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Tribology

2

Outline

Basics of Tribology: Introduction and definitions

The different types of friction and wear

Lubricants and surface treatments

Rheometry and Tribometry: The Rheo-Tribometer

Measurements on the Rheo-Tribometer• Stribeck curve• Static friction tests• Stick slip measurements

Rolling element bearing device

Measurements

Food tribology

3

Friction: Good or bad?

Problems related to friction A lot of energy is wasted by friction related heat production e.g in cars, wind

wheels etc. (No perpetum mobile) Friction reduces durability of material due to wear Functionality loss and annoyance due to noise

Huge sums of money are lost because of friction and wear

Benefits related to friction

Friction is required for functionality of material (screws, tires, brakes etc.)

Friction enables music (bow instruments)

Friction is required for a lot of todays applications

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Different Types of Friction

Static FrictionStatic frictional forces from the interlocking of the irregularities of two solid surfaces will increase preventing any relative motion until

some limit where motion occurs.

Sliding friction

Sliding friction is when two solid

surfaces slide against each other.

Rolling frictionWhen a body rolls on a surface, the force resisting the motion is termed rolling friction.

Static Friction > Sliding Friction > Rolling Friction

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Tribology: Friction-Coefficient

Friction Force FF

Load FL

Body 1

Body 2

Sliding

µ is dimensionless, often f is used instead of µ

Amontons Laws: FF is independent on area FF ~ FL

Range of Friction Coefficients:Dry sliding: 0.05 (PTFE under high loads) – 5.0 (gold sliding in vacuum). Steel: 0.3 - 0.6.

Lubricated sliding: 0.03 (hydrodynamic conditions) – 0.15 (boundary conditions)

Rolling friction: 0.002 (fully lubricated) – 0.05 (running dry)

Friction Coefficient:µ = FF / FL

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Parameters influencing friction

Tribological System

Environmental Conditions:

• Temperature

• Humitidity

• Time

• Etc...

Friction Partners

• Surface parameters

• Chemical reactivity

• Elasticity

• Etc...

System Factors:

• Relative Speed

• Direction of movement : • unidirectional• bi-, multidirectional

• Normal load

Lubricant

• Chemistry

• Viscosity

• Stability

• Etc...

7

Wear

Definition:

Wear is the erosion of a solid surface by the action of another surface.

There are four principle of wear process:

Adhesive wearWhen two solid surfaces slide over another the asperities are plastically deformed and eventually welded together. As sliding continues, these bonds are broken, producing cavities on the surface and abrasive particle which contribute to future wear of surfaces

Abrasive wear When material is removed by contact with hard particles, abrasive wear occurs. The particles either may be present at the surface of a second material or may exist as loose particles between two surfaces

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Wear

Corrosive wear Corrosive wear is deterioration of useful properties in a material due to reactions with its environment (e.g. oxidation).

Surface fatigue Surface fatigue is a process by which the surface of a material is weakened by cyclic loading.

Parameters influencing wear: Wear partners: Material, hardness, surface roughness, shape, friction coefficient Particles: Chemistry, particle size Load Movement: Sliding, rolling pushing, uni- or bi-directional Environmental conditions: Air, inertgas, vacuum, humidity Temperature Sliding distance

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Lubrication: The Stribeck curve / Speed ramp

1

2

3

4

5

Sliding speed

Fri

ctio

n c

oef

fici

ent

1 Static friction, no movement, no wear

2 Boundery lubrication, very low sliding speed, solid contacts, high wear

3 Mixed lubrication, moderate sliding speed, partial solid contact, moderate wear

4 Elasto-hydrodynamic lubrication, intermediate sliding speed, thin lubrication film

5 Hydrodynamic librication, high sliding speed, developed lubrication film, no wear

Richard Stribeck

1861-1950

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Static Friction, Boundery Lubrication

Direct solid contact of the friction

partners

No movement

No wear

Force is high enough to induce a very low speed

Solid friction, Stick slip Load is carried by the contact points (asperities)

and the shear is taken by the absorbed

lubrication molecules. No hydrodynamic pressure build up No lubrication film present Asperities are protected by adsorbed lubricant

molecules and/or a thin oxide layer. High wear

1) Static Friction 2) Boundery Lubrication

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Mixed Lubrication, Elasto-Hydrodynamic Lubrication

3) Mixed Lubrication

Low speed Low hydrodynamic pressure is build up in

the lubricant The loading is carried by a combination of

the hydrodynamic pressure and the

contact pressure between the asperities of

both surfaces. Lubricant film only in between contact

points Moderate wear

4) Elastohydrodynamic Lubrication

Intermediate speed Hydrodynamic pressure increases. Full, but still very thin lubrication film. Elastic deformation of the contact

points. Lubricant viscosity increase due to

increasing pressure. No wear

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Hydrodynamic Pressure

1) Solid contact no water flow under the skis

2) Water pressure under the skis builds up as the drainage of water from the skis is lower than the amount of incomming water

3) Water pressure increases further acting against the normal force resulting in decrease of friction

4) Water resistance increases with speed until the skis cannot hold anymore (crash)

Water ski:

Water resistance force

Normal force

Water pressureSolid friction

1 2

3 4 Speed:

1 < 2 < 3 < 4

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Hydrodynamic lubrication

5) Hydrodynamic lubrication

High speeds The surface asperities are completely

separated by a lubricant film. The load and hydrodynamic pressure

are in equilibrium Thick lubrication film No wear

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Types of Lubricants

Base oil

Additives:• Friction Improver• Dispersing Chemicals• Viscosity Index Improver• Foam Inhibiters• Oxidation Inhibitors• Pour point Improver• Wear Protection

Base oil

Thickener• Metallsoaps• Silicagel• Bentonite

Additives:• Friction Improver• Foam Inhibiters• Oxidation Inhibitors• Wear protection

Base oil

Thickener• Metallsoaps• Silicagel• Bentonite

Additives:• Friction Improver• Foam Inhibiters• Oxidation Inhibitors• Wear protection

Solid Lubicants• Molypdenum sulfite• Graphite• Coper

Oils Greases Pastes

Viscosity of the oil depends on application:

Low Viscosity for low loads and high sliding speeds

High viscosity for high loads, low sliding speeds and high temperature

Pour point has to be taken into account at low temperatures

Greases are used where oil is not applicable due to con-struction or low temperatures.

A grease acts like a sponge as the thinkener binds the oil and releases it under pressure.

Can only be used for mixed lubrication, no hydrodynamic lubrication possible. Useful for high loads.

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Types of Lubricants

Solid Lubricants

Solid lubricants produce sliding or seperation films on metall surfaces due to their structure and physical-chemical properties. They are used as powders.

Examples: Molypdenum sulfite, PFTE, graphite, coper

Solid libricants are oft used as primary film in combination with lubricants.

Solvents / Water

Solid Lubricants• Molypdenum sulfite• Graphite• Coper

Additives:• Dispersing agents• Corrosion inhibitors• Wetting agents

Adhesive agent:• Synthetic resins

Dry lubricants /Anti-friction coatings

Anti-friction coatings are surface fixed solid lubricants

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Tribological Problems

Theory

Analysis

Model

tests

Module

tests

Product

tests

Field

tests

Tribology

costs

Product

17

Tribometry, Modelling

The field test in the final device is the most accurate test for friction partners and lubricant. As this is a very expensive model systems are normally used in the first stage of development.

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Tribology Device

Ball-on-3-plates

Based on a cooperation between

Werner Stehr(Dr. Tillwich GmbH Werner StehrMurber Steige 2672160 Horb-Ahldorf)

and

Anton Paar

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Tribology Device: Ball-on-3-Plates Principleattached to a MCR Rheometer

Side View Top View

FN

Normal-force

M: Torque

M: Torque

FN

Normal-force

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Tribology Device: Ball-on-3-Plates PrinciplePeltier Bottom Plate and Peltier Hood

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Tribology Device: Ball-on-3-Plates Principleattached to a MCR Rheometer

Specifications

Rheometer Tribometer

Normal Force FN:0.01N - 50N Normal Load FL: 2 N - 70 N

Torque M: 0.1µNm - 200mNm Friction Force FF: 0.01 N - 44.5N

Rot. speed n: 10-6 - 3000 rpm Sliding speed sv : 1 10-8 - 1.41m/s

Temperature: -40°C - 200°C

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Friction Coefficient for a Steel Ball on POM:Dry, Penetrating Oil, Motor Oil

0.01

0.1

1

0 200 400 600 800 1,000 1,400mm/sSliding Speed vs

Reproducability: Good for dry and motor oilFor penetrating oil general behavoir is reproducable

dry

penetrating oil

motor oil

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Friction Coefficient for a Steel Ball on POM:Dry, Penetrating Oil, Motor Oil

Rotational speed: 0.001 - 3000 rpm; NF= 10N, Normal load NL= 14NLogarithmic scale for sliding speed

0.01

0.1

1

10-4

10-2

100

102

104

mm/sSliding Speed vs

Steel/POM CR dry

Friction Factor

Steel/POM CR penetrating oil

Friction Factor

Steel/POM CR motor oil

Friction Factor

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Stick Slip

Stick slip is usually an unwanted effect occuring at very low sliding speeds in the boundery lubrication regime. It leads to vibrations and noise

Examples: Noise of train in a turn, sound of a violin, window cleaner in dry conditions, wet finger on the rim of a crystall wine glass

Prevention of stick slip:• Adequate materials and surfaces• Use lubricants• Choose higher speeds• Dithering (Vibration with small amplitude and adequate frequency)

Static Friction > Pulling Force Static Friction < Pulling Force Sliding

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 120mmSliding Distance ss

Steel/rubber dry

Friction Factor

Steel/rubber, 10mPas silicone oil

Friction Factor

Steel rubber, lubricant grease

Friction Factor

Measurements on a Steel/Rubber System

Measuring conditions: Steel/rubber dry and lubricated, Normal load 14 N, 10 rpm

run in effects

Stick slip effects can be observed in dry and oil lubricated conditions but not when lubricated with grease.

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Tribology of Lubricants:2 Different Greases at 25°C and - 40°C

Stribeck curves: Friction coefficient as function of sliding speed

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.1 1 10 1,000mm/sSliding Speed vs

CSR NGLI 0 25°C

CSR NGLI 2 25°C

CSR NLGI 0 -40°C

CSR NLGI 2 -40°C

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Tribology of Lubricants:2 Different Greases at 25°C and -40°C

Static Friction: measured

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.0001 0.01 100mm/sSliding Speed vs

CS NLGI 0 w/o run-in 25°C

Friction Factor

CS NGLI 0 with run-in 25°C

Friction Factor

CS NLGI 0 w/o run-in -40°C

Friction Factor

CS NLGI 0 with run-in -40°C

Friction Factor

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Rolling-element Bearing measuring fixture

Measuring cones:Min. inner bearing diameter: 3 mmMax. inner bearing diameter : 25 mmMin. outer bearing diameter: 10 mmMax. outer bearing diameter: 42 mm

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Performance tests of rolling element bearingsLogarithmic speed ramp 0.1 to 3000 rpm

Load 10N, Speed ramp 0.1 to 3000 rpm, -40°C

0

5

10

15

20

25

30

35

40

mNm

M

1 10 100 1 0001/min

Speed n

CR NLGI 0 -40°C

M Torque

CR NLGI 1 -40°C

M Torque

CR NLGI 2 -40°C

M Torque

Best high speed performance for grease of NLGI class 2

Grease of NLGI class 0 with the highest friction at high speeds

Torque decrease at high speeds due to friction heating

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0,1

1

10

100

1 000

10 000

1/min

n

0 5 10 15 20 25sInterval Time tint

Roll out NLGI 0 -40°C

n Speed

Roll out NLGI 1 -40°C

n Speed

Roll out NLGI 2 -40°C

n Speed

Roll out NLGI 0 25°C

n Speed

Roll out NLGI 1 25°C

n Speed

Roll out NLGI 2 25°C

n Speed

Roll out NLGI 0 60°C

n Speed

Roll out NLGI 1 60°C

n Speed

Roll out NLGI 2 60°C

n Speed

Example: Roll out Test

- 40°C25°C

60°C

Load 10 N, Step to 3000 rpm for 10 seconds and then roll out (only roll out intervall plotted)

Roll out test proves the results of speed ramps

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Wood Polymer Composites (WPC)

Advantages WPC:

• Better humidity resistance

• Improved rigidity

• Smaller expension coefficients

The higher the wood fraction the lower the friction coefficient

Terrasses made of WPC tends to be more slippery the higher the wood content. Effect is even more pronounced when lubricated with water (rain).

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.00001 0.0001 0.001 0.01 0.1 10m/s

Sliding Speed vs

Stribeck PP 3

Friction Factor

Stribeck PP + 30% wood

Friction Factor

Stribeck PP + 50% wood

Friction Factor

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Food Tribology: Key Question

Is there any quantitative method to predict or determine mouthfeel?

Today the mouthfeel is determined by a sensory panel having several disadvantages:

Sensory panels are very expensive

Trained people are required

Time consuming

Limited reproducibility

Limited quantitative statement

The human factor!

It is impossible to avoid sensory panels for food design but if there would be a prescreening methode to determine mouthfeel the number of panels could be reduced which would save a lot of money.

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From Rheology to Tribology:Chewing, Swallowing, Drinking Mechanisms

Mouthfeel is more than just flow properties. It includes fracture and failure (large strain rheology), but it is also driven by friction and lubrication properties

In tribological experiments the soft texture of the mouth is represented by at least one elastomer friction partner.

Cargill discovered and patented some elastomers having specialized properties (elasticity, surface etc.) for food tribological measurements. (Patents: WO 2008/148538 A1, WO 2008/148536 A1).

In-mouth flow regimes during beverage consumption

Size

polymer

Controlling elements

Viscosity, diffusion, “solution-

dominated”

100 – 800 um

Rheology ZoneTribology Zone

Lubrication, solution limited, saliva limited,

“surface-dominated”

1 um

“creamy”, “slippery”

“thickness”, “mouth coating”Sensation

Composite behavior, related to phase ratio,

“particle-dominated”

Time course of drinking

Food Physics Methods

Model to Human Sensory Perception

“dry”,

“powdery”,

“gritty”,

“burn”

0 um

10 um

Yield stressrelated

Rheology Tribology

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Differentiation of Dairy Drinks

0.0

0.4

0.8

1.2

1.6

1 10 100 1000

Sliding Speed (mm/sec)

Fri

ctio

n F

acto

r

Fat Free Skim Milk

2% Reduced Fat Milk

Whole Milk

Heavy Cream

Half & Half

Drinking is a fast process performed at high sliding speeds.

Milk drinks having various fat contents can be tribologically differentiated

Strong stick slip effects observed at low sliding speeds for low fat milks

Applied Normal Force: 3 N

T = 20°C

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Conclusion

Features and Benefits

Tribology measurements on a MCR rheometer, i.e. the Rheometer as Tribometer

Low as well as high sliding speeds can be set very accurately (wide range) Stribeck curves Stick slip measurements Measurements in the boundery and mixed regime Long time measurement at desired speed

Possibility of force (torque) control Measurements of static friction coefficients

Temperature control by Peltier system from -40°C up to 200°C; with the additional Peltier hood uniform temperature distribution

Flexible choice of friction partners (Steel, polymers, elastomers etc.)

Rolling element bearing performance measurements

Food tribology