100

150

ISO3448

Viscosity

at40

◦ C,m

m2/s

±10

%32

4668

100

150

3246

68100

150

ISO3104

(IP7

1)at100

◦ Cmm

2/s

tobe

stated

tobe

stated

Pour

point∗,

◦ Cmax.

−9

−9

−9

−9

−9

−9

−9

−9

−9

−9

ISO3016

(IP1

5)Coppercorrosion,max.

1b1b

1b1b

1b1b

1b1b

1b1b

ISO2160

(IP1

54)

Rust

norust

norust

ISO/DP7120A

(IP1

35A)

Emulsion

characteristics

ISO/DP6614

(AST

MD1401)

Temperature,◦ C

norequirem

ent

5454

5482

82Tim

e(m

in)to

3mL

3030

3030

60Emulsion,m

ax.

Oxidatio

nstability

afteraging

at200

◦ CEvapor.loss,%

,max.

1515

1515

15no

requirem

ent

ISO/DP6617

Part1

(DIN

51352)

Increase

inCCRC,%

,max.

1.5

1.5

1.5

2.0

2.0

norequirem

ent

afteragingat200

◦ CEvapor.loss,%

,max.

notapplicable

2020

2020

20ISO/DP6617

Part2

(DIN

51352)

Increase

inCRC,%

,max.

notapplicable

2.5

2.5

3.0

3.0

3.0

Distill.residue(20vol%

)CRC,%

,max.

notapplicable

0.3

0.3

0.3

0.3

0.6

ISO/DP6616

with

ISO/DP6615

andISO3104

Ratio

ofviscosity

ofresidue

tothatof

newoil,max.

55

55

5

∗WhenVG32

orVG46

oilsareused

incold

clim

ate,pour

pointslower

than

−9

◦ Carerequired.


Table 30. The requirements of air compressor oils for screw compressors – ISO/DP 6521 – Draft 1983

Mineral oil-based lubricants for rotary screw compressors

Category ISO-L-DAH ISO-L-DAG TestMethod

Viscosity grades 32 46 68 100 150 32 46 68 100 150 ISO 3448Viscosity at 40 ◦C,mm2/s ± 10%

32 46 68 100 150 32 46 68 100 150 ISO 3104(IP71)

Pour point∗, ◦C max. −9 −9 −9 −9 −9 −9 −9 −9 −9 −9 ISO 3016(IP15)

Copper corrosion, max. 1b 1b 1b 1b 1b 1b 1b 1b 1b 1b ISO 2160(IP154)

Rust no rust no rust ISO/DP7120A(IP135A)

Emulsion characteristicsTemperature∗∗, ◦C 54 54 54 54 82 54 54 54 54 82 ISO/DP

6614(ASTMD1401)

Time (min) to 3mLemulsion, max.

30 30 30 30 30 30 30 30 30 30

Foaming characteristics ISO/DP6247(IP146)

Sequence I at 24 ◦C 300 300 300 300 300 300 300 300 300 300Tendency, mL, max. nil nilStability, mL, max.Oxidation stabilityEvapor. loss, %, max. to be decided to be decided to be establishedIncrease in viscosity, %Sludge, wt%Increase in acidity, %

∗ When VG32 or VG46 oils are used in cold climate, pour points lower than −9 ◦C are required.∗∗ Required only in those applications where condensation of atmospheric moisture is a problem. Where this does not apply,oils with dispersant additives, which tend to have poor water separating properties, may be used satisfactorily.

Table 31. Classification of important refrigerants and refrigerator oils.

ASHRAE name Trade name Chemical name / formula Refrigerator oila)

Chlorine-free refrigerants and refrigerator oilsR 134a diverse CH2FCF3 POE, PAGR 507 Solkane 507, AZ 50 R 125/R 143a POER 404 A diverse R 125/R 143a/ R 134a POER 407 C diverse R 32/R 125/ R 134a POER 410 A Solkane 410, AZ 20 R 32/R 125 POER 600a/R 290 isobutane/propane C4H10 / C3H8 MO / ABR 717 ammonia NH3 MO / PAO / ABR 744 carbon dioxide CO2 synth. oilb)

Drop-in refrigerantns and refrigerator oilsR 22 diverse CHClF2 MO / ABR 401 A MP 39 R 22/R 152a/ R 124 MO / ABR 401 B MP 66 R 22/R 152a/ R 124 MO / ABR 402 A/B HP 80/81 R 22/R 125/ R 290 MO / ABR 403 A/B 69 S/L R 22/R 218/ R 290 MO / ABR 408 A FX 10 R 22/R 143a/ R 125 MO / AB

a AB= alkylbenzene oil; MO=mineral oil; PAG= polyalkylene glycol; PAO= polyalphaolefins; POE= polyolester oilb Development product; POE, PAG, ...


Table 32. Overview of normally used refrigerator oils. The viscosity recommendations of the compressor manufacturers must be taken intoconsideration

Compressor type Refrigeranthydrocarbonsa (e.g.,R 290 R 600a)

Ammoniab CFC, HCFC, (e.g., R12, R 22)

CFC, HCFc (e.g., R134a, R 404a)

Drop-In (e.g., R402A, R 403A)

Hermetic compressors(e.g., pistoncompressors)

MO – MO POE MO/ABAB AB

(MO/AB)ISO VG 15-32 ISO VG 15-32 ISO VG 10-32 ISO VG 32

Open pistoncompressors

MO MO MO POE MO/ABAB AB ABPAO PAO, PAG MO/ABISO VG 46-100 ISO VG 32-68 ISO VG 32-68 ISO VG 32-68 ISO VG 32-68

Semihermeticcompressors

MO – MO POE MO/ABAB ABPAO MO/ABISO VG 46-100 ISO VG 32-68 ISO VG 32-68 ISO VG 32-68

Scroll compressorsMO – MO POE MO/ABAB ABISO VG 46-100 ISO VG 32-68 ISO VG 32-68 ISO VG 32-68

Screw compressors MO MO MO POE MO/ABAB AB ABPAO, PAG PAO, PAGISO VG 68-220 ISO VG 32-68 ISO VG 68-150 ISO VG 100-150 ISO VG 68

Turbo compressors MO – d MO POE MO/ABPAO, PAGISO VG 68-100 ISO VG 68-100 ISO VG 68-150 ISO VG 68

a PAG-lubricants are partly soluble with hydrocarbons (low viscosity reduction); MO, AB and PAO are highly soluble withhydrocarbons (high viscosity reduction).b MO, AB and PAO are not soluble with ammonia, PAG is (partly) soluble with ammonia, ISO VG 68 is used in piston compressors,up to ISO VG 220 is used in screw compressors, please use PAG compatible components, PAG is hygroscopicc PAG-lubricants are used in R 134a air conditioning systems of cars and trucks; POE and PAG lubricants are hygroscopicd Normally oil-freeMO=mineral oil; AB= alkylbenzene; MO/AB=mineral oil – alkylbenzene mixture; PAO= polyalphaolefin; PAG= polyalkyleneglycol; POE= polyol ester.

low- temperature R 22 systems and for drop-inrefrigerants such as 401 A/B, 402 A/B and R 22mixtures [260], [263].

Fully Synthetic Refrigerator Oils.Alkylbenzenes. Fully synthetic refrigerator

oils are based on chemically and thermallyhighly stable alkylbenzenes. Alkylbenzeneshave been used as refrigerator oils for a num-ber of years. Carefully selected and speciallytreated alkylaromatics are used. A number ofcomplex manufacturing stages ensure that theproducts are free of difficult-to-dissolve waxysubstances and other contaminants such as sul-fur. Alkylbenzene-based lubricants display ex-cellent solubility in CFC (HCFC) refrigerants(e.g., R 22, R 502) and mixtures thereof at evap-oration temperatures down to − 80 ◦C (e.g.,R 22).

ISOVG46 and 68 alkylbenzenes haveproved particularly suitable for use in heavy-

duty ammonia compressors with very high out-let temperatures. Compared tomineral oil-basedrefrigerator oils, alkylbenzenes form minimalamounts of coke and slash less when starting-up.

Alkylbenzenes are also increasingly beingused in hermetically sealed and semisealed com-pressors. Alkylbenzenes are increasingly gain-ing acceptance for use with drop-in refrigerantssuch as R 401 A/B, R 402 A/B, R 22 mix-tures and togetherwith propane/isobutane [262],[263].

Polyalphaolefins are recommended for am-monia compressors because of their good ther-mal stability. The formation of oxidation prod-ucts (coke) is avoided even at high compressoroutlet temperatures. Compared to mineral oilsand particularly in the case of screw compres-sors, the use of polyalphaolefins can reduce theamount of oil mist and oil vapor which collectsin oil separators. The amount of oil in the refrig-


erant vapor can also be reduced to a minimum.As a result of their chemical structure, polyal-phaolefins display good V –T behavior (highVI) and thus good cold flowing characteristics.The low pour point and low viscosity of theseproducts guarantee satisfactory oil return evenat evaporation temperatures of – 50 ◦C.

Polyalphaolefins of the viscosity gradeISOVG68 are generally used in screw and pis-ton ammonia compressors [261].

Polyol Esters. Previously used refrigeratorlubricants such as mineral oils, alkylbenzenes orpolyalphaolefins are not or not sufficiently solu-ble in the new chlorine-free refrigerant mixturessuch as R 134a, R 404 a and R 507. This led tothe development of synthetic polyol ester-basedrefrigerator oils which are soluble in FC andHFC refrigerants according to DIN 8960. Thechemical and thermal stability of these productsis excellent.

Legislation passed in 1991 progressivelybanned the use of CFCs in all refrigeration units.Since January 1995, new equipment must becompletely CFC-free. R 134a and R 22 shouldreplace CFCR12 in the long and short-term re-spectively. These substitute refrigerants are thussteadily gaining acceptance as is the demand forpolyol ester-based, synthetic refrigerator oils.Ester oils are suitable for all refrigerant sys-tems which use R 134a, R 404a, and FC andHFC mixtures. Products with the suitable vis-cosities (ISOVG15−220) are available for in-dustrial and household piston and screw com-pressors. The manufacturers’ viscosity recom-mendations should be observed.

Similarly to all ester oils, saturated and high-purity polyol ester oils can hydrolyze if theycome into contact with water in the compressor.It is therefore essential that these products areprotected from water, and moisture in general,during storage and use. Ester oils are ultra-driedand are filled into air-tight metal drums with awater content of < 100 ppm in a nitrogen atmo-sphere [261], [264], [265].

In addition to their properties described inSection 5.3.1.2 polyol esters have the followingspecific characteristics:

– Excellent solubility in FC and HFC refriger-ants

– Avoidance of oil build-up in the con-denser/evaporator

– Very good thermal and chemical stabilityeven in the presence of refrigerants

– Compatibility with all commonly-used seal-ing materials such as NBR, HNBR, EPDMand others

– The products are ultra-driedPolyalkylene Glycols. Along with the intro-

duction of R 134a refrigerants as a substitutefor R 12 in the area of vehicle aircon systems,most of such compressors are designed to usepolyalkylene glycol oils. These polyalkyleneglycols are not always compatible and misci-ble with normal lubricants based on mineraloil, alkylbenzenes or esters. Polyalkylene gly-cols are naturally polar and thus miscible withR 134a. These polar characteristics make poly-glycols very hygroscopic and this fact must betaken into consideration when handling thesespecialized lubricants. When filled, the watercontent of polyglycol oils should be< 1000 ppm(fresh oil < 500 ppm).

PAG-based refrigerator oils are ultra-driedbefore use [261].

Fully synthetic, polyglycol-based refrigera-tor oils (ISOVG68, ISOVG100) are also usedfor ammonia refrigerant systems.

In the past, ammonia system lubricants usedrelatively naphthenic mineral oils, alkylben-zenes or PAO. The problems of oil enrichmentand oil deposits in such systems are well known.Polyalkylene glycols on the other hand, dis-play excellent solubility in ammonia refriger-ants. Such good solubility will lead to new sys-temdesigns in the future including very low tem-peratures and dry evaporation. The carefully se-lected synthetic components display excellentV –T behavior and high thermal stability. Thewater content of polyglycol oils must be keptlow (about 300− 500 ppm). Mixing or contam-ination with mineral oil must be avoided (for-mation of reaction products!) [266].

Other Synthetic Fluids. In the past, polysili-cic acid, ester-based synthetic fluids were usedfor evaporation temperatures below − 120 ◦C.Products based on low-viscosity silicone oils(polydimethylsiloxane; PDMS) are also used.

Refrigerator Oils for CO2. The natural re-frigerant CO2 is gaining acceptance amongusers. The properties of CO2 – oil mixturesare not fully known. In principle, the solu-bility of CO2 in oil increases in the order


MO>AB>PAG>POE, i.e., at the same tem-perature and pressure, the most CO2 is dis-solved in ester oils and the least in mineral oils.Depending on the type of oil, carbon dioxidecauses varying degrees of viscosity drops. Interms of mass%, viscosity decreases in the or-derAB>MO>POE>PAG.According to this,the viscosity of polyglycol- based oils fall theleast. SuitableCO2 refrigerator oils are currentlybeing developed. Apart from solubility, thermalstability is also an important factor (outlet tem-peratures approx. 160− 180 ◦C) [267], [268].

10.2.4. Viscosity Selection

The guidelines for the selection of refrigera-tor compressor lubricants are much the sameas those applied to engineering in general, i.e.,fast running machines permit the use of lowerviscosities than slow running machinery. Highbearing loads require higher viscosities than lowbearing loads. In addition, refrigerator compres-sors require much lower viscosities than thosecalculated from the hydrodynamic lubricationtheory.

In the past, many refrigerator systems wereoperated with chlorinated refrigerants. Chlorinecompounds are excellent EP additives whichprotect against wear. Since the introduction ofchlorine-free refrigerants, this function must beperformed by the refrigerator oil.

The most important parameter for determin-ing the lubricity of oils or oil – refrigerant mix-tures is viscosity.

Mixture Concentration in Relationship toTemperature and Pressure. Figure 53 showshow much refrigerant is dissolved in a refrig-erator oil when saturated at a defined operat-ing condition (pressure, temperature). As satura-tion is time-related, the refrigerant concentrationshown by the diagram is, as a rule, greater thanthe actual value. It can be viewed as the maxi-mum concentration at any given operating con-dition. The viscosity which can be taken fromthe mixture concentration adds a safety marginto any bearing loading calculations.

Mixture Viscosity and Density in Rela-tionship to Temperature, Pressure and Re-frigerant Concentration. The precise refriger-

ant concentration in a system is given at a de-fined pressure and temperature as shown in Fig-ure 53. Figure 54 can be used to read-off thekinematic viscosity of the oil – refrigerant mix-ture at a defined temperature, defined pressureand defined refrigerant concentration.

Care should be taken that the oil does notreach its maximum viscosity at problematicpoints in the oil return circuit (e.g., upwardflows). It is also important that the conditions inthe compressor crankcase do not approach thoseindicated by the falling left-hand part of theV –Tgraph because the smallest temperature fluctu-ations can have a significant effect on viscosityin such conditions [261].

The density of an oil refrigerant mixture de-pends on the density – temperature behavior ofthe oil and the refrigerant.

Miscibility Gap, Solubility Threshold. R-refrigerants are among the most oil-soluble re-frigerants. However, not all are miscible in re-frigerator oils at all temperatures and in allconcentrations. If, for example, a fully homo-geneous oil – refrigerant mixture is cooled, apoint is reached when the mixture separatesinto two phases. The size of the miscibilitygap depends on the type of refrigerant andalso to a large extent on the type of refriger-ator oil. Refrigerant solubility is defined stati-cally in EDIN 51 514. For normal applications,the miscibility gap (with alkylbenzenes) is nota problem for a series of refrigerants suchas R 22. Some other refrigerants display pro-nounced miscibility thresholds. The miscibilitygap is of great importance to the refrigerant cir-cuit. If the oil – refrigerant ratio iswithin themis-cibility gap, problems can occur caused by oil-enriched phases depositing in collectors, con-densers, evaporators and the crankcase. Floodedevaporators require the largest possible quantityof refrigerant to be dissolved at evaporating tem-peratures without the phases separating.

Table 33. DIN 51 515 classification of turbine oils, Draft 1999

‘Normal’ turbineoils – steam turbineoils

‘High-temperature’turbine oils

Without EP DIN 51 515-1 DIN 51 515-2With EP DIN 51 515-1 DIN 51 515-2(FZG, Load Stagemin. 8)

Annex A Annex A


Figure 53.Mixture concentration in relationship to temperature and pressure (RENSIO Triton SE 55 – R 134a)

Figure 54.Mixture viscosity in relationship to temperature, pressure, and refrigerant concentration (RENSIO Triton SE 55 – R134a)


Table 34. Requirements of steam turbine oils, DIN 51 515 Part 1, Draft 1999

Test Limits Testing according tob Comparable ISOstandards

Lubricating oil group TD 32 TD 46 TD 68 TD 100ISO viscosity gradea ISO VG 32 ISO VG 46 ISO VG 68 ISO VG 100 DIN 51 519 ISO 3448Kinematic viscosity at 40 ◦Cin mm2/s min. max.

28.8 35.2 41.4 50.6 61.2 74.8 90.0 110 DIN 51 562-1 or DIN51 562-2 or DIN ENISO 3104

ISO 3104

Flash point (COC), ◦C min. 160 185 205 215 DIN ISO 2592 ISO 2592Air release propertyd at50 ◦C, minutes max.

5 5 6 no valuespecified

DIN 51 381 –

Density at 15 ◦C, g/mL max. to be specified by supplier DIN 51 757 or DINEN ISO 3675

ISO 3675

Pour point, ◦C ≤ −6 ≤ −6 ≤ −6 ≤ −6 DIN ISO 3016 ISO 3016Neutralization number, mgKOH/g

to be specified by supplier DIN 51 558 Part 1 ISO 6618

Ash (oxide ash), mass % to be specified by supplier DIN EN ISO 6245 ISO 6245Water content, mg/kg max. 150 150 150 150 DIN 51 777-1 ISO/DIS 12937Cleanliness level, min. 20/17/14 20/17/14 20/17/14 20/17/14 DIN ISO 5884 with

DIN ISO 4406ISO 5884 with ISO4406

Water separation ability (aftersteam treatment), max.(seconds)

300 300 300 300 DIN 51 589 Part 1 –

Copper corrosion, max.corrosiveness (3 h at 100 ◦C)

2-100 A 3 2-100 A 3 2-100 A 3 2-100 A 3 DIN EN ISO 2160 ISO 2160

Corrosion protection againststeel corrosiveness, max.

no rust 0 - A DIN 51 585 ISO 7120

Aging behaviorc (TOST)Time in hours to reach deltaNZ of 2.0mg KOH/g

2000 2000 1500 1000 DIN 51 587 ISO 4263

Foam: ISO 6247Seq. I at 24 ◦C, max. mL 450/0 450/0 450/0 450/0 –Seq. II at 93 ◦C, max. mL 100/0 100/0 100/0 100/0 –Seq. III at 24 ◦C after 93 ◦C,max. mL

450/0 450/0 450/0 450/0 –

a Middle point viscosity at 40 ◦C in mm2/s.b The oil sample has to be stored without any contact to light before doing the test.c The aging behavior test has to be done as a type test procedure because of the long testing time. TOST=Turbine OxidationStability Test.d The test at a temperature of 25 ◦C has to be specified by the supplier when the customer needs values at low temperatures.Annex A (normative) – for EP turbine oils.If the turbine oil supplies also a turbine gear set, the oil has to reach a load stage of min. 8 according to DIN 51 345, part 1 and part 2(FZG).

11. Turbine Oils

11.1. Demands on TurbineOils – Characteristics

The demands on turbine oils are defined by theturbines themselves and their specific operatingconditions. The oil in the lubricating and controlcircuits of steam and gas turbines has to fulfil theobjectives [271], [272]:

– hydrodynamic lubrication of all bearings andthe lubrication of gearboxes

– heat dissipation– functional fluid for control and safety circuits

– avoidance of friction and wear on gear toothflanks in turbine gearboxes andwhen the tur-bine is spooled-upApart from these mechanical – dynamic re-

quirements, the following physical – chemicalspecifications also have to be fulfilled by turbineoils [271], [272]:

aging stability for long operating periodshydrolytic stability (especially of the addi-tives used)corrosion protection even if water, steamand/or condensation is presentreliable separation of water vaporrapid air release and low foaminggood filterability and purity


Table 35. Requirements of high-temperature turbine oils, DIN 51 515 Part 2, Draft 1999

Test Limits Testing according tob Comparable ISOstandards

Lubricating oil group TG 32 TG 46ISO viscosity gradea ISO VG 32 ISO VG 46 DIN 51 519 ISO 3448Kinematic viscosity at 40 ◦C in mm2/smin. 28.8 41.4 DIN 51 550 in conjunctionmax. 35.2 50.6 with DIN 51561 or DIN 51562-1 ISO 3104Flash point (COC), ◦C, min. 180 200 DIN ISO 2592 ISO 2592Air release propertyd at 50 ◦C, max.minutes

5 5 DIN 51 381 –

Density at 15 ◦C, g/mL, max. to be specified by supplier DIN 51 757 ISO 3675Pour point, ◦C ≤ −6 DIN ISO 3016 ISO 3016Neutralization number, mg KOH/g to be specified by supplier DIN 51 558-1 ISO/DIS 6618Ash (oxide ash), mass% to be specified by supplier DIN EN 7 ISO 6245Water content, mg/kg, max. 150 150 DIN 51777-1 ISO/DIS 12937Cleanliness level, min. 17/14 17/14 DIN ISO 5884 with ISO 4406 ISO 5884 with ISO 4406Foam: ISO 6247Seq. I at 24 ◦C, max. mL 450/0 450/0 –Seq. II at 93 ◦C, max. mL 100/0 100/0 –Seq. III at 24 ◦C after 93 ◦C, max. mL 450/0 450/0 –Demulsibility, minutes to be specified by supplier DIN 51 599 ASTM-D 1401Water separation ability (after steamtreatment), seconds max.

300 300 DIN 51 589 Part 1 –

Copper corrosion corrosiveness, max. 2−125 A 3 2−125 A 3 DIN 51 759 ISO 2160Corrosion protection against steelcorrosiveness, max.

0 –A 5 0 –A 5 DIN 51 585 ISO/DIS 7120

Aging behaviorc

Time in hours to reach delta NZ of 2.0mg KOH/g, min.

3.000 3.000 DIN 51 587 ISO/DIS 4263

RBOT, minutes ≥ 800∗ ≥ 800∗ –Modified RBOT, % of time, ≥ 85% ≥ 85% – ASTM-D 2272minutes, in unmodified test –

∗ GE recommends only 450min.a Middle point viscosity at 40 ◦C in mm2/s.b The oil sample has to be stored without any contact to light before doing the test.c The aging behavior test has to be done as a type test procedure because of the long testing time.d The test at a temperature of 25 ◦C has to be specified by the supplier when the customer needs values at low temperatures.Annex A (normative) – for EP turbine oilsIf the turbine oil supplies also a turbine gear set, the oil has to reach a load stage of min. 8 according to DIN 51 345, part 1 and part 2(FZG).

These stringent demands on steam and gasturbine oils are met with carefully selected baseoils and the inclusion of special additives.

11.2. Formulation

Turbine oils used today contain special paraf-finic base oils with good V –T characteristics,antioxidants, and corrosion inhibitors. If gearedturbines require a degree of load-bearing capac-ity (e.g., failure load stage FZG9 –DIN 51 345),some mild EP additives are included.

These days, turbine base oils are created ex-clusively by extraction and hydration. Refiningand subsequent selective high-pressure hydro-

genation significantly determine and influenceturbine oil characteristics such as oxidation sta-bility, water separation, air release and foaming.This applies in particular to water separation andair release because these feature cannot be sub-sequently improved with additives. Steam tur-bine oils are generally made of special paraffinicbase oil cuts [270–272].

A combination of phenolic and aminic an-tioxidants (synergies) are added to turbine oilsto improve their oxidation stability [271]. Toimprove corrosion protection, non-emulsifiablecorrosion protection agents and nonferrousmetal passivators are used. These are not neg-atively affected by water or steam contamina-tion during operation and remain suspended. If


standard turbine oils are used in geared turbines,mild, long life, temperature and oxidation stableEP additives (organophosphorus and/or sulfurcompounds) are included. Furthermore, smallquantities of silicone-free antifoams and pourpoint depressants are also used in turbine oils[271–273].

Moreover, these additives must not detrimen-tally affect the air release (very sensitive) of theoil. All additives should be free of ash (e.g.,zinc-free). The purity of a turbine oil accordingto ISO 4406 should be 15/12 in the tank [274–277].Nocircuits,wires, cables or insulation con-taining silicone should come into contact withturbine oils.

11.3. Specifications

Special paraffinic mineral oils with additives arenormally used for gas and steam turbine lubri-cants. These serve to protect all shaft bearings inthe turbine and generator as well as the gearboxin corresponding configurations. They can alsobe used as hydraulic fluids in control and safetysystems. Fire-resistant, synthetic Type HFD-Rfluids [272] are normally used in hydraulic sys-tems which operate at pressures around 40 bar,if there are separate lubricating oil and controloil circuits (twin circuit systems).

DIN 51 515 (Lubricants and Control Fluidsfor Steam Turbines, issued 1986) is presentlybeing revised. Following ISO 6743, Part 5, Fam-ily T (Turbines) – Classification of Turbine Oils[278], the latest draft of DIN 51 515, dated 1999,classifies turbine oils as in Table 33.

Table 36. ISO 6743-5 classification of turbine lubricants, Draft 1999

Normal turbine oils High-temperatureturbine oils

Without EP ISO-L – TSA (steam) ISO-L – TGB (gas)ISO-L – TGA (gas)

With EP ISO-L – TSE (steam) ??∗

ISO-L – TGE (gas)

∗ Under discussion.

The demandsmade byDIN 51 515-1 – SteamTurbine Oils [278] and DIN 51 515-2 – High-Temperature Turbine Oils [278] are shown inTables 34 and 35.

ISO 6743-5 classifies turbine lubricants ac-cording to whether they are used in steam or gas

turbines and if they contain EP agents (Table 36)[279], [280].

Synthetic fluids such as PAO and phosphoricacid ester are also listed (see Table 37) [279],[280].

Table 38 [274–277] shows the principal de-mands made by the world’s leading turbinemanufacturers. The main differences betweenthese demands relate to the aging and/or oxi-dation tests.

11.4. Turbine Oil Circuits

The oil circuits play an especially importantrole in the lubrication of power station turbines.Steam turbines are normally fitted with pressur-ized oil circuits and separate lubricating and con-trol circuits as well as separate lubricating andcontrol oil tanks are used.

In normal operating conditions, the turbineshaft-driven main oil pump draws oil from thetank into the control and bearing supply circuits.Pressure in control circuits is normally between10 and 40 bar [269–272]. Oil tank temperaturesrange from 40 – 60 ◦C. The velocity of the oilin the feed circuit is about 1.5 – 4.5m/s [271].Cooled and passing through reduction valves,the oil reaches the turbine, generator and possi-ble gearboxbearings at a pressure of ca. 1 – 3 bar.The individual oil feeds return to the oil tank atatmospheric pressure. The lubricating oil circuitof a gas turbine is largely similar to that of asteam turbine.

Larger oil circuits are fitted with centrifugalby-pass filtering systems. These ensure that thefinest of contaminants are removed along withaging byproducts and sludge.

Oil aging is also influenced by the frequencywith which the oil is pumped through the circuit.If the oil is pumped too fast, excessive amountsof air are either dispersed or dissolved (problem:cavitation in bearings, premature aging etc.). Oiltank foaming can also occur but this generallycollapses rapidly.

11.5. Monitoring and Maintenance ofTurbine Oils

In normal circumstances, oil monitoring inter-vals of one year are perfectly acceptable. [270],


Table 37. Classification of lubricants for turbines, ISO 6743-5, Draft 1999.

Part 5: Family T (Turbines): Table – Classification of lubricants for turbines

General application Composition and properties Symbol ISO-L Typical application

1) Steam turbines, directly coupled orgeared to the load, normal service

Highly refined petroleum oil withrust protection and oxidationstability

TSA Power generation and industrial drives andtheir associated control systems, marinedrives, where improved load carryingcapacity is not required for the gearing2) Gas turbines, directly coupled or

geared to the load, normal serviceTGA

3) Steam turbines, directly coupled orgeared to the load, highload-carrying capacity

Highly refined petroleum oil withrust protection, oxidation stabilityand enhanced load carrying capacity

TSE Power generation and industrial drives andmarine geared drives and their associatedcontrol systems where the gearing requiresimproved load-carrying capacity.4) Gas turbines, directly coupled or

geared to the load, highload-carrying capacity

TGE

5) Gas turbines, directly coupled orgeared to the load,higher-temperature service

Highly refined petroleum oil withrust protection and improvedoxidation stability

TGB Power generation and industrial drives andtheir associated control systems wherehigh temperature resistance is requireddue to hot spot temperatures

6) Other lubricantsa) TSC – Synthetic steam turbine fluids with no specific fire-resistant properties (e.g., PAO)b) TSD – Synthetic steam turbine fluids based on phosphate esters with fire-resistant propertiesc) TGC – Synthetic gas turbine fluids with no specific fire-resistant properties (e.g., PAO)d) TGD – Synthetic gas turbine fluids based on phosphate esters with fire-resistant propertiese) TCD – Synthetic fluids for control systems based on phosphate esters with fire-resistant properties

[272], [274]. As a rule, these should be per-formed in the oil manufacturer’s laboratories.In addition, a weekly visual inspection of theoil should be performed to spot contaminationand impurities in the oil in good time. Filteringthe oil with a centrifuge in a by-pass circuit is areliable method.

11.6. Life of (Steam) Turbine Oils

Oil life of 100 000 h is not uncommon in largesteam turbines [270], [271]. However, the an-tioxidant level in the oil can fall to about20 – 40% of the fresh oil (oxidation, aging). Thelife of turbine oil depends heavily on operat-ing conditions such as temperature and pressure,oil circulation speed, filtering and the qualityof maintenance and finally, the amount of oiltopped-up (this helps maintain adequate addi-tive levels).

The temperature of the oil in a turbine de-pends on the bearing loading, speed, bearing di-mensions and the oil’s flow rate. Radiated heatcan also be an important parameter. The oil cir-culation factor, i.e., the ratio between flow vol-ume per hour and tank volume should be bet-ween 8 and 12 h−1 [271–273]. Such relativelylow oil circulation factors ensure that gaseous,

fluid and solid impurities can be efficiently sep-arated while air and other gases can be released.Furthermore, low oil circulation factors reducethe thermal loads on an oil (with mineral oils,oxidation speed doubles when the temperatureincreases by 8 – 10K).

11.7. Gas Turbine Oils – Applicationand Requirements

Gas turbine oils are used in stationary turbines.These produce either electricity or heat. Thecompressor fans generate pressures of up to30 bar which vent into the combustion chamberswhere gas is injected [271]. Depending on thetype involved, combustion temperatures of upto 1000 ◦C are reached (generally 800 – 900 ◦C)[271], [281]. Exhaust gas temperatures can reachabout 400 – 500 ◦C.

Commonmineral oil-based steam turbineoilsare used for conventional gas turbines. How-ever, since the temperature of some bearings ingas turbines is higher than in steam turbines,premature oil aging can be expected. More-over, hot-spots can occur around some turbinebearings and localized temperatures can reach200 – 280 ◦C [271] whereby the temperature ofthe oil in the tank remains at about 70 – 90 ◦C.

110 Lubricants and LubricationTable38.P

rincipaldemands

ofturbinemanufacturers

Siem

ensTLV

901304

(Sept.1996)

GeneralElectricGEK

101941

A(N

ov.1999)

GeneralElectricGEK

32568E(M

ay1999)

ABBHTGD90

117

V0001Q

ABBHTGD90

117

V0001Q

Testing

accordingto

DIN

ISO

Testing

accordingto

AST

M

Lubricatin

goilg

roup

Steam

andgasturbine

oils

aGas

turbineoilswith

EP/AW

additiv

eswith

bearingam

bientsabove

260

◦ Cb

Gas

turbineoilswith

bearingam

bientsabove

260

◦ Cc

Steam

andgasturbine

oils(w

ithandwith

out

EP/AW

additiv

es),ISO

VG32/46d

Steam

andgasturbine

oils(w

ithandwith

out

EP/AW

additiv

es),

ISOVG68

d

Kinem

aticviscosity

(at4

0◦ C

),mm

2(s

DIN

51562-1

AST

M-D

445

ISOVG32

28.8

−35.2

28.8

−35.2

28.8

−35.2

VG32:±

10%

VG68:±

10%

ISOVG46

41.4

−50.6

VG46:±

10%

Gravity,◦ A

PI–

29−

33.5

29−

33.5

––

AST

M-D

287

Airreleaseproperty

(at5

0◦ C

),min

≤4

5(m

ax.)

5(m

ax.)

≤4

≤7

DIN

51381

AST

M-D

3427

NeutralizationNum

ber,mgKOH/g

DIN

51558-1

AST

M-D

974

with

outE

P/AW

additiv

es≤

0.2

–0.2max.

0.3max.

0.3max.

with

EP/AW

additiv

es≤

0.3

0.2max.

–0.3max.

0.3max.

Water

content,mg/kg

≤100

––

––

DIN

51777-1

AST

M-D

1744

Foam

ing(at2

5◦ C

)50/0

max.

50/0

max.

450/0

450/0

–AST

M-D

892

Tendency,m

L≤

400(Seq.1)

50/0

max.

50/0

max.

50/0

50/0

Stability,s

≤450

50/0

max.

50/0

max.

450/0

450/0

Water

separability,s

≤300

≤300

≤300

DIN

51589-1

–Dem

ulsibility,min

≤20

≤30

≤30

DIN

51599

AST

M-D

1401

Density

(at1

5◦ C

),kg/m

3≤

900

≤900

≤900

DIN

51757

AST

M-D

1298

Flashpoint

VG32

and46:

VG68:

DIN

ISO2592

AST

M-D

92ISOVG32,◦ C

>160

215(m

in.)

215(m

in.)

≥185

≥205

ISOVG46,◦ C

>185

Pour

point,

◦ C≤

−6

−12

(max.)

−12

(max.)

<−6

<−6

ISO3016

AST

M-D

97Particledistributio

n(ISO

class)

≤17/14

18/15

18/15

ISO4406

–Color

≤2

max.2.0

max.2.0

––

DIN

ISO2049

AST

M-D

1500

Coppercorrosion,corrosiveness

≤2−

100A3

1B(m

ax.)

1B(m

ax.)

≤2−

100A3

≤2−

100A3

DIN

ENISO

2160

AST

M-D

130

Lubricants and Lubrication 111Table38.(Contin

ued)

Siem

ensTLV

901304

(Sept.1996)

GeneralElectricGEK

101941

A(N

ov.1999)

GeneralElectricGEK

32568E(M

ay1999)

ABBHTGD90

117

V0001Q

ABBHTGD90

117

V0001Q

Testing

accordingto

DIN

ISO

Testing

accordingto

AST

M

corrosionprotectio

nagainststeel,

corrosiveness

≤0–b

0–b(pass)

0–b(pass)

free

from

corrosionwhenusingartifi

cial

seaw

ater

din51

585

astm

-d665

agingbehavior,increaseof

neutralization

numberin

mgkoh/gafterh,tosttest

≤2.0(after

2.500h)

≤2.0(after

3.000h)

≤2.0(after

min.

3.000h)

<2.0(after

2.000h)

∗<

2.0(after

2.000h)

∗din51

587

astm

-d943

additio

nalrequirementson

turbineoils

forusein

gearboxes

with

outg

ear:

e

vg32:5

,l.s.

vg46:6

,l.s.

with

outg

ear:

e

vg68:7

,l.s.

din51

354

astm

-d1947

FZG-Test:A/8,3/90

Failu

reload

stage

≥8

≥8

–with

gear:

VG32:9

,L.S.V

G46:

9,L.S.

with

gear:

VG68:9

,L.S.

CarbonresidueRam

sbottom,%

0.1%

(max.)

(orequivalent)

0.1%

(max.)

(orequivalent)

––

–AST

M-D

524

Oxidatio

nstability

byrotatin

gbomb,

minutes

500(m

in.)

500(m

in.)

>300min.

>300min.

–AST

M-D

2272

Oxidatio

nstability

byrotatin

gbomb

(modified)

85%

(min.)of

timein

unmodified

test

85%

(min.)of

timein

unmodified

test

––

–AST

M-D

2272

N2-blow

nRBOT

VI

95(m

in.)

95(m

in.)

≥90

≥90

–AST

M-D

2270

Atomicem

ission

spectroscopy

AST

M-D

4951

Zinccontent

<5ppm

≤2ppm

≤2ppm

–

∗Neutralizationnumber<

1.8mgKOH/g,sludge

<0.4%

accordingto

DP7624.

Particledistributio

n(cleanlin

esslevels)accordingto

ISO4406:b

etween16/13and15/12dependingon

themanufacturer.

BaseOils:

aMineraloilsor

synthetic

oilswith

additiv

esto

improvecorrosionprotectio

nandagingstability

(additionally

EP/AW

additiv

esin

thecase

ofgearbo

xlubrication).PAOaccepted

forsteam

turbines,PAOnotacceptedforgasturbines.

bPetroleum

lubricatingoil:Sy

nthetic

Hydrocarbonswith

greaterhigh

temperature,oxidatio

nstability

andR&Oinhibitors,E

P/AW

additiv

es.

cPetroleum

lubricatingoil:Sy

nthetic

Hydrocarbonswith

greaterhigh

temperature,oxidatio

nstability

andR&Oinhibitors.

dRefinedmineraloilswith

aginginhibitorsandcorrosionprotectio

nadditiv

es(noEP/AW

additiv

es).

Other

importantspecifications

(examples):

Westin

ghouse:I.L.1250-5312

–Steam

Turbines;21

T0591

–Gas

Turbines;So

lar:ES9-224–Gas

Turbines.

eL.S.=

LoadStage.


The temperature of the oil reaching a bear-ing is mostly between 50 – 55 ◦C and the exittemperature about 70 – 75 ◦C [271]. The vol-ume of oil for a 40 – 60MW generator (GE)is about 6000 – 7000 L and its life is between20 000 and 30 000 h (in the case of a 40 – 60MWSiemens, 14 000 L and 40 000 – 80 000 h [274],[277]. Semisynthetic turbine oils (multiple hy-drated base oils) or fully synthetic turbine oilsbased on synthetic PAO’s are recommended forthese applications [271], [272], [276].

In civil and military aviation, gas turbines areused for propulsion. Because of the high tem-peratures encountered, special, low-viscosity(ISOVG10, 22) synthetic oils based on sat-urated esters (e.g., neopolyesters) are used inthese aircraft engines or turbines [281]. Avia-tion turbine oils containing special naphthenicbase oils (ISOVG15 – 32) with good low-tem-perature characteristics are also used [281].

11.8. Fire-Resistant, Water-Free Fluidsfor Power Station Applications

For safety reasons, fire-resistant fluids are usedin control and governor circuits which are ex-posed to ignition and fire hazards. In power sta-tions, this applies in particular to hydraulic sys-tems in high temperature zones such as near tosuperheated steam pipes. The fire-resistant hy-draulic fluids used in power stations are gener-ally water-free, synthetic fluids based on phos-phoric acid esters (type DFD-R HFD-R accord-ing to DIN 51 502 or ISO 6743-0, ISO VG32−68) and offer the following features [272]:

– fire-resistance– self-ignition temperature over 500 ◦C– auto-oxidation stable to surface tempera-tures up to 300 ◦C

– good lubricity– good protection against corrosion and wear– good aging stability– good demulsification– low foaming– good air release and low vapor pressure

Additives to improve oxidation stability (pos-sibly foam inhibitors) as well as rust and corro-sion inhibitors are sometimes used.According tothe 7th Luxembourg Report, the maximum per-missible temperature of HFD fluids in hydrody-

namic systems is 150 ◦C. Continuous temper-atures of 80 ◦C should not be exceeded in hy-draulic systems. These synthetic fluids are gen-erally used for control circuits, but in some spe-cial cases, also for the lubrication of plain bear-ings in turbines as well as other hydraulic cir-cuits in steamandgas turbine installations.How-ever, these systems must be designed for thesefluids (HFDR-compatible elastomers, paint fin-ishes and coatings). DIN 51 518 lists the mini-mum requirements which power station controlcircuit fluids have to fulfil. Further informationcan be found in guidelines and specifications re-lating to fire-resistant fluids.

11.9. Lubricants for Water Turbinesand Hydroelectric Plants

Operators of hydroelectric power stations haveto pay particular attention to the handling ofwater-polluting substances, e.g., the lubricantsemployed. The specific operating conditions ofthe hydroelectric plant need to be consideredwhen selecting lubricants. The lubricants mustdisplay goodwater and air release, low foaming,good corrosion protection, FZGWear protection> 12 in gearboxes, good aging resistance andcompatibility with standard elastomers [272].As there are no established standards for wa-ter turbine oils, the existing product specifica-tions for general turbine oils are adopted as ba-sic requirements. The viscosity of water turbineoils depends on the type and design of the tur-bine as well as its operating temperature andcan range from 46 to 460mm2/s at 40 ◦C. TypeTD and LTD lubricating and control oils accord-ing to DIN 51 515 are used. In most cases, thesame oil can be used for bearings, gearboxesand control equipment. In many cases, the vis-cosity of these turbine and bearing oils is bet-ween 68 and 100mm2/s. Water turbine oils aresubject to little thermal stress, and as oil tankvolumes tend to be high, the life of water tur-bine oils is very long. In hydroelectric power sta-tions, the oil sampling and analysis intervals canbe correspondingly long. Particular care shouldbe taken when sealing the turbine’s lubricatingoil circuit from possible water ingress. In recentyears, rapidly biodegradable water turbine oilsbased on saturated esters have proven successful


in practice. In addition, Type HLP 46 hydraulicoils, Type HEES 46 rapidly biodegradable flu-ids and NLGI grade 2 and 3 greases are used inhydroelectric plants [272].

12. Metalworking Fluids [51]

The great flexibility and versatility of the dif-ferent types of machining method are particu-larly significant in the metalworking industries.Although for some years now there has beena growing trend towards noncutting (forming)methods for workpiece quality reasons and tosave material and process costs, this has not ledto a recognizable decrease in the volume shareof lubricants. This is also apparent from the ma-chine tool statistics. The dramatic change pre-dicted in the 1970sdid not takeplace in the 1980sand 1990s.

Because of the particular significance of cool-ing for the cutting operation, this process iscalled cooling lubrication and the fluids used arecalled coolants. Apart from this term ‘coolant’which is commonly used in general practice,there are also numerous other terms for specificapplications such as, for example, cutting oils,grinding oils, reaming oils, deep hole drillingoils and honing oils.

The advantages of cutting fluids can be sum-marized briefly as follows: accelerated heat dis-sipation with increased tool service life to makehigher cutting speeds possible; lubrication bet-ween tool, chip and workpiece with reducedtool wear and improvement of the quality ofthe workpiece surface finish; lubrication of slid-ing points outside the actual cutting zone bet-ween tool, workpiece and chips; and improvedchip removal. Frequently less attention is paid tolubrication outside the tool – chip contact zonewhich, nevertheless, can be very important. Ex-amples are margins of twist drill and reamers aswell as the support and guide rails on deep holedrills and honing tools.

12.1. Mechanism of Action

When machining by cutting, a very reactive‘clean’ workpiece material surface is continu-ously generated which can tend to react adhe-sively on the cutting surface of the tool in the

workpiece material – tool contact zone. It mustbe assumed that this reactive surface not onlytries to saturate the free valances of the tool ma-terial but that other available substances are alsoinvolved which can be bonded by physical orchemical absorption, or chemical bonding. Theoxygen in the air plays a particular role. In nu-merous tests under vacuum it has been estab-lished that the tool wear is considerably reducedthrough the saturation of the surface opened upby cutting, compared to where a gas or gas mix-ture has been available at different partial pres-sures.

Coolant penetration into the contact surface[282] is via a network of capillaries which islinked to each other. The width of the capillarydiameters is 10−3 to 10−4 mm.

As far as the saturation process of the newlyformed surface is concerned, the surface isnever completely covered by lubrication effec-tive molecules; there is a clear gradient of sur-face activity in the direction of the tool tip, withlittle reaction at the outer range of the tool. Highnormal pressure also prevents lubricant transportwhen machining materials which are difficult tocut. The assessment of cutting tests with modelsubstances has shown that 30% saturation of thecontact surface between chip and tool lead to areduction in friction force by 75% [282].

Cooling. In view of the findings relating tocooling and lubrication, it is impossible to dif-ferentiate the two. The changes inmaterial prop-erties caused by temperature are closely associ-ated with the effect of friction. Cooling gainsincreasing significance for tool wear when themaximum cutting temperature approaches thesoftening point of the tool material.

On the one hand the cooling effect of the cut-ting fluid and the heat dissipated depends on itsthermal properties, especially the heat capacityand the heat transfer coefficient; on the otherhand the flowconditions and the heat transfer co-efficient play a significant role. The heat transfercoefficient can be influenced considerably by thesubstances active at the interface and by evapo-ration.

The high specific heat and high heat transfercoefficient of water give water-miscible cuttingfluids more favorable cooling properties thannon-water-miscible oils.


12.2. Water-Miscible Cutting Fluids

By volume this is the most significant groupof metalworking fluids in the USA and Europe(using as a reference the amount of concentratefrom which the users produce the water-mixedfinished products). In the Far East, especially inJapan, non-water-miscible products clearly out-weigh water-miscible products. Here the trendto increase the share of water-miscible prod-ucts is even more evident, whereas in Germany,a trend towards non-water-miscible products ismore apparent, not least because of the stringentlegislation covering storage and handling of lu-bricants of all kinds.

The question of the cutting performance ofwater-miscible cutting fluids, which has con-siderable significance, is not generally of pri-mary importance in their selection, as is thecase with non-water-miscible cutting fluids. Thesecondary demands put on these products arefrequently so much the focal point of discus-sions that their actual main tasks, such as cool-ing and lubricating, are overshadowed. Of ever-increasing importance are the problems of healthprotection in the workplace, microbiology, hy-giene and the disposal of used fluids.

12.2.1. Composition

The main ingredients of water-miscible can besummarized in the following groups [283].

1) Mineral oil hydrocarbons2) Synthetic hydrocarbons, synthetic esters,

fatty oils3) Emulsifiers (alkali and alkanolamine soaps,

naphthenates, sulfonates, sulfates, ethoxy-lates, sorbitan estes)

4) Corrosion inhibitors (alkali and alka-nolamine soaps, naphthenates, amines,amides, boron compounds)

5) Stabilizers, coupling agents (glycols, alco-hols)

6) Extreme pressure additives7) Antiwear additives8) Lubricating film improvers9) Antifoam agents10) Microbiocides11) Complexing agents (EDTA derivatives)

Even if mineral oils are the major con-stituents, then part or even the total mineral oil

content can be replaced by synthetic hydrocar-bons such as polyalphaolefins or alkyl benzenesby fatty oils or even synthetic esters. In manycases the use of synthetic hydrocarbon is re-stricted to specific areas of use but synthetic es-ters or natural fat oils are in regular use.

Basically, suitablewater-miscible cutting flu-ids can be produced both from paraffinic andnaphthenic oils; naphthenic oils, however, havea higher solubility for additives and more favor-able emulsifying properties. However, the useof naphthenic oils is declining because of theirhigher aromatic content compared with paraffinic oils. This is considered critical, particularlywith regard to health and safety at work.

12.2.2. Corrosion Protection and CorrosionTest Methods

Normally the corrosion protection afforded by awater-mixed cutting fluid is reduced as the de-gree of dilution increases. The quality of a prod-uct, as far as its corrosion protection propertiesare concerned, is, therefore, most frequently de-fined in laboratory application according to theconcentration which still gives no corrosion orachieves a specific degree of corrosion. Themostsignificant test methods are described in DIN 51360, Part 1 (Herbert corrosion test) or IP 125 aswell as in DIN 51 360, part 2 (chips/filter pa-per method). Similar testing methods are knownunder ASTM D-4627 and IP 287 whereas theFrench standard NF T 60-188 is more similar tothe Herbert test.

12.2.3. Concentration of Water-MixedCutting Fluids

The possibility to produce water-mixed prod-ucts from water-miscible coolants with varyingconcentrations for adaptation to different cut-ting processes gives great flexibility. Normally,water-mixed coolants with concentrations up toapprox. 30% can be applied without any par-ticular problems. However, if this concentrationrange is exceeded then the stability has to beassessed, especially in the case of conventionalemulsions. When working with hard water, limesoaps can arise when working far below this


concentration, which could lead to considerableirregularities in the course of machining; thisdepends to a great extent on the specific com-position of the coolants. Often, the lower con-centration limit is determined by the corrosionprotection properties. In this respect, attentionis drawn to the fact that concentrations below1.5% can only be checked with difficulty duringoperation because measurement of the concen-tration is a problem. Reducing the concentrationby only a few tenths of a percentage point canalready lead to a serious corrosion effect. In thecase of water-miscible coolants, which containmicrobiocides, the skin compatibility of theseproducts must be observed when using partic-ularly high concentrations; this also applies forother additives. Normally, cutting fluids are veryoften used for rough machining with a concen-tration of between 2 and 5%, but very frequentlyat 4%. In the case of special applications such asdeep hole drilling, reaming or generally for par-ticularly difficult cutting operations the use ofconsiderably higher concentrations is possible.

Frequently a concentration of between 1.5and 2.5% is selected for grinding operationswhich are to be carried out with water-mixedcoolants. In the case of high performance grind-ing (grindingwith high chip volume) higher con-centrations are also sometimes used.

12.2.4. Stability of Coolants

Stability is the most important property for theservice life of a coolant. Apart from the generalterms for stability, in coolant practice stability isalso defined according to the different stressesexperienced during service. For example, elec-trolytic stability, thermal and boiling stabilityand frequently also stability againstmicroorgan-isms.

12.2.5. Foaming Properties

In all metalworking sectors as well as in themetal cutting field, the formation of stable foamon the processing fluids is an undesirable phe-nomenon. Especially coolant containing surfac-tants, and these include all emulsions, tend, to agreater or lesser extent, to develop surface foam,under appropriate agitation.

To prevent foaming, the selection of a lowerfoaming coolant is the first of the measures tobe taken. Very low foaming products are espe-cially available amongst the more conventionalemulsions. One reason for this is that the surfac-tantmolecules (in this case essentially emulsifiermolecules and anionic corrosion inhibitors) aremainly bound on the oil –water interface, that isto say to the oil drop, and are not available to besited on the foam lamellae.

The effect of themost significant group of de-foamers for coolants, the silicones, is based ontheir particularly low surface tension and theirinsolubility in both oil and water.

12.2.6. Preservation of Coolants withBiocides

Biocides are used to reduce or avoid heavy mi-crobial activity. Biocides should fulfill the fol-lowing requirements:

– Effectiveness against as many types of mi-croorganisms as possible (broad spectrumeffective).

– Fast action: this has particular significance inthe case of systems with high drag-out rates;products which act slowly are taken awaywith the coolant before they become effec-tive.

– Long continuous effect: the biocide shouldnot lose its activity through reaction withother components or contaminants.

– High thermal stability.– Low influence on pH value of the coolant:different ways of assessing this exist. Inmany cases an alkaline biocide is desir-able. In the case of coolants in use, whichhave clearly suffered a drop in pH valuethrough microbial attack, it is expedient touse highly alkaline biocides for correctingpurposes. Over and above this, alkaline bio-cides quickly form an essential part of thereserve alkalinity of coolants. A high alkalireserve at pH < 9 is generally favorable.

– Oil solubility: in many cases use in coolantconcentrates (water-miscible coolant) is abasic requirement. The biocide is either dis-solved or homogeneously dispersed in theconcentrate. The use of biocides without sol-ubility in the concentrate is restricted to thetreatment of water-mixed coolants.


– No odor nuisance: no odor nuisance in work-shops, both with regard to the biocide’s ownodor and by generation of odor by a reactionwith microorganisms or components.

– Good skin compatibility: since biocides aregenerally based on a reaction with the pro-teins of the microorganisms, skin proteinsare frequently subject to the risk of attackfrom these biocides. However, risk of der-matitis begins, as a general rule, in concen-tration ranges above the usual concentrationsused in cutting fluids.

– Low toxicity: no toxic effects must occurwhen the biocides penetrate the body, eitherthrough the skin or through inhalation andswallowing of the coolant spray.

– No waste water hazard: biocides entersewage/waste treatment plants through thewaste water from the cutting fluids in suchlow concentrations that they can be biolog-ically decomposed without the sludge florabeing destroyed. The ability of biocides to bedecomposed is necessary in biological clar-ification plants in order to avoid burdeningpublic waters, especially through fish toxic-ity.

– High economy: the great expense of preser-vation of coolants mentioned at the begin-ning requires high economic efficiency inthese products. Both the recommended con-centration and the concentration applied inpractice should be considered when compar-ing costs. For better practical handling, thedilution of the active substance should be se-lected by the biocide manufacturers so thatthe normal concentration used is between 0.1and 0.2% in water-mixed coolants.

The most significant groups of substances usedas preservatives are:

– Aldehydes (formaldehydes)– O-Formals

alcohol + formaldehyde(semi formal of benzyl alcohol)

– N-Formalsamine + formaldehyde (hexa-hydrotriazines)methylbisoxazolidinesamide + formaldehyde (N-methylchloroacet-amide)

– Phenolic derivatives (o-phenylphenol)– Isothiazoles (benzoisothiazole)

chloromethyl, methyl, octylisothiazoles– Compounds derived from carbon disulfide(sodium-dimethyldithiocarbamate)

– Sodium pyrithione– IPBC (3-iodine-propinylbutylcarbamate)– Phenoxyethanol– Phenoxypropanol

12.3. Neat Cutting Fluids

Neat cutting fluids are mainly products such ashoning oils, deep hole drilling oils, grinding oilsand broaching oils which are not mixed withwater prior to use. Neat cutting fluids have thefollowing advantages over water-miscible flu-ids: they offer longer tool life and better sur-face finishes for difficult operations performedat low cutting speeds, the maintenance of theoil is much less complicated and the life of theoil is significantly longer. The contamination ofhydraulic and slideway oils with neat oils is lessof a problem than with water-miscible products.Moreover, leaks of hydraulic oils and other ma-chine lubricants are easier to cope with if com-patible oils are used.

Their principal disadvantage compared towater-miscible fluids is the poorer cooling of-fered by oils, particularly in high speed cuttingoperations. An exception is high speed grindingwhere the superior lubricity of oil reduces theamount of heat generated. Another disadvantageis the combustibility of oils as well as the explo-sion risks of oil mist and vapor. As the viscosityof oils is higher than water-miscible metalwork-ing fluids, drag-out losses on chips and compo-nents are also higher.

12.3.1. Specifications

Neat metalworking oils are classified accordingto ISO 6743/7, DIN 51 385, or DIN 51 520 onthe basis of the additives used (Table 39).

12.3.2. Composition

Most metalworking fluids are based on petro-leum hydrocarbons and contain additives to im-prove lubricity, wear protection and other ad-ditives to manipulate secondary characteristics


such as foaming, misting, corrosion protec-tion, dispersion, oxidation stability and flushing.Of the petroleum hydrocarbon base oils, low-aromatic, paraffinic solvent extracted or hydro-cracked products are preferred. Recent trends inGermany are heavily focused on ester based flu-ids.

Apart from sulfurized fatty oils and polysul-fides, sulfur – phosphorus compounds and purephosphorus compounds are often used as EP ad-ditives in advanced cutting oils. The use of zinc-and other heavymetal-based additives in cuttingoils is declining due to environmental andwaste-water considerations. Chlorinated paraffins arestill used as universal EP agents throughout theworld but combinations of sulfur additives andester oils are being increasingly used in Ger-many and Western Europe as substitutes. Thereasons for these developments are the consid-erably higher disposal costs for products con-taining chlorine in Germany.

Table 39. Classification of neat metalworking oils according to ISO6743/7, DIN 51 385 or DIN 51 520

DIN 51385Code letters

ISO 6743/7Code letters

Metalworking oils containingmineral oils without antifrictionadditives or EP agents

SNO MHA

Metalworking oils containingantifriction additives

SNP MHB

Metalworking oils containing EPagents, chemically inactive

SNPA MHC

Metalworking oils containing EPagents, chemically active

SNPB MHD

Metalworking oils containingantifriction additives and EP agents,chemically inactive

SNPC MHE

Metalworking oils containingantifriction additives and EP agents,chemically active

SNPD MHF

12.4. Application

12.4.1. Machining with GeometricallyDefined Cutting Edges

Turning produces parts which are round inshape. Cylindrical and conical surfaces are gen-erated from a rough cylindrical blank which ro-tates about its longitudinal axis and against acutting tool. Most turning operations use single-point tools. Normally, a tool holder contains anindexable insert withmultiple cutting edges. For

turning operations, water-mixed metalworkingfluids are used only when running automaticlathes; neat oils are often preferred.

Drilling. The most common shape found ina manufactured part is a circular hole, and manyof these holes are produced by drilling. Sincethe chips are formed within the part, the flutesor grooves normally serve two purposes. In ad-dition to providing a conduit for the removalof chips, they also allow the cutting fluid toreach the tool–workpiece interface. Most drillsare made from HSS which demands the appli-cation of a cutting fluid [284].

Water-mixed metalworking fluids are usedwhere high cooling properties are required. Re-cent developments show that this operation maybe run also withMinimumQuantity Lubrication(MQL) or by using dry cutting technology. Bothtechniques depend on thematerialmachined andthe machine tool designed for this application.

Milling. A variety of milling processes isavailable where end milling, slab milling, andface milling are included. High metal removalrates are possible since the tools have multipleteeth and each tooth produces a chip. In mostapplications, the workpiece is fed into a rotatingtool. The feed motion usually is perpendicularto the tool axis, and cutting occurs on the cir-cumference of the tool.

When machining cast iron, dry milling hasbeen state-of-the-art for many years. High ten-sile steels and aluminum alloys are lubricatedwith water-mixed metalworking fluids but re-cent developments show that MQL can in somecases provide further advantages froma cost per-spective.

Gear Cutting. The most important machin-ing operations are gear hobbing, gear shapingand gear shaving. The different forms of teethand various types ofmachineswith different toolkinematics very frequently lead to different oilrecommendations by the machine tool maker.Nevertheless, in this case the users very clearlywant to use one and the same product for sev-eral gear cutting applications. Today in large se-rial automobile gearbox production this has al-ready been generally achieved and all hobbingand generating shaping operations are carriedout with just one oil.


As far as tool cooling is concerned, the toolcutting edges only cut for a comparatively shorttime and sufficient time is available for cool-ing. As a consequence sufficient cooling is pro-vided by neat cutting oils.Although todaywater-miscible coolants are available and can achievegood results, the majority of machine tools forhobbing and shaping are not designed for water-miscible products. The latest trends are towardsdry machining and the basis for this is coatedtools and chip transportation with compressedair.

Oils with viscosities of ca. 40mm2/s at 40 ◦Care particularly suitable for gear shaving witha higher module (m> 2.5); recommended forsmaller modules are less viscous oils with vis-cosities between 15 and 25mm2/s at 40 ◦C.

Deep Hole Drilling. Current deep holedrilling techniques, previously employed for themost part only in the armaments industry, havebeen steadily modified and are now used in theautomotive, shipbuilding, aircraft, machine andtool industries, in industrial plant construction,and for hydraulic and pneumatic equipment.

As one of the process parameters, the cuttingfluid can make a considerable contribution toincreased economic viability, assuming that allother influencing factors have been optimized.Its principal task is to cool and lubricate the cut-ting edges. Chemically active EP additives in-corporated into the cutting fluid and polar effec-tive substances help to reduce wear and friction.

In addition to the actual cooling and lubricat-ing functions, the cutting fluid, which is fed tothe tool at high pressure, must also carry awaythe swarf from the cutting point. The high-pres-sure supply system requires not only a carefullydesigned fluid circuit (with a recirculation rateno higher than 6 h−1), but also for products withlow foam and in particular low oil-mist charac-teristics.

Threading and Tapping. Frequently water-miscible lubricants are used in the production ofexternal threads. The focus is clearly on neat cut-ting oils for tappingwhere thematerials to be cutare difficult or most difficult (material groups 2and 3, section 1); typical tapping and threadingoils of this type are sulfur active oils with vis-cosities between 15 and 40mm2/s at 40 ◦C; theproportion of polar substances is also important.

Tight drill holes and the low pitch of the inter-nal threads call for less viscous oils because ofthe favorable flushing properties. Here again therisk of cutting edge build-up is great because ofthe generally low cutting speed, and damage tothe cutting edge can result.

Frequently recommended where tapping iscarried out and on difficult and most difficult-to-cut materials are thread cutting pastes (some-times dilutablewithwater up to 1 : 5) or oils con-taining solvent. In particular, productswith chlo-rinated hydrocarbons, more especially 1,1,1-tri-chloroethane as solvent, are easy to apply bybrush or in drops. Today for environmental rea-sons such products have been replaced by lowviscosity hydrocarbon solvents and/or other syn-thetic fluids.

Broaching. Far more than in many othercutting operations the main consideration inbroaching is tool wear or tool life. The reason forthis is the fact that the broaching tool is a verycomplicated part and, the tool is manufacturedfrom one single part. The material is predomi-nantly high speed steel; carbide metal as a toolmaterial is only used for gray iron machining.In the case of broaching several teeth engage atthe same time and very frequently the chipwidthis large. Chip removal can be very problematicwhich then generally also calls for relatively lowviscosity oils.

Temperature sensitivity of the broaching toolcombined with cutting from the solid workpiecewhich follows each tooth without any soft ini-tial cut, call for relatively low cutting speeds. Inbroaching mostly neat cutting oils are used. Atvery high broaching speeds and high stock re-moval rates good results are achievedon freema-chining steels by using water-miscible coolants,more especially EP emulsions and syntheticproducts free of mineral oil with relatively highconcentrations (10 to 35%); in this case, com-pared with neat oils, the improvements of thecooling effect and of chip removal are impor-tant factors.

12.4.2. Machining with GeometricNon-Defined Cutting Edges

In all abrasives according toDIN69111 the abra-sive grains are bondedwith each other to the base


material. The purpose of the bond is to secure theabrasive grains until they become dull and thenrelease them. A differentiation is made betweenresinoid ceramic and metallic bonds because ofthe difference in their elasticity, temperature re-sistance, heat dissipation and dressing capacity.Their resistance to chemicals is particularly im-portant as far as the coolants used are concerned.

Resin bonded grinding wheels are not stableto hydrolysis and can lose their tensile strengthwhen water-mixed coolants are used [285].

Ceramic materials are highly resistant tochemicals and temperature but are brittle andglass-like in character. Metallic bonds are be-coming more and more important as far as su-perabrasives are concerned because of their highresistance to wear.

Grinding Fluids. The grinding process, as aprecision machining process, is used frequentlyfor the final stage machining of workpieces. Asa result, high demands are set on component ac-curacy. Particularly significant in this respect isthe coolant.

Its first task is to reduce the friction betweenthe abrasive material and surface of the work-piece in the contact zone and, as a result, lowerthe temperature in the grinding area. It is neces-sary to reduce the contact zone temperature toincrease the tool life of the grinding wheel andprevent the peripheral zones of the workpiecefrom being affected in any way.

A further task of the coolant is, of course, toprevent thermal damage to either the workpieceor grinding wheel by dissipating the heat gener-ated. Flushing the grinding wheel clean is an im-portant criterion as far as tool life is concerned.Here the viscosity and quality of the base oil aswell as the coolant additives play a decisive role.

Whether a water-miscible coolant or astraight oil is suitable for a process also depends,apart from the above-mentioned criteria, on theperipheral system. Here the machine tool equip-ment plays a particularly decisive role with re-gards to extraction, sealing, wiring and type offilter system.

Honing Oils. During honing, there is a verylarge contact zone between tool and workpiecewith relatively low pressures. This means thatthe heat formation is relatively low compared

with grinding so that the coolant does not to haveto dissipate any great amount of heat.

An important criterion when honing is flush-ing the honing stone to remove the stock re-movedmaterial and thus to ensure that the work-ing surface retains its grip and is self sharpening.

As a consequence, very lowviscosity oils bet-ween 2 and 10mm2/s at 40 ◦C are used whenhoning. These are applied, as a general rule, atlow pressure but at a high flow rate through ringnozzles. To achieve the component accuracy re-quired, the honing oil is kept at a constant tem-perature by chillers.

Apart from straight oils, water-mixedcoolants are also used, especially when hon-ing cylinder liners with diamond honing stonesin the automobile industry. Reasons for this areto be found frequently in the process line, be-cause previous machining is generally carriedout with water-mixed fluids. This leads to re-duced process costs when uniform media areused.

Lapping Oils. Unlike grinding and honing,lapping is a precision finishing processwith non-bonded abrasive. In lapping, the workpiece sur-faces are machined by the friction between theworkpiece surfaces and the appropriate countersurface in the form of a working disc. The stockremoval is by means of a lap (grit and liquid)which is applied between theworkpiece and lap-ping wheel surface.

The purpose of lapping oil is to induce ho-mogeneous mixing of the grain. However, lap-ping powder agglomeration must certainly beavoided. The lapping oil has an important taskto perform in the operation, in that it lubricatesproperly to prevent cold welding between work-piece and working disk. The lubricating filmmust not be applied too thickly as otherwise hy-drodynamics will prevent stock removal.

In the case of lapping oils a distinction ismade between the very low viscous oils, whichcan only maintain the lapping powder in sus-pension for a short period of time, and thethixotropic, high viscous products, which sus-pend the lapping powder for days on end. Theadvantage of low viscous oils is the very goodwashing behavior, which is reflected in cleansurfaces. Due to the lack of carrier capacity forthe lapping powders these products are only used


on machines where the lapping medium tank isequipped with an agitator.

12.5. Storage

Whereas neat cutting oils can be stored for anumber of years, water-miscible fluids can bestored for between 6 months and maximum 1year. Water-miscible products should be storedbetween + 5 ◦C and + 40 ◦C. If the ambient stor-age temperature falls below 0 ◦C, the concen-trate should be stirred (rolling the drum suffices)and warmed before use.

Containers should, preferably, be stored in-doors. If drums are stored outdoors, they shouldbe placed horizontally to avoid water and/ormoisture seeping past the bung when the drum‘breathes’.

Ideally, cutting fluids should be constantlymonitored. Most importantly, solid and liquidimpurities such as chips, abrasion residue andtramp oils should be removed by various cen-trifuges and/or filters because only clean cuttingfluids increase efficiency and reduce scrap. Thecondition of the emulsion in the tanks should bechecked at least weekly, better daily. Such mea-sures can significantly increase the life of thecutting fluid.

Water-miscible cutting fluids require muchmore monitoring than neat oils. Compared toneat cutting fluids, their change intervals aresometimes very short. The maintenance mea-sures required for small tank capacities are oftenmore complex than for larger central systems.

12.6. Environmental Aspects

Maintenance procedures allow cutting fluids toremain in use for long periods of time. Never-theless, a point can come at which the level ofsolid and fluid contaminants has reached such astage that the fluid has to be disposed of and thesystem refilled with fresh fluid.

This poses no particular problem in the caseof neat cutting and grinding oils which may ei-ther be regenerated on-site and used for lessdemanding applications, used as heating oil orcommercially disposed of.

The situation is totally different for water-miscible products which, containing an average

of 3% organic compounds, are water-pollutingsubstances and cannot thus be discharged intorivers or water purification plants. In most cases,expensive treatment is thus necessary. The var-ious separation processes available are not al-ways ideal for the different types of cutting fluidin use. Moreover, the evaluation of the waterphase remaining after separation depends on notjust national legislation but also local waste wa-ter regulations.

Among these processes are:

– Electrolyte separation (e.g., salt and acidsplitting)

– Splitting of emulsions with adsorbents (e.g.,adsorption with amorphous silica and withmetal hydroxides)

– Thermal methods (e.g., immersion heating,thin-film evaporation, incineration)

– Ultrafiltration

12.7. New Trends in Coolant Technology

Oil Instead of Emulsion. These days, the‘oil instead of emulsion’ trend is seen as an an-swer to a number of problems. Not only are costbenefits realizable, but environmental, safety-at-work and technical performance of oils aresuperior. Another major aspect is the almostunlimited life of oils compared to the fluidchange cycles of water-miscible fluids (6 weeksfor individually-filled machines and 2 – 3 yearsmax. for central systems). As regards pure ma-chining performance, oils can satisfy more than90% of all machining operations.

Replacing emulsions with oils offers betterlubricity, improved surface finishes and signifi-cantly longer tool life. A cost analysis performedin a gearbox plant generated the factor 2 as anaverage of all machining operations [287].

The only disadvantages of machining withoils are in processes which generate a lot of heat.With a four-fold reduction in cooling, tool andmachining problems can occur. Critical opera-tions include the manufacture of carbide drillsfrom solid stock.

To perform these processes with oil, the vis-cosity has to be reduced to a minimum. Thisgenerates thefirst conflict between technical per-formance and safety-at-work. The evaporationof conventional oils based on paraffinic solvent


raffinates rises almost exponentially to fallingviscosity. At the same time, the flash point de-creases. This problem can only be solved by theuse of base oils which combine the advantagesof a high flash point with low evaporation at verylow viscosities.

In response to these requirements, the firstcutting oils based on hydrocracked oils with es-ters appeared at the end of the 1980s with pureester oils established on the market in the early1990s.

The trend towards machining with neat oilspresented a range of cost-reduction possibilities.One analysis by a leading German machine toolmanufacturer showed that an average of sevendifferent lubricants are used in every machinetool. Apart form the leakage problem and the in-compatibility of some lubricants, costs are alsogenerated by the stocking of all necessary lu-bricants. Incorrect application of lubricants orgrade confusion can cause considerable damageto machines or even production stoppages.

One possible solution is the introduction of‘multifunctional’ products which satisfy a widerange of machining operations at the same timeas fulfilling the demands of advanced machinetool lubrication.

Amachine tool fluid family contains the sameor compatible additives in lubricants of differentviscosities.

The advantages are:– the unavoidable leaks of hydraulic fluids andslideway oils have no negative effect on thecutting fluid

– manufacturing quality remains constantwithout complex analytical measures

– tramp oils function as cutting fluids and thusdo not create additional costs

– higher process reliability, good machiningresults and reduced tool wear all serve tolower manufacturing costs

– universal application

Minimum Quantity Lubrication. Theever-changing legislation and an ever increas-ing awareness of the environment have led to achange in the production processes used up tonow and especially in the production auxiliariessuch as coolants.

The analysis of an automobile manufacturer[288] published early in the 1990s revealed that aconsiderable proportion of the process costs are

caused by the application of metal working flu-ids. In this respect the costs for coolant concen-trates themselves played an insignificant role.The actual costs are caused by system costs, per-sonnel costs for the care and monitoring of ma-terials, the high personnel and investment costsassociated with splitting plants associated withwater purification systems and last but not least,the costs for disposal.

This has led to much more attention beingpaid to the difference between dry machiningand minimum quantity lubrication (in minimumquantity lubrication the coolant volume flowdoes not exceed 50mL/h) [289]. The drastic re-duction in the amount of coolants used as a re-sult of these new technologies only offers a con-siderable potential for savings in process costs.However, dispensing with coolants also meansthat the primary functions of the coolants suchas cooling, lubrication and chip transportationhave to be replaced by other suitable technicalsolutions.

By means of a proportioning device a smallamount of coolant spray (max. 50mL/h) is ap-plied to the machining point. Of all the dosingequipment on the market only two systems haveproved successful [290]. The pressure systemshave foundwidespread use,whereby differencescan be made in this case between two strategiesfor preparing the mix.

All minimum quantity coolant systems haveone thing in common. The coolant is dispersedinto more or less fine drops which reach the ma-chining point as an aerosol. However this causesthe first problems with regards to toxicologi-cal and workplace hygiene. Although for someyears now, in the case of conventional coolantflooding at the workplace, efforts have beenmade to minimize spray development by newtechnologies and low emission coolants, spraysare now being intentionally developed for min-imum quantity lubrication which are releasedinto the workshop atmosphere.

Today greases and oils as well as esters andfatty alcohols are in use, apart from conven-tional mineral oils and water-miscible coolants.Since minimum quantity lubrication is a matterof pure total loss lubrication and the coolant isfrequently completely dispersed in the workingarea in the form of vapors and mist, particularattention should be paid to safety aspects. Es-ter oils and fatty alcohols with additives with


low toxicological values have proved particu-larly successful in this respect.

Natural oils and fats have the disadvantage ofpoor oxidative stability. Ester oils and fatty al-cohols should be given preference in the case ofprocesses where high heat is developed to avoidresidues on workpieces and machines.

13. Forming Lubricants

According to DIN 8582 a distinction betweensix main groups of manufacturing processes ismade: primary shaping, forming, separating, as-sembling; changing properties of materials andcoating. The processes listed under forming(DIN 8583 forming under compressive condi-tions, DIN 8584 forming under combination oftensile and compressive conditions, DIN 8585forming under tensile conditions. DIN 8586forming under bending conditions, DIN 8587forming under shearing conditions), all referto noncutting production. Over and above this,forming technology also includes some sub-divisions under severing operations (DIN 8588severing operations, shear cutting) and jointing(DIN 8589 jointing by forming).

Apart from the differentiation according toDIN 8582, in practical applications the areaof forming technology is normally divided intosheet-metal forming and extrusion forming.Thisconventional distinction is based on the factthat unlike extrusion, sheet metal forming work-pieces are produced with nearly uniform metalthickness over the complete product. Addition-ally, extrusion forming uses much higher forcesthan are requiredwhen forming sheetmetal [51].

13.1. Sheet Metal Working Lubricants

Sheet metal forming processes have not onlygained considerable importance in the thin sheetmetal processing sector but significant progresshas also been made in materials which in earliertimes could not be processed by forming. Espe-cially noteworthy in this respect is the formingof stainless, austenitic and ferritic, sheet metalwhich initiated great activity in both the devel-opment of lubricants and materials for the tools.

During large scale production, for examplein the production of parts for vehicle bodies,

the greatest expense in development over re-cent years as far as lubricants are concernedwas not for the actual lubrication but for ful-filling new secondary requirements of the lubri-cant. Advancements in production technologiesmade it necessary for lubricants to fulfill specificrequirements for removability, new applicationmethods, corrosion protection, behavior duringsheet metal adhesive bonding, welding, temper-ature resistance, compatibility with special coat-ings along with evolving workplace health andsafety concerns.

13.1.1. Deep Drawing

During deep drawing, hollow bodies are pro-duced frommetal blanks using punches, dies andblank holders. In no other forming operation arethe friction and, as a result, the lubricating con-ditions so complex. In one and the same drawingoperation, a particularly low coefficient of fric-tion can be required in one area and a particularlyhigh coefficient of friction in another.

The drawing force in the flange necessary toform the sheet metal is applied by the punchin the base of the cup and transferred fromhere through the wall into the flange (area Cin Fig. 55). This transmission of force calls forthe highest possible coefficient of friction onthe punch edge: neither the punch nor the sheetmetal blank should be lubricated in this area.This friction area requires lubrication in theboundary mode with a high coefficient of fric-tion and antiwear behavior. This is possible withlow viscosity drawing oils, polar substances orEP additives. Moreover, in this case the thick-ness of the lubricant film must not be excessive.It should be somewhere in the range of the sheetmetal roughness Ra. A high coefficient of fric-tion along with antiwear performance can alsobe achieved in some cases by lubricants witha high filler content. These filling materials orsolid lubricants should be blunt in structure butnot molybdenum disulfide or graphite.

Since the best possible lubrication and low-est coefficient of friction possible are required onthe punch in the blank holder area of the sheetmetal flange, a compromise frequently has tobe made concerning the lubrication. It is oftennot practicable to lubricate one side of the sheet


Figure 55. Friction areas when deep drawing a cupA) Friction area between sheet metal blank and holder and sheet blankdie; B) Friction area between sheet metal blank and the die radius, C) Friction area between sheet metal blank and the punchedges1) Punch; 2) Blank holder; 3) Die; 4) Cup; 5) Flange area; 6) Cup wall; 7) Base of the cupFges =Total drawing force;FN =Blank holder force

differently from the other, especially with smalldrawn parts.

The cupbottomand the connection to thewallhave the least yield during deep drawing. Theyield increases with frame height and, as a con-sequence, also the yield strength. If the drawingforce transferred from the base of the cup is toohigh the sheetmetal will tear at the point with thelowest strength resulting in bottom tearing, themost significant and most difficult deep drawingfault to be analyzed [291]. Here, the lubricanthas the important task of reducing the consid-erable frictional component of the total drawingforce in the region of the blank holder and thedie radius.

The friction situation in the deformation areais explained by studying the friction in the regionof the blank holder and the die radius (areas Aand B in Fig. 55).

As a result of the tangential pressure stressesand radial tensile stresses in the flange, thereis a tendency towards material compression onthe outer rim. Because of this thickening of thesheet metal the blank holder can only transferthe blank holder force to a limited section of thesheet surface on the blank holder side, whichleads to high surface pressures and difficult fric-tion conditions. As a result of the lack of contactby the blank holder in the other areas, there is

also the risk of fold or wrinkle formation. Thiswrinkling also creates special friction conditionssince the backs of thewrinkles causemore or lessline contact with the blank holder, producing lo-cal areas of high surface pressure.

However, in the case of high viscosity oilsand drawing pastes, if the lubricant film appliedis excessive, there is the risk that hydrostatic ef-fects on the blank holder side of the flange cancause reduction of sheet metal contact with theblank holder resulting in wrinkling.

The friction conditions on the die are lesscomplex than in the blank holder area. Here thesurface pressure canbe relatively accurately esti-mated and the coefficient of friction determinedin deflection strip drawing tests. In this case theaim of lubrication is to achieve a minimum co-efficient of friction with minimum wear. Evenexcessive lubrication of the sheet metal surfaceon the die side normally does not create a prob-lem.

The geometrical design of the die has a sig-nificant influence on friction. If very small dieradii are necessary for technical reasons, EP lu-bricants have proven to have a positive influenceon the workpiece surface quality as well as diewear in spite of the high surface pressures occur-ing. However, this does not seem to bring abouta reduction of the drawing force.


13.1.2. Stretch Drawing and a Combinationof Stretch and Deep Drawing

In stretch drawing the blank sheetmetal clampedfirmly on both sides is stretched by tensile stress,generally by means of an upwards travelingstretch drawing punch. Here the thickness of thesheet metal is reduced unlike during deep draw-ing where the thickness of the sheet metal re-mains mostly unchanged. The goal is uniformdeformation which is favored by the lowest pos-sible friction over the entire stretch area. Thereduction in sheetmetal thickness leads to a con-siderable increase in surface area which has tobe taken into consideration when choosing thetype and amount of lubricant to be applied.

13.1.3. Shear Cutting

According to DIN 8580 shear cutting is a sev-ering process in the separating group. Whereasstamping is a relatively old method of sheetmetal working, fineblanking has gained partic-ular significance in the last two decades. Lubri-cation has a greater significance in fineblankingthan in normal stamping as far as the quality ofthe workpiece and tool wear are concerned. Thisnormally requires higher performing lubricants.

Stamping. A significant characterization ofstamping is that one third of the sheet metalthickness is a sheared edge and about two thirdsa torn zone. An important stamping method is,for example, making holes for the production ofpierced plate for protective grids and for manydecorative applications or for the production ofscreens and filters. The sheared area of stampedparts is subdivided into three areas; the formingarea, the cut area and torn area.

Efforts are being made to reduce the numberof lubricants used for stamping. This reductionin types of lubricant also makes it necessary forthe plants to select lubricants based on the mostdifficult operations, materials and sheet metalthicknesses. Today the majority of companiestry to make due with a maximum of three lu-bricants in the production of stamped parts bycombining blanking and forming operations.

The lubricants used for stamping are dividedinto 7 or 9 groups and can be described as fol-lows:

– Group 1: oils, 50 to 180mm2/s, 40 ◦C; theoils are either completely or to a considerableextent active substances. Used as substancesare EP materials (containing chlorine, sul-fur and phosphorus), polar substances (P)such as synthetic esters, natural fatty oils orfatty oil derivatives. Group 1 is designatedEP+++P+++.

– Group 2: oils, 30 to 110mm2/s, 40 ◦C; des-ignation EP++P++.

– Group 3: oils, 30 to 110mm2/s, 40 ◦C; theoils are either completely or to a considerableextent polar substances, designation P+++.

– Group 4: oils, 30 to 110mm2/s, 40 ◦C; apartfrom considerable proportions of polar sub-stances the oils contain lower concentrationsof EP substances, designation EP+P+++.

– Group 5a: oils, 15 to 110mm2/s, 40 ◦C; theoils are mixtures of polar substances withmineral oil hydrocarbons, designation P++.

– Group 5b: oil,< 15mm2/s, 40 ◦C; these oilsare mixtures of polar substances with min-eral oil hydrocarbons, designation P+.

– Group 6a: water-miscible compounds.Mainly, these are products with a greaseconsistency, possibly also water-miscibleoils with considerable concentrations of po-lar substances and medium to low EP lev-els, designation EP+P+++. The products areused as undiluted pastes or diluted as lean as25 %. They are frequently diluted to a ratioof 1 : 3.

– Group 6b: water-miscible compounds.These are simple products without spe-cific combinations of active substances. Theproducts are used undiluted to 15 % but arefrequently diluted with water at a ratio of1 : 5.

– Group 7: corrosion protection oil. This is asurface protection oil which is already ap-plied by sheet metal producers and is alsoto serve as a lubricant for further processing(mill oils, slushing oils, prelubes).

Most lubricants in Groups 1 to 5 may be di-luted with petroleum hydrocarbon solvents. Thesolvent serves mainly to facilitate application.The difficult-to-cut austenitic materials with asheet thickness up to 1.5mm require a lubricantfrom Group 2 and lubricants for thicker sheetmetals are in Group 1. Ferritic CrNi sheet met-als require lubricants in Group 2 and for thick-


nesses in excess of 8mm lubricants in Group1 are selected. Depending upon thickness, thelubricants in the Groups 4, 2 or 1 are requiredfor carbon steels. Lubricants in Group 5b aresuitable for electrical quality sheet whereas thelubricants in Groups 3 to 5b are used for alu-minum, non-ferritic metals and their alloys.

Fineblanking. The most significant differ-ence between a part produced by stamping anda part produced by fineblanking is in the qualityof the cut surface, which in many applicationsserves as gearwheels. Significantly higher shapeaccuracy is achieved by, among other things, avery small die clearance, which in fineblankingis typically only approx. 0.5%of the sheet thick-ness. In stamping this clearance is approx. 10 to15% of the sheet thickness. The cutting surfaceshave no fracture area and the workpieces haveonly an insignificant burr.

When fineblanking, unlike stamping, thesheet is clamped on all sides by a vee-ring. Thespecific tool structure is significant for lubrica-tion. Lubricating chambers are formed by spe-cial chamfering on parts of the tool [292].

In fineblanking, in general, oils with higherconcentrations of additives and oils with re-active EP substances are used because thereis a strong tendency towards adhesive wearwhen fineblanking difficult-to-cut, thick materi-als with unfavorable structure. Oils with a highchlorine content are frequently used for difficultfineblanking operations [292].

Working with reactive EP substances, espe-cially chlorine compounds, can lead to consid-erable corrosion problems on fineblanked parts,particularly on the cut surface. Because chlorinecompounds cause environmental problems thereis a clear trend towards chlorine-free products.The extensive use is of chlorine-free products isalready well established in the area of low andmedium thickness sheet metal.

13.1.4. Choice of Lubricants

Before Forming. In this field of applicationa great number of different lubricants are avail-able.

Table 40 shows different types of forming lu-bricants and their preferred applications.

Table 40. Different types of sheet-metal-forming lubricant

Lubricants Main applications Remarks

Neat lubricantsVanishing oils stamping, drawing,

pressingflash point 40 – 65◦C

deep drawing of thinsheet metalpunching andcalibration

Oils containing polaradditives

lamella and diskplate stamping

low viscousapplications somelubricants mineral oilfree, biodegradable,compatible withprelubes

embossing,stamping, drawing,pressingcar bodymanufacturing, deepdrawing

Oils containingEP-additives(chlorine free)

severe stamping,pressing, deepdrawing

lubricants can causecopper staining

fineblankingdrawing and ironing

Oils containingEP-additives(chlorine containing)

severe stamping,pressing, deepdrawing

high carbon steel,stainless steel, somelubricants do notcause coppercorrosion

fine blankingdrawing and ironing

Drawing compoundswithout solid matteradditives

severe drawing, deepdrawing

low to high carbonsteel

Drawing compoundscontaining solidmatter and/orEP-additives

most difficultdrawing, deepdrawing

mostly pastylubricants

WatermisciblelubricantsOils drawing emulsions

or solutionssynthetic,semisynthetic orconventionalformulatedlubricants

secondary hydraulicfluid forhydroforming

Drawing compoundswithout solidlubricants

severe drawing, deepdrawing

sometimes pastylubricants

Drawing compoundscontaining solidlubricants

most difficultdrawing, deepdrawing

often pasty lubricants

After Forming. As mentioned before, steelsheet metal and strip are generally providedwitha filmof corrosion protective or prelube oil in thesteel rollingmill. The range of products have notonly proved successful as corrosion protectiveoil but also for the further processing in light to


medium severity drawing operations without anadditional lubricant having to be applied.

The remaining film on the parts drawn in thisway provides good corrosion protection.

If an additional lubricant (spot lubricant) isapplied for forming this can be left in manycases on the workpiece after the operation toprovide corrosion protection during intermedi-ate storage or before further processing. How-ever, in themajority of cases, it will be necessaryto remove the lubricants immediately after pro-cessing or intermediate storage. This is done ei-ther by washing or, in the case of heat treatment,during the annealing operation wihtout previouscleaning.

13.1.5. Sheet Metal Forming in AutomobileManufacturing

The pressing of car body parts is one of the mostimportant sheet metal drawing processes. Thecorrosion protection oil applied by the steelmillsplays a part in every sheet metal forming opera-tion and also makes up the largest proportion oflubricants used.

Prelubes. The idea to combine the corrosionprotection properties of a corrosion preventiveoil with the lubricity of a drawing oil led tothe development of the prelubes. Prelubes haveexisted on the American market for more than20 years, they were introduced in Europe in theearly 1990s. Applied at the finishing lines of thesteel mills, they serve as the forming lubricantin the press shops. The use of prelubes in steelmills reduces the number and quantity of spotlubricants for additional press shop oiling dra-matically.

Among the many specifications for mill oils,the comprehensive standard of the Associationof German Automobile Manufacturers (VDA)[293], [294] describes all relevant requirementsof a mill-applied lubricant, defines all importantproperties and lists all test procedures.

Skin Passing. To achieve the desired qual-ity, the final surface finish of sheet steel is oftenrolled-on wet. Water-based solutions are nor-mally used for plain cold rolled steel but low-viscosity oils are used for zinc-coated sheet

including electro-galvanized or hot dip galva-nized. As a result of the increasing use of coatedsheet steel in recent years, the compatibility ofskin-pass oils is becoming an increasingly im-portant factor. If the skin-pass and the subse-quently applied corrosion protection oil are notcompatible, the oil film can rapidly separate.

Additional Lubrication. Many drawing op-erations require additional spot lubricationwhich is applied in the press shop. These higherviscosity drawing lubricants lose out when pre-lubes are applied. Ultimately, they will becomeunnecessary if the lubricity of the mill appliedoil on the panels is adequate. If in spite of theprelube, difficult deep drawing still requires theuse of additional spot lubricants at critical points,these must be process-compatible. As such spotlubricants are again applied on top of the otheroil layers, a thixotropic formulation is also ad-visable to avoid run-off.

13.2. Lubricants for Wire, Tube, andProfile Drawing

According toDIN 8584 these processes are clas-sified under the manufacturing processes withforming under a combination of tensile and com-pressive conditions. Wire and rod drawing isapplied to produce semifinished products withsmooth surfaces and close tolerances for themost varied application purposes. Solid or hol-low bodies are generally drawn through a sta-tionary drawing tool (drawing die, drawing noz-zle, drawing ring) to reduce the cross section orchange the cross section geometrically.

13.2.1. Wire Drawing

In no other forming process the lubrication, themachine and tool technology are so closely asso-ciatedwith eachother as in the case ofwire draw-ing. Three different types of lubrication technol-ogy are applied:

– Dry drawing: (inwire drawing this termdoesnot mean the absence of lubricant but theuse of solid, not liquid lubricants) with drydrawing soaps in the drawing box. These lu-bricants only lubricate the drawing die andhardly ever have a cooling function.


– Lubricated drawing with pasty or high vis-cosity lubricants: which means drawinggreases, drawing oils or drawing emulsions.

– Wet drawing: Wet drawing is generally donewith oil-in-water emulsions, seldom withnon-water-miscible oils, however, quite re-cently, also with special, highly viscous,degradable ester oils.

In wet drawing the task of the lubricant is tolubricate and cool the drawing tools and keep themachines clean. A further criterion for lubricantselection is the thickness of the wire.

Lubricants for Copper Wire Drawing.Water-mixed lubricants are used exclusively.Just in the first draft of coarse wire, a speciallubricant of high viscosity is frequently usedin the lubricated drawing or, alternatively, theconcentrate used in a following wet drawing isused in a predrawing block to ensure more fa-vorable lubricating conditions at the given lowdrawing speed. The lubricant used for this ap-plication must be compatible with the lubricantused afterwards.

Emulsions are the most important of all thelubricants in wet drawing. Apart from these, sur-factant solutions, free ofmineral oil and fatty oil,are significant. To a small extent other syntheticsolutions are also used. The three groups of lu-bricants (see Table 41) can be characterized onthe basis of the most important ingredients.

Table 41. Lubricants for wet drawing copper wire

Type of lubricant Ingredients

Emulsions hydrocarbons, mineral oilnatural fatty oilssynthetic estersnonionic surfactantsanionic surfactantsstabilizers, inhibitorsantifoam agentsother additives

Surfactant solutions alkali soapsalkali salt of sulfated fatty oilsnonionic surfactantsother additives

Other synthetic solutions polymersorganic saltsinorganic saltsother additives

In practice these differ mainly through theirdegree of dispersion or their solubility propertiesin water.

The emulsions can vary from relativelycoarsely dispersed emulsions with an averagedrop size of approx. 5µm to the finely dispersedemulsions of an average drop size under 1µm.

Lubricants for Steel Wire Drawing. Thesame basic technical requirements apply for wetdrawing as is the case for copper wire drawing.The thicker the wire is for drawing, the greaterthe need for effective lubricating polar and EPsubstances. This applies for the emulsions usedin the fine wire sector as well as for the drawingoil in the medium wire section.

Occasionally pasty drawing greases con-taining fillers are used in the lubricated drawingbefore wet drawing. The combination of semi-plastic lubricant carrier layers with circulatingoil lubrication is also a type of lubricated draw-ing [295]. However, the major portion of steelwire in coarse and medium draft is drawn withdry drawing soaps.

Dry drawing soaps are composed of alkali,alkaline-earth, and group 13 metallic soaps,sodium, calcium, and aluminum soaps are themost important. The soap content of the stearatescan be between 20 to 80%. Used as furthersubstances in the content are natural and syn-thetic (under certain circumstances also chlo-rinated) waxes, polymers, inorganic solid sub-stances such as lime, borax, soda, talcum, tita-nium oxide as well as molybdenum disulfide,graphite and sulfur. Generally alkaline-earth andgroup 13 metallic soaps are not dissolvable inwater, but on the other hand sodium soaps arevery soluble inwater. This iswhy alkali stearatescan be also applied by immersing thewire coil inthe hot aqueous soap solution with subsequentdrying, besides being used in the drawing box.

Dry drawing at high speed calls for lubricantswith high melting soaps due to the high devel-opment of local heat. Generally, stearates havea higher melting point than oleates.

Nevertheless, technical drawing soaps are notonly selected according to their melting point.More important are generally the lubricatingproperties, the matching grain size distribution,the flowability in the drawing box, the protec-tion against wear for the drawing dies, the fur-ther processing of the semifinished products andthe corrosion protection, etc.

In practice, it is the grain distribution andflowability of the dry drawing soap which are


decisive for lubrication. Coarse grains flow bet-ter than fine grains but are not so easily pickedup by the wire. The degree can be determinedby screen analysis. The resistance of a drawingsoap to abrasion is also important. It dependsupon the composition, production process andthe milling and must not be too high so that thewire can take enough lubricant with it.

Lubricants for Aluminum Wire Drawing.To some extent, drawing compounds with a highcontent of drawing effective (polar) substancesare still found in the non-slip single draft. Oth-erwise almost only very age-stable non-waterdrawing oils in a wide range of viscosities areused. For drawing coarse wire the oil viscosi-ties can be between 300 and 1000mm2/s at40 ◦C, in medium wire drawing between 50 and100mm2/s and in finest and super-finest wiredrawing oils with very low viscosity are fre-quently used, for example, 2 to 10mm2/s at20 ◦C. The higher the viscosity, the more im-portant preheating the oil becomes at start-up.

In the case of aluminum wire drawing veryfine abrasion particles occur. Normal oil filtra-tion, as it is used in other applications, faces greatdifficulties in the case of aluminum wire draw-ing so that the particles can normally only beremoved by sedimentation at stand-still. Alto-gether in this way very long oil service lives (asa general rule several years) are achieved.

Lubricants for Wire Drawing from OtherMaterials.

Stainless Steel. For wire drawing of stainlesssteel, oxalate coating and also plastic films, forexample from chlorinated polymers or lead lay-ers are applied as a lubricant carrier. Lubricationin the drawing dies can be provided by dry draw-ing soaps or with EP oils (highly additivated,generally containing chlorine) [295].

Medium and fine wire is drawn with EP oils,finest wire also with water-miscible lubricants.

Nickel behaves in a similar way to stainlesssteel. Coarse wire with sodium stearate is drawnwith non-water-miscible EP oils, which must befree of corrosive sulfur. During the annealing(> 800 ◦C in reducing atmospheres) the lubri-cant residue must not have a detrimental effecton the nickel surface.

Tungsten wire, which is required for the pro-duction of incandescent bulbs, is produced by a

specialmethod (→Lamps,Chap. 5.1.).Aqueousdispersions of colloidal graphite [296] are usedas lubricants. In the case of tungsten wire draw-ing the oxide layer serves as a lubricant carrierand improves the adhesion of the graphite [297],[298].

13.2.2. Profile Drawing

In the case of wire and rod drawing the pre-liminary materials will normally have a cir-cular cross section. Where profile drawing isconcerned, an extrusion molded, generally hotrolled initial material is available which alreadyhas a similar profile to that of the finished mate-rial.

Profile drawing is dominant in all kinds ofsteel working but has much less significance byvolume in nonferrous metals.

Theprofiles are drawnondrawingbenches, tosome extent at > 150 m/min, with drawing car-riageswhich aremovedbychains, ropes, toothedracks or hydraulic plungers.

Lubricants for Profile Drawing. Drydrawing soaps in drawing boxes or even watersoluble drawing soaps (applied by immersingthe preliminarymaterial in sodium stearate solu-tion, for example) are used for lubrication. How-ever, oil circulation lubrication has the greatestsignificance for completely flooding the intakeside of the drawing tool – not least because of theeasier insertion of the workpiece in the drawinghole during discontinuous working. The draw-ing oils usually have a viscosity between 50 and120mm2/s.

Drawing oils with a high percentage of ad-ditives are used depending upon the degree ofdifficulty. In this case the same selection cri-teria are valid as in the case of wire drawing.Drawing oils containing chlorine are generallyremoved before intermediate annealing becauseof the development of corrosive gases. Specialemulsions containing solid film lubricants anddrawing grease are suitable for particularly pol-ished drawn surfaces.

13.2.3. Tube Drawing

The great significance of tube drawing becomesclear when taking into account that about one


seventh of the steel produced worldwide is usedfor the production of seamless or welded tube.Moreover, the production of tube made of stain-less steel, titanium, aluminum, and especially ofcopper has considerable significance, both com-mercially and technically.

Wherever possible, tube is produced by cast-ing or forming methods in the required dimen-sions and tube quality. Precision tube with de-fined, closer tolerances, especially with definedsurface finishes, can only be produced by coldforming. Cold drawing of tube, apart from othercold formingmethods such as cold pilger rollingor cold rolling, is the last link in the productionchain. As is the case with profile drawing, tubedrawing is performed out on drawing benches.Long lengths of tube, especially copper tube,are drawn on drum drawing machines which inprinciple work in the same way as single wiredrawing machines.

With regards to the tribological demands, thedrawing method is the primary factor [299]. inthe case of hollow drawingwithout an inner tool,only external friction occurs and as a result onlylubrication from the outside is necessary. In thecase of various mandrel drawing processes aconsiderable improvement in the inner surfacefinish and a defined wall thickness are achievedwith a cross section reduction per draft of up to50%.

Lubricants forDrawing of Steel Pipes. Wetdrawing with EP oils does play a role for lessdifficult forming operations, as is the case withprofile drawing to produce blank drawing.

A special form of wet drawing is carriedout with so-called reactive forming lubricants.These oils contain organic and inorganic deriva-tives of phosphoric acid applied initially in aheated immersion bath. For example, bundlesof tubes are immersed after pickling or inert gasannealing for some 10 to 15min in heated reac-tive oil at 40 to 70 ◦C where they are left a cer-tain period of time before being drawn with theresidual oil film. The phosphate layer remains asa rule on the pipes. Although the ferrous phos-phate coating produced in this way cannot bestressed to the same degree, tribologically, aszinc phosphate coatings, or even saponified zincphosphate coatings, the method has gained verystrongly in importance for cost reasons. High al-loyed materials as specified for rod and profile

drawing are drawn with special highly viscousoils with chlorine additive (300 to 1000mm2/s,with a Cl content from 30 to 70%). In this casethe inner lubrication frequently turns out to be aproblem. Apart from these, certain polymer andcellulose ether films have proved to be suitable.

Emulsion and water-mixed drawing greaseshave only certain significance when blank draw-ing with a low degree of deformation.

Lubricants for Drawing of TitaniumTubes. These are important as capillary tubesand for pipeline construction in the aircraftindustry. Lubricants containing chlorine havegood drawing properties which, however, can-not be used for applications in aircraft construc-tion because of the risk of crack corrosion. Thesuitability of titanium for phosphatizing is con-siderably less than that of steel. Tribologicallyeffective coating calls for multistage treatmentwith acid. In this case there is a risk of hydrogenembrittlement.

Lubricants for Drawing of AluminumTubes. The same working principles apply asfor drawing aluminum wire. Strong polar form-ing oils are used mainly for this.

Lubricants for Drawing of Copper Tubes.Copper tube has gained outstanding significancein the refrigeration and air conditioning instal-lation branch. Particularly rational drum draw-ing machines with high drawing speeds (up to35mm/s) are used with flying mandrels. The re-quired quantity of lubricant is supplied bymeansof grease filler for inner lubricating. Oil circula-tion lubrication is used for the outside, if nec-essary, also with emulsions (frequently 30 to40%).

13.2.4. Hydroforming

The full name of ’internal high pressure form-ing’ commonly is replaced by the term ‘hydro-forming’.

First applications using internal pressure areknown sincemore than 100 years for the bendingof tubes. In the 1960s the production of T-fittingsbecame a typical application. Then exhaust com-ponents, motor cradles and other structural com-ponents for automobiles were manufactured. In


the past ten years the hydroforming technol-ogy grew considerably and it will gain moreand more in importance. This is due to the gen-eral demands in saving energy and raw mate-rial cost, improved mechanical equipment andprocess control, replacement of complex struc-tural components manufactured in short processchains (often with additional operations inte-grated in the hydroforming tools) and to the con-tributions of an intensive research [300].

Hydroforming is regularly made on tubularcomponents where straight or preformed pre-products are expanded in a split tool by applyinghydrostatic pressure [301]. The stages of this op-eration are as follows. The workpiece is placedinto the die, the die halves close, the mandrels(rams or sealing plungers) move in a close po-sition to the workpiece and the pressure fluidis filled into the workpiece while air escapes.The mandrels now seal perfectly and the inter-nal high pressure is built up. The pressure in-creases and the workpiece is shifted and formedin axial and/or radial direction. Finally when theworkpiece is already very close to the shape ofthe dies a calibration pressure of about 2000 to2500 bar is applied by which the workpiece isaccurately adapted to the contours of the dies.

Lubricants for Hydroforming. Lubricantsare used in operations before hydroforming,e.g. for the bending of the tube and the preform-ing of the workpiece. The remaining lubricantsometimes suits also for the hydroforming pro-cess itself. Additional lubrication (sometimescalled ‘external lubrication’) is necessary inmany other cases depending on the shape andthe geometry of the workpiece, especially whenlittle radii of curvature and difficult formablematerials have to be processed. Thus the rangeof lubricants is wide but intricate. It includesdrawingoils and compounds aswell aswaxfilmsand solid film lubricants [302]. In respect to thepressure conditions lubricants of high polarity incombination with antiwear or EP additives haveto be used. The selection is preferably done inaccordance with the application conditions. Itis clear that the removability of the residues ofthe lubricant becomes more and more essentialsince this is the relevant parameter of the generalprocess compatibility.

Another essential parameter is the secondaryhydraulic fluid. This functional fluid is the pres-

suremedium in hydroformingwhile the primaryhydraulic fluid is used in bed and slide and tomove the sealing plungers aswell as the counter-pressure cylinders. As secondary hydraulic flu-ids aqueous solutions or emulsions are in use.The solubility of the hydroforming lubricant inthe secondary hydraulic fluid is one of the fun-damental features of the hydroforming process.

Where oils are used as external lubricants twoalternatives were successfully established in thepractice depending both on the constructive con-ditions of the tank-side equipment and the de-greasing facilities after hydroforming: the useof rather soluble and more often the use of in-soluble (demulsifying) lubricants.

Additional operating steps are necessarywhen solid film lubricants are used. The par-ticular application properties of this type of lu-bricants and the subsequent degreasing possibil-ities must be adequately arranged.

13.3. Lubricants for Rolling

Based on the quantitative production of semifin-ished goods, rolling is one of themost significantprocesses in forming technology. Cast slabs,blooms, billets, and wire can be processed tosemifinished products through rolling.

The main forming takes place in the hot-forming temperature range, to exploit firstly, theadvantages of lower rolling forces and, secondly,to obtain semifinished products with soft struc-tures suitable for further processing. Semi-hotforming has only been used to an insignificantdegree in the rolling sector. In contrast, coldrolling is a process with surprising significance,more especially in view of the increasing de-mands being put on forming accuracy and thequality of the surface finish.

Of all the products manufactured by rolling,in type and quantity the flat products have thegreatest significance, in both the hot and coldrolling sectors. These processes are followed bythe important hot rolling processes for tube, sec-tion andwire production and the cold rollingpro-cess for producing taps and spiral drills. Apartfrom this, teeth and thread are rolled – even onone-off work pieces – and there are a numberof smooth rolling methods for surface finishing.Sheet and strip is made up to > 90% from fer-rous metals, aluminum sheet accounts for only


approx. 5 to 8%, with copper materials in theregion of 2 to 3%.

Wherever possible today, hot rolled strip isused instead of cold rolled sheet because of thelower price.

The use of high quality hot rolling oils in theforming process instead of water cooling hascontributed to a large degree to improving thesurface quality and the formability of hot strip.

In the past years a continuous further devel-opment of lubricants has occurred. Although thelubricants for the various processes are funda-mentally different they have many common fea-tures for the technology and tribology of rolls,especially for the behavior of the lubricant in theroll gap.

Up to the 1930s, the standard roll stand forthe cold rolling of sheet metal was the four-high reversing roll-stand. This was followed bythe building of tandem roll stands, in the initialstages predominately three four-high roll stands,followed by four four-high roll stands in the1950s and, finally, from the 1960s onwards fiveand six four-high roll tandem lines [303].

Today’s five or six-stand tandems run at 1500to 2000 m/min. Above all, modern roll standscan be run in allworking phases at a higher speedwith high mechanical safety. The reason for thisis the enormous progress made in measuring,testing and control technology.

Unlike other forming processes the frictionbetween tool (work roll) and workpiece (rolledmaterial) when rolling must be neither too highnor too low, so that the friction necessary forfeeding can be applied.

Figure 56 shows the rolling process as a sim-ple model. The sheet to be rolled runs at thespeed v1 into the roll gap of the rolls turning ata peripheral speed v3. During this operation thethickness of the sheet is reduced from s1 to s2.

Reducing the roll gap leads mainly to an ex-tension of the rolling stock length but not to anincrease in width. Accordingly, the initial speedof the sheet must be lower, and the final speedhigher, than the peripheral speed of the rolls. Asa result sliding friction occurs. The difference inspeed between v1 (run-in) and v3 (rolls) givenas a percentage, is called backward slip, and thedifference between v2 (run-out) and v3 (rolls) iscalled forward slip.

In the contact area of the friction arc there isa point in which the speed of the rolls and sheet

is the same, the neutral point N (no-slip point),at which there is no slip. As a result of varyingfriction, for example, caused by different lubri-cation, the N position within the friction arc canbe moved. By improving the lubrication, N canbe moved towards the roll gap exit. It is possibleto reduce the pressure tension through an analog-running tensile stress, namely by involving reeltension. The tribological ratios are improved ingeneral through this measure, which increasesthe degree of deformation efficiency so that theundesirable widening of the strip can be almostcompletely avoided.

Lower friction has the effect of lower rollwear and prevents the occurrence of heat stripesand similar rolling faults. However, the rollingoperation is only possible with a minimum offriction, to safely avoid firstly, so-called refusalswhen starting the rolling operation, and sec-ondly, to prevent roll slipping (and, as a result,further rolling faults such as, for example, un-even surface roughness, rubble marks, etc.) dur-ing the rolling operation.

13.3.1. Rolling Steel Sheet

Hot Rolling. Slab produced by means ofcontinuous casting or continuous casting areproduced on hot roll stands. Special hot rollingoils are used in the production of high qualitystrip on hot wide band lines.

These lubricants comprise thermally stablebasic materials and are different from the usualemulsified rolling oils for cold rolling in thatthey cannot bemixedwithwater. Hot rolling oilsare generally, together with the necessary cool-ing water, applied to the work rolls by meansof a separate spray system and reduce the fric-tion between work rolls and roll bite but alsobetween work and back-up rolls. In this waythe tribological relationships are improved in thelong-term and lead, more especially, to reducedroll wear. Parallel to this, an effective reductionin thickness can be achieved by lowering therolling force, in other words, the forming per-formance increases despite lower consumptionof drive energy.

The most significant reason for the use of hotrolling oils is, however, the improved quality ofthe surface finish down the entire length of thestrip.


Figure 56. Principle of the rolling processv1 is the initial and v2 the final speed of the sheet, v3 is the peripheral speed of thesheet, s1 is the initial sheet metal thickness and s2 the sheet metal thickness after rolling.

Hot rolling oils contain, besides mineral ba-sic oils, polar and EP active substances, whichform a wear-resistant ‘protective skin’ on thework rolls. The consumption of hot rolling oilfor steel is between 15 and 80 g/t.

Cold Rolling. The final thicknesses, whichare frequently between approx. 1.1 and 0.4mmput highest demands on the cooling oil emul-sions, particularly in this sector. The rolled stripwidths are between 600 and 1800mm.

The rolling speeds are between 600 to2000m/min, depending upon the design of thetandem line, and even higher in individual cases.

Rolling Emulsions. In rolling, the emulsionhas to take over the very important function ofheat transfer apart from its tribological task. Thecomposition of this lubricant for rolling dependsto a great extent on the circulation systemand theother conditions and parameters for the rollingoperation.

Rolling oil bases are mineral oil hydrocar-bon substances or synthetic hydrocarbon ma-terial in combination with polar and EP activesubstances and more especially, a specificallymatched emulsifier system. Rolling oil saponi-

fication numbers for rolling sheet are frequentlybetween 30 and 130mg KOH/g, which is equiv-alent to 15 to 65% fat oil or ester components.Different synthetic esters are used, frequentlypolyol esters, but also vegetable and animal fats[304].

Free fatty acids indeed increase the lubricat-ing effect but they must not be too high in pro-portion because of possible reaction with sol-uble soaps. The hydrolysis resistance of estercomponents must be high in order to keep thedevelopment of soaps low. The acid value (orneutralization number) of rolling emulsions isabout 3mg KOH/g, equivalent to approx. 1.5%acids.

Rolling emulsions are mixed with fully soft-ened water, generally between 1 and 3%. Theemulsion temperature is frequently between 35and 55 ◦C. The consumption of precoat oil andemulsion concentrate is together between 0.25and 1.00 kg/t sheet.

The emulsion stability is important for therolling process. Stabile emulsions are generallyused in high volume circulating systems, andmetastable emulsions in smaller circulating sys-tems. In the case of metastable emulsions one


advantage is the easier flotation of leakage oil,abrasion and dirt; stable emulsions are generallyeasier to control.

As a general rule, the emulsion is checkedseveral times a day during operation and partic-ular attention is paid to the degree of dispersion,the cleanness of the sheet and the risk of cor-rosion. The pH values are frequently between6.0 and 7.0, the chloride content is not to exceed20, the sulfate content 40 or the iron content100mg/L.

Re-rolling. In so-called skin pass rollingmills the strip must be re-rolled after anneal-ing to improve the flatness and achieve therequired material properties (forming changedtensile strength, tensile yield strength etc.). Thereduction in thickness is only about 1 to 3%.The strip is rolled dry or wet with special tem-per rolling oils, which, in particular, increasethe strip cleanliness. Skin pass fluids have alsoproved to be successful for providing a uniformmat finish, because the surface of the temperrolling roll closes more slowly and does not pickup dirt or friction particles.

Skin pass fluids are synthetic aqueous solu-tions and mainly contain surfactants as well asorganic and inorganic acid derivatives as corro-sion protection components. Normally these arefree of sodium nitrite and must be compatiblewith the subsequently applied corrosion protec-tion oils and prelubes.

Finest Sheet Cold Rolling. Rolling is car-ried out with semistable high fat content emul-sions by the so-called direct application of fatoil dispersions. In the case of direct application,natural fats such as palm oil or animal talloware often prepared as a 10 to 20% dispersionin completely softened water in large mixers.The drops of fat oil are approx. 50µm in size.The fat and water are separated during the re-turn run, treated and used again. Recovered fatcan be used for rolling up to an acid number ofapprox. 20 mgKOH/g.

Where semistable emulsions are used initialconcentrations of 3 to 6 % in fully softened wa-ter are employed. The size of the drops in suchemulsions is between 10 and 50µm.

Cold Rolling of High Alloy Steel Sheet.Higher alloyed steels, especially high grade

stainless steels, are rolled on multiroll standswith a preference for lubricants which do notmix with water.

The rolling speeds, which depend upon thealloy and material thickness, can be up to500m/min, in which case, for example, highgrade stainless steel hot strip be reduced by up to85% in eleven passes without intermediate an-nealing. Frequently, however, annealing, pick-ling and re-rolling have to take place in order toachieve the required strip quality.

The selection of the rolling oils has a decisiveinfluence on the surface finish of the sheet. Matsurfaces are produced when the viscosity is toohigh even when using ground or polished workrolls. This is almost always undesirable in thecase of high grade stainless steels. This is why inpractice preference is given to mineral oils withviscosities of ca. 15 to 20mm2/s at 20 ◦C whichhave the closest possible boiling range [305].

13.3.2. Rolling Aluminum Sheet

Hot Rolling. Depending upon the alloy andrequired reduction in thickness the blocks arepreheated to 450 to 580 ◦C before hot rolling.With a rolling speed between 180 and 300m/minin the reversing process and appropriately higherspeeds in tandem stands, the material is rolledwith stable emulsions in an initial concentrationbetween 2 to 6% and an upstream temperatureof 35 to 60 ◦C. The temperature in the last pass,depending upon the alloy, is still 280 to 230 ◦C.

The concentrates comprise mineral oil hy-drocarbons or synthetic hydrocarbonswith polaradditives (ester, fatty acids, fatty alcohols), sur-factants and further antiwear substances, whereneeded.

Fatty acids form aluminum soaps which cancause too high slip and tarnishing. This is whythey are only to be used in low amounts.

Decisive for the lubrication and quality of thesurface finish is the development of a uniform,fine coating of aluminum pick-up (roll coating)on the work rolls. If the coating is too thin,the friction conditions are unfavorable and fre-quently unstable. On the other hand a roll coat-ing which is too thick causes uncontrolled re-lease of parts of this coatingwhich, in turn, leadsto pick-up rolling and surface faults. To preventthis, excessive pick-up is removed from thework


rolls during the rolling process [306] by meansof brush rolls. Exact control of the temperatureof the work and back-up rolls is accomplishedby means of specific emulsion dosing to obtaina favorable hot strip profile.

Cold Rolling. After soft annealing at tem-peratures from 370 to 430 ◦C, finish rolling iscarried out on cold wide strip lines at speeds upto 2700m/min. The sheet thickness is between0.1 and 3.0mm but the major amount of rolledstrip is about 0.6mm thick and achieved in twoto seven passes.

Despite the advantages of better heat dissipa-tion, lower fire risk and less expensive healthprotection, water-miscible rolling oils [307]have been unable to establish themselves forthis application, because of, for example, patchforming as a result of residue moisture in theround coil, and hydrogen embrittlement of thework rolls can only be controlled to a deficientdegree.

When making up strip rolling oils, the mainattention has to be paid to the selection of thebase oil. Particularly suitable are paraffinic hy-drocarbons with a viscosity which is not toohigh, in order to avoidmat surface finishes. Con-sequently, closely cut base oils with flash pointsabove 85 ◦C and a viscosity from 2 to 4mm2/sat 20 ◦C are mainly considered. The most pop-ular additives are straight-chain alcohols, acids,and esters with chain lengths from 10 to 14 car-bon atoms as well as oxidation inhibitors. Theactivation level seldom exceeds the 5% mark.

In principle, foil rolling oils differ from striprolling oils only by a lower additive content, oc-casionally also by lower viscosity and preferablyby a very low aromatic content to ensure com-pliance with the stipulations for use in the foodsector.

As is the case with hot rolling, to avoid spot-ting synthetic or barely-synthetic special lubri-cants and hydraulic oils are also used when coldrolling on the roll stand and in the rolling oper-ations.

13.3.3. Rolling of Other Materials

Copper and its alloys are generally rolled in ahot roll process with water, i.e., without spe-

cial lubricants, althoughoccasionallywith stableslightly fatty emulsions.

Generally stable emulsionswith fatty oils andsynthetic esters are also used for cold rolling.Under all circumstances, staining of the bright,mostly decorative surfaces by the emulsion or itscontents has to be avoided.Consequently specialattention must be paid to ensure the absence ofsubstances containing sulfur [308].

Fatty oils are used for cold rolling of titanium.Zinc is rolled in a semihot state at 200 ◦C,

very often dry, or cold with a slight lubricationusing low viscosity oils with polar substances oremulsions. By heating to between 120 to 150 ◦Cwhen cold rolling, a large part of the lubricant isvolatilized so that no further cleaning is neces-sary.

Tin and lead are generally cold rolled. Lowviscous special lubricants are used for specialalloys (solders and similar materials). Tungstenand molybdenum rolling has to be carried outat high temperatures under inert gas due to sen-sitivity to oxidization at high temperatures. Al-ternatively coating with glazes is also possible.Semi-hot or cold rolling is carried out with solidmatter content or strongly polar oils with addi-tives.

13.4. Lubricants for Solid MetalForming

Under this heading, a large number of formingmethods are included, most important are extru-sion and forging.

Mainly processes are classified on materialflow and the type of the tool used. However,there are frequently no clear differences betweenforging and extrusion.

Important processes include:

a) Upsetting is a process with compression bet-ween flat dies.

b) Extrusion. During extrusion processes, theworkpiece is placed in a container and com-pressed by means of ram movement. Ex-truded products can be hollow or solid. Ac-cording to thematerial flow, forward (direct),backward (reverse) and cross procedures aredistinguished.

c) Impression die forging is the dominant hotforging method economically. This is also


reflected in the significance of this group oflubricants.Closed die forging is a special form of im-pression die forging. Here the filling of thedie is not supported by the development ofa flash. There is no possibility for excessivematerial to escape, and vent holes are pro-vided for vapor generated from the lubricantto escape.

d) Open die forging is different from impres-sion die and closed die forging in that themetal is never completely enclosed as it isbeing shaped by the dies.

Cold Extrusion of Steel (Extrusion Oils).Lubricating Oils. Of particular significance

is the lubrication with EP oils, especially whenworking with wire without surface pretreatmentof the blank metallic shearing surfaces. In thecase of toolswith extremely long service life, theforming is possible with the majority of highlyactive EP substances. An example of this is theproduction of inner square socket-head screws.

Apart from lubricating properties, the extru-sion oils must have high thermal stability; theoil is overheated locally on the extrusion bodiesand tools and temperatures over 90 ◦C can occurin the circulating system. The extrusion oil alsohas an essential cooling function, especially inthe case of high speed automatic machines. Noadhesive residue must remain as a result of thetemperature stress as these could cause irregu-larities in operation.

The preferred viscosity range of extrusionoils is between 35 and 65mm2/s at 40 ◦C. Selec-tion criteria for the initial viscosity are the work-piece temperatures, size of the extruded parts,machine pumping facilities and the specific cri-teria for the transfer between theworking stages,as well as the thickening effects as a result of en-richment by abraded phosphate particles wherephosphatized material is concerned.

EP additives containing sulfur and chlorineplay a role in extrusionoils.Additives containingchlorine have lost their significance in Europedue to the environmental problem described ear-lier. Apart from the sulfurized fatty oils there arealso other products in the polar additive sectorsuch as thermally stable synthetic esters (alsoas base oils). Sulfurized mineral oils, sulfurizedfatty oils and polysulfides play a special roleas sulfur carriers. Zinc dialkyldithiophosphate

smoothes the surface roughness of the work-pieces [309]. Alkyl and arylphosphoric acid es-ters and even phosphoric acid partial esters arealso used [310]. Because the thermal stress isconsiderable when using chlorinated products,particular attention must be paid to their stabil-ity and the risk of corrosion on machines andextruded parts. A degreasing of the parts di-rectly after extrusion is recommended when us-ing strongly chlorine containing oils.

For the most important areas of application,extrusion oils are classified by type and amountof additive into four classes [311]

– standard screws and high tension hexagonalbolts: oil with polar additives and EP addi-tives on a phosphorus basis (also suitable fornonferrous metals)

– larger size bolts and hexagonal nuts pro-duced from zinc phosphatized wire on mul-tistage presses: oils with polar EP additives,copper active

– cylinder bolts with an inner hexagon recessor inner toothing made of wire on multistagepresses (larger dimensions): oils with an ac-tive proportion of polar additives and EP ad-ditives on a sulfur basis, copper active

– high tensile rust and acid resistant steels sub-ject to high deformation: maximum alloyedoils with a very high share of EP additiveson a (chlorine) sulfur and phosphorus basis,copper active

Thermally stable, biodegradable ester oilswith excellent tribological characteristics willgain more significance in the future.

Phosphate Coatings and Soap Lubricants.Phosphatization has remained the most impor-tant method of surface treatment for cold extru-sion of steel until today and has undergone fur-ther technical development in conjunction withother lubricating systems. Particularly favorableis the ability of the layers of zinc phosphate togo along with large expanding surfaces withoutbreaking away when extruding.

The high resistance to pressure, shearing andadhesion during extrusion of zinc phosphatecoatings are considered as positive.

A fine crystalline coating approx. 5 to 7mmis to be used for oil lubrication on multistagepresses. Layers, which are too thick, lead toproblems due to formation of layers of phos-phate on the tools. This system is preferred for


parts with low piece weights such as screws,nuts, bolts or spark plug bodies produced onmultistage presses.

Alkali soap in connection with a zinc phos-phate coating has gained particular importance.In this case, not only the absorptive and chemi-cally absorptive binding of the lubricants to theworkpiece surface is neutralized but there is alsoconsiderable chemical conversion of the soapswith the zinc phosphate.In the aqueous solutionthe alkali soap reacts with the zinc phosphatecoating according to the following formula:

Zn3(PO4)2 + 6CH3(CH2)nCOONa→3 Zn[CH3(CH2)nCOO]2 + 2Na3PO4

zinc phosphate + sodium soap→ zinc soap +

sodium phosphate

Some 50 to 60% of the phosphate coating canbe converted depending on temperature, dwelltime, concentration and the developing phos-phate layer. The type of soap also plays a consid-erable role in this respect. Salts of stearic acid aremore reactive than the salts of oleic and palmiticacid.

The zinc soap developed by the reaction leadsto lower coefficients of friction when formingand the firm anchoring with the phosphate layeris particularly favorable for the lubricating op-eration.

The reactive soaps are supplied by the manu-facturers in powder or flake form and mixed bythe user with water to provide 2 to 10% soap so-lutions, depending upon the degree of difficulty.The parts are soaped by immersion over a pe-riod of 2 to 6min at bath temperatures from 60to 85 ◦C [312–314].

Solid Lubricants. Although soap lubricantsin conjunction with zinc phosphate coatingsare preferred for medium to larger sized parts,structure-effective solid lubricants, especiallymolybdenum sulfide, are used for smaller parts.

As a general rule the solid lubricants are ap-plied to a phosphate layer as dry powder lubri-cants by tumbling or from aqueous suspensionsby immersion or spraying. Application by im-mersion follows in dipping baskets or rotatingscreen tubs [311], [315–318].

In a few cases, solid lubricants can be appliedas suspensions in mineral oils, synthetic oils, oreven solvents. In special cases, this method can

solve wetting problems better than aqueous sus-pensions. In general, however, the aqueous dis-persions offer considerable advantages as carrierfluids as a result of their environmental friend-liness, especially when compared with organicsolvents, and can also be better integrated in thewet treatment of workpieces.

Warm Extrusion and Forging. The yieldstress decreases with the temperature increasein the workpiece material and as a result lowerforming forces and higher degrees of worka-bility can be achieved. This effect is utilizedto some extent through the existing formingheat (up to 250 ◦C) even when cold extruding(without preheating the blanks), especiallywhenworking on high speed multistage presses.

Warm forming has the greatest significancefor high alloyed steels and specialmaterials. Themethod is applied when the parts cannot be coldformed or when it is possible to ensure moreeconomical production by the warm process byless forming stages.

A phosphate layer is expedient at tempera-tures under 400 ◦C – as when cold extruding.

In the temperature range up to 350 ◦C zincphosphate coatings serve as carrier layers; themost important fluids are mineral oil based oilswith a high flash point, polyglycols and poly-butenes; they can be applied as loss lubricationand at temperatures up to about 300 ◦C as cir-culating lubrication. Solid lubricants can also besuspended in the above-mentioned oils; in thiscase mainlyMoS2 and graphite. AqueousMoS2and graphite suspensions are also suitable.

The lubricantsmostly used in the temperaturerange 350 to 500 ◦C are graphite dispersions inwater or organic carrier media. The decompo-sition of synthetic polymer oils is intentionallyaccepted, if necessary. In this case thermal re-sistance limits are set for MoS2 lubricants.

In the temperature range 500 to 600 ◦C aque-ous graphite dispersions are given preferenceand applied by spraying. Decomposition of thesolid lubricants graphite and MoS2 can be pre-vented when these are mixed with boric oxidepowder [319].

At temperatures > 600 ◦C the lubricants andthe application technology are similar to thoseapplied for the hot forging process. In addition tographite, zinc sulfide has also proved successfulas a solid lubricant because of its particular re-


sistance to temperature in individual cases; it isalso used in operations with graphite and watersoluble organic carriers such as polyglycols.

Hot Forging. The hotworkpiecematerial (inthe case of steel approximately 1200 ◦C) flowsinto the cavities of the die during the formingoperation. This is carried out to some extent ata high relative speed which depends, firstly, onthe speed of the striking tool (hammer or forgingpress) and, secondly, on the die.

In hot forging of steel workpiece temper-atures are between ca.1100 and 1200 ◦C anddie temperatures preferably between 150 and300 ◦C (maximum up to 500 ◦C). The tempera-ture of the lubricant layer is between 725 and775 ◦C at a die temperature of 250 ◦C and aworkpiece temperature of 1200 ◦C.

Graphite preparations have gained outstand-ing significance as lubricants. Colloidal prepara-tions in oil and in water as well as pasty graphitelubricants are used.

Graphite preparations are generally appliedas water-based suspensions. The wetting prob-lems given in the case of aqueous graphite sus-pensions, especially at high forging tempera-tures, can be solved by incorporating an or-ganic phase with appropriate surface active sub-stances, either as an emulsion phase or even asa real solution.

The propellant effect can be improvedthrough inorganic salts (alkali carbonate andbicarbonate) or through organic substances inaqueous solution or by dispersion. Bondingagents frequently on silicate or borate basis areused to form surface films.

If the die temperature is clearly under 200 ◦Cor even under 150 ◦C when the lubricant is ap-plied itmaywell be that nofilmcan be formed bythe aqueous dispersion, depending on the avail-able evaporation time. In such cases organic car-riers can be used which either evaporate morequickly or form an oily film. When selectingsynthetic polymer oils, care must be taken thatno toxic monomers develop during the depoly-merization process during contact with the hotforging. In individual cases pasty graphite prepa-rations are still being used.

Aluminum Forging. Because of their lowweight and excellent mechanical properties,parts produced by massive forming from alu-

minumand aluminumalloys are gaining increas-ing importance. Because of the major adhesionproblem, the technology in the usual mechani-cal presses with high deformation rates has stillnot been completely developed. Themost exten-sively used lubricants are water-based graphitedispersions [320–323].

14. Lubricating Greases

14.1. Introduction

Definition. Lubricating greases can be de-fined as solid to semifluid products that repre-sent dispersion of a thickening agent in a liquidlubricant. They usually contain additional com-pounds to impart special properties, and usu-ally the thickening agent is a metallic soap. Itis not easy to classify greases as either liquids orsolids – the transitions are fluid.

Greases in general contain from65 to 95wt%base oils, from 5 to 35 wt% thickeners and from0 to 10 wt% additives.

History. It can be speculated that grease-likelubricants were already known to the Sumeri-ans and used by them in their wheeled vehi-cles from ca 3500 to 2500 b.c. [324]; it hasalso been stated that as early as 1400 b.c. theEgyptians used greases made from olive oil ortallow and lime to lubricate the axles of theirchariots [325], although only pig fat was re-ported for that purpose by ancient authors suchas Dioscurides and Plinius Secundus [326].Probably the first grease of the industrial agewaspatented by Partridge [327] in 1835; it was acalcium grease, also made from olive oil or tal-low.Greases based onmineral oils and thickenedby soaps were probably first proposed ca 1845byRaecz [328], and a sodium greasemadewithtallow was patented by Little [329] in 1849.

Much general information on greases is givenin two encyclopedic monographs, published in1937 and 1954 [330], [331].

Classification. Greases are named after theindustry in which they are used, e.g., steel millgreases, after the kind of application, e.g., wheelbearing greases, after their prevalent applicationtemperature, e.g., low-temperature greases, or


after their range of application, e.g., multipur-pose greases. The meaning of the latter namehas changed with the years [332] and the othernames do not say very much about the qualityof the performance of the greases in question.

Even today, greases are named accordingto the consistency classes defined by the USNational Lubricating Grease Institute (NLGI)in 1938 and in accordance with the cone-penetration method developed in 1925 [333](Table 42).

Table 42. Classification of greases by NLGI numbers.

NLGI number Appearance ASTM workedpenetration(1/10mm)

Application

000 445−475 gear greases00 Semifluid 400−4300 355−3851 Soft 310−3402 Creamy 265−295 greases for

bearings3 220−2504 175−2055 130−1606 Soap-like 85−115 block greases

Nowadays the performance of lubricatinggreases is described by standards such as ISO6743-9 [334] or DIN 51825 [335] definingmainly consistency, upper and lower operatingtemperature, water resistance, and load-carryingcapacity, and for automotive greases by ASTMD 4950 [336], which has been followed by theconsideration of reference greases [337] and theintroduction of the NLGI Certification Mark[338].

But to some extent greases are best judgedsimply by the chemical and physical proper-ties of their base oils and the agents thickeningthem – naturally the consistency of greases in-creases with increasing thickener content, andwith it some of their properties change. Theseproperties at the same time best reflect the rea-sonable limits of practical applications.

14.2. Components of Greases

14.2.1. Thickeners

Thickeners not only transform liquid lubricantsinto consistent lubricants, they change the prop-erties of the liquid lubricants also (see Sec-

tion 14.8). Commercially important thickenersare listed in Table 43 [339].

14.2.1.1. Simple Soaps →Metallic Soaps

A maximum thickening effect is achieved withcarboxylic acids having 18 carbon atoms. Thus,soaps are usually prepared both from vegetable-derived 12-hydroxystearic acid [340], [341] andfrom animal- or vegetable-derived stearic acid[342], or from their esters, usually their glyc-erides, and from the hydroxides of elements ofthe alkali and alkaline-earth metals. Soaps, bygelling their base oils give greases most of theirunique properties. They are not only present ascrystallites and dissolved molecules, but aboveall in a separate phase represented by agglomer-ates called fibrils or fibers (see Section 14.4).

Soap Anions. The carbon chain length of acarboxylic acid affects the solubility and sur-face properties of a soap. Longer and shortercarbon chains than 18 carbon atoms reduce itsthickening capacity. Increasing the chain lengthincreases the solubility in the base oil, shorten-ing the chain length reduces it. A branched alkylchain lowers the melting point of a soap and re-duces its thickening effect. Unsaturated carbox-ylic acids are more soluble in mineral oils andalso reduce the thickening effect and lower thedropping point. Their use is limited because oftheir lower oxidation stability. Hydroxyl groupsincrease the melting point and the thickening ef-fect of a soap because of its increased polarity.

Soap Cations. The soap cations also areresponsible for essential properties of soapgreases. The cations govern the thickener yield,the dropping point, the water resistance, and,to some extent, the load-carrying capacity of agrease (see Section 14.8).

In 1996 greases based on simple soaps stillaccounted for more than 70% of known worldproduction [343]. Lithium soaps were the mostimportant, with ca 50%, then the calcium,sodium and aluminum soaps. The importanceof the latter has steadily decreased during recentdecades.


Table 43. Competitiveness of thickeners

I II III IV V VI VII VIII IX X XI XII Sum

Lithium12-hydroxystearate

2.5 1.0 2.0 1.5 2.0 2.0 2.5 1.5 2.5 2.0 1.0 3.0 2.0

Calcium12-hydroxystearate

3.0 1.0 3.0 1.0 1.5 1.0 2.5 1.0 2.0 2.0 1.0 3.0 1.8

Lithium complexes 1.5 2.0 1.5 2.0 1.5 2.0 2.0 2.5 1.5 2.0 2.0 2.5 1.9Calcium complexes 2.0 3.0 2.0 2.0 1.0 1.5 1.5 3.0 1.5 2.0 2.0 2.0 2.0Aluminum complexes 2.0 2.0 2.0 2.5 1.5 2.0 2.0 2.5 2.0 2.0 2.0 2.5 2.1Inorganic thickeners 1.5 1.0 1.0 3.0 3.0 1.0 3.0 1.0 3.0 3.0 2.5 3.0 2.2Polyureas 1.0 1.5 1.5 2.5 2.0 1.5 2.5 2.0 3.0 3.0 1.0 2.0 2.0Terephthalamates 1.5 1.5 1.5 1.0 2.5 1.5 2.0 1.0 2.5 2.0 1.0 2.0 1.7Calcium – sulfonatecomplexes

2.0 3.0 2.0 2.0 1.0 2.0 1.5 3.0 1.0 1.0 2.0 1.5 1.8

Carbamate-likethickeners

2.0 1.5 2.0 2.0 2.0 2.0 2.5 1.5 2.0 2.0 1.0 2.0 1.9

I = high temperature, II = low temperature, III = aging, IV = compatibility, V = oil loss, VI = toxicity, VII = tackiness,VIII = flowability, IX = load, X = shear, XI = friction, XII =wear.1.0 = excellent, 2.0 = average, 3.0 = poor

Lithium Soaps. Lithium soap-based greasesare usually prepared by reacting lithium hydrox-ide as a powder or dissolved in water with 12-hydroxystearic acid or its glyceride in mineraloils or synthetic oils. The production tempera-ture is between 160 and 250 ◦C, depending onthe base oil and the type of reactor in use. Thedropping point of a mineral oil-based NLGI 2grease is usually between 185 and 195 ◦C. Forsuch a multipurpose grease ca 6 wt% of soapis required with a naphthenic oil, ca 9 wt%with a paraffinic oil, and ca 12 wt% with apolyalphaolefin, and a kinematic viscosity ofca 100mm2/s at 40 ◦C. The thickening effectdepends not only on the carbon distribution(Caromatic +Cnaphthenic +Cparaffinic) in a baseoil, but also on its viscosity.

The fiber size in lithium 12-hydroxystearategreases is usually between 0.2× 2 and0.2× 20µm. Good multipurpose properties,e.g., a high dropping point, good water resis-tance, good shear stability – which has beenrelated to hydrogen-bonding of the hydroxylgroups – and good response to additives arethe reasons why lithium 12-hydroxystearate-based greases have now been the most popularfor more than half a century. They have founda wide range of applications from EP greasesbasedonoilswith kinematic viscosities of ca 200to 1000mm2/s at 40 ◦C for heavy loads, throughmultipurpose greases based on mineral oils withkinematic viscosities from 60 to 120mm2/s at40 ◦C for all kinds of bearings, greases made

with diesters or polyalphaolefin oils and kine-matic viscosities of 15 to 30mm2/s for high-speeds, to those with oil-insoluble polyalkyleneglycols for gears. The lower and upper applica-tion temperature limit for a lithium soap thick-ened grease, as for all other greases, dependsmainly on the physical properties of its baseoil (see Section 14.3). Oil separation has beendiscussed as a criterion for both lower and upperapplication temperature limits [344]. In recentyears attempts have been made to improve thestructural stability of lithium soap-based greasesby use of reactive polymers [345], [346].

Calcium Soaps. Greases based on calcium12-hydroxystearates (anhydrous calcium soaps)are produced in the same way as lithium soap-based greases, but at temperatures between 120and 160 ◦C. The size of their fibers lies betweenthose for lithium soaps and hydrated calciumsoaps. The greases can be used up to 120 ◦C.Their dropping point is between 130 and 150 ◦C,depending on their base oil. They usually have avery good corrosion resistance and good oxida-tion stability; when prepared from suitable baseoils they are possibly the best low-temperaturegreases.

Calcium soaps mainly based on stearic,palmitic, or oleic acid still are called hydratedcalcium soaps. For these greases the cost of rawmaterials, and the performance levels, are low-est. Calcium soap greases are prepared by neu-tralization of a slurry of calcium hydroxide inwater with fatty acids or fats in mineral oil. Sta-


ble greases can only be obtained in the presenceof some water, usually ca 10 wt% of the soap.The water content is usually adjusted in a sec-ond step in a stirring, or cooling vessel. The fibersize is usually ca 0.1× 1µm. When the wateris removed, the grease structure collapses. Thedropping point of this type of grease is there-fore only 90 to 110 ◦C and the upper applicationtemperature limit is only ca 80 ◦C.

The greases have very good water resistanceand good adhesive properties. Because theirmanufacture is rather demanding in relation tothe performance obtained, their importance isdeclining rapidly.

Sodium Soaps. Compared with lithium andcalcium 12-hydroxystearate greases the impor-tance of greases based on sodium soaps is nowa-days low, although in the formof semifluid prod-ucts they still are of some interest for the lu-brication of gears. Sodium greases made fromfatty acids or fats have dropping points of ca165 to 175 ◦C. Their upper temperature limitis ca 120 ◦C. Products are available with shortand long fiber structures; the latter reaching upto 1× 100µm and are responsible to some ex-tent for the rather high load carrying capabil-ity in gears. Although the greases have extraor-dinary good corrosion-preventive properties ifonly small amounts of water are present, theirmain disadvantage is the solubility of the sodiumsoaps in larger amounts of water; this leads firstto gel formation, which dramatically increasesthe apparent viscosity, and later to breakdownof the whole structure.

Aluminum soap-based greases are usuallyproduced with pre-manufactured aluminumsoaps, usually aluminum stearate. They havedropping points of up to 120 ◦C, their uppertemperature limit is 80 to 90 ◦C – above 90 ◦Cthey tend to gel. Typical of this soap is a par-ticle size of < 0.1 × 0.1µm, which is to someextent responsible for the low shear stabilityand the pronounced thixotropic behavior of theproducts. Aluminum greases are usually verytransparent and smooth. They have good wa-ter resistance and adhesive properties, but havebeen widely replaced by lithium greases, part-ly because, for consistent products, aluminumgreases must not be stirred during the finishingprocess but poured into pans and left for severalhours to cool.

Cation Mixed Soaps M1X/M2X. Mix-tures of soap greases with different cations,mainly lithium – calcium, calcium – sodium,and sodium – aluminum are called mixed-soapgreases. Their properties largely depend onthe relative proportions of the components.Lithium – calcium greases have better water re-sistance than pure lithium greases and oftenbetter shear stability. As long as the proportionof calcium soap does not exceed 20 wt% theirdropping points, 170 to 180 ◦C, are close to thatof the lithium soap alone, and their wear andfriction performance are better than that of purelithium grades [347]. Some calcium – lithiumgreases perform better than pure calcium 12-hy-droxystearate greases.

Lithium – calciumgreases have become quitepopular as specialized multipurpose greases.Greases mainly based on sodium-aluminumstearate, have been used as substitutes forlithium greases. Cation mixed soap greases areusually manufactured by a one-step process, be-cause the stability of mixtures of finished prod-ucts are not always satisfactory.

AnionMixedSoapsMX1/MX2. Because ofthe animal and vegetable origin of the acidiccomponents most simple soap-based greases arealready anion mixed soap based. Neverthelessthe fine-tuning of multipurpose greases and spe-cialized multipurpose greases is, especially forthe comparatively pure 12-hydroxystearic acid,often achieved by substituting small amounts ofthe main acid by a complementary acid, e.g.,behenic, naphthenic, or stearic acid.

14.2.1.2. Complex Soaps

Simple soaps can form complex soaps with saltsof other acids. Examples are inorganic acids,e.g., boric and phosphoric acid, or with short-chain carboxylic acids, e.g., acetic acid, or withdicarboxylic acids, e.g., azelaic [348] and se-bacic acids [349]. The addition of complemen-tary salts always leads to an increase in thedropping point from ca 50 to 100 ◦C and to re-duced oil separation, primarily because of theincreased thickener concentration, and, thus, forthe same reason, to reduced suitability for low-temperature application. Because of their supe-rior properties complex soap-based greases have


become very popular and nowadays account forca 20% of all greases sold.

Lithium Complex Soaps. Apart from theirhigher upper-temperature limit, between 160to 180 ◦C, some lithium complex soap-basedgreases perform similarly to the correspond-ing simple soap-based products. Of the manyknown formulations the most widespread isbased on 12-hydroxystearic acid and azelaicacid, introduced in 1974 [350]. The first com-plex was based on 12-hydroxystearic acid andacetic acid and was patented as early as 1947[351]. The lithium complex soaps with the bestload-carrying capacities contain boric or phos-phoric acid. The fiber size of such a complexsoap does not differ much from that of the sim-ple soap and it does not change significantlywithcommon shear. In addition to azelaic and boricacids, other acids have also been examined sys-tematically

Lithium 12-hydroxystearate+Lithium adipateLithium azelateLithium dimerateLithium sebacateLithium terephthalamate...Lithium borateLithium phosphate

With a share of about 10%, lithium com-plex greases are the most widespread complexgreases.

Calcium Complex Soaps. All commercialcalcium complex greases contain acetic acid as acomplementary acid. The complex was first de-scribed in 1940 [353]. Calcium complex greaseshave good shear stability and water resistance,a low oil separation, and good load-carrying ca-pacity. Their upper temperature limit is 160 ◦C.Because of the formation of ketones, from about120 ◦C pronounced hardening can occur. It has,nevertheless, been possible to retard hardening

with the aid of polymeric structure-modifiers[354].

Calcium Sulfonate Complex Soaps. Com-petitive greases based on this complex were firstoffered in 1985 [355]. Initially they comprisedin situ-produced overbased calcium sulfonatesand the calcium salts of other sulfonates, of12-hydroxystearic acid, and of boric acid [356].The complex could be improved by replacingcalciumborate byphosphate [357].Greases con-taining calcium acetate instead are also possible[358]. The greases are extraordinarily corrosionprotective and resistant to shear and have load-carrying capacities that can be matched onlyby greases of other soaps with very many addi-tives. Their dropping points start at ca 220 ◦C,but their upper temperature limit is ca 160 ◦C.Some grades can, nevertheless, survive temper-atures up to 250 ◦C for several hours. It can beexpected that these greases will become evenmore important in the future, although, becauseof their high thickener content their low-tem-perature pumpability is limited.

Aluminum Complex Soaps. Today onlyone of the possible aluminum complexes is stillwidely used. It comprises aluminum stearate andbenzoate, and was first patented in 1952 [359].Aluminum complex greases of that kind havegoodwater resistance and good low-temperatureproperties. Their importance has decreased inrecent years, although attempts have been madeto make them more attractive again. This mightbe possible for food grade and biodegradablegreases (see Section 14.10).

Other Complex Soaps Sodium complexsoap-based greases have been used becauseof their suitability for high relative velocities,but like the simple soaps they lost importancebecause of their limited water resistance; bar-ium complex soaps have been replaced almostas completely as the simple soaps.


Titanium complex greases were patentedin 1993 [360]. They are based on 12-hy-droxystearic and terephthalic acid.Of their prop-erties [361] good load-carrying capability ismost worthy of mention.

14.2.2. Other Ionic Organic Thickeners

Ofmanypossible soap-like salts only the sodiumand the calcium salts of stearylamidoterephthal-ic acid are used on an industrial scale.

(14.1)

They were patented in 1954 [362] and proposedfor multipurpose greases in 1957 [363]. Greasesof this kind have dropping points of up to 300 ◦Candupper temperature limits of up to 180 ◦C.Al-though they have the thickening effect of simplesoap greases they behave like complex greases;this makes them valuable wide-range greases.The thickeners are among the most expensiveand are preferably used with synthetic base oils.Complex soaps comprising terephthalamate andbenzoate have been described and complexes ofaluminum stearate with terephthalamates havealso been investigated [364].

14.2.3. Nonionic Organic Thickeners

Of a rather large number of theoretically suit-able compounds, only the oligoureas, commonlycalled polyureas, have gained real industrial im-portance.

Diureas and Tetraureas. Oligourea thick-eners were introduced in 1954 [365]. The re-action products of one molecule of di-4,4′-isocyanatophenylmethane (MDI) or other diiso-cyanates with twomolecules ofmonoamines arecalled diureas.

(14.2)

Tetraureas are the reaction products of twomolecules diisocyanate with onemolecule of di-amine and two molecules of monoamine.

(14.3)

Depending on the performance requiredaliphatic or aromatic amines or mixtures ofboth are used. With excess diisocyanate three-dimensional structures are built along biuret-likebridges.

(14.4)

The upper temperature limit of oligourea greasesis not determined so much by the stability ofthe thickener, the decomposition of which usu-ally starts slightly below 250 ◦C, as by the sta-bility of the base oil. These greases are, there-fore superior to soap-based greases when the ap-plication temperatures exceed 180 ◦C. When anoligourea grease based on polyalkylene glycolsis overheated, ideally, only gaseous products areformed. Although tetraureas also have some ad-vantages [366], the trend in polyureas will prob-ably be toward diureas [367]. Oligourea com-plex greases containing calcium acetate wereintroduced in 1974 [368]; others containing car-bonate [369], [370] and other complementarysalts followed and are still favored for some ap-plications. Although soap-based greases cannotcompete with oligourea greases at higher tem-peratures, below 180 ◦C lithium complexes, forexample, have at least equal performance [371].

Carbamate-like thickeners are related to theoligoureas and simple soaps, and have propertiesintermediate between those of these groups.

(14.5)

Other Nonionic Organic Thickeners.Polymerized perfluorinated hydrocarbons,usually micronized poly(tetrafluoroethylene)


(PTFE) powders, are the choice for greases thatmust function at temperatures above 220 ◦C,with an upper temperature limit of about270 ◦C. For applications of this kind their liquidoligomers or preferably corresponding perflu-oroalkylene ethers must be chosen as base oils[372–374]. Polymers such as polyamides orpolyethylenes are mainly used as additives.

14.2.4. Inorganic Thickeners

To be suitable for use in lubricating oils inor-ganic thickeners must be treated with between 5and 10 wt% reactive organic compounds. With-out this treatment they would act like fillers,co-thickeners, and solid lubricants, that only atconcentrations above ca 40 wt% yield grease-like pastes.Apart from these hydrophobic agentsadditional polar activators, e.g., acetone, etha-nol, or, for safety reasons, propylene carbonate[375], are needed to accomplish gelation. Theyare used at a level of 10 wt% relative to thethickener. The thickeners themselves are stableup to temperatures of> 300 ◦C, but the resultinggreases or gels are mainly used for applicationsup to 200 ◦C that do not require too pronouncedshear stability. This is partially because of thediameter of the primary parts, only ca 0.05µm.The tendency of inorganic thickened greases tohardening and oil separation during storage, andtheir sensitivity to polar additives can to someextent be overcome by addition of functional-ized polymeric agents [376].

Clays. Bentonite-type aluminosilicates,mainly the smectites montmorillonite and hec-torite [377], are the most important inorganicthickeners. They are usually treated with qua-ternary ammonium compounds, e.g., trimethylstearyl ammonium chloride, and the activatorsmentioned above.

Highly dispersed silica is usually preparedby flame hydrolysis of silicon tetrachloride(→Silica, Chap. 6.1.1.); it is made more suit-able as a thickener by treating it with, e.g.,silanes, silazanes, or siloxanes [378]. One ad-vantage of these products is the low temperature-dependence of their consistency [379]. Withsuitable base oils and activators they form whiteto transparent gels that are used in food or med-ical applications.

14.2.5. Miscellaneous Thickeners

In principle inorganic and organic pigmentsof all kinds can be used as thickeners or co-thickeners or at least as fillers. The transition tolubricant additives is fluid. Only the inorganicmaterials soot and expanded graphite [380] andthe organic phthalocyanines [381] are some-times used on an industrial scale.

14.2.6. Temporarily Thickened Fluids

Under defined conditions liquids and suspen-sions of solids in liquids have significantly in-creased viscosities.

Some liquid crystalline systems are suitableas lubricants under the influence of changes inpressure [382], or temperature [383]. Some ofthe solutions that can form liquid crystals in lim-ited concentration ranges [384] are comparablewith consistent lubricants and some liquid crys-tals in concentrated point contacts are even su-perior [385].

Electrorheological or electroviscous fluids,i.e., suspensions of micronized highly polariz-able and hydrophilic porous solids, are charac-terized by an enormous increase in apparent vis-cosity, when subject to electric fields. Originallysilica in silicone oil was usedwithwater as a pro-moter, later polyurethanes without a promoter inhydrocarbons. The first practical applications goback to 1942 [386]. electrorheological fluids areincreasingly used in hydraulic valves, shock ab-sorbers, and clutches [387], [388].

Magnetorheological fluids [389], suspen-sions of micronized transition elements, mainlyferrites, behave similarly in magnetic fields.Both electrorheological and magnetorheologi-cal fluids have also been named ‘smart fluids’.They contain between 20 and 60% solid parti-cles which formmore or less branched chains inthe applied fields; they therefore act as Binghamplastics (see Section 14.7).

14.3. Base Oils

In principle all the lubricating oils already de-scribed in Chapters 4 and 5 can be used as base


oils for greases, but in general only oils withkinematic viscosities from 15 to 1500mm2/s at40 ◦C are used. The oils with the lowest vis-cosities and the best low-temperature propertieshave the lowest operational temperatures at thehighest speeds. The oils with the highest vis-cosities have the best performance at the lowestspeeds and the highest loads, the lowest evapo-ration loss, the strongest adhesion and the bestwater or solvent resistance.

14.3.1. Mineral Oils (see Chap. 4)

Oils with low VIs usually require less thickenerthan those with high VIs and the same kine-matic viscosity. The latter can be used over awider temperature range and are, therefore, pre-ferred nowadays. In principle the thickening ef-fect (yield) depends on the difference betweenthe solubility parameters of the base oil andthickener [390–393]. Because oil separation de-pends on the concentration of thickener, at agiven temperature oil separation is most pro-nounced for greases based on low VI oils, e.g.,aromatic oils or alkylbenzenes. Oil separationgoes through a minimum with increasing kine-matic viscosity for a given kind of base oil. Thiscan easily be understood when only the depen-dence of particle interactions on the size of theinvolved particles is considered. Some of thespecific properties of greases also depend on theratio of the temperature and pressure coefficientsof the viscosity of their base oils. If all this istaken into account, the effect of even a smallchange in base oil composition on the perfor-mance of a grease should not be underestimated[394].

14.3.2. Synthetic Base Oils (see Chap. 5)

Because of their cost, synthetic oils are usedin greases only when the performance requiredcannot be achievedwithmineral oils [395]. Eventoday greases of this kind represent less than 5%of overall grease consumption.

Synthetic Hydrocarbons. Polyalphaolefins[396] are the ideal base oils for wide temper-ature-range greases. They are usually used inthe same viscosity range as their mineral oil

equivalents, but whereas the upper kinematicviscosity border of the former is ca 50mm2/sat 100 ◦C, polyalphaolefins and similar prod-ucts are available to ca 2000mm2/s at 100 ◦C.In contrast with mineral oils they are preferablyshrinking rather then swelling polymeric sealingmaterials. Usually, therefore, esters with soft-ener properties must be added. Greases madefrom polyalphaolefins with standard viscositiesare usually meant for lifetime lubrication, thosemade frommixtures with mineral oils (semisyn-thetic oils) and are used in specialized greasesto increase lifetime.

Polybutenes and comparable polymers areusually applied to increase the viscosity of stan-dard base oils; occasionally they are used as abase oil component, or even the only base oil.

Other Synthetic Base Oils. High molecu-lar-mass esters made from diols or poly-ols and dicarboxylic acids with viscosities> 2000mm2/s at 40 ◦C are used as base oils ingreases that must be solvent resistant.

In contrastwith other greases silicone greases[397] are better distinguished according to theproperties of their base oils [398]. The kine-matic viscosities of silicone oils for greasesrange from ca 75 to ca 10 000mm2/s at 40 ◦C.Dimethylsilicone oils are physiologically inert;when thickenedwith highly dispersed silica theyare mainly used as sealants or in applicationswhere specific electrical and thermal conductiv-ity is important.

Partially phenylated dimethylsilicone oilshave very good low-temperature properties andare used with all types of thickener, depend-ing on their (often military) application. Par-tially fluorinated dimethylsilicone oils are usu-ally thickened with PTFE powders.

Apart frombeing suitable for temperatures upto 270 ◦C perfluorinated ethers thickened withPTFEpowders are the only greases that canworkin the presence of aggressive chemicals such asoxygen or chlorine. This is also their main ad-vantage over the corresponding greases based onpartially fluorinated silicone oils.

14.4. Grease Structure [399–402]

Greases have a more or less distinct latticestructure, soaps therefore form a framework of


string-like aggregates comprising micelles andcrystallites that are called fibrils or fibers, thebase oil is embedded in this framework. Theoil is probably bound in three ways: firstly bymolecular attraction between polar thickenerand oil areas; secondly by capillary forces; andthirdly by mechanical occlusion. In soap-basedgreases small changes in the concentrations ofthe free acids and bases have a large effect onthe shape and size of the aggregates. Even to-day it has not been definitively decided whethersoap-based greases fit into the system of col-loids (→Colloids) or are of a more crystallinenature. Macroscopically the thickener particlesare agglomerates of crystallites andmicelles anddissolved components the size of which is lim-ited by the method of homogenization. The na-ture of grease-like products formed with inor-ganic thickeners, pigments, or ferrites, is, how-ever, not as difficult to describe. Here the lawsof colloidal chemistry and physics can usuallybe applied fully [403].

The structures of liquid crystals are rather dif-ferent (→Liquid Crystals) [404]. Study of thesehas revealed much of interest in recent years[405].

14.5. Additives

Most of the additives already described in Chap-ter 6, can be used in greases also, and in the sameway, although in general at clearly higher con-centrations (Table 44). Interactions of thickenersand additives must therefore always be consid-ered.

Table 44. Usual grease additive levels (%)

Antioxidants 0.10−1.00Corrosion inhibitors 0.50−3.00EP/AW additives 0.50−5.00Metal deactivators 0.05−0.10Solid lubricants (black) 1.50−3.00Tackifiers 0.10−1.00

Structure Modifiers. Greases always con-tain compounds that affect the structure of thethickener itself to the same extent as they act asadditives for the greases as a whole. These com-pounds are mainly water, glycerol, excess fattyacid, and excess alkali metal/alkaline-earth hy-

droxides. Interactions between structure modi-fiers and additives, e.g., that between EP addi-tives and glycerol [406] or stearic acid [407],can severely affect the performance of a grease.Another structure modifier of greases that caneasily be overlooked is the necessarily incorpo-rated air. Though the air content must be limited,of course – often < 3% [408] – without air thetransport of a grease through a long pipe wouldbe almost impossible.

Antirust Additives (Corrosion Inhibitors).The most efficient corrosion inhibitor for a soapbased grease is, when practicable, excess ofthe metal hydroxide. Sodium or calcium sul-fonates, among the most powerful antirust addi-tives, are also strong structure modifiers and cancause problems with most greases, especiallygreases made with inorganic thickeners. Imida-zoline and sarcosine derivatives and alkenyl suc-cinic acid esters are suitable inhibitors for soap-based greases; disodium sebacate is mainly usedin gel greases. Carbodiimides have been recom-mended for use in ester-based greases, primarilyto prevent hydrolysis [409].

Extreme Pressure and Antiwear Addi-tives. Compounds containing heavymetals suchas antimony or lead are, for reasons of toxic-ity, no longer acceptable as heavy-duty and an-tiwear additives, and even the use of zinc is dis-puted today, because of water pollution prob-lems. Ashless additives, e.g., dithiocarbamates,dithiophosphates, and thiadiazole derivatives areslowly taking over [410] – the last also havecorrosion-inhibiting properties [411] – and theimportance of ashless alkylaminophosphorodi-thioates [412] as grease additives is also ex-pected to increase.

Bismuth is one exception among the heavymetals. Additives containing this element havesome of the properties of its toxic neighbors, butwithout their disadvantages [413], [414].

Solid Lubricants (see Chap. 15). Because oftheir nature greases can contain these, or otheradditives that are insoluble in the base oil or baseoil mixture, in rather high concentrations. Formost applications an average particle size of themicronized solids of between 5 and 15µm is ac-ceptable.


Graphite and molybdenum disulfide are theoldest, the most common, and the by far bestinvestigated solid lubricants for greases. Manymanufacturers use them as mixtures, becausethey are believed to perform better. Fullerenes[415] have been tested successfully as solidlubricants [416], [417] but are still extremelyexpensive. The performance of molybdenumdisulfide has been improved by the addition ofzinc dithiophosphates [418] and by admixtureof polyamide powders, urea resins, or salts ofcyanuric acid derivatives. Admixture with PTFEpowders does not give comparable results [419].Molybdenum dithiophosphates and dithiocarb-amates, rather than the disulfide, are of growinginterest for joint greases [420].

Polytetrafluoroethylene, although rather ex-pensive, is still the most common polymericsolid lubricant in greases. But high and ultra-high molecular-mass polyolefin powders are be-coming increasingly competitive.

Inorganic white solid lubricants, e.g., the hy-droxides and diphosphates of some of the alka-line earth metals, improve the EP/AW proper-ties of common greases and are especially suit-able when the greases are subject to oscillations[421].

Friction Modifiers. In liquid lubricants car-boxylic acids and their salts act as friction mod-ifiers [422] and as additives for the boundarylubrication regime [423]. In greases that containlarge amounts of these compounds, solid lubri-cants are sometimes called frictionmodifiers, in-stead [424], but sometimes polymeric additivesalso [425].

Some properties of greases, for examplemechanical stability, can, to some extent, beimproved only by use of polymeric additives;some properties, e.g., resistance against aggres-sive reagents cannot be improved by use of ad-ditives.

14.6. Manufacture of Greases

The properties of greases, especially metallicsoap-based greases, depend not only on theircomposition but also, and to nearly the sameextent, on the way in which the thickeners areprepared and dispersed.

14.6.1. Metal Soap-Based Greases

Batch Production with Preformed MetalSoaps. Greases can be prepared by dissolutionor dispersion of preformed metallic spoaps ina suitable base oil with subsequent heating andcooling. But because of the higher costs of thethickener thismethod can be recommended onlyfor highly sophisticated synthetic greases withprecise chemistry or for functional base fluidsthat would react with the water or, even worse,with the steam generated during the neutraliza-tion process.

Batch Production with Metal Soaps Pre-pared In-situ. In general fatty acids or theirglycerides or even theirmethyl esters are reactedwith aqueous solutions or suspensions of the de-scribed metal hydroxides in part of the base oil.Each batch of a grease is produced by followinga ten-point schedule.

1) dissolution or dispersion of the fatty acidsin one third to two thirds of the base oil attemperatures up to 90 ◦C;

2) addition of themetal hydroxides as a solutionor suspension in water;

3) heating to temperatures between 115 to 150–or under pressure at 180 to 250 ◦C – depend-ing on the type of reactor;

4) dehydration of the soap by heating to 180 to200 ◦C or by pressure release;

5) crystallization during cooling to 150 to130 ◦C combinedwith addition of more baseoil;

6) addition of additives at temperatures below80 ◦C;

7) (pre)filtration and homogenization with oneof several possible devices;

8) adjustment of the specified consistency orflow pressure;

9) (end)filtration and deaeration; and10) packing into containers

Different kinds of reactor or reactor combi-nations are used (Fig. 57). Single reactors areusually open vessels heated by gas, steam, orheating oil. Mixing is achieved with single ac-tion, double action, or counterrotating stirrers,usually designed as planetary stirrers, althoughturbines or dispenser disks can also be used.


Scrapers are always necessary, because of theneed to move crystallizing material away fromthe walls to prevent a drop in heat transfer anddegradation of the forming product. Sometimesthe vessels can be used for evaporation or usedunder pressure. Since 1890, all in all the de-sign has not changed and systematic optimiza-tion has not been reported [426]. During the re-action stage open reactors must not be heatedmuch above 120 ◦C, because of the possibilityof excessive foaming. Sometimes defoamers, forexample low-viscosity dimethylsilicone oils, areadded in small quantities during this stage.

Double or multiple reactors, the first beingonly a melting vessel, a Stratco contactor, or anautoclave – the last type of reactor can also beheated inductively – have some advantages incomparison with single and open reactors. Theneutralization reaction is controlled more easilyand the reaction time is shorter when the dehy-dration process is optimized [427]. The secondvessel is either a finishing or a diluting vessel ifseveral different grades are to be produced fromone base grease. In each instance themain inten-tion is to put the maximum thermal andmechan-ical energy into the reactingmaterial in themini-mum time.One further advantage of themultiplereactor process is that the first reactor never getscontaminated with additives. A third reactor isusually used for black, colored, orwhite greases.

When base oils such as silicone oils used forthe in-situ production of soap greases, the thick-eners are sometimes formed and dispersed in thepresence of solvents which are subsequently re-moved by extraction or evaporation [428].

Continuous Production. Technology forthe continuous manufacture of greases was firstdescribed in 1969 [429]. In principle a produc-tion unit consists of three elements only:

1) A tubular reactor – in which measured quan-tities of the base grease components are in-tensely mixed at pressures up to 2MPa andtemperatures up to 210 ◦C

2) A vacuum chamber in which dehydration ofthe product takes place

3) Afinishing segment – usually a staticmixer –in which oil and additives are added and ho-mogenization takes place

The advantages of such a unit are its com-pact size (a few hundred liters only), its minimal

energy consumption, and the uniformity of theproduct; the disadvantages are the difficulty ofproducing different ranges of products and thecapacity of such units – far bigger than the needfor common greases nowadays.

The finishing equipment comprises suitablepumps, filtration units, homogenization units(mills are still common although high-pressureinjection homogenizers are preferable), deaera-tion units (vacuum pumps), and filling deviceswith geometries suitable for the types of con-tainers to be filled – in a typical grease planttheir range is usually from 400-g cartridges tocontainers holding 1 t.

14.6.2. Oligourea Greases

Oligoureas are usually prepared in a two-stepprocess. In the first step the diisocyanate andthe amine are separately dissolved or dispersedin appropriate amounts of the base oil. In thesecond step one of the two solutions or disper-sions – usually that containing the diisocyanate –is added to the other portion [430]. The reac-tion is rather exothermic and, because of thetoxicity of diisocyanates, the vessels must besealed against the atmosphere during this stepand subsequent heating. Because of the suddenand enormous increase in consistency very pow-erful stirring devices are necessary. To completethe reaction and for optimization of the thick-ener structure the greases are heated to 180 ◦C.Oligourea greases made in this way are prefer-ably homogenized with kneaders or high-pres-sure injection homogenizers.

14.6.3. Gel Greases

For production of gel greases the raw materialsare warmed to 60 to 80 ◦C only, to acceleratethe activation and improve the gelation process.This is usually conducted in the same type of sin-gle reactor used for the one-step production ofmetallic soap-based greases. The consistency re-quired of the product can best, and inmany casesonly, be achieved with the aid of high-pressureinjection homogenizers or rotor – stator colloidmills [431], or, for polyester-based greases, withannular gear colloid mills [432].


Figure 57. Reactors for the manufacture of greases

Table 45. Twenty-four properties of greases.

Twelve phenomena Twenty-four properties

High temperature Maximum thermal stabilityMinimum evaporation lossMaximum viscosity

Low temperature No (regular) crystallizationMinimum viscosity

Aging Maximum oxidation resistanceResistance to changes instructure

Compatibility No reaction with nonferrousmetalsMaximum corrosion inhibitionMaximum polymercompatibilityImmiscibility with foreignliquidsDeflection of foreign solidmatter

Oil loss Optimum oil lossa

Toxicity No toxicityBiodegradability

Tackiness Optimum tackinessa

Flowability Optimum relaxationa

Maximum pumpabilityLoad Optimum elasticitya

Maximum lubricating filmthicknessMaximum emergency runningproperties

Shear Maximum mechanical stability,orOptimum relaxation timea

Friction Minimum, or optimum frictiona

Wear Minimum wear.

a According to application.

Table 46. Important ASTM test methods for lubricating greases.

ASTM D 0128-95 AnalysisASTM D 1092-93 Apparent viscosityASTM D 1263-94 Leakage tendencies of

automotive wheel bearinggreases

ASTM D 1264-96 Water washout characteristicsASTM D 1478-91 Low-temperature torque of ball

bearing greasesASTM D 1742-88 Oil separation during storage

(air pressure method)ASTM D 1743-94 Corrosion preventive propertiesASTM D 1831-88 Roll stabilityASTM D 2509-93 Load carrying capacity, Timken

methodASTM D 3337-94 Life and torque in small

bearingsASTM D 3527-95 Life performance of automotive

wheel bearing greasesASTM D 4049-86 Resistance to water spray

14.7. Grease Rheology (see Chap. 3)

Because of their complex rheological properties[433] greases have been described as both solidand liquid or as viscoelastic plastic solids [434].Beyond the yield point, where flow occurs understress, greases have an apparent viscosity whichdepends on shear rate, temperature, and sheartime, and, to some extent, even on the mechani-cal pretreatment. This can be explained in simpleterms, as, for example, in the NLGI LubricatingGrease Guide [435]. However, when the mathe-matical background of plastic flow according to


Bingham, pseudoplastic flow according to Ost-wald, and liquid flow according to Newton istaken into account the subject [436], [437], avery sophisticated treatment is necessary [438].Controlled stress rheometers seem to be the bestsuited devices for a better insight into greaserheology today [439] but will maybe be assistedby velocity imaging nNMR spectrometers in thenear future [440].

It cannot surprise that the thickener has someinfluence on the rheological properties of agrease [441]. Of all the thickeners compared va-rieties of highly dispersed silica seem to respondbest rheologically to both low and high temper-atures [442]. For a series of military greases aquasi linear relationship was observed betweenapparent viscosity and low-temperature torque[443]. With perfluorinated polyethers thickenedwith PTFE it was possible to show correlationbetween rheological and lubrication properties[444].

New attempts to gain deeper insight into thedynamic behavior of greases include the exami-nation of possible chaotic states [445], the obser-vation of temperature-related changes with theaid of refractive index measurements [446], andthe consideration of rheological wear [447] orthe change of the storage modulus of a greaseunder stress [448].

14.8. Performance

The performance of a grease is, again, proba-bly best judged in terms of the thickener [449].From a more theoretical view there are twelvemain phenomena which must be considered. Ofthese twelve, all but two – tackiness and toxic-ity – are related either to pressure or to temper-ature. The action or effect of pressure or tem-perature result in 24 requirements (Table 45). Itis out of question for a real grease to satisfy allthese requirements. The suitability of a greasefor both high- and low-temperature applicationsis, for example, difficult to achieve, although thelower and upper temperature limits of a greaseexceed those of its base oil, by analogy withRaoult’s law. This means that the thickener actsas an impurity that, on the one hand, hinderscrystallization and thus lowers the pour point ofthe oil yet, on the other hand, reduces its vaporpressure. The effect can be increased by use of

the kinds of polymer employed as pour pointdepressants and viscosity index improvers in lu-bricating oils. The temperature-dependence ofthe apparent viscosity of a grease – at constantshear rates – is less pronounced than that of itsbase oil.

Table 47. Important DIN (ASTM) test methods for lubricatinggreases.

DIN 51350-4 (ASTM D 2596-97)Testing in the Shell four-ball tester, determination of the weldingload of consistent lubricantsDIN 51350-5 (ASTM D 2266-91)Testing in the Shell four-ball tester, determination of the wearparameters of consistent lubricantsDIN 51801 (ASTM D 566-97), replaced by DIN ISO

2176Determination of dropping point of greasesDIN 51802Testing of roller bearing greases with regard to theircorrosion-inhibiting properties, SKF-Emcor methodDIN 51804-1 (ASTM D 217-97), replaced by DIN ISO

2137Determination of cone penetration of greases with hollow coneand solid coneDIN 51804-2 (ASTM D 1403-97), replaced by DIN ISO

2137Determination of cone penetration of greases with one-quarterconeDIN 51805Determination of yield pressure of lubricating greases,Kesternich-methodDIN 51807-1Test for the behavior of greases in the presence of water, statictestDIN 51808 (ASTM D 942-90)Determination of oxidation stability of greases, oxygen methodDIN 51810Determination of flow behaviour of greases in the rotaryviscometerDIN 51811 (ASTM D 4048-86)Testing of corrosive effects of greases on copper, copper strip testDIN 51817Determination of oil separation from greases under staticconditionsDIN 51821-2Test using the FAG roller bearing grease testing apparatus FE 9

14.8.1. Test Methods

Many test methods are used today; all are meantto judge the single or combined and more or lesscomplex properties of greases. The most impor-tant ASTM and DIN methods are described inTables 46 and 47. Complete collections are pub-lished regularly [451], [452], the same is true forthe FrenchAFNOR, the English IP, the JapaneseJIS, and some other national collections. A de-scription of RussianGOST and former East Ger-man TGL methods is given in [450]. The devel-


opment of international standards (EN and ISO)is slowly proceeding.

The methods themselves are described inChapter 16.

14.8.2. Analytical Methods

Elemental analysis of greases is nowadays per-formed by spectroscopic methods, e.g., X-ray fluorescence spectrometry (XRF), induc-tively coupled plasma atomic emission (ICP),or atomic absorption spectrometry (AAS) [453],with attention being directed mostly to methodsof preparation [454], [455]. IR spectroscopywasintroduced as ameans of identifying greases andtheir components ca 40 years ago [456–458].Its use has steadily grown since and been ex-tended to questions of structure, development,and manufacture also [459]. NMR has also beenused to investigate structural questions [460] andelectron microscopy has enabled not only thescrutiny of soap fibers [461] but also study ofthermal changes in greases [462].

The use of thermogravimetry (TG) is usuallylimited to investigations of base oils [463], butthe other thermoanalytical method, differentialscanning calorimetry (DSC) [464], [465], or acombination of both, is expected to become avaluable tool in the analysis of grease and an-tioxidant analysis.

Chromatographic methods, e.g., gas (GC)and high-pressure liquid chromatography(HPLC), are mainly used to identify the compo-nents of liquid or liquefied grease [466], [467].

14.9. Applications

The selection of a grease for a particular ap-plication is always a compromise between thedemands of a customer and the circumstancesthe grease must face during its operational life –temperature, speed, load including centrifugalforces and vibrations, and relubrication inter-vals.

14.9.1. Roller Bearings

The servie life of roller bearings is connectedwith that of the grease used, especially under ex-

treme conditions [468]. Sophisticated test equip-ment has been developed, mainly to ensure bet-ter lifetimepredictability for the selectedbearingtogether with the selected grease [469], [470].The test rigs SKF R2F and later FAG FE 9 havefound great acceptance inGermany [471], [472].

Clean production and low-noise greases aremeeting reliability and lifetime requirements ofroller bearings. The stringent requirements ofhigh-precision bearings, for small bearings invideo and audio applications, and bearings formilitary use have been established for years[473], [474].

In general greases used in roller bearingssuch as ball-, deep groove ball-, thrust ball-,spherical-, taper-, cylindrical- or needle rollerbearings must have good working stability. Thiscan be checked by the prolonged penetration testand the Shell-roller test (ASTM 1831) [475].Conventional lithium greases in the NLGI class2 are recommended for most types of bearingat working temperatures up to 120 ◦C. Greasesof the NLGI class 1 are preferred for needlebearings. For bearings exposed to temperaturesabove 120 ◦C complex soap or polyurea thick-ened greases are preferable. For bearings thatmust operate under high load and/or low speedthe base oil viscosity must be 200mm/s2 mini-mum at 40 ◦C. Greases for cold climates or foraerospace or military use have to ensure perfor-mancedown tobelow−70 ◦C.Low-temperatureperformance can be checked with the low-tem-perature torque (ASTM 1263), the low-temper-ature penetration (AFNOR NF T 60-171) andthe flow pressure (DIN 51805) tests [476–478].Greases of that kind need base oils with suffi-ciently low pour points. In many military ap-plications long-life properties are also required;these can be fulfilled by use of synthetic baseoils only [479]

Because plain bearings are often exposedto moisture or water, calcium soap-thickenedgreases are recommended. When open housingsare used in a dusty atmosphere frequent relubri-cation makes it possible to wash out the contam-inated grease.

Relubrication Intervals. The lubricationinterval tf is based on the F10 value of a stan-dard grease as agreed in DIN 51 825 [480] undernormal environmental conditions with temper-atures up to 70 ◦C and a mean bearing load of


P/C < 0.1. Figure 58 and Table 48 takes intoconsideration the type of bearing and the speed.For each 15K temperature increase the relubri-cation interval is said to be reduced by 50%.Severe working conditions reduce the lubrica-tion interval tf to a reduced lubrication intervaltfq.

tfq = f 1 × f 2 × f 3 × f 4 × f 5 × tf (16.1)

Table 48. Relationship between bearing type and correction factorsfor relubrication intervals (GFT Worksheet 3)

Bearing type kf

Deep grove ball bearings single row 0.9−1.1double row 1.5

Angular contact ballbearings

single row 1.6

double row 2Spindle bearings α= 15 ◦ 0.75

α= 25 ◦ 0.9Four point bearings 1.6Self-aligning ballbearings

1.3−1.6

Thrust ball bearings 5−6Angular contact thrustball bearings

double row 1.4

Cylinder roller bearings single row 3−3.5a

double row 3.5full complement 25

Cylindrical roller thrustbearings

90

Needle roller bearings 3.5Tapered roller bearings 4Barrel roller bearings 10Spherical roller bearingswithout lips (E-design)

7−9

Spherical roller bearingswith center lip

9−12

a kf = 2 for radial load or increasing thrust load and kf = 3 forconstant thrust load.

In relubrication it is usually impossible to re-move the used grease. Consequently the relubri-cation interval tfq must to be reduced by 30 to50%.

14.9.2. Cars, Trucks, Construction Vehicles

Most modern cars do not need relubrication,with the exception of door hinges, lock mecha-nisms, and battery poles. But among the approx-imately 30 hidden greases (Fig. 59) in a moderncar [481], [482] only the constant-velocity (CV)joint greases are required in substantial quan-tities. Although improved conventional lithium

greases containing molybdenum disulfide arestill in use [483], lithium complex or polyureagreases are already preferred in some moderncars and this usage will increase in the future[484]. Most of the greases used in cars, forexample the greases for CV joints, hub units,starters, alternators, seat adjustments, clutchesrelease bearings, belt – pulley bearings, windowlevers and windshield wiper gears, are specifiedand approved by the large motor companies anddeveloped in close cooperation with the greasemanufacturers. For the same application, how-ever, different motor companies have differentgrease specifications and approvals; for exam-ple, most European and U.S. motor companiesprefer lithiumcomplexgreases in the frontwheelbearings, whereas Japanese manufacturers pre-fer polyurea greases.

ASTMD 4950 (see below) describe the min-imum requirements of current greases in auto-motive service – fill applications for passengercars, trucks, and other vehicles operating undervarious service conditions [485].

Chassis grease classifications – intended useof chassis (L) classified greases:

LA ClassificationChassis components and universal joints un-der mild duty– Frequent relubrication– Noncritical applicationsLB ClassificationChassis components and universal joints un-der mild to severe duty– Prolonged relubrication intervals– High loads– Severe vibration– Exposure to water or other contaminants

Intended use of wheel bearing (G) classifiedgreases:

GA ClassificationService typical of wheel bearings operatingunder mild duty– Frequent relubrication– Non-critical applicationsGB ClassificationService typical of wheel bearings operatingunder mild duty/moderate duty– Normal urban, highway and off-highwayservice

GC Classification


Figure 58. Re-lubrication intervals

Figure 59. Greases hidden in a modern car

Service typical of wheel bearings operatingunder mild duty/severe duty– High bearing temperatures– Frequent stop and go service (buses, taxis,police)

– Severe braking service (trailer towing,heavy towing, mountain driving)

Lithium-basedmultipurpose greases have re-placed several other greases for the relubri-

cation of trucks and construction equipment.Conventional lithium soap based greases thatrequire frequent relubrication are still in usefor the wheel bearings of trucks and trailers.Modern trucks and trailers with prolonged oil-drain intervals require lithium complex greaseswith semisynthetic or fully synthetic base oils.Lithium greases containing black solid lubri-cants are recommended for 5th-wheel applica-


tions, for chassis points, and for plain bearings ofconstruction equipment. Many trucks and busesand much construction equipment uses central-ized lubricating systems, designed for semifluidgreases of NLGI-class 00 or 000, for onboardrelubrication. Other systems require greases ofNLGI class 2. Lithium greases optimized forlow temperature applications, good pumpabil-ity, and low oil separation are recommended.

14.9.3. Steel Mills

In Europe calcium complex and sometimespolyurea greases based on mineral oil are usedfor lubricating continuous casting equipment.The U.S. market prefers aluminum complex andpolyurea greases and Japanese equipmentmanu-facturers mainly recommend polyurea greases.Some roller bearingmanufacturers equip contin-uous castings with double- or triple-sealed bear-ings that are greased for life, preferablywith syn-thetic polyurea greases. When the sealing of thebearings is not perfect relubrication has a clean-ing function. Contaminated grease that could re-sult in limited lifetime is squeezed out.

Conventional EP lithium greases, calciumcomplex greases, calcium sulfonate complexgreases, lithiumcomplex greases, and aluminumcomplex greases, all based on mineral oilsare used in hot rolling equipment. Most cus-tomers require a minimum base oil viscosity of200mm/s2 at 40 ◦C [486]. The rotating shift sys-tem is typically used for hot-rolling lubrication.In general the bearings are not regreased duringoperation, only during maintenance.

Many different greases, e.g., conventional EPlithium greases, lithium complex greases, cal-cium complex greases, and calcium sulfonatecomplex greases, all based on mineral oil, areused in cold rolling equipment. Modern greasesuse a lithium – calciummixed soap for improvedwater resistance.

14.9.4. Mining

Open pit mines which use wheel bucket excava-tors andbelt systems to ship the spoil and coal areespecially large grease consumers. Lithium orlithium – calciummixed-soap-based greasewithbase oil viscosities> 350mm/s2 at 40 ◦C and an

effective EP and AW additive package are used.Black solid lubricants are also recommended.Because the equipment is exposed to dust or wa-ter and mud, the sealing efficiency of the bear-ings determines their lifetime and must be sup-ported by a grease.

Kilometers of wire rope are used on the ex-cavators or drag lines and in the undergroundmines. During production of the wire ropes lu-bricants are applied to ensure corrosion protec-tion and to minimize friction of the single wireswhen the rope stretches under load.

Lay-up lubricants based on wax resin are ap-plied during manufacture to ensure lubricationof the individual wires and stands.

During operation in some countries the wireropes aremaintained by cleaning the surface andrelubrication. In many countries bitumen-basedproducts are still in use, but are being replacedby bitumen-free greases or even biodegradablelubricants.

Winch greases, preferable lithium complexgreases with improved water-resistance, ensureprolonged winch lifetimes up to several years.

14.9.5. Railroad, Railway

Depending on the design of the driving systemof the locomotive a gear oil or a gear grease is re-quired. The poor sealing properties of these kindof gears leads usually to leakage; this is usuallyminimized by use of bitumen-based products.Modern greases are thickened with lithium orsodium soap and based on mineral oils of upto 2000mm/s2 at 40 ◦C. Traction motor geargreases are not described in terms of consis-tency; the apparent viscosity is usually checkedwith a Brookfield viscometer, for example witha number 3 spindle at 93 ◦C and at 4 rpm andgiving results of 5000 to 10 000mPa · s [487].

The axle bearings are greased with conven-tional lithium EP greases. The development ofready-to-build in axle boxes and the increasingspeed of modern trains have led to improvementof the high-temperature performance and life-time of greases.

Switch lubrication, wheel flange lubrication(mainly used in Europe), and rail track-side lu-brication in curves, used mainly in the USAand Canada, cause environmental problems. Forthese applications biodegradable greases based


on esters have better wear-protection and con-sumption performance as conventional greases[488–490]. These greases must be sprayable be-cause of the means of application.

14.9.6. Gears

Fibrous sodium soap greases of NLGI-class 0,00, or even 000 consistency are used. Althoughthe load-carrying capacity, antiwear properties,and adhesion of these greases is especially good,lithium-based greases and greases based on syn-thetic hydrocarbons or polyglycols are gainingin importance

Girth gear drives are a type of drive widelyused for many large systems in the primaryindustries [491]. These open gears require anadhesive grease based on high-viscosity oils.Such greases are usually sprayed on the teethby means of a centralized lubricating system.

14.9.7. Food-Grade Applications

Greases for lubricating machines used in foodprocessing must fulfil specific requirements re-lating to food legislation, human health protec-tion, taste, and odor. These properties must beapproved by internationally accepted tests andorganizations [492].

The FDA lists ingredients permitted for usein food-grade grease formulations (21 CFR§178.3570); white oils are listed in 21 CFR§178.3620. The USDA (United States Depart-ment of Agriculture) has approved the finishedproducts. For white and synthetic oil-basedproducts maximum contamination of 0.1 g/kgfood is allowed. Since USDA stopped ist activ-ities in 1999 a working group of ELGI, NLGIand the European Hygienic Equipment DesignGroup ( EHEDG) was founded with the aim toinstall an ISOStandard for foodgrade lubricants.

14.9.8. Textile Machines

Because greases could contaminate textiles, theyare formulated with white oils and thickenedwith water-soluble sodium soaps, and are thusremovable by washing.

14.9.9. Applications with PolymericMaterials

There are three ways in which greases and poly-mericmaterials can come into contact: polymerscan be contained in greases as thickeners, assolid lubricants, and as additives; they can act assealants; and they can form one or even both ofthe solid partners in friction couples. Althoughbase oils are the primary consideration [493],when the interaction of greases with sealants isimportant [494], [495] – the influence of addi-tives, and among these especially EP additives,must always be considered [496]. Even migra-tion of soap molecules into a polymer can occur[497].

An early test method for the action of lubri-cants on polymeric materials was the pendulumaccording to Barker [498]. During the automa-tion of this process a more meaningful frictionand wear tester was developed [499], [500]. Itproved also to be necessary to test the tendencyof a polymer to form cracks under the influenceof its lubricant and tension [501].

The lubrication of polymers or polymers andmetals as friction couples is one field in whichsilicone greases and, better, fluorinated siliconegreases, or, even better, greases based on per-fluoropolyalkyl ethers are of some advantage.Although silicone greases made with highly dis-persed silicic acid should be used as general lu-bricants and as sealants [502], silicone greasesmade with the usual thickeners are sometimespreferable to other synthetic greases. With cal-cium stearate or 12-hydroxystearate, for ex-ample, they are permitted in food applications[503].

14.10. Ecology and the Environment

Pre-industrial greases consisted of tallow orvegetable oils and their reaction products withlime; they were therefore, not merely ecolog-ically compatible lubricants, but even rapidlybiodegradable. When mineral oils took over,most of the thickeners and some of the ad-ditives remained ecologically acceptable. Theecological and environmental aspects of mod-ern base oils have been described in detail inChapter 7. Of all the vegetable oils that could


be chosen as base oils for greases, in CentralEurope only rape seed oil has been introducedon a technical scale [504] and only for loss-lubrication applications. Initially calcium 12-hydroxystearate was chosen as the most suitablethickener [505]. Greases of that kind proved su-perior to lithium- andmineral oil-based general-purpose grades (Fig. 60). The limitations of veg-etable oil-based greases soon led to the intro-duction of transesterified vegetable oils [506]and synthetic esters (see Chap. 5). Because ofindustrial demands greases based on calcium12-hydroxystearate were soon replaced by lithiumand lithium – calcium greases [507–509]. Poly-urea greases [510], aluminum complex greases[511], and titanium complex greases [512] fol-lowed.

Figure 60. Friction – load behavior of rapeseed oil-basedcalcium 12-hydroxystearate greases

The biodegradability of greases essentiallyreflects the biodegradability of their base oils.In round robin tests greases based on rape seedoil and calcium 12-hydroxystearate achievedbiodegradability close to 100%. Methods en-abling the use of emulsifiers led to significantlylower results [513].

The elimination of toxic components such aspolycyclic hydrocarbons from base oils is al-most complete. This is also true for chlorinesubstitution. The substitution of antimony andlead cations and nitrite anions in additives is notyet complete, because it is not easy to main-tain the same performance level. In Germany the

demand for environmentally harmless products[514] became an additional aspect of the per-formance of greases; changes in the demandsfor water pollution class (WGK) 0, that cameinto force in 1999 aggravated the situation. Themost suitable antioxidants are only accepted inconcentrations which are too low, hydrocarbonsincluding polyalphaolefins, white oils, and low-viscosity polybutenes are not accepted and themost suitable soaps are also excluded.

Although experiences with biodegradablegreases are promising [515], [516], some prob-lems, mainly relating to legal aspects of wastemanagement [517] and the balance between eco-logical requirements and economical possibili-ties [518], remain unsolved. Therefore the thirdand possibly most important aspect of applyinggreases in a manner that is both environmentallyresponsible and economical is to minimize theamount of lubricant necessary and to maximizelifetime by using the most inert materials [519],[520].

14.11. Grease Tribology

General aspects of friction and wear [521], andof lubricating oils and their viscosity [522],have been described in detail in Chapters 2 and3. Recent tribological research on greases hasdealt with the time-dependence of their behav-ior under stress as a fourth factor in additionto temperature, pressure, and thickener prop-erties [523]. Behavior in EHL contacts, wherethe thickness of grease films can exceed thatof their base oil under comparable conditions[524], [525], and the behavior under starved con-ditions, when the load is supposed to be carriedby thickener particles deposited in the center ofthe contact [526], has also been of concern. Soapdeterioration is another important subject [527],[528].

15. Solid Lubricants

Solid lubricants are required for lubrication un-der extreme conditions where the bearing sur-faces in tribological contact must still be effec-tively separated. The life of lubricated machineparts depends on the functional and tribologicaldesign and optimization of lubricants as calcu-lable functional elements. To make a systematic


choice of a suitable lubricant it is absolutely nec-essary to understand the relationship betweenfriction,wear, and lubrication and the interactionbetween the elements of the tribological systemand the specific properties of each element.

Special attention must be given to the differ-ent stress factors and the structure of the tribo-logical system (see Chap. 2).

Products containing solid lubricants are oftenused to solve problems, particularly in boundaryand mixed frictional states when high specificloads are applied to sliding surfaces and at verylow hydrodynamically effective speeds but alsoin critical applications where, e.g., the lubricantmust perform over a wide temperature range orunder extreme temperature conditions, for ex-ample in aviation and in rocket technology.

Dry lubrication with solid lubricants is alsorequired in nuclear reactors, in high vacuum ap-plications, in aggressive environments, and inapplications where contamination by lubricat-ing oils or greases cannot be tolerated.

15.1. Classification

A solid lubricant is often defined as any solidmaterial which reduces friction and/or wear ofcontacting surfaces in relative motion. Varioussystems are used to classify the different types ofsolid lubricant. An arbitrary, but useful, classi-fication is into structural lubricants, mechanicallubricants, soaps, and chemically active lubri-cants.

15.1.1. Class 1: Structural Lubricants

The most widely used solid lubricants aregraphite and molybdenum disulfide. Their sat-isfactory lubricating properties are assumed toresult from their layered lattice structures. Inaddition to these two substances, other solidsare used, for example metal halides and sulfides,which have, in themain, inherent lubricant prop-erties, a lamellar hexagonal crystal structure, andare usually anisotropic (Table 49).

In contrast to graphite and molybdenumdisulfide these are not yet extensively used inindustry.

Table 49. Structural lubricants.

Name Formula

Graphite CGraphite fluoride (CFx)nMolybdenum disulfide MoS2

Molybdenum diselenide MoSe2Tungsten disulfide WS2

Tungsten diselenide WSe2Niobium disulfide NbS2

Niobium diselenide NbSe2Tantalum disulfide TaS2

Tantalum diselenide TaSe2Titanium disulfide TiS2

Titanium diselenide TiSe2Titanium telluride TiTe2Cerium fluoride CeF3

Barium hydroxide Ba(OH)2Cadmium chloride CdCl2Cobalt chloride CoCl2Zirconium chloride ZrCl2Lead chloride PbCl2Lead iodide PbI2Boron nitride BNSilver sulfate Ag2SO4

Borax Na2B4O7

Talc Mg3(OH)2Si2O10

Mica KAl2(Si3Al)O10(OH)2

15.1.2. Class 2: Mechanical Lubricants

There are different types of substances withinthis class whose lubricating effect is based ondifferent physical and mechanical properties orspecial conditions. These lubricants are dividedinto self-lubricating substances, substances thatneed a supporting medium to create lubricatingproperties, and substances with indirect lubri-cating properties based on their hardness.

Self-Lubricating substances can be classi-fied as organic compounds, metal films, chemi-cal surface layers, and glasses.

Organic Compounds. The most importantself-lubricating organic substances are liste be-low:

Linear polymers (thermoplastic types)Poly(tetrafluoroethylene) (PTFE)Fluoroethylene – propene (FEP) copolymersPerfluoroalkoxy polymers (PFA)Polyethylene (PE)Polypropylene (PP)Polyurethane (PU)PolyamidePolyacetals


PolyterephthalatePolysiloxanesNylonCross-linkedpolymers (thermosetting types)Phenol – formaldehyde resinUrea –melamine – formaldehyde resinEpoxy resinPhenolic resinUnsaturated polyester resin

Polyimides

The sliding characteristics of polymers de-pend on their chemical nature and the matingpartner. The permissible load is a function ofheat dissipation. Temperature changes have littleinfluence on the friction characteristics of poly-mers. Addition of MoS2, graphite, and metalpowder improves the frictional characteristicsof the polymers and increases their hardness.Graphite also increases the elasticity module ofPTFE.

Metal Films. Important self-lubricatingmetal films include:

LeadTinSilverIndiumBariumGoldAluminumNickel (Ni and Ni –Cr alloys)Copper (Cu and Cu alloys)Zinc (Zn and Zn alloys)

Friction can be reduced by the coating of thebody material surfaces with a thin film of a softmetal, because the friction depends on the shearstrength of the soft metal film. The durabilitydepends on the film hardness, homogeneity, andadhesion. The lubricating effect of soft metallayers is limited by their melting point.

Chemical Surface Layers (ConversionFilms). In addition to the naturally occurringoxide films present on the surface of most basematerials exposed to air, other solid lubricantfilms can be formed by chemical or electro-chemical action on the metal surface.

Chemical surface coatings such as zinc, iron,or manganese phosphates behave similarly tosoft metal coatings but consist not of a metal butof metal salts. Bonderization (phosphate treat-ment) creates a thin, microcrystalline, stronglyadhering phosphate layer on the metal surface;

this reduces the coefficient of friction and thedanger of seizure during the running-in period.The lubricating efficiency of the layer, which isnormally 2−5µm thick, is based on its lowershear strength in comparison with the metal.

Glasses (→Glass). The lubrication proper-ties of glass depend on the composition. The co-efficient of friction at a given temperature is afunction of the viscosity, the thermal conduc-tivity, the rate of shear, the area of shear, thecapacity to dissolve different amounts of oxidefrom the surface of the material to be lubricated,and the contact angle between the glass and thematerial, because this determines the capacity ofthe glass to wet the material.

The importance of glasses used as lubricantsis specially to be seen in metal-forming op-erations with operating temperatures up to ca.1500 ◦C.

Substances with lubricating propertiesthat need a supporting medium can be classi-fied as inorganic compounds andmetal powders.

Inorganic Compounds (Table 50). The coef-ficient of friction of natural oxide films on met-als, which are usually approx. 10 nm thick, hasbeen investigated. It is generally accepted thatthe oxide film reduces surface damage, makessliding smooth, and often reduces friction.

Table 50. Inorganic compounds needing a supporting medium

Metal sulfides ZnS, SnS2, FeS, ...Metal fluorides CaF2, LiF, ...Metal phosphates Zn2P2O7, Ca3(PO4)2,

Fe2P2O7, ...Metal hydroxides Ca(OH)2, Mg(OH)2, Zn(OH)2, ...Metal oxides PbO, ZnO, FeO, Fe2O3, ...

Sulfides, fluorides, phosphates and hydrox-ides are claimed to act as a supporting agentor a catalyst by producing friction- and wear-reducing layers. Calcium hydroxide, for exam-ple, supports the production of a layer of Fe3O4iron oxide on the rubbing surface of steel.

The process of formation of these layers de-pends on the chemical composition of steel and,in particular, on its surface chemistry.

Phosphate layers, by galvanic techniques, aremainly used as precoatings for dry-film appli-cation and as lubricant carriers in cold metal-forming processes. In addition to acting as alubricant carrier, the phosphate coating can be


plastically deformed with the steel slug and,therefore, in conjunction with the lubricant, pre-ventsmetal –metal contact and thus reduces sur-face friction and wear.

The three main types of phosphating solu-tions contain zinc, iron, and manganese phos-phates, and of these the zinc phosphate is prob-ably the most widely used.

Metal Powders. In contrast to structural lu-bricants and self-lubricating mechanical lubri-cants, the lubricating properties of the othermechanical lubricants are mainly based on thesupporting effect of a carrier substance or abinder. The main purpose of these substances,which include Pb, Sn, Zn, Cu, Ag, and In, is toimprove the adhesion and cohesion properties ofthe non-self-lubricating mechanical lubricants.

Substances with Indirect LubricatingProperties Based on their Hardness (Phys-ical Vapor Deposition, Chemical Vapor De-position, and Diamond-like Carbon Lay-ers). Vapor-deposited (VD) coatings of oxides,borides, nitrides, or silicates, for example TiN,TiNC, CrN, ZrN, and AlTiN, usually have quitehigh coefficients of friction but can preventseizure even at high temperature and offer ex-cellent wear resistance.

Another relatively new form of thin-filmlayer with proven good tribological proper-ties is diamond-like amorphous carbon (DLC)(→Thin Films, Chap. 2.3.). Hardness valuesfor DLC range from 500 to 13 000HV de-pending on the amount of hydrogenation, from50% to zero. In addition to hard coatings thereare also soft physical vapor-deposition coat-ings containingMoS2 orMoS2 –metal compos-ites. These films, normally deposited by varioussputtering technologies like ion-beam-assisteddeposition (IBAD) or closed-field-magnetronsputtering (CFMS), afford excellent tribologi-cal properties in terms of wear-resistance andlow coefficient of friction. They are used for cut-ting and forming tools, ball bearings, and slidingcontacts with steel and ceramics.

15.1.3. Class 3: Soaps

Soaps often give the lowest coefficients of fric-tion obtainable with solid lubricants but in gen-

eral cannot be used above their melting pointsor at high loads.

The main function of soaps in lubricationtechnology is in the preparation of greases (seeChap. 14). The main use of soaps as lubricantsin their own right depends on their formation insitu on a metal surface, by the chemical attackof a fatty acid on the metal.

15.1.4. Class 4: Chemically ActiveLubricants

This category includes EP andAWadditives anda variety of chemicals which interact with themetal surface to produce a lubricating or pro-tecting layer (see Chap. 6).

15.2. Characteristics

15.2.1. Crystal Structures of Lamellar SolidLubricants

Graphite (→Carbon, Chap. 3.1.). In thehexagonal graphite structure each carbon atomis linked to its nearest neighbors by sp2 typeσ bonds. The strength of the latter is increasedabove the normal single covalent C –C bond bythe interaction of non-localized electrons asso-ciated with the 2pz orbital of the planar car-bon atoms. It is this extra bonding which givesgraphite its thermal stability. The planes them-selves are held together largely by van derWaalsforces which are much weaker than the planarbonding forces and the strength of bonding to asubstrate.

The low friction of graphite is not basedsolely on its crystal structure. It also dependson adsorbed films, particularly of water vapor,which provide surfaceswith lowcohesion. Thus,graphite develops its most favorable lubricatingproperties in the presence of moisture.

Molybdenum Disulfide (→MolybdenumandMolybdenumCompounds, Chap. 7.3.). Thecrystal of molybdenum disulfide has a hexago-nal structure with trigonal symmetry. Each Moatom is surrounded by a trigonal prism of sulfuratoms.

The lubricating effect of MoS2 is also basedon its crystal structure. The weak bonds (van derWaals forces) between the sulfur layers enable


easy movement of the lamellae over each other,resulting in a low friction between the slidingparts. On the other hand, the ionic bond betweenMo and S gives the layers a high strength so thatthey can resist the penetration of surface asper-ities.

15.2.2. Heat Stability

Heat stability is required if thermal decompo-sition of the solid lubricants is to be avoided,especially in the presence of oxygen-, carbondioxide-, orwater-containing atmospheres. Veryoften the decomposition products are either cor-rosive vapors or abrasive solids.

When heated in air, graphite forms carbondioxide above 450 ◦C; this escapeswithout leav-ing any grinding particles behind. MoS2 is at-tacked by oxygen even at 20 ◦C. In air, appre-ciable oxidation to molybdenum trioxide occursabove 400 ◦C and effects the lubricating prop-erties and the adhesion of MoS2 to metal sur-faces. When oxidation can be excluded, MoS2retains its lubricating properties up to 800 ◦C.Lead monoxide is an excellent lubricant up to700 ◦C. Metal-free phthalocyanines have beenestablished as very efficient lubricants up to tem-peratures of 800−900 ◦C.Glass has proved to bean efficient lubricant at extreme loads and tem-peratures up to 1200 ◦C.

Other solids, for example metal oxides, sul-fides or phosphates, also afford excellent heatstability. Without a supporting liquid, however,these solids provide no lubricating properties butbuild up separating layers only. The lubricationeffect is, therefore, limited by the heat stabilityof the carrying substance.

The melting point of a solid lubricant can beregarded as a physical aspect of heat stability.

15.2.3. Thermal Conductivity

Frictional heat, developed by rubbing of mate-rial surfaces at the high spots, is also transmittedto the solid lubricant, which must be capable ofdissipating this heat as quickly as possible – orlocal welding of the material might occur, de-spite the presence of the solid. This is particu-larly true of plastic solid lubricants, which allhave poor thermal conductivity.

15.2.4. Adsorbed Films

Adsorbed film of gas or vapor might be attachedto the external surface of the solid or chemicallyto an internal surface (intercalation). Physicallyor chemically adsorbed molecules on the exter-nal surface of the solid can influence:

– The sintering of the aggregates of the parti-cles under the action of heat;

– The ease of breakdown of particles undershear stress; and

– The wear properties of the edges of lamellarsolids.

The chemisorbed molecules within the parti-cles can affect:

– The decomposition or oxidation temperatureof the solid;

– The mechanical properties of the solid; and– The conductivity (thermal and electrical) ofthe solid.

15.2.5. Chemical Stability

Chemical stability is closely linked with heatstability. When solid lubricants are to be usedintermittently over long periods in relatively in-accessible places, tendency to corrosion can beserious. Corrosion can occur in three ways:

Interaction of the solid with the metal sur-face;Action of heat and/or the atmosphere on thesolid to give decomposition products, whichthen react with the metal surface; andAction of heat and/or the atmosphere on ma-terial chemisorbed on to the solid lubricantproducingmaterial which has a corrosive ac-tion on the metal.

15.2.6. Particle Size

The smaller a nonporous particle, the more ac-tive it is. Although such particle activity is re-quired in someapplications there are otherfields,for examplemanymetal-forming operations, forwhich it is desirable to have a relatively large par-ticle size range to give the required film proper-ties. With graphite and molybdenum disulfide,relatively coarse particle sizes are advantageousbecause this results in optimum resistance to ox-idation.


15.3. Products Containing SolidLubricants

15.3.1. Powders

To ensure that solid lubricants in the form ofpowder give sufficient coverage in a tribologicalsystem.

The following requirements must be fulfilled

1) The level of adhesion between the lubricantfilm and the surface of the material must begreat enough to ensure that this lubricant filmadheres to this surface when it is subjectedto friction.

2) The internal cohesion of the film must besufficiently large that the film does not splitwhen subjected to friction.

3) The adhesion between the particles and lay-ers in the shearing direction should be assmall as possible to keep the resistance tofriction low.

These main requirements can be met onlyby self-lubricating dry lubricants especially byMoS2 lattice; it is, as a result, the most com-monly used. Other solid lubricants which areapplied in powder form are organic compoundssuch as PTFE and graphite, although these fulfilthe requirements listed above to a limited extentonly.

Before the solid lubricant powder can be ap-plied, the surface of the material must be thor-oughly cleaned. Roughing the surface mechani-cally or with phosphates improves adhesion andtherefore the lifetime. The lubricant can be ef-fectively applied by simple rubbing with cloths,sponges, brushes, or polishing pads or polishingbuffs, by applying it using suitable carriers, andcoating by cathode evaporation in an ultra-highvacuum.

Solid Lubricants in Carrying Media.Most substances used as solid lubricants requirea carrying medium, a bonding agent and/or pre-treatment of the material surface, to help create,or improve, their adhesive and film-creatingproperties. The substances used as bondingagents are: organic binding agents, resins; inor-ganic binding agents, silicates; and oils, greases,water.

It is also advantageous to pretreat the sur-faces by: degreasing; sandblasting, corroding,

etching; phosphating, anodizing; and activating(e.g., low-pressure plasma).

15.3.2. Dispersions and Suspensions

Dispersions and suspensions in carrying liquidswith low volatility are mainly used in areas inwhich, for tribological reasons, a dry lubricat-ing film should be created, but where effectiveapplication of a powder is not possible for tech-nical reasons. The same types of solid lubricantare used here in the same way as for powders.

Dispersions and suspensions of solid lubri-cants in water are usually used to coat masselements for cold and hot forming. The mostcommonly used substances here are salts, spe-cial white solid lubricants, and graphite.

Dispersions and suspensions in oils also actas aids in forming techniques, and they are alsoused as additions in gear- and oil-lubricating sys-tems. The solid lubricants used here in form-ing techniques are the same types as those usedas dispersions and suspensions in water. MoS2plays a dominant role as an additive to lubricat-ing gears and for general use in oil lubricatingsystems.

15.3.3. Greases and Grease Pastes

Addition of solid lubricants to greases is pri-marily intended to have a positive effect on theircapacity to absorb pressure, and the ability towithstand wear and tear, and friction. The spe-cific advantages of solid lubricants in compar-ison with oil-soluble, chemically reactive addi-tives is that they react neutrally to many typesof plastic and elastomer, perform well at hightemperatures and have good safety reserves withregard to emergency running properties, whichcome into action when the layer of grease col-lapses. The use of black solid lubricants suchas graphite and MoS2 also have a beneficial ef-fect on running-in processes and the capacity towithstand wear and tear in boundary and mixedfriction areas. Where oscillating movements orvibration is involved, white solid lubricants havethe advantage. These effects are illustrated inFigs. 61 and 62.

Solid lubricants in greases are used at concen-trations of 1−3%.When this proportion rises to


Figure 61. Lithium grease with molybdenum disulfide, showing the irregular pattern of the wear profile, the chipping of thematerial, and serious wear. The maximum wear depth exceeds 13µm

over 10% they are known as grease pastes, be-cause the solid lubricant at this concentration hasa noticeable thickening effect.

High-performance lubricating greases usu-ally contain combinations of two or more sub-stances, which have a synergistic effect. This ap-plies both to the combination of graphite andMoS2 and to mixtures of different inorganicsolid lubricating substances, for example cal-cium hydroxide, zinc phosphate, or iron sulfide.

15.3.4. Pastes

Pastes are solid lubricants in a carrying oil. Toachieve the correct consistency, the proportionof solid lubricant must be at least 40%. To re-duce the undesirable ‘bleeding’ effect, modernformulations often include a small amount ofsoap.

Black pastes usually contain graphite orMoS2 and combinations of both. Because oftheir high load-bearing capacity, these pastes areparticularly useful when movement with very

slow relative speeds is involved, e.g., assemblyprocesses, and running-in processes.

Pastes prepared from combinations of whitesolid lubricants are especially suitable for usewhere oscillating movements or vibration is in-volved. White pastes are particularly good atpreventing the wear and tear which results fromvibration and are proven to afford excellent pro-tection against fretting corrosion.

Another area where pastes are traditionallyused is for lubrication of screw connections.Special screwsmade of steelswhich are resistantto high temperatures, based on chrome/nickel al-loys, are prone to seizure because an oxide layeris not created.

The pastes used in this particular applicationarea are usually made up of combinations ofsolid lubricants and specialmetal powders.Morerecent developments show that the use of for-mulations which do not contain metal, and arebased on white solid lubricants, perform wellwhen used at high temperatures.


Figure 62. Lithium grease with white solid lubricants showing the smooth pattern of the wear profile. The maximum weardepth is only 5.9µm, the wear pattern is uniform, and there is no chipping of material. Scanning electron photomicrographs(50×magnification).

15.3.5. Dry-Film Lubricants

Lubricating varnishes or dry-film lubricants aresuspensions of solid lubricants and other addi-tives in a solution of inorganic or organic bindingagents. The most commonly used solid lubri-cants are MoS2, graphite and PTFE. The mostimportant additives are pigments which protectagainst corrosion. The main types of bindingagent used are organic resins (acrylic, phenol,epoxy, silicone, urethane), cellulose, and inor-ganic silicates and phosphates. Hydrocarbons orwater are used as solvents. Dry-film lubricantscan be used in a variety of ways; these dependprimarily on the number, shape, and/or partic-ular requirements with regard to partial coat-ing. They are applied by dipping centrifuges anddrums and by various types of spraying pro-cedure. The hardening process depends on thetype of binding system and happens at ambi-ent temperature or in an enameling stove. Afterthey have hardened lubricating varnishes form ahighly adhesive, dry film of lubricant. The pro-

portion of solid lubricants in the layer created inthis way can be as high as 70%.

Solid lubricantswith a layer structure (MoS2)have a floating effect in the wet film, wherebythe layers arrange themselves horizontally as thefilm dries and settle on top of each other in indi-vidual layers. This is how a separating layer iscreated between the base unit and the opposingunit of the tribological system. In an ideal situ-ation this layer is between 10 and 15µm thick.When placed under pressure, the texture of thislayer becomes compressed and creates an ex-tremely smooth, shiny film surface. PTFE lu-bricating varnishes are usually applied in muchthinner layers, e.g., 3 to 5µm thick.

The performance of dry-film lubricants de-pends on:

1) Their composition, in particular the type ofbinding agent, the ratio of solid lubricants tothe binding agent, the types of solid lubri-cants, and the film thickness;

2) The properties of the substrate to whichthe lubricating varnish is applied, in par-


ticular the cleanliness, roughness (porosity;Fig. 63), and hardness of the surface; and

3) Environmental (local) conditions, in particu-lar the demands, placed on the lubricant (e.g.,load), the type and speed of movement, thetemperature, atmospheric pollution (humid-ity, dust, radiation), atmospheric pressure,and the surrounding media (gases, liquids).

The pretreatment of the substrate surfaceplays a particularly important role in the suc-cessful use of lubricating varnishes – as alreadymentioned, the roughness of the surface andthe way in which the crystallites are arrangedhave a considerable impact on their lifetime.Various pretreatment procedures can be used toachieve the best possible surface. Sandblasting,phosphating, anodizing, atramentizing, etching,or corroding are the most suitable for metallicsubstrates. Plastic and elastomer surfaces canbe pretreated by chemical and mechanical pro-cesses and by energetic activation procedures.

15.4. Industrial Uses of ProductsContaining Solid Lubricants

Because of the continuously increasing demandsput on modern lubricating systems, almost allformulations contain a certain proportion ofsolid lubricants. Significant fields of applicationare the lubrication of machine elements and inprocess auxiliaries for various production tech-nologies such as, for example, the production ofglass or metalworking.

Solid lubricants play a significant role inmet-alworking, especially in forming. Their maintask is to generate an effective separating layerbetween the tool and the surface of the work-piece and thus act against the adhesion forcesbetween the materials. Solid lubricants, in com-bination with their carrier medium and other ad-ditives, also reduce friction or generate a definedcoefficient of friction, and keep the process tem-perature constant. Through their wear protec-tion function they increase the service life oftools, protect against corrosion, ensure a stipu-lated surface quality and the accuracy to gage ofthe workpieces.

Solid lubricants are used in the area of non-cutting forming, especially cold and hot forging,pressure die-casting and drawing, and cold ex-trusion operations.

15.4.1. Screw Lubrication

The machine element most used is the screw.The selection of the most suitable screw lubri-cant depends upon the material or the surfaceof the screw and the thread partner, and on theconditions of use, for example temperature, hu-midity, and the influence of chemicals, radiation,and the constructive requirement according toa stipulated coefficient of friction or minimumpretensioning force, even after the screws havebeen tightened or loosened many times.

Special high-temperature pastes are used forthe lubrication of screw connections at highworking temperatures. These pastes comprise abasic oil which is mixed with different solid lu-bricants and can contain additional additives. Informulation or selection care must be taken toensure that the components do not contain sul-fur, zinc, lead, and aluminum, which can leadto damaging changes to the screw material, e.g.,stress crack corrosion at high temperatures.Theyshould also be free of health-hazardous nickel.

Particularly effective in high-temperature ap-plications are greases and pastes combined withwhite solid lubricant prepared from metal hy-droxides, phosphates, and sulfides.

Formany screw lubrication applications, spe-cial dry-film lubricants are also available as analternative to greases and pastes. The advantageof lubricating varnish is its simple applicationin immersion centrifuges or spray drums. Thehandling of coated screws is clean and suitablefor robot systems.

Special areas of application for lubricatingvarnish and slide films are thread-cutting screws,thread-shaping screws, thread-grooving screws,all kinds of wood screws, and screws made ofplastic.

15.4.2. Roller-Bearing Lubrication

For lubrication of roller bearings greases orpastes containing solid lubricants must alwaysbe used when reliable separation of the rollerbody and bearing ring at low speeds and highloads cannot be ensured by grease alone. Overand above this, greases or pastes containing solid


Figure 63. Relative lifetime of MoS2 solid-film lubricants as a function of surface roughness

lubricants have good emergency running prop-erties and afford high safety under shock and im-pact stress. Greases or pasteswithwhite solid lu-bricants are preferable for use in roller bearingsemployed in oscillating operations. Lubricatingvarnishes are the best solution for applications ina vacuum, for example, in air and space travel,nuclear engineering, extreme temperatures, orfor microsetting movements.

Special constructions also enable solid lubri-cants to be incorporated in the bearing material;this enables the production of maintenance-freebearings.

15.4.3. Slide Bearing, Slide Guideway, andSlide Surface Lubrication

Here, reliable separation of the material partnersmust be ensured at all times to comply with theprimary demand put on the lubricant for reli-able load transfer at lowest wear rates. Whereas,as a general rule, the operating point of a fric-tion bearing is in the region of hydrodynamiclubrication and therefore the viscosity of the lu-bricant is the most important key value, slideguideways and other slide surfaces (for exam-ple, infeed wedges) are mainly in the bound-ary or mixed friction range. The best solution

for these operational requirements is lubricationwith PTFE belts and slide liners based on epoxyresin with the incorporation of solid lubricantssuch as MoS2, graphite, or polyurethane. Pastesor lubricating varnishes containing solid lubri-cants are likewise suitable and offer themselvesfor the running-in phase and the warm-up andcool-down phases in normal operation. In thisway the boundary andmixed friction zone can besafely worked through without wear occurring,even with pure oil lubrication, and emergencyrunning properties can be achieved.

15.4.4. Chain Lubrication

When selecting a suitable lubricant special con-siderationmust be given not only to the differentchain-drive construction types, but also to thematerial pairs, the operating parameters, and theambient conditions. For solid, highly viscous lu-brication care must be taken to ensure adequatecreep or penetration to get to the narrow gaps,for example, between the bolt and bush. The lu-bricant must also have an appropriate adhesioncapacity so that it is not spun off at high chainspeeds or high centrifugal forces at the turningpoint (pinion and wheel).


15.4.5. Plastic and Elastomer Lubrication

In plastic and elastomer lubrication the mate-rial compatibility plays a particularly importantrole. Swelling, shrinking, or embrittlement arethe most serious changes which elastomers canexperience when they come into contact withlubricants. Solid lubricants have limited influ-ence on elasticity module, tensile strength, andhardness. On top of reducing friction and pro-tection against wear, the separating effect of thelubricant also plays an important role. The ma-jority of requirement profiles can be covered ad-equately by lubricating varnishes and films con-taining solid lubricants.

In plastic lubrication the special features ofsubstances, i.e., interaction between the plasticand the lubricant, must also be taken into con-sideration. Particular attention must be paid tothe tendency to develop stress cracking. Oils,greases and pastes containing solid lubricants,and also lubricating varnishes and slide films,can be used successfully for lubricating plas-tic and steel, and plastic – plastic pairs. Therunning-in process must also be given enoughattention in the use of plastic materials, and asa general rule, can be positively influenced bysolid lubricants.

16. Testing and Analysis

16.1. Base Oil Categories andEvaluation of Various Petroleum BaseOils

The American Petroleum Institute (API) andthe Association Technique de l’Industrie Eu-ropeenne des Lubrifiants (ATIEL) have classi-fied base oils according to their chemical com-position. Initially there were four groups; afterthe introduction of VHVI oils in Europe thiswas increased to five. The most important rea-son for these groups was the necessity to regu-late base oil interchangeability for engine oils.(Chap. 8). The classification of petroleum baseoils (Groups I to III) considers three parameters:saturates content, sulfur content, and viscosityindex. Table 4 shows this classification. Accord-ingly, Group I oils are solvent extracted HVIoils (SN oils), Group II oils are hydrogenated

or hydrocracked oils (as the sulfur content of< 300 ppm shows) and Group III products areVHVI oils manufactured by severe hydrocrack-ing and or wax isomerization (VI> 120, sulfur< 300 ppm).

Table 51 shows typical data of various hydro-cracked base oils (HC-oils), in comparison withsolvent refined oils and polyalphaolefins.

HC-II oils are classified as very high viscosityindex (VHVI) oils. HC-III oils are also known asextra or ultra high viscosity index (UHVI) oils.They are generally made by hydrocracking andisomerization slack waxes followed by solventdewaxing.

Table 52 shows typical hydrocarbon compo-sitions of HC oils in comparison with conven-tional solvent refined oils and polyalphaolefins[64].

HC-II and HC-III oils are being increas-ingly compared with synthetic polyalphaolefins(PAO); the close similarity of the performancesof some hydrocracked oils and PAO, and theirsignificantly lower cost, make them increasinglyattractive for lubricant formulations.

Figure 64 illustrates the development ofVHVI and UHVI oils (HC-II and HC-III oils)in Europe, Asia, and North America. WhereasShell and BP in France and Union Fuchs in Ger-many were the only manufacturers of VHVI oilin the 1980s, by the mid-1990s similar activi-ties had spread fromWestern Europe around theworld [66].

16.2. Laboratory Methods for TestingLubricants

The properties of lubricants are mostly deter-mined by standardized test methods.

The most common laboratory test meth-ods are listed in the following sections andan overview of the corresponding internationalstandards is given e.g., AFNOR [529], ASTM[530], DIN [531], IP [532], and ISO [533].

16.2.1. Density

For the determination of density of lubricantsdifferent test equipment is in use such as hy-drometer, pycnometer, hydrostatic balance ordigital densimeter.


Table 51. Typical data for 4mm2/s base oils (HC oils) in comparison with conventional solvent refined oils and polyalphaolefins

Solvent refined HC-1 HC-2 HC-3 Polyalphaolefins

Viscosity, mm2/ s at 100 ◦C 4 4 4 4 4Viscosity index 100 105 125 130 125Volatility, Noack evaporation loss,wt%

23 18 14 13 12

Pour point, ◦C −15 −15 −18 −20 −65API group I II III IV

Table 52. Typical hydrocarbon composition of HC oils in comparison with conventional (4mm2/s) solvent refined oils and polyalphaolefins

Solvent refined SN 100 HC-1 HC-2 HC-3 Polyalphaolefins

n- and Isoparaffins, % 25 30 55 75 96Monocycloparaffins, % 20 35 24 15 4Polycycloparaffins, % 30 34 20 10 –Aromatics, % 24 0.5 0.3 0.1 –Thiophenes, % 0.5 – – – –

Figure 64. Development of VHVI and UHVI oils (HC-II and HC-III) in Europe, Asia and Northern America

16.2.2. Viscosity

For the determination of viscosity capillary vis-cometers, falling ball and rotary viscometers areused.

Capillary viscometers (DIN 51562/ASTMD 445) are generally used for most of the au-tomotive and industrial lubricants for determi-nation of the kinematic viscosity. The most fre-quently used viscometers are Ubbelohde fortransparent liquids, and Cannon – Fenske vis-cometers (DIN 51366).

Rotary viscometers use the torque on a ro-tating shaft to measure a fluid’s resistance toflow. The shear rates can be changed by mod-ifying the speed of rotation, rotor dimensionsand the gaps between rotor and stator.

The CCS apparent viscosity is measured in arange between 2.000 and 20 000 mPa · s at tem-peratures between −5 and −30 ◦C (ASTM D2602/DIN 51377). The CCS apparent viscositycorrelates excellently with low temperature en-gine cranking.

TheMRV (ASTMD3829) is applied tomea-sure the borderline pumping temperature and is


related to the mechanism of pumpability of en-gine oils.

The Brookfield viscometer (ASTM D2983/DIN 51398) is used for the determinationof the lowshear rate viscosity of automotivefluidlubricants in a temperature range between−5 ◦Cto +40 ◦C.

The Ravenfield viscometer (ASTM D 4741)allows the determination of dynamic viscosityof engine oils at 150 ◦C using a high shear ratetapered/plaque viscometer.

16.2.3. Refractive Index

The refractive index (DIN 51423/ASTM D1218) is used for the characterization of thestructure of petroleum hydrocarbons like thedensity. Furthermore, it is mostly used as a prop-erty for fast identification of a product.

16.2.4. Structural Analyses

The amount (%) of aromatic and paraffinic hy-drocarbons in a mineral oil can be determinedby the carbon distribution method developed byBrandes [534]. The method determines the in-frared absorbency at wavelengths of 1620 and724 cm−1, which correspond to the stretchingfrequencies of the aromatic and paraffinic hy-drocarbons respectively. However, this methodgives no information on isoparaffins and doesnot take into consideration sulfur content.

According to DIN 51378/ASTM D 2140 thekinematic viscosity at 40 ◦C, the density at 20 ◦Cand the refractive index as well as the sulfurcontent are determined. From these characteris-tics the percentage of aromatic, naphthenic andparaffinic hydrocarbons is determined with orwithout sulfur correction.

For improved characterization chromato-graphic, gas chromatographic and NMR spec-troscopy methods as well as element analysesof the narrow distillate fractions have to be car-ried out.

16.2.5. Flash Point

For the determination of the flash point closedand open cups are used. The open cup flash point

is described according to DIN ISO 2592/ASTMD 92. The closed cup method is mainly used forlubricants with a flash point > 79 ◦C.

For lubricants and mixtures with solvents theflash point is determined by closed cup meth-ods according to Abel – Pensky (DIN 51755) orPensky –Martens (DIN 51758/ASTM D93).

16.2.6. Surface Phenomena

During storage or application lubricants are ina continuous contact with air or even with wa-ter, which might lead to formation of foam oremulsion. For evaluation of the behavior of baseoils and lubricants the following methods areapplied.

Air Release. For the determination of air re-lease behavior (DIN 51381/ASTM D 3427)compressed air is blown through the test oilwhich is heated to a defined temperature. Af-ter the airflow is stopped the time required forthe air entrained in the oil to be released is de-termined. This is the time at which the densityis narrowed to 0.002 g/mL of the density of thepure oil.

Demulsibility. For the determination ofdemulsibility (DIN ISO 6614, ASTM D 1401)40mL lubricant and 40mL of distilled water arestirred at a defined temperature in a graduatedcylinder. After stirring the time required for theseparation of oil from the water is determined. Ifafter 30min standing the separation is not totallycompleted the volume of oil, water and emulsionis reported.

Foaming Characteristics. The foamingcharacteristics (ASTMD892) are determinedbyblowing for 5min a constant rate of air throughthe oil. The volume of the foam is measuredimmediately and after settling for 10min.

16.2.7. Cloud Point, Pour Point

The pour point (ASTM D 97) is the lowest tem-perature expressed as amultiple of 3 ◦C atwhichthe test fluid is observed to flow when it iscooled and examined under defined conditions.


The cloud point (ASTMD 2500) is the low tem-perature behavior where an oil starts to becomecloudy when it is cooled under defined condi-tions.

16.2.8. Aniline Point

The aniline point (DIN 51775/ASTM D 611)is the temperature at which equal volumes ofhydrocarbon oils and aniline separate in twophases. This value gives an indication on thestructure of mineral oils. The higher the tem-perature of separation the more paraffinic is theoil.

16.2.9. Water Content

For water contents ≥ 0.05% ASTM D 95 isused. This test procedure involves azeotropic re-flux distillation of the oil with xylene with waterseparating from the reflux distillate.

For lower water contents between 50 and1000 ppm the Karl Fischer method (DIN51777/1/ASTM D 1744) is more accurate.

16.2.10. Ash Content

For unused oils the determination of the ash con-tent (DIN EN 7/ASTM D 482) gives an indica-tion of purity.

The content of acid constituents is charac-terized by the determination of sulfated ashfrom unused lubricating oils containing addi-tives. Those additive concentrates contain a va-riety of metals. As this is a time consumingmethod it has more or less been replaced by de-termination of elements by atomic absorption,X-ray fluorescence or emission spectroscopy.

16.2.11. Acidity, Alkalinity

The neutralization number of base oils is de-termined by titration against a color indicator(ASTMD 974). When the oil contains additivessuch as dispersants or detergents, the end pointof titration is difficult to observe, potentiometrictitration is used.

Another method is the total acid number(TAN/ASTMD 664). The acidity of unused oils

and fluids is normally derived from the typeand concentration of specific additive materialwhereas the acidity of used oil is of interest tomeasure the degree of oxidation of the fluid.

The TBN characterizes the alkaline reserve.It is particularly used for engine oils where byacidic combustion products use up the alkalinereserve.

16.2.12. Aging Tests

Aging tests are carried out on base oils as well asfully formulated products to test the efficiency ofadditives. The different oxidation stability testsare the most common ways for testing the agingproperties of lubricants. There is a big varietyof standardized test methods. The majority ofthese tests is based on exposing the test fluids tooxygen or air at relatively high temperatures inpresence of catalytic metals to increase oxida-tion rates and to reduce the testing period.

The turbine oxidation stability test (TOST)is used to evaluate the oxidation stability of in-hibited steam turbine oils in presence of oxygen,water, copper and iron catalyst at 95 ◦C. The testis continued until the TAN measured reaches atleast 2.0mgKOH/g. The number of test hoursrequired for the oil to reach 2.0mgKOH/g is the‘oxidation lifetime’ of an oil.

The oxidation stability by rotary bomb test(ASTM D 2272) oxidizes the oil at 150 ◦C inpresence of water, metallic copper catalyst andoxygen at 620 kPa pressure. The time is regis-tered to reach a specific pressure drop and thisis an indication of the oxidation stability.

The aging test according to Baader (DIN51554) is an oxidation test at atmospheric airand intermittent immersion of a copper spiral ata test temperature of 95 ◦C. After a given timethe saponification number in mgKOH/g is mea-sured.

In the oxygen bomb method a lubricatinggrease is oxidized in a bomb heated to 99 ◦C andfilledwith oxygen at 758 kPaoverpressure.Aftera defined time the pressure drop is recorded. Thedegree of oxidation after a given period of timeis determined by the corresponding decrease ofoxygen pressure.


16.2.13. Hydrolytic Stability

The hydrolytic stability of additives is com-monly tested in the beverage bottle method(ASTM D 2619). The method differentiates therelative stability of fluids in the presence of wa-ter under the conditions of this test.

16.2.14. Corrosion Tests

The detection of copper corrosion (ASTM D130/DIN 51759, Part 1) describes a simple pro-cedure for investigating the aggressiveness of thefluid towards a polished copper strip at a temper-ature for a time selected for the fluid under test.

The rust preventing characteristics of inhib-ited mineral oils (ASTM D 665/DIN 51585) inthe presence of water is used for evaluating cor-rosion preventive properties of steam turbine oilsand hydraulic oils.

For the evaluation of rust preventive proper-ties of metal corrosion inhibitors under high hu-midity (ASTMD1748/DIN 51359), steel panelsare prepared to a prescribed service finish anddipped in a test oil.

16.2.15. Oil Compatibility of Seals andInsulating Materials

An essential requirement for lubricants is thatthey should be compatible with seal materials asthey are frequently in prolonged contact at ele-vated temperatures. In all the test methods ap-plied, well defined test specimens are immersedin the lubricant for a defined time at definedtemperatures. After the test period the volumechange, weight loss, and change of mechanicalproperties are evaluated.

16.2.16. Evaporation Loss

The volatility of oils is determined by evapo-ration loss expressed as a percentage by massof nonadditive and additive-type lubricating oils(ASTM D 5800/DIN 51581, Part 1). The pro-cedure according to Noack has been developedfor motor oils but it can also be applied to otherlubricants. A well defined air stream is directedover the oilwhich is heated for 1 h to 250 ◦C.Theevaporation loss is established by weighting.

16.2.17. Analysis of Grease

The consistency of lubricating greases is mea-sured by penetration of a standard cone (ASTMD 217/ISO 2137). Penetration is the depth intenths of millimeters where a standard cone as-sembly sinks into the grease.

The dropping point of lubricating greases(ASTMD 566/ISO 2176) is the point at which aconventional soap-thickened grease passes fromsemisolid to a liquid state under the conditionsof the test or the temperature at which a nonsoap-thickened grease rapidly separates oil.

16.3. Mechanical –Dynamic TestingMethods for Lubricants

Mechanical – dynamic lubricant testing has be-come an essential element in the development ofmodern lubricants. In this respect, standardizedtribological, mechanical – dynamic testers andtest methods play a decisive role in the develop-ment of lubricants. A variety of so-called house-internal methods complete the range of today’stribological, mechanical – dynamic tests.

16.3.1. Tribological System Categorieswithin Lubricant Tests

Today’s wide range of tribological, mechani-cal – dynamic testers and testing methods in-cludes small laboratory instruments and fieldtests under real conditions. The various typesof lubricant test are classified, for example, bythe German DIN 50322 [535].

16.3.2. Standardized and NonstandardizedTest Methods for Lubricants

The most important test methods, used world-wide by many laboratories, are, above all, theISO, CEC, ASTM, IP and DIN standards. Of-ten, these standardized test methods will includeoverlapping among the procedures or even tech-nically equivalent methods.

The common laboratory testers used in to-day’s lubricant development and the tests per-formed with these testers [536,536,537], arelisted in Table 53.


Table 53. Common laboratory testers and test standards.

Tester Application Test method

Roll stability test apparatus standard test method for roll stability of lubricatinggrease

ASTM D 1831-94

SKF (EMCOR) Anti-rust rigdetermination of rust prevention characteristics oflubricating greases

IP 220/93DIN 51802

Water-washout apparatusstandard test method for determining the waterwashout characteristics of lubricating grease

ASTM D 1264-93IP 215/93DIN 51807 Part 2

ASTM Torque test apparatus standard test method for low-temperature torque ofball bearing grease

ASTM D 1478-91

IP Low temperature torque apparatus determination of low-temperature torque oflubricating grease

IP 186/93

ASTM Leakage testing apparatus standard test method for leakage tendencies ofautomotive wheel bearing greases

ASTM D 1263-94

FAG Noise testing apparatusnoise inspection of lubricating greases(computer-aided)

FAG-QV3.102FBMGG 11

SKF Noise testing apparatusnoise inspection of lubricating greases(computer-aided)

SKF specificationMVH 90 B

Sonic probe type apparatus standard test method for sonic shear stability ofpolymer-containing oils

ASTM D 2603-91

Shell delimon rheometer standard test method for determining the deliveryresistance using the shell delimon rheometer

E-DIN 51816, part 1

Evaporation loss tester standard test method for engine and hydraulic oilsfor the determination of the evaporation loss andsediments

WOLF specification

Incline tribometer standard test method for determining frictioncoefficients

H. Schmidt Specification

16.3.3. Common Mechanical –DynamicTesters

Four-Ball Apparatus. The four-ball appara-tus is oneof theoldest andbest-knownmodel testbenches for liquid and solid lubricants. Becauseit furnishes very precise lubricant key values,it is used all over the world. In this apparatus,a roller-bearing ball rotates under pressure andat a constant speed on three fixed steel balls ofthe same type in an oil bath or lubricated witha solid lubricant (Fig. 65). The gradual increasein pressure enables the determination of weldloads, supplies key values concerning loadabil-ity, or enables allows the determination of thefriction or start-up behavior in relation to thelubricant. In addition, the application of rela-tively small loads over a longer period of timeenables the determination of key wear protec-tion values and the friction values of lubricants.During these tests, the surface of the ball sur-face will produce wear impressions which whenmeasured will lead to information on the effectsof additives.

In the late 1990s several adapters have beendeveloped for the four-ball apparatus to furnishinformation on the pitting load capacity and the

shear stability of polymer-containing lubricants.Further modifications of the adapter testers en-able the determination of friction torque andtemperature in the immediate proximity of theouter ring of the roller bearings.

Figure 65.Diagram of the test using the four-ball, apparatus

As ever, today’s numerous specifications forgear and hydraulic lubrication and for all types


of greases and pastes contain key values for theminimum requirements for lubricants.

Reichert Friction Wear Tester. The Re-ichert friction wear tester (Fig. 67) plays animportant role in the determination of the wearrates of water-containing and non water-con-taining coolant lubricants.

Figure 66. Reichert friction wear tester

By means of a double lever handle system,a firmly clamped cylindrical roll is pressed,under load, against a slip ring rotating cross-directionally to the test roll. Approximately thelower third of the test ring is dipped into the testfluid. After a walkway of 100 m at a constantrotational speed of the rotating slip ring, the el-liptical wear mark produced on the test roll ismeasured.

Falex Tester. The Falex tester (Fig. 66) isused for testing lubrication oils and greases. It isused worldwide to determine the friction, loadstage, and wear key values of a lubricant.

Figure 67. Falex teste

A soft steel shaft, either greased or in an oilbath, rotates under pressure between two hard-ened steel prisms. At constant rotational speedthe test load is increased in three steps. If a lubri-cant withstands the applied load, the wear-lossof the softer shaft is measured.

Timken Tester. The Timken tester (Fig. 68)is particularly suitable for determination of thekey values of greases, lubrication oils, and solidlubricants such as pastes. The apparatus tests theload stage and the wear behavior of the lubri-cants under mixed-lubrication conditions.

In the metallurgical and steel industry theTimken tester is still particularly important allover the world.

The lubricant test is conducted in a frictionsystemcomprising a cuboid block and a test ring.The tester is run with a lubrication oil circula-tion system, which can be adjusted variably, anda grease feeder with a feed volume of 45 g/min.The test force, which is gradually increased,is brought into the friction system via the testblock. Individual weighing of the test block andthe test enables determination of the wear loss ina load stage. The test period for each load stageis 10 min at a rotational speed of 800min−1.

Translatory Oscillation Apparatus (SRV).The translatory oscillation apparatus (Fig. 69) isused to test lubrication oils, grease, and solidlubricants. It is used worldwide to record thefriction value, wear behavior, and load stage ofa lubricant in key figures. The lubricant test isconducted under mixed-lubrication conditions.


Figure 68. Timken tester

This, in particular, enables the testing of wear-protective and scuffing-preventing additive sys-tems which are part of EP and AW active sub-stances.

Test pieces made of roller bearing steel areinserted in the test chamber of the oscillationapparatus and wetted with the lubricant to betested.

A great advantage of the tester is the possi-bility of performing systematic studies simplyand quickly, with only small quantities of lu-bricant. These variations of testing conditions,which vary from the test standards, do, however,require sufficient experience with this tester andthe interpretation of its results.

FZG Gear Test Rig. The FZG gear test rig(Fig. 70) is a so-called component test rig witha direct relationship to the machine or transmis-sion system. The FZG gear test rig is one of themost important tribological test benches for gearlubrication and hydraulic oils worldwide. It ispredominantly used to determine the maximumload of lubricants for all types of gear drive.

The test bench consists of a bed plate (a) withtwo firmly mounted gears which are coupled toeach other through shafts. During operation theflanges of the load clutch (b) are connected witheach other by means of screws. If these screwsare loosened and one load clutch flange is lockedusing the lock pin (c), the second load clutchflange can be twisted by a lever arm with weightpieces, thus initializing a torque in the test bench.If the twoflanges of the load clutch are rescrewedto each other and the load lever is removed, theinitialized torque in the shafts is sustained. Ac-

cordingly, the forces also strain the toothing inthe test and drive gears.

The FZG gear test is used for the load stagetest (FZG test method A/8.3/90), micropittingtest (GF-C/9(10)/8.3/90), pitting load capacitytest (P-C/9(10)/8.3/90), wear load capacity test(ASTM D 4998 – 95), and gear efficiency test(VW-PV 1454).

FAG FE9 Roller Bearing Grease Tester.The FAG FE9 roller bearing grease tester(Fig. 71) has beendeveloped for component testsfor greases under realistic conditions. The ser-vice life of the grease is determined from the op-erating temperature, the load, and the rotationalspeed of the test bearing.

Test methodA/1500/6000 has been standard-ized according to DIN 51821, part 2. The angu-lar contact ball-bearing in the tester is filled withapproximately 2mL grease. The test-bearingis loaded with an axially aligned test force of1500N at a rotational speed of 6000min−1. Thetest temperature can be selected from within therange 120 – 200 ◦C (maximum) and is appliedto the test bearings by means of heating ele-ments. The high test temperatures in the greasedtest bearing facilitate oxidation of the lubricants.Failure of a lubricant in the test bearing resultsin a clear increase of the bearing friction.

FAG FE8 Roller Bearing Grease Tester.The FAG FE8 roller bearing grease tester(Fig. 72) is one of the most versatile componenttesters for lubricants. It tests the suitability oflubricants and lubrication oils with regard to thefriction and long-term wear protection behavior


Figure 69. Diagram of the linear oscillation tester (SRV) and possible test sample geometries

Figure 70. Diagram of the FZG gear test rig

of the lubricants in roller bearings for almost allapplications.

Two test bearings of the test head of theFE8 are rigged, axially using disk springs, backto back. The test bearings are lubricated withgrease or, by means of a circulation lubricationsystem, oil. The drive of the test bearings is ef-fected through the inner races of the test bearingon the drive shaft. The test bearing’s outer racesare fixed in the test head box. Bymeans of a wirerope the test head is fixed to a load cell whichrecords the dynamic friction torque during thetest run. An outer radiant heater, placed on thetest head as a heating cap, enables tests at bear-ing temperatures up to 200 ◦C.

The bearing load is equivalent to 10, 20, 50,or 80 kN. The engine speed can be varied from7.5 to 3000min−1.

Synchronizer Rig SSP 180. The synchro-nizer rig SSP 180 is a component tester devel-oped to test the different types of synchronizersystem normally used in the automotive industryand found in all kinds of manual transmission.

The durability to be determined depends notonly on the synchronizer component systemused but above all on the lubricant used. Nowa-days numerous lubricants are perfectly suitablefor use in one tribological system but cannot beused in another. Thus, this component test andthis relatively new tester play an important role


Figure 71. Test bench of the FAG-FE9 roller bearing grease tester. A) Open tester bearers; B) Covered tester bearings withoutgrease reservoir; C) Covered tester bearings with grease reservoir

Figure 72. Test bench diagram of the FAG FE8 roller bearing grease tester


in the development and pre-selection or screen-ing of new lubricants and additives for manualtransmissions.

Diesel Injection Nozzle. This tester is, onthe one hand, used in laboratories to test liquidlubricants and, on the other, enables a lubricanttest with an aggregate. This test method has beendeveloped to determine the change in viscosityof polymer-containing hydraulic and engine oilsfor in relation to the temperature.

For a specified number of run-through cycles,a sample (170mL) of lubricant is subject to ashear stability test in the tester, which consistsmainly of a two-cylinder pump and an injectionnozzle set for calibration oil shear stability. Be-fore and after the shear stress the kinematic vis-cosity is determined at 40 ◦Cwith hydraulic oilsor 100 ◦C with engine oils.

17. Economic Aspects

In 1999, 37.3× 106 t of lubricants were con-sumed worldwide (56% automotive lubricants,29% industrial lubricants, 5%marine oils, 10%process oils). Of total industrial oils, 38% werehydraulic oils, 7% industrial gear oils, 33%other industrial oils, 17% metalworking flu-ids (including temporary corrosion preventiveswhose multipurpose function often includes lu-brication) and 5% greases.

Table 54 shows the 1999 per capita consump-tion for various regions. North America andWestern Europe make up 40.6% of world lu-bricant consumption even though these regionsonly account for 10.7% of the world’s popula-tion [538].

Table 54. 1999 Per capita consumption of lubricants (kg/a)

North America 31.5Australia/Oceania 17.4Western Europe 15.3Central/Eastern Europe 11.9Latin America 5.8Asia 3.2Africa 2.2World 6.1

Since 1975, quantitative lubricant demandhas significantly detached itself from gross na-tional product and also from the number of reg-istered vehicles. This quantitative view gives an

inadequate impression of the significance of thelubricants business today. In almost all areas,products now have a longer life and offer greaterperformance, i.e., specific lubricant consump-tion has declined but specific revenues have in-creased noticeably. This is also confirmed by thevolumetrically very important group of engineoils: the doubling of requirements with extendedoil change intervals in recent years have quadru-pled the cost of such oils.

As about 50% of the lubricants sold world-wide end in and thus pollute the environment,every effort is made to minimize spillages andevaporation. An example is diesel engine par-ticulate emissions, about a third of which arecaused by engine oil evaporation.

A further incentive to reduce specific con-sumption is the ever-increasing cost of disposalor recycling of used lubricants. But this againcreates new demands on lubricants because re-duced leakage losses means less topping-up andless refreshing of the used oil. The new oils musttherefore display good aging stability.

Bearing in mind the growth potential in Asiawhere per capita consumption of lubricants insome areas is extremely low (1999: India 1.1 kg,China 2.5 kg) and a continuing reduction in vol-umes or stagnation in Western industrializedcountries, overall global growth is forecast. Thishas been estimated to be 6.5%between1997 and2002 [539].

Worldwide, there are 1700 lubricant manu-facturers ranging from large to small. On onehand there are vertically-integrated petroleumcompanies whose main business objective is thediscovery, extraction and refining of crude oil.Lubricants account for only a very small part oftheir oil business. At present, there are about200 such national and multinational oil com-panies engaged in manufacturing lubricants. Incontrast, the 1500 independent lubricant compa-nies view lubricants as their core business.Whilethe large, integrated companies focus on high-volume lubricants such as engine, gear and hy-draulic oils, many independent lube companiesconcentrate on specialties and niche business.

Less than 2%of lubricantmanufacturers pro-duce more than 60% of global lubricant vol-umes. Tables 55 and 56 show the world’s largestmanufacturers of lubricants and industrial lubri-cants.


Table 55.World’s largest lubricant manufacturers (2000)

Company Country

1 Exxon Mobil USA2 Shell UK / Netherlands3 Chevron Texaco Caltex USA4 BP CASTROL UK5 SINOPEC / CNPC China6 Pennzoil Quaker State USA7 Total Fina Elf France8 Nippon Mitsubishi Oil / Fuji

Kosan / Koa OilJapan

9 Lukoil Russia10 Valvoline USA11 SUN USA12 IDEMITSU Japan13 Fuchs Germany14 Indian Oil India15 AGIP / Petrogal Italy16 Repsol –YPF Spain

Table 56. World’s largest manufacturers of industrial lubricants(2000)

Company Country

1 Exxon Mobil USA2 Shell UK / Netherlands3 Sinopec / CNPC China4 Chevron Texaco Caltex USA5 BP Castrol Germany

The independent lubricant manufacturersrarely operate base oil refineries. They buytheir raw materials from the chemical and oleo-chemical industry and their mineral base oilsfrom the large petroleum companies.

The production of simple lubricants normallyinvolves blending processes but specialties of-ten require the use of chemical processes suchas saponification (in the case of greases), es-terification (when manufacturing ester base oilsor additives) or amidation (when manufacturingcomponents for metalworking lubricants). Fur-thermanufacturing processes include drying, fil-tration, homogenizing, dispersion or distillation.

Towards the end of the 1990s, the petro-leum industry was affected by a wave of merg-ers which will continue. These created new andlarger lubricant structures at the merged com-panies. The principal reasons for these mergerswere economic factors in crude oil extractionand refining which resulted in lower refiningmargins.

Grease Market. In 1995 37.1× 106 t oflubricants were used worldwide. Of these1.2× 106 t were greases [541]. The NLGI hasreported that 710 830 t of grease were producedin 1999 [542] and it must be considered that onlyca 60% of the worlds greasemanufacturers con-tributed to this figure.

Conventional lithium greases (52.8%) arethemost used. Lithium complex greases accountfor 14.1% and are followed by conventional cal-cium greases with 9.7% (Table 57).

Table 57. Global share of thickeners (1999)

Thickener system %

Conventional lithium soap 52.8Lithium complex soap 14.1Conventional calcium soap 9.7Aluminum complex soap 4.7Polyurea 4.3Organophilic clay thickeners 3.5Calcium complex soap 3.2Sodium soap 2.5Anhydrous calcium soap 2.0Other metallic soap 1.2Other thickeners 2.0

Although lithium soap based greases are themost used products worldwide, there are sig-nificant differences among local markets withregard to other grades of grease. In the USand Canadian market lithium complex greases(29.6%) are the most used. Aluminum complexgreases (6.8%) are also above the global av-erage. In Europe the calcium complex greases(9.4%) are ahead of lithium complex grades(8%). In Japan the polyurea greases (14.6%) arebehind conventional lithium greases (58.2%);both figures are above the global figures (Ta-ble 58).

Table 58. Local share (%) of high-temperature thickeners (1999)

Thickeners Global USA Europe Japan

Lithiumcomplex

14.1 29.6 8.6 0.2

Aluminumcomplex

4.7 8.0 4.7 2.7

Polyurea 4.3 7.6 1.7 14.5Organophilicclay

3.5 6.4 2.2 0.5

Calciumcomplex

3.2 2.2 6.6 0.3


The main consumer market is the automo-tive market, with ca 50%. The grease used areconventional lithium greases, conventional cal-cium greases, and often sodium greases for re-lubrication of trucks, construction, farming, andforestry equipment, and old-fashioned cars.

The industrial grease market is divided into awide range of different and tailor-made greases.Big consumers are steel mills, mines, especiallyopen-pit mines, railroad companies, motor com-panies, andmanufacturers of all kind of machin-ery; they rely on many suppliers. Often thesesuppliers emphasize the performance of a widevariety of greases. Among industrial grease cus-tomers the roller bearing industry is very im-portant; in addition to high-quality conventionallithium greases they increasingly need specificgreases to fulfil their customers’ requirements.

In Germany products used in switcheshave been completely replaced by ester basedbiodegradable greases. Trials with biodegrad-able wheel-flange greases in Austria, TheNetherlands, and Germany [543] have been suc-cessfully completed. Testing in Canada of abiodegradable grease for rail track-side lubrica-tion proved its superior load-carrying and wear-protection performance [544].

18. Disposal of Used Lubricating Oils

Figure 73 shows the fate of lubricants sold ev-ery year in Western Europe. Only 49% are col-lectable and only 28% are actually collected[545]. (The chart also includes process oilswhich are not lubricants.) To ameliorate thissituation, the following objectives should beachieved: the intensive gathering of collectableoils and an improvement in the environmentalcompatibility of the lubricants.

18.1. Possible Uses of Waste Oil

Used lubricants represent a problem for the en-vironment. Their ecologically-compatible use istherefore an important environmental protectionmeasure.

The demand that new lubricants should bemade of used products is based on the erroneousnotion that re-refining can restore the originalcondition of a lubricant. In fact, lubricants lose

value during use and re-refining, at best, can onlyrestore the value of base oils. In the case of con-ventional mineral oils, this value is only slightlyhigher than that of fuels or heating oils. This isalso the reason why re-refining is hardly eco-nomical without legislative provisions or subsi-dies. From a global competition point of view,other disposal options include the direct incin-eration of untreated waste oils, the simple pre-treatment (cleaning) and alternative uses suchas flux oils for bitumen or for the manufactureof secondary feeds for catalytic crackers and asblending stock for high-sulfur fuels.

18.2. Legislative Influences on Waste OilCollection and Reconditioning

The EC Directive 87/101 contains a recom-mendation to all member countries concern-ing the regeneration of used oils insofar aseconomic, technical and organizational condi-tions allow. Emission thresholds for incinerationplants (< 3MW) make the burning of untreatedused oils difficult. However, in some Europeancountries, incineration in smaller incinerationplants is still possible. Legislation permits incin-eration in high-temperature furnaces and by thecement manufacturing industry. In some coun-tries, fuels and heating oils reclaimed from usedlubricants are not taxed and are thus subsidized.

The polychlorinated biphenyl (PCB) prob-lem which surfaced in Europe in 1983 sig-nificantly influenced European legislation onwastes, i.e., two categories of waste oil were cre-ated: Used oils which contain more than 0.2%chlorine cannot be re-refined and are subject toexpensive disposal procedures. This in turn haspromoted the development of chlorine-free lu-bricants.

Re-refined used oils are subsidized in Italy.In Germany, themanufacturers of lubricants (in-cluding distributors) have transferred their legalrequirement to properly dispose of waste oilsto collection organizations. In 1999, these re-ceived about 90 per tonne from the lubricantconsumers.

In the USA, state law on this subject differs.Since 1986, used oils have been classified as haz-ardous wastes in California and other states havesince followed. In some states, the collector is


Figure 73. Lubricating oil supply, use and disposal in Western Europe (CONCAWE 1996) [545]

paid up to 20 cents per gallon by the oil user, inother states the collector has to pay.

Viewed globally, some extremely differingsituations exist. While some countries do notregulate the collection and disposal of used oiland used oil is generally not collected, othercountries can point to high collection and dis-posal rates (in 1996, 99% of used oils were col-lected or properly incinerated in Germany; butonly 60% in the USA).

18.3. Re-refining

The re-refining of used oils to lube base oilsstarted in 1935 [546]. The principal reasonswhy re-refining was unable to find acceptancewere: high process costs and therefore high sell-ing prices compared to relatively low virginoil prices, inadequate removal of carcinogenicpolycyclic aromatics, the negative image of suchoils inmostmarkets and the increasing complex-ity of base oil blends in engine and other lubri-

cants. In Western Europe, only 7% of base oildemand was satisfied by re-refined products in1998.

Numerous re-refining technologies havebeendeveloped over the last 20 years. Many werepatented but only few were suitable for large-scale application [547–552].

In general, the process stages, given below,are common to all the different methods.

1) Separation of larger solid impurities alongwith most of the water. This is normallyachieved by sedimentation.

2) Separation of the volatile parts (fuel residuesin engine oils, solvents and low boiling-point lubricant components), normally atmo-spheric distillation. The separated light hy-drocarbons can usually be used in-house forenergy generation.

3) Separation of the additives and aging by-products. This can occur by acid refining,solvent (propane) extraction, vacuum distil-lation or partly also by hydrogenation.


4) Finishing process to separate any remainingadditives, aging byproducts and refining re-action products. This is normally done by hy-drofinishing,with absorbents such as bleach-ing clay or mild, selective solvent extraction(e.g., furfural).

Sulfuric Acid Refining (Meinken). Thesulfuric acid refining process was mostly de-veloped by Meinken. Compared to older acid-based methods, various process stages reducethe amount of acidic sludge and used bleachingclay generated as well as increasing the lube oilyield.

Due to the acidic sludge problem, acid refin-ing has largely been replaced by other methods.However, numerous such plants were still in op-eration in 1999. Figure 74 shows the flow sheetof a Meinken plant.

Propane Extraction Process (IFP, Snam-progetti). Of the principal extractive refiningprocesses, the IFP (Institut Francais du Petrole)technology is worth mentioning. This processinitially used propane extraction together withacid refining and later together with hydrofinish-ing. Propane extraction is also used by Snampro-getti (Italy) as the main refining step before andafter vacuum distillation. Figure 75 shows theprocess with propane extraction [553], [554].

Mohawk Technology (CEP–Mohawk).The Mohawk Process (subsequentlyCEP –Mohawk) using high pressure hydro-genating was introduced in the USA at the endof the 1980s. The process begins with thin-filmvacuum distillation of the waste oil (after flash-ing the light hydrocarbons and water). This isfollowed by hydrogenation of the distillate at6900 kPa over a standard catalyst. Special stepsrealized catalyst life of 8 to 12 months, whichwas essential for the economy of the process.

A marked reduction in the amount of wa-ter which must be treated as effluent as well asthe cheaper materials for construction (absenceof corrosion) are further advantages. The Mo-hawk process which is based on the KTI processhas been licensed for Evergreen Oil (USA andCanada).

KTI Process. The KTI (Kinetics Technol-ogy International) process combines vacuum

distillation and hydrofinishing to remove mostof the contamination and additives. The key tothe process is the thin-film vacuum distillationtominimize thermal stress throughmild temper-atures not exceeding 250 ◦C.

The hydrofinisher removes sulfur, nitrogenand oxygen. The yield of finished base oils ishigh (82% on a dry waste oil basis). Figure 76shows the flow chart of this process.

PROPProcess (Fig. 77). ThePROP technol-ogy was developed by Phillips Petroleum Com-pany. The key elements of the process are thechemical demetallization (mixing an aqueoussolution of diammonium phosphate with heatedbase oils) and a hydrogenation process. A bedof clay is used to adsorb the remaining traces ofcontaminants to avoid poisoning of the Ni/Mocatalyst.

Safety Kleen Process. The Safety Kleenprocess uses atmospheric flashing for removingwater and solvents, a vacuum fuel stripper, vac-uum distillation with two thin-film evaporators,and a hydrotreater with fixed bed Ni/Mo cata-lysts. When using high severity the hydrotreatercan reduce the content of polynuclear aromat-ics; it also removes higher boiling chlorinatedparaffins.

Figure 78 shows a simplified block diagram.In 1998 the Safety Kleen process was used inthe largest waste oil re-refinery in the world(East Chicago, Indiana, USA, plant capacity250 000 t/a).

DEA Technology (Fig. 79). The best resultswith regard to the technical and environmentalquality of the re-refined oil and the eliminationof PAH are provided by a combination of thinfilm distillation followed by selective solventextraction. In this process, the distillate fromvacuum thin-film distillation towers equipmentat the re-refinery (Dollbergen/Germany) are fi-nally treated in a lube refinery solvent extractionplant followed by hydrofinishing (DEA, Ham-burg/Germany). After this extraction process,the PAH content is lower than that of virgin sol-vent neutrals [554].

Other Re-refining Technologies. Vaxon(Enpotec fabrication facilities in Denmark)


Figure 74. Flow sheet of sulfuric acid re-refining (Meinken process) [553]

Figure 75. Re-refining by propane extraction (IFP, Snamprogetti)

Figure 76. Flow chart of the KTI Process: thin film evaporator (TFE) with hydrotreatment


Figure 77. Flow chart of the PROP Process

Figure 78. Safety Kleen process


Figure 79. Introduction of selective solvent extraction in the re-refining process (DEA / Mineralol-Raffinierie Dollbergen,Germany)

uses three or four vacuum cyclone evapora-tors and finishing treatment with chemicals forre-refining of lubricating oils.

The key step in the ENTRA technology is thespecial vacuum evaporation in a vacuum lineartubular reactor (single tube). After continuousevaporation bymeans of rapidly increasing tem-perature, vapor condensation is performed byfractional condensation. Complete dechlorina-tion can be achieved with metallic sodium. Claypolishing is used as a finishing process.

The thermal deasphalting (TDA) process hasbeen developed byAgip Petroli/Viscolube usingthe technology of PIQSA Ulibarri in Spain. Theprocess is based on chemical treatment to facil-itate subsequent deasphalting.

The deasphalting process is combined withhigh fractionating efficiency (TDA unit). Fin-ishing can be performed by clay treatment orhydrofinishing.

19. Toxicology and OccupationalHealth

19.1. Safety Aspects of HandlingLubricants (Working Materials)

Lubricants are working materials when person-nel contact with these substances via contactwith the skin or clothes or by inhalation or swal-lowing. Toxicological, medical and industrialhygiene aspects thus have to be considered.

19.1.1. Polycyclic Aromatic Hydrocarbons(PAK, PAH, PCA)

The largest proportion of raw materials in met-alworking lubricants are high boiling point pe-troleum cuts which contain very few polycyclichydrocarbons (PAH).


Neat metalworking oils are in the forefront ofthe PAH discussion because these can directlycontact a comparatively large number of peo-ple via the inhalation of oil mists. Efforts werealready made in the past to evaluate the cancerrisks posed by neat cutting oils by determiningtheir concentration of polycyclic aromatic hy-drocarbons.

19.1.2. Nitrosamines in Cutting Fluids

Discussions on this subject were triggered bythe use of alkali metal nitrites as corrosion in-hibitors. The most prominent was and is sodiumnitrite, a widely available and cheap substancewith good inhibitor properties. Reaction of sec-ondary amines with nitrites can lead to the for-mation of nitrosamines, 80% of which are car-cinogenic. Sodium nitrite can be found, aboveall, in low-mineral oil (semisynthetic) or hy-drocarbon-free (fully synthetic) solutions. Theseconcentrations are so high that water-miscibleapplication concentrations of 0.05 to 0.2% canbe found. If other, mostly organic inhibitors areused, significantly lower concentrations can beused. Apart from the use of sodium nitrite inwater-miscible cutting fluid concentrates, it canalso be directly added to water-miscible cuttingfluids by consumers.

While the carcinogenic effect of some ni-trosamines has been known since themid-1950s,it was assumed that the reaction between nitriteand amines could not take place in cutting flu-ids because these were alkaline and the reactionrequires an acidic environment. However, if cut-ting fluid mists are swallowed, nitrosamines canbe formed in the acid milieu of the stomach.

High-performance liquid chromatographyand other analytical methods were used to provethe presence of between 0.02 and 2.99% ofdiethanol nitrosamine in cutting fluid concen-trates of fully synthetic products. The pH of theproducts was between 9 and 11.

The nitrosamine discussions have led a num-ber of cutting fluid manufacturers to developnitrite-free cutting fluids.However, itmust be re-membered that in the past, a much larger propor-tion of water-miscible cutting fluids were free ofnitrite.

19.2. Skin Problems Caused byLubricants

In the metalworking industry, a large proportionof the problems are caused by contact with lubri-cants. Chip-forming machining operations withwater-miscible and neat cutting oils are in theforefront because of the large number of contactpossibilities.

Oil acne (particle acne) is one of the mostcommon skin problems caused by contact withneat cutting and grinding oils. Usually it is notthe oil itself but small particles such as metalfragments which are the cause. This is why thisproblem is often referred to as particle acne. Oilacne can appear wherever oil directly contactsthe skin and this can include oil-stained cloth-ing. Body areas particularly at risk are the lowerand upper arms, the backs of hands, the face,thighs and waist.

Improvements in machine tools such as ma-chine encapsulation, fume extraction, the avoid-ance of oil mists and general automation meansthat personnel have less contact with oil and thecases of oil acne have fallen significantly in re-cent years. The frequency of oil acne cases infactories is a measure of the hygiene standardsin force and the personal cleanliness of machineroom personnel.

Oil Eczema. The term ‘oil eczema’ encom-passes several skin problems. These look likescaly or wet reddened areas, sometimes with acracked surface.

Acute toxic eczema is caused by the directeffect of the substance on the skin and causes aparticular appearance.

Degenerative eczema is the most importantskin problem caused by water-miscible cut-ting fluids. Long-term contact between the cut-ting fluid and the skin produces signs of de-generation and a breakdown of the skin’s de-fenses. Key factors are the alkalinity of the fluid,the long-term wetness and specific product in-gredients and particularly boundary-active sub-stances from emulsifiers.

In microbiological eczemas, pathogenic andapathogenic germs play a part. There are con-siderable differences in opinion concerning thesignificance of microbiological eczemas in met-alworking. One question is whether the large


number of germs in water-miscible cutting flu-ids promote this type of eczema. Some experi-ence indicates that no hygienic risk is posed bygerm numbers of 105 or 106 m/L if no particu-larly pathogenic germs are present.

Long-termdegeneration, for example, causedby exposure to alkalis in water-miscible cuttingfluids can ease the ingress of allergens into theskin and thus start the oversensitivity process.

Allergic skin diseases are relatively seldom inmetalworking companies. However, the increas-ing number of chemically active substances incutting fluids may increase the oversensitivityof skin. Such substances are often found in bio-cides, EP additives, and corrosion inhibitors.

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461. M. D. Foster, M. R. Funk, L. Irwin, N. Riszo,Extracting, imaging and quantifying soapfibers in grease, NLGI Spokesman 60 (1996)24 – 25.

462. C. Araki, H. Kanzani, T. Taguchi, A study onthe termal degradation of lubricating greases,NLGI Spokesman 59 (1995) no. 8, 15 – 23.

463. H. Kopsch, E. Wedepohl, Zur Prufung vonSchmierstoffen mit thermoanalytischenMethoden, Tribologie und Schmierungstechnik30 (1983) 132 – 138.

464. M. J. Pohlen, DSC – A valuable tool for thegrease laboratory, NLGI Spokesman 62 (1998)no. 4, 11 – 16.

465. V. J. Gatto, M. A. Grina, Effects of base oiltype, oxidation test conditions and phenolicantioxidant structure on the detection andmagnitude of hindered phenole/diphenylaminesynergism, Lubrication Engineering 55(1999) 11 – 20.

466. T. A. Norris, Chromatography I and II,Lubrication 1979, 65, 1 – 12, 13 – 24.

467. W. Holweger, U. Jauch,Hochdruckflussigkeitschromatographie,Tribologie und Schmierungstechnik 34 (1987)352 – 355.

468. D. Loderer, Lifetime lubrication of rollingbearings under extreme conditions, NLGISpokesman 60 (1996) no. 8, 8 – 13.

469. E. Kleinlein, Grease test system for improvedlife of ball and roller bearings, LubricationEngineering 48 (1992) 916 – 922.

470. A. Kemble, Evaluation of industrial bearinggrease performance, Eurogrease (1998)July/Aug., 10 – 25.


471. DIN 51 806, Test Using SKF-R2F RollerBearing Grease Test Machine, 1988.

472. DDIN 51 821, Test Using the Roller BearingGrease Testing Apparatus FE 9, 1990.

473. E. Beghini, Cleanliness and Its Performance toBearing Performance, ELGI Preprint, 1992.

474. H. Komiya, Effect of contaminant in lubricanton noise of ball bearings, NLGI Spokesman 56(1992) 94 – 100.

475. ASTM D 1831 Annual Book of ASTMStandards, Vol. 05.01, 1996.

476. ASTM D 1263 Annual Book of ASTMStandards, Vol, 05.01, 1996.

477. NF T 60-171 AFNOR, Vol. 7 2, Paris, 1988.478. DIN 51 805, Testing of lubricants,

determination of flow pressure of lubricatinggreases, Kesternich method, 1974.

479. V. Stepina, V. Vesely, Lubricants and SpecialFluids, Tribology Series 23 (1992) 672 – 696.

480. DIN 51 825, Type K Lubricating Greases,1990.

481. D. M. Jahn, Grease Applications – AutomotiveNLGI Grease Training Course, 1998.

482. W. H. Dresel, H. Glusing, H. Junemann,Hidden Uses of Greases in Cars, NLGIPreprint, 1987.

483. G. Fish, P. Ratcliffe, CV Joint Lubricationfrom the 50’s to the Future, Institution ofMechanical Engineers, 1994.

484. J. Richter, Optimizing efficiency indices ofconstant-velocity joints with low frictionlubricants, Eurogrease (1999) March/April,34 – 39.

485. ASTM D4950-95, Standard Classification andSpecification for Automotive Service Greases,1995.

486. C. Mishra, O. Naruyan, D. Singh, Selection ofLubricants for Steel Plant Application:Practics in Tata Steel, Greasetech India, II 1(1999) 16 – 26.

487. W. A. Christianson, Traction motor gearlubrication requirements and development,Lubrication Engineering 35 (1978) 303 – 308.

488. W.-D. Abel, Einige Aspekte zurSpurkranzschmierung bei der DB AG, 10thInternational Colloquium Esslingen, 1996,Vol. I 4.4, 187 – 212.

489. R. L. Goyan, R. E. Melley, P. A. Wissner, W.C. Ong, Biodegradable lubricants, LubricationEngineering 54 (1998) 10 – 17.

490. R. P. Heckler, Application Experience withBiodegradable Greases, Greasetech India, II2 (1999) 5 – 9.

491. B. Biehl, Instandhaltung vonZahnkranzantrieben unter tribologischen

Gesichtspunkten, Zement-Kalk-Gips 43(1990) no. 11, 534 – 538.

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493. K. Nagdi, Einwirkung von Mineralolen aufGummi-Werkstoffe fur Dichtungen,Maschinenmarkt 80 (1974) 118 – 121; K.Nagdi, Wechselbeziehungen zwischensynthetischen Flussigkeiten undDichtungsmaterialien, ArbeitskreisSchrifttumsauswertung Schmierungstechnik1989, 17-89.

494. ASTM D 4289-91, Standard Test Method forCompatibility of Lubricating Grease withElastomers, Annual Book of ASTMStandards, Vol. 05.03, 1991.

495. DIN 53538-1, Standard-Referenz-Elastomere,Acrylnitril-Butadien-Vulkanizat (NBR)peroxidvernetzt, zur Charakterizierungflussiger Betriebsmittel hinsichtlich ihresVerhaltens gegen NBR, 1992; ISO 6072,Fluidtechnik, Hydraulik, Vertraglichkeitzwischen Elastomerwerkstoffen und Fluiden,1986.

496. S. Beret, T. J. Boersig, P. K. Wong, Elastomercompatibility – base oil and additive effects,NLGI Spokesman 60 (1996) no. 5, 21 – 25.

497. G. Dornhofer, Die WechselwirkungSchmierstoff-Kunststoff dargestellt an Handvon Beispielen aus der KFZ-Ausrustung, inGleitreibung und Gleitverschleiß beiKunststoffen, Esslingen, 1997, ConferenceCompendium.

498. G. E. Barker, et al., The ComprehensiveLaboratory Testing of Instrument Lubricants,ASTM Bulletin 3 (1946) 26 – 35.

499. M. Tillwich, W. Stehr, W. H. Dresel, Prufgeratfur Langzeitversuche zur BestimmungtribologischerWerkstoff-Schmierstoff-Kenndaten, inKunststofflagern, sowie Uberprufungausgewahlter Reibsysteme auf tribologischeReaktionen, Tribologie, vol. 5, Springer,Berlin 1982, pp. 125 – 185.

500. W. Stehr, Precision bearings and theirtribological simulation, JSL 7 (1990)297 – 309.

501. W. Kaufmann, Bestimmung derChemikalienbestandigkeit von Kunststoffenunter mechanischer Spannung, Kunststoffe 65(1975) 155 – 157.


502. W. Noll, Chemie und Technologie der Silicone,2nd ed., Verlag Chemie, Weinheim 1968, pp.408 – 410.

503. H. Wieczorek (ed.): Kunststoffe imLebensmittelverkehr, 44th ed., CarlHeymanns, Koln 1993.

504. A. Hubmann, Rapsol – ein alternatives Basisolfur Schmierstoffe, Mineraloltechnik, 1989, 34,9; A. Hubmann, Chemie pflanzlicher Ole, 9thInternational Colloquium Esslingen, 1994, volI 2.1, 1 – 15.

505. W. H. Dresel, Schmierfette auf pflanzlicherBasis, in Biologisch abbaubare Schmierstoffeund Arbeitsflussigkeiten, Esslingen, 1990,Conference compendium 6; W. H. Dresel,Schmierfette auf pflanzlicher Basis, inW. J.Bartz (ed.): Biologisch schnell abbaubareSchmierstoffe und Arbeitsflussigkeiten, expert,Ehningen 1993, pp. 239 – 258; W. H. Dresel,Biologically degradable Lubricating Greasesbased on Industrial Crops, Industrial Cropsand Products 2 (1994) 281 – 288.

506. H.-G. Schmidt, Modified Natural BaseOils – Their Properties and Use forLubricating Greases, ELGI Preprint, 1993;H.-G. Schmidt, Komplexester aus pflanzlichenOlen, 9th International Colloquium Esslingen,1994, vol I 2.2, 1 – 29.

507. E. M. Stempfel, L. A. Schmid, Biodegradablelubricating greases, development, application,trends, NLGI Spokesman 55 (1991) 313 – 321;E. M. Stempfel, L. A. Schmid, Biologischabbaubare Schmierstoffe – EineStandortbestimmung, speziell am Beispiel vonSchmierfetten, 8th International ColloquiumEsslingen, 1992, vol I 12.1, 1 – 20; E. M.Stempfel, Practical experience with highlybiodegradable lubricants, especially hydraulicoils and lubricating greases, NLGI Spokesman62 (1998) 8 – 23.

508. D. Loderer, E. M. Stempfel, Lifetimelubrication of rolling bearings with rapidlybiodegradable lubricating greases, NLGISpokesman 61 (1997) no. 2, 16 – 20.

509. P. Legiza, K. Nahal, Some experience withbiodegradable lubricants, SyntheticLubrication 13 (1997) 347 – 360.

510. R. Hiss, A biodegradable vegetable polyureagrease, NLGI Spokesman 57 (1993) 188 – 191.

511. C. Kyriakopoulos, Aluminum complex greasesformulated with biodegradable base oils, NLGISpokesman 59 (1995) no. 2, 10 – 23.

512. A. K. Bhatnagar, et al., Titanium complexgrease – some new findings, NLGI Spokesman59 (1995) no. 2, 10 – 18; A. K. Bhatnagar, et

al., Ecofriendly titanium complex grease,NLGI Spokesman 61 (1997) no. 8, 22 – 28.

513. L. Ulm, et al., Biodegradability ofwater-resistant greases, in Proceedings of theInternational Conference on Tribology, GozdMartuljek, 1998, 104 – 114.

514. T. Mang, Environmentally harmless lubricants,NLGI Spokesman 57 (1993) no. 6, 9 – 15.

515. R.-P. Heckler, Application Experience withBiodegradable Greases, Greasetech India, II2 (1999) 5 – 6.

516. E. Kleinlein, W. H. Dresel, N. Geheeb, R.Kuhl, D. Sohn, Walzlagerschmierung,GfT-Arbeitsblatt 3, Moers, 1993, 35; E.Kleinlein, W. H. Dresel, N. Geheeb, R. Kuhl,D. Sohn, Rolling Bearing Lubrication, GfTWork Sheet 3, Moers, 1994, 35.

517. U. J. Moller, Entsorgungsmoglichkeiten furbiologisch schnell abbaubare Schmierstoffe,9th International Colloquium Esslingen, 1994,Vol II 13.1, 1 – 9.

518. W. J. Bartz, Lubricants and the Environment,Eurogrease (1996) May/June, 15 – 52.

519. W. H. Dresel, Grundlegende Aspektezukunftsorientierter Schmierfette, Tribologieund Schmierungstechnik 40 (1993) 176 – 182;W. H. Dresel, R.-P. Heckler, Some aspects oftomorrow’s greases, NLGI Spokesman 58(1994) 17 – 24.

520. G. Dornhofer, Lebensdauerschmierung vonKfz-Aggregaten unter Berucksichtigung derUmweltschonung, Tribologie undSchmierungstechnik 41 (1994) 86 – 90.

521. F. P. Bowden, D. Tabor, The Friction andLubrication of Solids, Oxford UniversityPress, London 1954; F. P. Bowden, D. Tabor,Reibung und Schmierung fester Korper,Springer, Berlin 1959.

522. A. Bondi, Physical Chemistry of LubricatingOils, Reinhold, New York 1951, pp. 19 – 100.

523. E. Kuhn, Tribologische Beurteilung deszeitabhangigen Verhaltens von Schmierfetten,Tribologie und Schmierungstechnik 43 (1996)245 – 247; E. Kuhn, Lubricating grease as anactive element of the tribological system,Eurogrease (1998) March/April, 13 – 19.

524. P. M. Cann, A. A. Lubrecht, An Analysis ofthe Mechanisms of Grease Lubrication inRolling Element Bearings, 11th InternationalColloquium Esslingen, 1998, Vol I 6.1,561 – 569.

525. S. Hurley, P. M. Cann, Grease composition andfilm thickness in rolling contacts, NLGISpokesman 63 (1999)no. 4, 12 – 22; P. M.


Cann, S. Hurley, Grease composition and filmthickness in rolling contacts, Eurogrease(1999) May/June, 7 – 25.

526. P. M. Cann, Grease Lubricant Films in RollingContacts, Eurogrease (1997) Nov./Dec.,6 – 20.

527. S. Hosoya, Deterioration of lithium soapgreases and functional life in ball bearings,NLGI Spokesman 53 (1989) 246 – 251.

528. P. M. Cann, The influence of temperature onthe lubricating behavior of a lithiumhydroxystearate grease, Eurogrease (1995)Jan./Feb., 25 – 32.

529. AFNOR Produits petroliers et lubrifiants,Tome 1 and 2, L’Imprimerie Chirat, 42540St-Just-La-Pendue 1992.

530. ASTM Volumes 5.01, 5.02, 5.03, AmericanSociety for Testing and Materials, 100 BarrHarbor Drive, West Conshohocken, PA 19428.

531. DIN Handbuch 2-1, 2-2, Beuth Verlag GmbH,Berlin, 1997.

532. IP Standard Methods for Analysis and Testingof Petroleum and Related Products, JohnWiley & Sons, Chichester 1999.

533. DIN Handbuch 2-3, Beuth Verlag GmbH,Berlin, 1997.

534. G. Brandes, Brennst. Chem. 37 (1956) 263– 267.

535. DIN 50322, Kategorien derVerschleißprufung, Beuth Verlag, Berlin.

536. H. Junemann, Tribometrie – Pruf- undMeßtechnik, Expert-Verlag, Boblingen 1997.

537. DIN-Taschenbuch 192, Schmierstoffe,Eigenschaften und Anforderungen, 4th edn,Beuth Verlag, Berlin, 1995.

538. M. Fuchs, The World Lubricants Market, 8thInternational Conference on IndustrialTribology, Calcutta, 1997.

539. M. Fuchs, The Global and RegionalLubricants Markets, Conference ′′Revitalizingthe Lubricants Business in the 21st Century′′

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541. M. Fuchs, The global lubricating greasemarket, NLGI Spokesman 60 (1996) no. 5,25 – 40.

542. J. A. Lurz, Grease Production Survey Reportfor the Calendar Years 1999, 1998, 1997 and1996, NLGI, Kansas City, 2000.

543. W.-D. Abel, Biologisch schnell abbaubareWeichenschmierfette (Prufung undAnwendung), 11th International Colloquium,Esslingen, 1998, vol. I 2.7, 225 – 236.

544. R. L. Goyan, R. E. Melley, P. A. Wissner, W.C. Ong, Biodegradable lubricants, LubricatingEngineering 54 (1998) July, 10 – 17.

545. Collection and Disposal of Used LubricatingOil, CONCAWE Report 5 (1996).

546. C. Kajdas, Used Oil Re-refining: Overview ofCurrent Technologies Used, 3rd EuropeanCongress on Re-refining, Lyon, 1996.

547. D. J. McKeagan, Economics of Re-RefiningUsed Lubricants, Lubrication Engineering(1992) 418 – 423.

548. P. F. V. C. Oosterkamp, KTI Re-RefiningTechnology, UNIDO Workshop, Karachi,1992.

549. D. W. Brinkman, Large Grassroots LubeRe-Refinery in Operation, Oils and GasJournal (1991) 60 – 63.

550. D. Peel, Greening of America, World BaseOils ’96 Conference, Elgin, 1996.

551. C. Schoen, EinrohrreaktorverfahrenzurAufarbeitung von Altoel und fluessigenAbfallstoffen, Description of the ENTRATechnology, ENTRA Ingenieur- und HandelsGmbH, Achern, 1996.

552. F. D. Giovanna, P. Tromeur, G. Cohen,Successful Re-Refining in Practice,International Used Oil Conference, Orlando,1998.

553. Waste Oil Re-Refining, HydrocarbonProcessing 59 1980 143.

554. D. Klamann, Lubricants and Related Products,VCH Verlagsgesellschaft, Weinheim 1984.

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Ullmann's Encyclopedia of Industrial Chemistry || Lubricants and Lubrication