Post on 15-Dec-2015
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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
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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...
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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
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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 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