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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
4 Different Types of Friction Static Friction Static 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 friction When a body rolls on a surface, the force resisting the motion is termed rolling friction. Static Friction > Sliding Friction > Rolling Friction
5 Tribology: Friction-Coefficient Friction Force F F Load F L Body 1 Body 2 Sliding µ is dimensionless, often f is used instead of µ Amontons Laws: F F is independent on area F F ~ F L 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 = F F / F L
6 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 wear When 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
8 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
9 Lubrication: The Stribeck curve / Speed ramp 1 2 3 4 5 Sliding speed Friction coefficient 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
10 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 Friction2) Boundery Lubrication
11 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
12 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 pressure Solid friction 12 3 4Speed: 1 < 2 < 3 < 4
13 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
14 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 OilsGreasesPastes 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.
15 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
16 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.
18 Tribology Device Ball-on-3-plates Based on a cooperation between Werner Stehr (Dr. Tillwich GmbH Werner Stehr Murber Steige 26 72160 Horb-Ahldorf) and Anton Paar
19 Tribology Device: Ball-on-3-Plates Principle attached to a MCR Rheometer Side ViewTop View F N Normal- force M: Torque F N Normal- force
21 Tribology Device: Ball-on-3-Plates Principle attached to a MCR Rheometer Specifications RheometerTribometer Normal Force F N :0.01N - 50NNormal Load F L : 2 N - 70 N Torque M: 0.1µNm - 200mNmFriction Force F F : 0.01 N - 44.5N Rot. speed n:10 -6 - 3000 rpmSliding speed s v : 1 10 -8 - 1.41m/s Temperature: -40°C - 200°C
22 Friction Coefficient for a Steel Ball on POM: Dry, Penetrating Oil, Motor Oil Reproducability: Good for dry and motor oil For penetrating oil general behavoir is reproducable dry penetrating oil motor oil
23 Friction Coefficient for a Steel Ball on POM: Dry, Penetrating Oil, Motor Oil Rotational speed: 0.001 - 3000 rpm; N F = 10N, Normal load N L = 14N Logarithmic scale for sliding speed
24 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 ForceStatic Friction < Pulling Force Sliding
25 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.
26 Tribology of Lubricants: 2 Different Greases at 25°C and - 40°C Stribeck curves: Friction coefficient as function of sliding speed
27 Tribology of Lubricants: 2 Different Greases at 25°C and -40°C Static Friction: measured
28 Rolling-element Bearing measuring fixture Measuring cones: Min. inner bearing diameter: 3 mm Max. inner bearing diameter : 25 mm Min. outer bearing diameter: 10 mm Max. outer bearing diameter:42 mm
29 Performance tests of rolling element bearings Logarithmic speed ramp 0.1 to 3000 rpm Load 10N, Speed ramp 0.1 to 3000 rpm, -40°C 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
30 Example: Roll out Test - 40°C 25°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
31 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).
32 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.
33 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). RheologyTribology
34 Differentiation of Dairy Drinks 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
35 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