34110418 Lubrication Industrial Gears

8/3/2019 34110418 Lubrication Industrial Gears

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INTRODUCTION

Industrial gears have been in use formany years. One of the first gears was around wooden disk with wood pegs extend-ing from the side of the disk. As years wentby gear teeth cut into the outer edge of thedisk replaced the pegs. Today, gears are

used in virtually all areas of industry andmanufacturing. Precision gears are used inthe manufacture of microchips, in the oper-ation of automobiles, and to position thetapes in our VCRs. Precision, durability,accuracy, and reliability are the require-ments made on gear installations today.Whether transferring large amounts ofhorsepower in an industrial application ormaking precision movements inside amicrochip tool, gears require high quality

lubrication to insure smooth, noise-free, effi-cient operation. The wide ranges of appli-

cations gears encompass demand that theybe manufactured in a variety of methodsand designs, as well as a variety of materi-als. Each of these applications creates adifferent set of conditions for lubrication.

The information presented in this articlediscusses gear design, describes the typesof gears, and discusses the different typesand applications of lubricants required ingear use. Also discussed in the article arethe industrial applications of gears, lubrica-tion system problems, and gearfailures/causes.

TYPES OF GEARS

Gears are mechanical devices used totransfer rotational power or energy from oneplace to another. This transfer may be rota-

tional or linear in its output. Rotational ener-gy output can be varied in regards to output

INDUSTRIAL GEARS

AND LUBRICATION

A Technical Publication Devoted to the Selection and Use of Lubricants

Published By

Texaco, Inc.

2000 Westchester Avenue

White Plains, NY 10650

LUBRICATION

Volume 86 Number 6 July, 2000

To request a new subscription or to report a change of address (enclose mailing label),

please write to: Robert J. Taylor, Texaco, Inc., 1111 Bagby Street, Houston, TX 77002;

or by e-mail: [email protected]

Copyright2000 by Texaco, Inc. All Rights Reserved.

Materials may not be reproduced or reprinted without written permission of Texaco, Inc.

TECHNICAL EDITOR: LYNNE L. MEGNIN

MANAGING EDITOR: AUGUST H. BIRKE

PRODUCED BY: BAKER PRINTING, BAKER, LA


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speeds, torque, and direction of rotation.For example, a large gear driving a smallgear will produce a higher speed in the out-put, but a lower torque than the input.However, a small gear driving a large gear

will have a lower speed than the input, but ahigher torque in the output. In either case,the small gear is called a pinion and thelarge gear is called a bull gear, ring gear, ora gear wheel. Throughout this article, thevarious types of gears, each with its ownapplication and purpose, will be briefly dis-cussed.

Spur gears, often found in machine lathesand mills, are the least complicated and

lowest cost to manufacture. A spur gear hasstraight teeth cut parallel to the axis of thegear. They are the easiest to lubricate andare found in three configurations: external,internal, and rack and pinion, as shown inFigure 1.

The external configuration of the spurgear is of typical gear design with teeth cutacross the outer perimeter of the gear body.Another configuration, the internal gear, isused in special applications such as plane-tary gear sets. The rack and pinion, whichis also a type of spur gear, is uniquebecause it transforms rotary power into lin-ear power. Rack and pinion configurationscan be found in automotive steering andvarious manufacturing plants. In this design,the spur meshes with the gear teeth on aflat rack. Regardless of configuration, spur

gears are operated under moderate loads atmoderate speeds because only one pair ofgear teeth is in contact at one time duringoperation; meaning the entire load isagainst a single pair of teeth. A disadvan-

tage of spur gears is that they are relativelynoisy in operation because the entire face ofthe gear tooth comes into contact with themating tooth at the same time.

A modification of spur gears, the helicalgear, is designed to provide high-speedtransmission between parallel shafts.Helical gears, shown in Figure 2, have teethcut at an angle to the axis of the gear ratherthan parallel to the axis. When the teeth of

a helical gear are meshed with a matinggear, there is more than one pair of teeth incontact at a time allowing the helical gear tocarry a larger load and operate at higherspeeds than the spur gear. The helical gearalso operates with much less noise than thespur gear. Helical gears are commonlyfound in pump drives and centrifugal com-pressor drives.

Drawbacks in the use of helical gears arethe cost of production and the axial thrustproduced by the gears. In helical gears, theaxial thrust tends to push the two matinggears apart because of the angle cut on thegear teeth; therefore, a thrust bearing orsome means of preventing axial movementmust be used. A solution to the unwantedaxial thrust is a variation on the basic helicaldesign called a herringbone gear, shown inFigure 3.

Figure 1 - Spur Gears

EXTERNAL

INTERNAL

RACK & PINION

Figure 2 - Helical Gear


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In a herringbone gear, the teeth are cut at

an angle, but the angle begins in the middleof the gear and extends in opposite direc-tions to the edge. The appearance of thegear teeth is similar to the letter V. Theadvantage of the V-shaped design is theelimination of axial thrust while maintainingthe high speed and high load capabilities.However, the herringbone design can causelubrication problems. Lubricants canbecome compressed at the center of thegear face. When mating teeth come into

contact, the center of the V is closed andlubricant can become trapped in the space.To eliminate this condition a lubricant chan-nel is cut through the center of the gear faceproviding a path for the lubricant to escape.If the two helices are not joined in the cen-ter it is called a double helical. This geome-try is easier to machine and facilitates theflow of lubricant while maintaining thecapacity for high speed/high load. Both her-

ringbone and helical gears are typicallyused in pump and compressor drives, andelectric generators.

A modification of the spur gear is thebevel gear. Bevel gears, shown in Figure 4,are typically used to transfer motion ofshafts that are at right angles to each otherand of shafts where the centerlines inter-sect. The gear teeth of bevel gears are cuton an angular surface of a truncated cone.Simple bevel gears, which are typically

found in the intersecting shaft system ofconveyor drives, have straight teeth that

radiate from the point of the cone. At highspeeds, bevel gears have the same disad-vantages as straight spur gears, which arenoisy during operation and carry moderateloads. If the two gears are the same sizethey are called miter gears.

Spiral bevel gears, shown in Figure 5,have gear teeth cut at an angle on a radialline. The spiral bevel gear permits gradualtooth engagement with multiple tooth con-tact. Spiral bevel gears have greater load

and speed ratings and operate with lessnoise than straight bevel gears.Applications involving spiral bevel gearsinclude multi- and single-cylinder enginesand large electric motors.

Figure 3 - Herringbone GearFigure 4 - Simple Bevel Gear

Figure 5 - Spiral Bevel Gear


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Worm gear, shown in Figure 6, is another

type of commonly used gear. Worm gearsare typically found in small enclosed drives,as well as turn-table drives, drum drives,and shear and press brakes. The rotatingshafts are at right angles that are offset, andperpendicular to each other. The smallergear is referred to as the worm and theother gear is called the wheel. There arebasically two types of worm gear designs,the non-throated and throated designs.Non-throated worm gears permit only sin-gle-tooth contact, which reduces the loadsapplicable to this design. When wheel teethare curved, partially enveloping the wormgear (single-throated or self-enveloping),the contact area is increased, boosting theload carrying capabilities of the set. If boththe wheel and worm are curved (double-throated or double-enveloping), the contactarea is even greater permitting extremelyhigh reduction ratios with smooth quiet

operation. The disadvantage to the double-throated worm gear design is the slidingcontact that occurs between the gears.Special lubricants, such as synthetics andcompounded oils, are required to reducefriction.

Hypoid gears, shown in Figure 7, are amodification of a spiral bevel gear. Theaxes of a spiral bevel gear set are in thesame plane. By contrast, the axes of thehypoid gear set are in different planes. Thehypoid gears, called pinion and ring gears,permit high load capacity, reduced tooth

breakage and quiet operation. Because themovement of hypoid gears is strictly slidingone against the other, lubrication require-ments are unique. A high level of anti-scuff,extreme pressure (EP) additive in the lubri-

cant is required to reduce friction betweenthe gear teeth.

Another collection of gears, called plane-tary gears, provides an efficient means ofpower transmission in a compact design.The driven and driving shafts of planetarygears are concentric. Planetary gears havemany applications including automotivedrives, industrial drives, and wind turbines.Planetary gear sets can utilize spur or heli-cal gear tooth forms, and are suitable forinstallations that require the following:

An increase or decrease in thespeed of the driven component

A change in direction of rotation ofthe output

A torque increase or decrease Input shafts and output shafts on

the same axisPlanetary gears, shown in Figure 8, are

similar to the configuration of the solar sys-tem in that the planet gears turn on theirown axis while they rotate around a central-ly located sun gear. The planet pinionsmesh with the inside gear teeth of the ringgear. When the gear set is assembled, thesun gear, the planet pinions, and the ringgear are constantly in mesh. The planetpinions are mounted on shafts in a carrier

Figure 6 - Worm Gears

Figure 7 - Hypoid Gear


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assembly and rotate on their axis to walkaround the sun gear or the ring gear.

When power is applied to one of themembers of the planetary system (e.g., thesun gear), the entire planetary system willrotate as a unit. If a force is applied torestrain one of the other two planetarymembers (the carrier assembly) to preventit from rotating, the other planetary member(the ring gear) will rotate and become thepower unit. If no force is applied to restrainany member, the planetary gear system isin a neutral mode.

ARRANGEMENTS OF GEARS

This section will examine the various geararrangements that are used to transmitpower from the driver to the driven unit.Applications for the various gear units range

from industrial size, large horsepowerequipment, often found in mining and con-struction applications, to small equipmenttypically found in chip manufacturing toolsand sewing machines. As long as the gearsoperate within the recommended speed andhorsepower parameters, the applicationsfor the gear units can be widespread. Thefollowing arrangements will be discussed:

Parallel Axis Single ReductionSingle and Double Helical Gear Unit

Parallel Axis Double ReductionSingle and Double Helical Gear Unit

Spiral Bevel Gear Unit (TripleReduction System)

Right Angle Reduction Gear Unit Epicyclic Reduction Gear Unit

(Planetary Gear explained in previ-ous section)

PARALLEL AXIS SINGLE REDUCTION

SINGLE AND DOUBLE HELICAL GEAR

UNIT

Gear units are classified as parallel axiswhen the input shafts and the output shaftsare parallel and in the same plane. Gearunits are classified as single reduction whenthe reduction in speed occurs through oneset of gears. Figure 9 shows the internalgear layout of a parallel axis single reduc-tion single helical gear unit. Because singlehelical gears are the principal type of gearused in this arrangement, the configurationis classified as single helical.

Figure 8 - Planetary Gear

Figure 9 - Parallel Axis Single Reduction Single Helical Gear Unit


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Figure 10 shows a cut-away view of aparallel axis single reduction double heli-cal gear unit. Because double helicalgears are the principal type of gear used inthis arrangement, the configuration is clas-sified as double helical. Parallel axis sin-gle reduction single and double helicalgear units have the following features:

Speed reduction ratio of approxi-mately 10:1

Horsepower range of up to approx-imately 30,000 hp

PARALLEL AXIS DOUBLE REDUCTION

SINGLE AND DOUBLE HELICAL GEAR

UNIT

Much like the single reduction gearunits, the double reduction gear units areclassified as parallel axis when the input

shaft and the output shaft are in the sameplane. Gear units are classified as doublereduction when the reduction in speedoccurs through two sets of gears (doublesets of gears). Figure 11 shows a cut-

away view of a parallel axis double reduc-tion single helical gear unit. Because sin-gle helical gears are the principal type ofgear used in this arrangement, the config-uration is classified as single helical.

Figure 12 shows a view of a parallel axisdouble reduction double helical gear unitwith the upper half of the gear unitremoved. Because double helical gearsare the principal type of gear used in this

arrangement, the configuration is classi-fied as double helical. Parallel axis doublereduction single and double helical gearunits have the following features:

Speed reduction ratios of 5:1 up to40:1

Horsepower range of approximate-ly 10,000 hp

SPIRAL BEVEL GEAR UNIT (TRIPLE

REDUCTION SYSTEM)

The spiral bevel gear unit, shown inFigure 13, is a left-hand spiral bevel gearunit with a right angle output. The spiral ofthe pinion gear is the one that is common-ly specified when a spiral bevel gear unit

Figure 10 - Parallel Axis Single ReductionDouble Helical Gear Unit

Figure 11 - Parallel Axis Double ReductionSingle Helical Gear Unit

Figure 12 - Parallel Axis Double ReductionDouble Helical Gear Unit


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is identified, and the smaller gear of theset is called the pinion gear. When look-ing at the face of the pinion gear in theunit, the teeth are curved away from theaxis in a counterclockwise direction.Therefore, the pinion gear is a left-handspiral bevel gear because of the counter-clockwise direction. The spiral bevel gearunit can be used for shafts that are mount-ed at any angle as long as the centerlinesof the shafts intersect. Because thereduction of speed occurs through threesets of gears (two sets of helical gears andone set of bevel gears), the gear unitshown in Figure 13 could be classified asa triple reduction unit. Spiral bevel gearunits have the following features:

Speed reduction ratio of 6:1 up to500:1

Horsepower range of up to approx-imately 3,000 hp

RIGHT ANGLE REDUCTION GEAR UNIT

Figure 14 shows a right angle reductiongear unit. The output shaft is at a rightangle, or 90 degrees, to the input shaft. Aright angle reduction gear unit could con-sist of any of the following units:

A bevel gear unit (straight beveled,spiral beveled, or Zerol, but not amiter gear unit. Miter gears do notprovide speed reduction)

A single helical gear unit with both

gears cut to the same hand and ahelix angle of 45 degrees

A worm gear unit (shown in Figure14)

The right angle reduction gear unit,shown in Figure 14, is called a doublereduction helical-worm gear drive. Thedrive consists of a helical gear unit and aworm gear unit. The input shaft is con-nected to the helical gear units piniongear. The helical gear unit provides thefirst reduction in speed. The larger gear ofthe helical gear unit is connected to theinput of the worm gear unit or the worm.The worm drives the worm gear that isconnected to the output shaft. The wormgear unit provides the second speedreduction and an output shaft rotation at

Figure 13 - Spiral Bevel Gear Unit (Triple Reduction System)

Figure 14 - Right Angle ReductionGear Unit


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right angles to the input shaft. Doublereduction helical-worm gear drive unitshave the following features:

Speed reduction ratios of 6:1 up to290:1

Horsepower range of up to approx-imately 25 hp

For horsepower ranges of 25 hp up to3,000 hp, spiral bevel-helical gear unitsare used. Speed reduction ratios for spiralbevel-helical gear units range from 6:1 upto 500:1.

GEAR DESIGN ANDCONSTRUCTION

The design and selection of gears is acomplex procedure using gear geometry,shown in Figure 15. The speed and loadof the gear application and the type ofenvironment the gear will operate in dic-tate the gear type. Materials of construc-tion and manufacturing procedures arealso determined by the gear application.Lubrication requirements will be deter-mined by the same criteria as gear selec-

tion, speed, load, and environment. Someof the most commonly used gear terminol-

ogy and nomenclature are presented inTable 1.

The present form of involute gear toothhas evolved through studies and experi-ments over several centuries. In order to

obtain quiet, vibrationless, and efficientoperation, the pitch line velocities of mat-ing gear teeth must be equal. This con-cept has become a fundamental law ofgear design. To fulfill this law, the involuteor cycloid gear profile is used. The invo-lute profile, shown in Figure 16, is formedby points from the arc of a circle moving

Figure 15 - Gear Geometry

Figure 16 - Involute


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away from the circle. These pressureangles can range from 14.5 to 30 degrees.The primary advantages of an involute areits capability of transforming constantangular velocity of a gear tooth to constant

linear velocity of a tooth follower. Involutegears have no fixed pitch diameter untilthey are mated with another gear. This

allows the gear to be operated over a widerange of center distances without disturb-ing the constant rotational velocity of thegear, provided at least one pair of teethstay in contact at all times.

The diametral pitch of the gear is also aconsideration in gear design. In anymatched set of gears the diametral pitch of

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Table 1 - Gear Terminology


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the gears must be the same. Generally,whole pitch numbers are used to reducethe number and type of cutting tools in themanufacturing process. The most com-mon pitches range between 2 (coarse) to

64 (fine). Standards for manufacturing ofgear sets have been published by theAmerican Gear Manufacturers Association(AGMA) and should be consulted for spe-cific and complete information.

The transfer of power in a set of gears ismade at areas of mutual contact. In spur,helical and bevel gears, the opposingtooth surfaces move over each other in asequence of sliding-rolling-sliding motion

to create the mutual contact. The purerolling motion only occurs for a moment atthe pitch line of the teeth. In hypoid andworm gears, the sliding motion of thegears provides the mutual contact.

The sliding velocity of mating gear teethdepends on the pitch line velocity and thedistance between the contact point andthe pitch line. Maximum velocity will occurat the tip as the teeth come into contactand again when they disengage. At nor-mal operating speed and under full load,the sliding action causes the lubricant tobe wiped away creating a severe lubrica-tion requirement. When the teethapproach the pitch line, contact-slidingvelocity reduces as rolling action increas-es. At this point, the remaining lubricant issubjected to maximum pressure forces.The repeated sliding-rolling-sliding motionis responsible for much of the adhesive

wear that occurs near the tip and root ofthe tooth. Because the repeated slidingaction causes the greatest amount ofwear, high-speed gears are designed withlarge numbers of teeth. The larger thenumber of gear teeth results in less slidingaction between any two teeth, and pro-vides better load-sharing capacity.

The amount of load a gear set is going tocarry is also a factor in designing the gear.Along with load, the application must beconsidered. Is the set to be housed in asealed enclosure, or will it be open to the

surrounding environment? Is the nature ofthe surrounding environment dusty, corro-sive, or yielding high levels of heat orcold? Each of these factors must be con-sidered in the design of the gear set. For

best performance and load carrying ability,gear sets should be designed so that morethan one set of the teeth is in contact at atime. The greater the number of teeth incontact at a given moment, the better theload is distributed. This permits the gearsto operate smoothly and reduce wear.

Backlash is an important design param-eter and significantly impacts gear lubrica-tion (see Figure 15). Backlash is the

designed space that exists between theunloaded side of the meshing gear teeth toavoid binding during operation. Thedesigned space insures that contact willonly occur on the meshing surfaces of theteeth. The space also provides a place forthe oil film to develop and a place to han-dle changes in deflection and thermalexpansion of the material. Insufficientbacklash can cause interference resultingin excessive temperature rise, noise, over-loading, and adhesive wear. In addition,backlash can be too great due to inade-quate design or excessive wear.

GEAR MATERIALS ANDMETHODS OF MANUFACTURING

Gear materials, or metallurgy, and thefinish of the gear tooth surface must beconsidered in the selection of gear sets.

As with most application requirements inindustry, the environment the equipmentoperates in, the maintenance requirementthe load carrying requirements, and thecost of manufacturing all play a part in theselection of gears. Gear manufacturingmaterials range from the latest thermo-plastics to carbon steel to exotic alloys.Each specific application determines thebest choice of material. The deciding fac-tor may be wear resistance, while in othercases it may be strength and/or cost.Depending on the type of gear set, the


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material selection will have to take intoconsideration the type of lubricant that canbe used in the application. Applicationsfor the following gear materials are basedprimarily on load and speed requirements.

To a lesser, but important degree, theenvironment, frequency of maintenance,and lubrication used impact the applica-tion the gear is used in.

STEEL

Steel is a commercial alloy of iron con-taining combined carbon in amounts rang-ing from 0.25 wt % carbon to a maximum

of about 1.7 wt % carbon. Although not allsteels are hardened, they are usually heattreated to some extent to obtain the bestcombination of machinability and thedesired properties for use in gearing.Small quantities of alloying elements, suchas chromium, manganese, nickel, andmolybdenum, may be added to the steel tomodify its properties. If maximum strengthis desired, the machined gear can behardened by heat treatment through sev-eral methods, such as carburizing, nitrid-ing, flame or induction hardening, or com-bination heat treatments like carbo-nitrid-ing. Each of these treatments, in combi-nation with various steel compositions,gives different results, and the final choicedepends on the intended use of the fin-ished gear.

CAST IRON

Like steel, cast iron is an alloy of ironthat is used in the manufacture of gears.The major difference is the carbon con-tent. The typical range for cast iron is from1.7 wt % carbon to approximately 4 wt %carbon. Along with the carbon contentbeing higher, the carbon molecule isgraphite and only a minor portion is com-bined with iron. The free graphite in thestructure reduces the strength of the metalbut allows the gear to operate with lowamounts of lubricant. Because of the

strength loss when compared to steel, thegears are not typically used in high speedor high load applications.

NON-FERROUS

Bronze, an alloy consisting of 90% cop-per and 10% tin, is the most common non-ferrous metal used to manufacture gears.However, other materials are also usedsuch as aluminum, zinc, manganese, lead,silicon, iron, and nickel. Each of theseelements varies the strength and loadbearing capabilities of bronze as neces-sary for the gear application. Bronze,

which boasts a modest load carrying andspeed rating, handles sliding loads partic-ularly well so it is used in the manufactureof worm wheels. The use of other non-fer-rous metals for gears is usually based onthe special properties of the material suchas their die-casting or forming properties.Although die-casting is used extensively,these gear types are usually restricted tosmall machines where power transmissionand durability requirements are moderate.

NON-METALLIC

The first non-metallic gears weredesigned for noise reduction and made ofrawhide. Later, a fiber material withimpregnated plastic was developed.Today, the primary non-metallic materialused is plastic and made from a variety ofdifferent types of plastic such as nylon,

acetal, ABS (acrylonitrile butadienestyrene), or polycarbonates. Noise reduc-tion is the basic reason for using plasticgears. The standard tooth form requiresmodification to accommodate the deflec-tion that will occur. The modified toothform, as well as advances in manufactur-ing processes and availability of plasticwith varying properties, has increased theapplications in which plastic materialgears can be used. Small high-speedunits like power hand tools, home appli-ances, instruments and automotive parts


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are some of the applications using plastic

gears. When fillers, such as polytetrafluo-roethylene, molybdenum disulfide, and sil-icones are used the tensile strength and

lubricity of the gears are enhanced while

increasing wear resistance. Advances intechnology have made it possible to man-

ufacture gears of plastic up to 75 inches indiameter or larger. The casting of plastic

gears in large diameters from the varyingmaterials provides high strength,resilience, and wear resistance. In some

cases, molybdenum is added to increasethe self-lubricating properties of nylon.

SURFACE FINISH

The gear finishing process has a signifi-cant effect on the life of a gear. Theprocess must be chosen with recognitionof the gear metallurgy and the applicationfor which the gear is intended.

Surface finish of gear teeth is related tothe lubrication condition that exists in a setof running gears. The ratio of the oil filmthickness to the composite surface rough-ness of the gears, designated lambda,

quantitatively describes the lubricationstatus of the gear set. This is expressedin the equation = h/s, where is thelambda parameter, h is the oil film thick-ness and s the composite surface rough-

ness of the gears. Reducing surfaceroughness increases ; therefore, mini-mizing adhesive wear (scuffing).Reduction in roughness can be achievedby grinding, lapping or honing. It is gener-ally conceded that for values greaterthan four, ideal lubrication conditions exist.Equations and charts are available fordesign engineers that enable them to cal-culate for a given set of gears.

Table 2 relates finishing procedures tosurface texture. The break-in, or run-inperiod, is also an important factor inobtaining a smooth surface finish. Insome cases, special lubricants are neededfor the run-in period. In addition, surfacetreatments, such as phosphating,Tufftride, NoSkuff, sulfurizing, or platingwith tin, silver or copper, may be used toprovide a sacrificial surface. The surfacefinish will directly effect the coefficient offriction of the gear tooth surface.

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Table 2 - Effect of Gear Finish on Surface Texture


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LUBRICATION

Other manufacturing practices used toextend gear life include hardening the sur-face of one gear and pinion, or selecting asuitable combination of materials. Twotypes of hardening are case-hardened and

through-hardened. Case hardened refersto hardening (a ferrous alloy) so that thesurface layer is harder than the interior ofthe material, while through-hardenedrefers to hardening (a ferrous alloy) so thatthe material hardness is the samethroughout the material. For example, asteel worm that has been grounded usedwith a bronze wheel for higher gear ratios(above 1.5) use pinions that are harder

than the gear. Shot peening has also beenfound to improve gear quality by work-hardening the tooth surface.

SPEED, LOAD, AND DIRECTION

When selecting a gear set, several con-siderations are necessary for efficientservice. These include the speed of oper-ation, the load placed on the gear drive,and whether or not the drive will have tochange direction. When evaluating theabove considerations, there are manycombinations that influence the designparameters of the gear set selection.Each application presents its own uniquerequirements for gear selection.

Speed of operation determines the styleof gear to be used. High-speed gears,3,500 feet per minute pitch line velocity orover, require a specific lubrication method.

High-speed gears require that the lubri-cant be applied under pressure and direct-ly into the meshing gear teeth. Splash orbath lubrication will not perform as well inhigh-speed applications because foamingor churning of the lubricant heats the oiland dissipates power. The gear may rejectthe oil before it reaches the mesh pointcausing insufficient oil film and excessivegear wear. In all cases, the equipmentmanufacturer has the best knowledge ofthe gear set and their recommendationsfor lubrication should be followed.

The load placed on a gear set by thedriven equipment is an important factor inthe design and selection of gears. Toaccommodate greater loads, more toothcontact, a wider gear face, increased shaft

diameters, greater center distance, andlarger bearings within a stronger case arerequired. For high load applications multi-ple tooth contact is desired. The materialused in the construction of the gear is alsoa consideration for high load carryingapplications. Lubrication requirementsinclude oils that will stand up under high-pressure conditions created by the load onthe teeth. Gear lubricants in this category

should have a high viscosity and be veryadhesive to the gear teeth surface.

As the operational speed of the gearincreases, inaccuracies in the gear setbecome more critical. One of the mostobvious problems with gear sets is thehigh noise level. In addition, dynamicloads on teeth that are caused by theseerrors may be a substantial part of thetotal transmitted load. A maximum contin-uous speed of 110% of the rated speed ofthe pinion should be attainable when avariable speed driver is used in conjunc-tion with the gear set. This rating is toinsure that the gear unit is capable ofoperation up to its trip speed without detri-mental effects.

Pitch line velocity, which takes intoaccount the size as well as the rotationalspeed, is the usual measure of gearspeed. The unit that is normally used to

measure this velocity at the tooth mesh isfeet per minute.

OPERATING ENVIRONMENT

The environment in which gears operatealso affects the design and manufacture ofgear sets. Excessive hot or cold ambienttemperatures, high systemic tempera-tures, unusually wet or dry environments,and particulate concentrations of abrasivematerials in the atmosphere all affect theperformance of the equipment. Each of


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these factors must be considered whendesigning a gear drive and selecting theproper method of lubrication. An opengear design may work in one application;but in a different environment, the gear

design (or similar term) may require anenclosure to protect against contamina-tion.

SAFETY

As with all operations and equipmentinstallations, safety is of primary concern.Protection of personnel from injury, as wellas protection of operating equipment,

must be a part of the design considera-tions of all gear sets. Equipment guardsand enclosures must be designed to pro-tect personnel from contact with movingparts and at the same time allow theequipment to accomplish the designedtask. Because the gear is a rotationalpiece of equipment, open gear sets musthave guards to protect personnel againstcontacting moving parts, and to preventapplied lubricants from slinging off ontowalk areas creating a slipping hazard.Proper gear material, size, housingdesign, bearing loads, and lubrication areall conditions that must be consideredwhen determining the safe installation andoperation of any gear set.

INDUSTRY APPLICATIONS OFGEARS AND LUBRICATION

EARTH HANDLING EQUIPMENT

The operating parts of earth-handlingequipment such as shovels, draglines,backhoes and cranes are usually exposedto conditions that will contaminate thoseparts with water, abrasive dust and dirt.Lubricants with high adhesive qualitiesare typically used to aid in protecting andmaintaining lubrication of the movingparts. The adhesive qualities of the lubri-cant provide a thick coating of oil orgrease that minimizes the effects of con-

taminants. This reduces wear and helpsprevent rusting of exposed parts. Thistype of lubricant also helps assure effi-cient lubrication for long periods of time.The thick coating also helps reduce oper-

ating noise. Because earth-hauling equip-ment typically operates on very uneventerrain, shock loads can be excessive.Highly adhesive lubricants aid in reducingthe effects of shock loading on the equip-ment.

PAPER MILLS

Chemical mixers, digesters, agitators,

and pumps are some of the operatingequipment using gear drives in the paperindustry today. Each of these devicespresents a unique set of conditions forboth gear types and lubrication require-ments. The use of caustic and acidicprocess fluids, high load conditions, andcontinually wet environmental conditionspresent application, design, and lubrica-tion challenges.

SUGAR MILLS

In sugar mills, gears operating thegrinders are quite heavily loaded andsome initial pitting of new gears may occurduring break-in. This initial pitting usuallysubsides following the break-in period.Some mills have completely open gearingwith no slush pans. Gear designs that areopen are lubricated with heavy residual

type lubricant that is normally appliedmanually. Other mills are equipped withenclosed gear cases using slush pans thatthe gears dip into. A less viscous typegear oil is used to permit self-lubricationduring operation. Greater care is taken insugar mills as in all food processing facili-ties to insure that accidental contamina-tion of product by the lubricant does notoccur and create toxic materials. Othertypes of equipment used in sugar milloperations include tractors, harvesters,elevators, conveyors, cane carriers, cane


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crushers and grinders, granulators, anddryers.

CEMENT MANUFACTURING

Cement processing involves high shockload and driver load applications. Theenvironment is filled with abrasive dustthat will rapidly increase wear of movingparts. Crushers, grinders, kilns and dry-ers all present special application anddesign requirements for gears and lubrica-tion.

There are three basic types of crushers:roll, jaw, and gyrator. These are used for

crushing rock in cement making as well asother industrial applications. Rock isreceived in sizes up to four feet in lengthand reduced through the crusher to fourinches or less. Along with the tremendouspressures required to accomplish thisreduction, large amounts of heat are alsopresent. Gears, bearings, and housingsare exposed to high temperature, heavyshock loads, and considerable amounts ofdust and abrasive grit. Lubrication of thisequipment is often handled by a circulat-ing system made up of a pump, sumptank, filter assembly, and a heat exchang-er designed to keep the lube oil at approx-imately 130F.

Grinders in the cement industry consistof three basic designs: ball, rod, and peb-ble mills. These grinders are typicallyused to reduce the crushed rock to veryfine sizes. The mill usually contains,

around 40 to 45% by volume, steel balls orrods within the cylinder that revolve hori-zontally on its axis. The crushed rock isplaced inside the mill and rotated, causingthe balls or rods to strike the rock, reduc-ing it in size. As the material tumblesthrough the mill, the load on the drivervaries. This is the source of shock loadingin a grinder. A ring gear that is located atone end of the mill is used to rotate themill. Normally, a guard or set of guards isused to protect personnel and keep loosematerial from entering the gear area.

Adhesive residual type gear oil is used asthe lubricant.

In the cement industry, kilns and dryersare large diameter cylindrical devices,usually ten feet in diameter and 200 to

400 feet in length, used to calcine or dehy-drate cement. They are mounted horizon-tally, rotate at about one rpm (revolutionsper minute), and operate with internaltemperatures around 2,700F. This lowrotational speed is accomplished througha speed reducer and pinion gear arrange-ment. In order for a lubricant to be effec-tive, it must be able to resist oxidation atthese high temperatures. Additionally, the

lubricant must adhere to the gear teeth toprevent rusting from water washing andprovide protection from particulates.Other gear-operated equipment associat-ed with cement processing includescranes, conveyors, elevators, shovels,pumps, pulverizers, centrifuges, andthickeners.

MINING

Mining has several special gear applica-tions used throughout the industry inequipment such as hoists, crushers, shak-ing screens and conveyors, shuttle cars,cutters, locomotives, and loaders. Thegears used in operating mine hoists areamong the largest used in mining opera-tions. If the gears are open, they can belubricated with residual type oil; but ifclosed, the gears usually dip into a reser-

voir of gear oil or heavy cylinder oil. Theunit is usually housed in a building so con-tamination problems are greatly reduced.

Mining machines, used underground,operate several sets of gears to gatherand convey the mined product from themine. The equipment has a drive unit,gathering unit, and conveyor unit built intoa single machine. The machine operatesin extremely dusty and wet conditions.Some of the more adverse conditions ofoperation are high temperature, dust andwater contamination, and high shock load


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operation. Each of these conditions offersa special need for lubrication of themachine to prevent premature breakdownof equipment.

POWER GENERATION

One type of power generation is the useof wind turbines to produce the rotationalenergy to drive a generator. Worldwide,wind turbine applications have rapidlyincreased over the past several years.The use of a gear drive in a turbine con-verts the 60 rpm operation of the rotor to1,800 rpm rotational speed at the genera-

tor. The gear drive is housed inside thecovering of the turbine assembly and,except for heat, is protected from environ-mental conditions. The lubrication require-ments in gear drives for wind turbines gen-erators are low maintenance, long-life oilsand greases.

Cogeneration is another type of powergeneration. This field of power generationhas seen a change from the traditionalpower plant to a system of cogeneration.Natural gas fired turbines develop thepower necessary to produce rotationalenergy to operate the generators. Geardrives are used to transfer the power pro-duced by the turbine to the generator. Thetypical application employs a reductiongear system that provides stable continu-ous power output. The reduction gearassembly is typically located on the com-pressor end of the turbine to reduce its

exposure to the high temperatures pro-duced at the turbine end of the unit. Theenvironment is relatively clean in compari-son to many industrial applications.Overall loading is moderate and shockloading is very low. Average input speedsrange around 10,000 to 15,000 rpm, andoutput speeds are typically 1,800 rpm.These speeds are necessary to producepower outputs at 60 Hz. The relativelyhigh input speed requires some form ofheat exchanger on the lube oil system tocool the oil.

GEAR LUBRICATION ANDLUBRICANT SELECTION

LUBRICATION REGIMES

Lubrication is primarily concerned withreducing frictional resistance, whichoccurs at the interacting surfaces of twosolids when one is moved relative to theother. Any material introduced betweentwo such surfaces to accomplish a reduc-tion in friction is called a lubricant. Thelubricant may also serve to remove heatgenerated by the surface interaction andflush away contaminants.

There are three regimes of lubrication,which are illustrated in Figure 17 anddescribed as:

Fluid Film Boundary Mixed Film

Fluid film lubrication is maintained aslong as there is an uninterrupted film oflubricant between the moving surfaces.This is determined by the ratio of filmthickness to composite surface rough-ness, previously defined as . Film thick-ness is a function of the viscosity of thelubricant, speed of the moving surfaces,and the load. Fluid film conditions aregenerally considered to exist at greaterthan four. There is no metal-to-metal con-tact and consequently there should beessentially no wear.

At the other extreme, under severeoperating conditions, the lubricant oil film

does not exist, and a thin film of molecu-lar dimension referred to as the boundarylayer maintains surface separation. Inthis regime, is much less than two andthe chemical nature of the lubricant-metalsurface interface predominates.

In many cases, conditions are such thatmixed lubrication exists, wherein the loadis partially supported by pressure devel-oped in the lubricant film and partially bycontact of the surfaces. Under these con-ditions, the fluid film is pressurized but istoo thin to avoid contact of the highest


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asperities of each surface. In otherwords, is about two, and lubrication is

only partially provided by the boundaryfilm. This complex lubrication regime mayrequire special lubricants that containanti-scuff (EP) additives.

LUBRICANT SELECTION

There are a variety of lubricants avail-able for use with gear drive applications.Considering the wide range of gear appli-cations and the environment they operatein, the selection of the proper lubricantbecomes very important to the long-term

efficient operation of the device.Residual oils and heavy mineral oils can

be used on spur gears with relatively lowrpm ratings (


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As previously noted, mineral oils ofintermediate viscosity are often blendedwith small amounts of fatty oils or otherpolar materials such as vegetable oils.This will provide improved lubrication for

gears operating under mixed-film condi-tions where plain mineral oils will not suf-fice and the higher load-carrying capacityof EP (anti-scuff) products is not required.These oils are particularly suitable forbronze-on-steel worm gear sets.

Sulfurized esters and organic sulfur-phosphorus compounds have been foundto be effective EP (anti-scuff) additives forgear lubrication. These additives produce

a protective film that protects againstscuffing of the gear surface. EP (anti-scuff) additives are especially effectivewhere steel-on-steel surfaces areinvolved and where spot temperatures arehigh enough for a chemical reaction totake place. The additives are not particu-larly effective when steel is in contact withsofter metals such as bronze, brass, bab-bitt, cadmium or aluminum. Under heavyloads, these softer materials deform,increasing the load-bearing areas wheretemperatures may not rise high enough tosupport the chemical reaction of the EPadditives. The additives can also be tooactive with both ferrous and non-ferrousmaterials. In such cases, acceleratedcorrosive wear may occur. If pressure orshock loads are extremely high, there is apoint at which EP agents can no longer beeffective. Excessive wear and pitting will

occur at this point. EP lubricants cannotcompensate for design or mechanicalinadequacies, and under these conditionsthe use of EP lubricants will only postponethe final failure of the gear set.

Greases are frequently used on opengears where it is impossible to make anenclosure that will retain oil. Grease isalso necessary in applications whereaccessibility or ease of recharging oil isdifficult. In applications where grease isused in a circulation system, the grease isfluidized or softened to permit flow.

Synthetic oils are manufactured chemical-ly to produce a product with special prop-erties that will improve performance inboth severe and normal operating condi-tions. The stability of synthetic oils is

much better than those of petroleumbased oils and allows them to withstand awider temperature range of operation.They have higher viscosity index valuesand in some cases have a greater loadcarrying capacity with better lubricity.However, synthetics are not a cure-all forthe gear industry. Each lubricant has itsown set of limitations and restrictions. Aswith all lubricant applications, the user,

manufacturer, and lubricant suppliershould be consulted and information coor-dinated to insure that the proper lubricantfor the application is selected.

AGMA DOCUMENTS USED FOR

LUBRICANT SELECTION

The American Gear ManufacturersAssociation (AGMA) has produced a lubri-cant guideline specification (ANSI/AGMA9005-D94) that provides lubricating guide-lines for enclosed and open gearing thatare installed in general industrial powertransmission applications. This standardis not intended to supplant specificinstructions from the gear manufacturer,but to be used as a guideline by designersand manufacturers of industrial powertransmission gears. The standard can beused as a guide in applications where

manufacturer specifications or recom-mendations do not exist.

The AGMA has also developed a speci-fication standard concerning gear failure,wear and the terminology associated withthese conditions. The information in thestandard is based on use and operation ofsteel gears, but the conditions describeddo apply to gears of other materials aswell. ANSI/AGMA 1010-E95 providesnomenclature for general modes of geartooth wear and failure. Because of themany interpretations surrounding gear


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failure, the standard does not define thoseterms. The application and operation ofthe gear set govern whether or not a gearhas actually failed. When attempting todetermine the state of wear existing in an

application of gears, this standard will behelpful in evaluating the condition of thegear. The actual solution to a gear failurerequires detailed investigation by theuser, and analysis by specialists to deter-mine the actual cause and the correctiveaction.

In addition to ANSI/AGMA 1010-E95,ANSI/AGMA 925-AXX, Effect ofLubrication on Gear Surface Distress, is

an excellent resource in troubleshootingand analysis of gear wear and failure.

RELIABILITY MAINTENANCEFOR INDUSTRIAL GEARS

Proactive and/or predictive mainte-nance are essential to the long-term effi-cient use of industrial gears. A plannedsystematic approach to monitoring andmaintaining gear installations has provento extend the life and efficiency of gears.As with all changes in process conditions,there are parameters that, when moni-tored, define the change as it occurs.Operating variables like temperature,vibration and mechanical movement areindicators of changing conditions in a geardrive unit. Lubricating fluid cleanliness,fluid analysis for contaminants, and visualinspections of fluid reservoirs are just a

few points to look at in a systematicapproach to maintenance. Although wearwill occur in mechanical equipment opera-tion, a system of proactive or predictivemaintenance will allow the point of failureto occur under planned conditions.Planned down time is much more afford-able when requirements for productpreparation have been made and every-one is aware the unit will be offline for aspecific period of time. Reduced productloss, lower hazardous material genera-tion, and avoidance of critical process

conditions aim to save valuable companyresources.

Another type of reliability maintenanceis preventive maintenance, which useshistorical data to predict failure of system

components. From the prediction, a pointin time prior to the failure is chosen totake the equipment off-line and to performmaintenance. Using this approach, thefailure is avoided and less unplanned out-ages occur. In efforts to make the opera-tion of gear sets more efficient, there arespecific tasks to be completed.Monitoring the condition of lubricating oilis important to preventing failures. Oil

samples should be analyzed for contami-nants, build up of corrosive substances,water content, and abrasive particulate ona scheduled basis. Installation of newequipment or the replacement of existingequipment should be tracked on a com-prehensive quality assurance program toestablish baseline-operating parametersfor future reference.

Lube oil system contamination is a largecontributor to premature gear failure. It isvery important to keep the lubricating oilreservoir clean. Protecting the vent areafrom dust and particulate matter, pressur-izing the system in high dust areas, orusing loop filtration are a few ways to pro-tect the lubricant and reduce sources ofwear in gear sets.

Production costs, environmental con-cerns, and handling and storage of haz-ardous waste are just a few reasons to

reclaim and/or refortify lubricating oils.Whether to dispose of, reclaim, or reforti-fy lubricating oils can be determined bythe close monitoring of the lubricantscondition through Reliability MaintenancePrograms.

WEAR AND GEAR DISTRESS

Gear wear is defined as a change ingear tooth surface involving the removalor displacement of gear constructionmaterial due to mechanical, chemical, or


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electrical action. Mild wear is consideredto be normal wear in many applications.Moderate wear, and sometime severewear, can also be considered normal insome situations.

Gear distress can involve wear, defor-mation, and/or fracture. Modes of wearare a description of how and what hap-pens to a gear during operation. Themode can also indicate why the wearoccurred. Modes of wear include abra-sion, adhesion, corrosion, polishing, scal-ing, cavitation, electrical discharge, ero-sion, and scuffing. Each has its ownunique characteristic cause and effect on

the mating faces of the gear.Abrasive wear, shown in Figure 18, is

the most common form of wear and usu-ally results from the presence of foreignmatter in the lubricant. The type of lubri-cant in use is of little concern at this pointand the obvious remedy is to clean the

lubricant and system. Introduction ofclean lubricant into a contaminated sys-tem is of minimum benefit and very costly.If abrasive wear is left undetected, it willeventually become destructive, causing

gear failure. Polishing is fine scale abra-sion caused by contamination of the lubri-cant with fine particulates .

Adhesive wear, shown in Figure 19, iswear caused by sliding contact of matingtooth surfaces under boundary lubricationconditions. Affected areas of the gearteeth will have the appearance of a sur-face that was welded to its mating surfaceand then torn loose leaving a rough or

matted surface. During break-in, adhesionis mild. Severe adhesion is called scuff-ing. Addition of anti-scuff (EP) additivesto the gear oil reduces the coefficient offriction and decreases scuffing.

Corrosive wear, shown in Figure 20, iscaused by chemical attack on the tooth

Figure 18 - Severe Abrasive Wear


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surface. Sources of corrosion includechemical contaminants, water, overlyaggressive additives in the lubricant, andproducts of hydrolysis or oil degradation.Unlike other forms of wear, corrosion can

affect any surface that comes in contactwith the corrosive agent, not just theactive tooth profiles. This characteristiccan be helpful in diagnosing a corrosionproblem. If the corrosion produces a fine

Figure 19 - Adhesive Wear

Figure 20 - Extensive Corrosive Wear


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abrasive material, polishing can occur andthe evidence of corrosion on the matingsurfaces will be gone. However, the pitsextraneous to the mating surfaces of thegear teeth can still carry evidence of the

corrosion. Chemical analysis of the oilplus knowledge of the operating systemcan also be helpful in determining thepresence of corrosive wear. If the lubri-cant is found to be corrosive, it should beremoved from service immediately andthe system thoroughly flushed andcleaned.

Contact fatigue is the natural agingprocess of a gear tooth. During its life, a

gear goes through millions of cycles ofelastic deformation. This cyclical stresscauses fatigue cracks to form at or nearthe surface of the active flank of the geartooth. When the crack grows to the extentthat a piece of the surface material is sep-arated, a pit forms. On softer gears, suchas through-hardened gears, the size ofthe pits is on the order of millimeters. This

is called macropitting or classical pitting,and is shown in Figure 21. The edges ofmacropits will be sharp and angular.During the break-in period, macropits mayform along the pitch line. This is known

as initial pitting and can be considerednormal in many gear applications. If pitscontinue to form, however, it is called pro-gressive pitting and corrective action isnecessary. If the loading on the gear isuneven or if there are defects in the activeflanks, premature contact fatigue pittingmay occur at those points.

When many pits coalesce into a largepit, spalling occurs. Spalling, which can

cover a very extensive area of a toothflank, often masks the original source ofthe distress. If the original distress moderemoves or weakens part of the gear sur-face, it may promote premature contactfatigue. Similarly, if the loading is uneven,premature fatigue could occur where theload is concentrated. If spalling occurs,you still may be able to discern the origi-

Figure 21 - Progressive Macropitting


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nal distress mode by the location of the

spalling, the condition of the area near thespall, or by clues extraneous to the geartooth.

When the gear is surface-hardened, pit-ting may occur on a much smaller scale,typically about 20 microns deep. This iscalled micropitting. The affected surfaceappears as if it has a frosted or matte fin-ish. Micropitting propagates by the sameprocess as classical pitting, and usuallyappears early in the life of case hardenedgears. Quite often, micropitting stopsafter break-in because of the improvedsurface finish afforded by break-in.However, micropitting can escalate intofull scale pitting, leading to destruction ofthe gear teeth.

Since the pits are too small to be distin-guished with the naked eye, they appearas a gray stain. This coloration is due tosome of the reflective light absorbed by

the pits. Micropitting is sometimesreferred to as frosting, gray staining, or

peeling. These are all referred to as small

Hertzian contact fatigue pits.Micropitting is sometimes confused with

polishing. A polished surface will beshiny, while micropitting, by contrast,leaves a matte finish.

Another type of gear distress is plasticdeformation. Plastic deformation occurswhen the load exceeds the yield strengthof the metal. If compressive loads arehigh or vibration causes peak loads, thetooth surface can become peened orrolled. The process is the same as thehead of a cold chisel after repeated blowsby a hammer.

Rippling, shown in Figure 22, is a formof plastic deformation that is caused byshearing stress at the surface of the metalas well as compressive stresses. Thegear surface appears as waves in a fishscale pattern. The flow of the pattern willbe in the direction of the line of force.

Rippling has most commonly been foundon spur gear sets highly loaded and oper-

Figure 22 - Rippling


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ating at relatively slow speeds.Ridging, shown in Figure 23, is plastic

flow due to high spots on a gear plowingover the mating surface. Ridging mayoccur on hardened hypoid gears whenlubricants having anti-weld or anti-scuffingproperties are used. In one sense, ridging

is evidence that the lubricant has per-formed as intended since the pressureshave exceeded the yield point of the steelwithout causing rupture of the lubricantfilm. However, under these conditions, itis likely that some wear will occur simulta-neously.

Gear teeth are loaded like a cantileverbeam. If this load is above the yieldstrength of the metal, plastic deformation

will occur in the form of bending. If theload is below the yield strength, bendingfatigue may occur. The origin of a bend-ing fatigue crack will be at a weak spot,such as an inclusion, a grinding crack or apit, and/or an area of stress concentra-tion, such as at the root. When the crackpropagates to a critical point, fracture willoccur. A fatigue crack can usually betraced to its origin by observing the frac-ture face and noting the beach marks. If

a tooth breaks due to pitting, the fracture

starts in the middle of the tooth near oneof the pits. Tooth breakage due to suddenoverload is called tooth shear. It does nothave the characteristic beach marks,although the fractured surface is usuallyquite rough and originates in the rootarea.

SUMMARY

The design, selection and application ofindustrial gears is a science in and ofitself that is constantly evolving throughadvances in gear technology, as well aschanges in application. For proper instal-lation and efficiency of operation, the sys-tem must meet all the requirements of the

driven equipment, environmental con-cerns, lubrication requirements, and con-struction costs.

The information presented in this articleis an outline of the basic criteria neces-sary to operate and maintain industrialgears drive. The information presented isin no way intended to be complete. Forcomplete design and selection data, theprofessional organizations referencematerial and standards mentioned in the

article should be consulted.

Figure 23 - Ridging

Documents

34110418 Lubrication Industrial Gears