2 Outline Basics of Tribology: Introduction and definitions The different types of friction and wearLubricants and surface treatmentsRheometry and Tribometry: The Rheo-TribometerMeasurements on the Rheo-TribometerStribeck curveStatic friction testsStick slip measurementsRolling element bearing deviceMeasurementsFood 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, windwheels etc. (No perpetum mobile)Friction reduces durability of material due to wearFunctionality loss and annoyance due to noise Huge sums of money are lost because of friction and wearBenefits related to frictionFriction 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 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 frictionSliding 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
5 Tribology: Friction-Coefficient Friction Force FFLoad FLBody 1Body 2SlidingFriction Coefficient:µ = FF / FLµ is dimensionless,often f is used instead of µAmontons Laws:FF is independent on areaFF ~ FLRange of Friction Coefficients:Dry sliding:0.05 (PTFE under high loads) – 5.0 (gold sliding in vacuum). Steel:Lubricated sliding:0.03 (hydrodynamic conditions) – 0.15 (boundary conditions)Rolling friction:0.002 (fully lubricated) – 0.05 (running dry)
6 Parameters influencing friction Friction PartnersSurface parametersChemical reactivityElasticityEtc...LubricantChemistryViscosityStabilityEtc...Tribological SystemSystem Factors:Relative SpeedDirection of movement :unidirectionalbi-, multidirectionalNormal loadEnvironmental Conditions:TemperatureHumitidityTimeEtc...
7 WearDefinition: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 surfacesAbrasive wearWhen 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 WearCorrosive wearCorrosive wear is deterioration of useful properties in a material due to reactions with its environment (e.g. oxidation).Surface fatigueSurface 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 coefficientParticles: Chemistry, particle sizeLoadMovement: Sliding, rolling pushing, uni- or bi-directionalEnvironmental conditions: Air, inertgas, vacuum, humidityTemperatureSliding distance
9 Lubrication: The Stribeck curve / Speed ramp 1Friction coefficient524Richard Stribeck3Sliding speed1 Static friction, no movement, no wear2 Boundery lubrication, very low sliding speed, solid contacts, high wear3 Mixed lubrication, moderate sliding speed, partial solid contact, moderate wear4 Elasto-hydrodynamic lubrication, intermediate sliding speed, thin lubrication film5 Hydrodynamic librication, high sliding speed, developed lubrication film, no wear
10 Static Friction, Boundery Lubrication Direct solid contact of the frictionpartnersNo movementNo wearForce is high enough to induce a very low speed Solid friction, Stick slipLoad is carried by the contact points (asperities)and the shear is taken by the absorbedlubrication molecules.No hydrodynamic pressure build upNo lubrication film presentAsperities are protected by adsorbed lubricantmolecules and/or a thin oxide layer.High wear
11 Mixed Lubrication, Elasto-Hydrodynamic Lubrication Low speedLow hydrodynamic pressure is build up inthe lubricantThe loading is carried by a combination ofthe hydrodynamic pressure and thecontact pressure between the asperities ofboth surfaces.Lubricant film only in between contactpointsModerate wearIntermediate speedHydrodynamic pressure increases.Full, but still very thin lubrication film.Elastic deformation of the contactpoints.Lubricant viscosity increase due toincreasing pressure.No wear
12 Hydrodynamic Pressure Water ski:4Speed:1 < 2 < 3 < 4312Water resistance forceNormal forceWater pressureSolid frictionSolid contact no water flow under the skisWater pressure under the skis builds up as the drainage of water from the skis is lower than the amount of incomming waterWater pressure increases further acting against the normal force resulting in decrease of frictionWater resistance increases with speed until the skis cannot hold anymore (crash)
13 Hydrodynamic lubrication High speedsThe surface asperities are completelyseparated by a lubricant film.The load and hydrodynamic pressureare in equilibriumThick lubrication filmNo wear
14 Types of Lubricants Oils Greases Pastes Base oil Base oil Base oil Additives:Friction ImproverDispersing ChemicalsViscosity Index ImproverFoam InhibitersOxidation InhibitorsPour point ImproverWear ProtectionAdditives:Friction ImproverFoam InhibitersOxidation InhibitorsWear protectionAdditives:Friction ImproverFoam InhibitersOxidation InhibitorsWear protectionThickenerMetallsoapsSilicagelBentoniteThickenerMetallsoapsSilicagelBentoniteBase oilBase oilSolid LubicantsMolypdenum sulfiteGraphiteCoperViscosity of the oil depends on application:Low Viscosity for low loads and high sliding speedsHigh viscosity for high loads, low sliding speeds and high temperaturePour point has to be taken into account at low temperaturesGreases 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.Base oilCan only be used for mixed lubrication, no hydrodynamic lubrication possible. Useful for high loads.
15 Types of Lubricants Solvents / Water 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, coperSolid libricants are oft used as primary film in combination with lubricants.Dry lubricants /Anti-friction coatingsAdditives:Dispersing agentsCorrosion inhibitorsWetting agentsSolid LubricantsMolypdenum sulfiteGraphiteCoperAdhesive agent:Synthetic resinsSolvents / WaterAnti-friction coatings are surface fixed solid lubricants
17 Tribometry, ModellingThe 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 StehrMurber Steige 2672160 Horb-Ahldorf)andAnton Paar
19 Tribology Device: Ball-on-3-Plates Principle attached to a MCR Rheometer Side ViewTop ViewM: TorqueFNNormal-forceM: TorqueFNNormal-force
21 Tribology Device: Ball-on-3-Plates Principle attached to a MCR Rheometer SpecificationsRheometer TribometerNormal Force FN: 0.01N - 50N Normal Load FL: 2 N - 70 NTorque M: µNm mNm Friction Force FF: N NRot. speed n: rpm Sliding speed sv : 1 m/sTemperature: °C - 200°C
22 Friction Coefficient for a Steel Ball on POM: Dry, Penetrating Oil, Motor Oil Reproducability: Good for dry and motor oilFor penetrating oil general behavoir is reproducabledrypenetrating oilmotor oil
23 Friction Coefficient for a Steel Ball on POM: Dry, Penetrating Oil, Motor Oil Rotational speed: rpm; NF= 10N, Normal load NL= 14NLogarithmic scale for sliding speed
24 Stick Slip Static Friction > Pulling Force Static Friction < Pulling Force SlidingStick slip is usually an unwanted effect occuring at very low sliding speeds in the boundery lubrication regime. It leads to vibrations and noiseExamples: Noise of train in a turn, sound of a violin, window cleaner in dry conditions, wet finger on the rim of a crystall wine glassPrevention of stick slip:Adequate materials and surfacesUse lubricantsChoose higher speedsDithering (Vibration with small amplitude and adequate frequency)
25 Measurements on a Steel/Rubber System Measuring conditions: Steel/rubber dry and lubricated, Normal load 14 N, 10 rpmrun in effectsStick 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
29 Performance tests of rolling element bearings Logarithmic speed ramp 0 Performance tests of rolling element bearings Logarithmic speed ramp 0.1 to 3000 rpmLoad 10N, Speed ramp 0.1 to 3000 rpm, -40°C Best high speed performance for grease of NLGI class 2Grease of NLGI class 0 with the highest friction at high speedsTorque decrease at high speeds due to friction heating
30 Example: Roll out TestLoad 10 N, Step to 3000 rpm for 10 seconds and then roll out (only roll out intervall plotted)25°C- 40°C60°C Roll out test proves the results of speed ramps
31 Wood Polymer Composites (WPC) Advantages WPC:Better humidity resistanceImproved rigiditySmaller expension coefficientsThe 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 expensiveTrained people are requiredTime consumingLimited reproducibilityLimited quantitative statementThe 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 propertiesIn 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/ A1, WO 2008/ A1).
34 Differentiation of Dairy Drinks Applied Normal Force: 3 NT = 20°CDrinking is a fast process performed at high sliding speeds.Milk drinks having various fat contents can be tribologically differentiatedStrong stick slip effects observed at low sliding speeds for low fat milks
35 Conclusion Features and Benefits Tribology measurements on a MCR rheometer, i.e. the Rheometer as TribometerLow 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 speedPossibility of force (torque) control Measurements of static friction coefficientsTemperature control by Peltier system from -40°C up to 200°C; with the additional Peltier hood uniform temperature distributionFlexible choice of friction partners (Steel, polymers, elastomers etc.)Rolling element bearing performance measurementsFood tribology