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Friction in Elasto HydrodynamicallyLubricated contacts
The influence of speed and slide to roll ratio
• Marcus Björling– Prof. Roland Larsson (supervisor)
– Dr. Pär Marklund (co-supervisor)
• Industrial Partners– Vicura AB
– Scania
– Volvo Construction Equipment
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3D friction map
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Elasto Hydrodynamic Lubrication (EHL)
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Elasto Hydrodynamic Lubrication (EHL)
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Project aims
• Improve the understanding of friction in EHL
• Find a good way to map out system performance
• Improve efficiency of machine components working in EHL
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Conformal and non conformal contacts
Lubricant
Small contact areaHigh contact pressure
Large contact areaLow contact pressure
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Typical contact pressure in rolling element bearings, 1-4 GPa!
30-120 cars on one coin
coin8
Viscosity increase several orders of magnitude
water
Mineral oil
Ester oil
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Example of EHL contact
u1
u2
w
inlet outlet
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Example of EHL contact
inlet outlet
u1
u2
h(x)
w´
h min
h c
inlet outlet
u1
u2
w
Film pressure
Dry contactHertz pressure
Film pressure
Dry contactHertz pressure
h c
ViscosityEntrainment speedPressure-viscosity behaviour…
Not to scale!
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EHL shear profile
u2
u1
u1=u2 No sliding
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EHL shear profile
u2
u1
u1<u2 sliding
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Limiting shear stress
Shear stress
Shear strain rate
Linear
Non-Linear Limiting shear stress
Thermal
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Friction versus Slide to Roll Ratio (μ-slip)
μ
SRR
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Lubrication regimes
Full film lubrication
Mixed lubrication
Boundary lubrication
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Stribeck curve
Boundary Mixed Full film
Film thickness
μ
EHL
Hydrodynamic
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EHL - Summary
• EHL = Elasto Hydrodynamic Lubrication • Non-conformal surfaces → small contact region• High contact pressures, 1-4 GPa • Solidification of lubricant
• The surfaces are deformed• Thin lubricant films <1μm (10-1000 nm)
• Examples: ball bearing, gears, cam followers
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Application example
Action line coordinate
dedendum addendum
Pitch point
Entrainment velocity
Slide to roll ratio (SRR)
Effective radiusGear tooth load
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Wedeven Associates Machine (WAM) no. 11
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Experimental setupEntrainment speed
inlet outlet
u1
u2
h(x)
w´
h min
h c
inlet outlet
u1
u2
221 uuUe
+=
0.34-9.6 m/s
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Experimental setupSlide to roll ratio (SRR)
inlet outlet
u1
u2
h(x)
w´
h min
h c
inlet outlet
u1
u2
eUuuSRR 21 +=
0.0002-0.49
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Experimental setupLoad
inlet outlet
u1
u2
h(x)
w´
h min
h c
Dry contact
inlet outlet
u1
u2
w
Dry contactMaximum Hertzian pressure 1.7 GPaMean Hertzian pressure1.13 GPa
w=200 N
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3D friction map
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µ-slip curve
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Stribeck curve
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3D friction map
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2D contour map
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Schematic 2D map of EHL friction regimes
M T
NL
LEntrainment speed
SRR
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3D map – EHL friction regimes
Thermal
Mixed
Linear Non-Linear
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Test parameters
• Temperatures – 40 and 90°C
• Viscosities – low and high
• Base oil type – mineral and ester
• EP additive content – 0 and 2 %
• Surface roughness – rough and smooth
• Coating – uncoated and DLC coated
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Contact friction, low viscosity oil
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Contact friction, high viscosity oil
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Low viscosity vs high viscosity mineral oil
High viscosityLow viscosity
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Difference in friction coefficient, low – high viscosity
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Difference in friction coefficient, low – high viscosity %
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Difference in friction coefficient, 40° – 90° %
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Difference in friction coefficient, mineral – ester %
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Difference in friction coefficient, rough – smooth %
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–
40
– %
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– %
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1D Simulation modelPrinciple
Lubricant
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1D Simulation modelSchematics
SteelCoating/Steel
Lubricant
Steel
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Effect of DLC coating on oil film temperature increase
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Influence of several parameters on coefficient of friction
1. Different results depending on surface roughness, temperature etc2. Given comparable viscosities
www.ltu.se/lib search ”Marcus Björling”Licentiate thesis:
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Application example
Action line coordinate
dedendum addendum
Pitch point
Entrainment velocity
Slide to roll ratio (SRR)
Effective radiusGear tooth load
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Where to operate?
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Friction power MW/m²
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In conclusion
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Future work
• Correlate WAM tests with tests with actual gears
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Power re-circulating test rig
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Future work
• Correlate WAM tests with tests with actual gears
• Model the circular contact
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Model vs experiment
ExperimentModel
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Future work
• Correlate WAM tests with tests with actual gears
• Model the circular EHL contact
• Model gear contact friction
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Thank you for your attention!
• Volvo Construction Equipment• Scania• Vicura AB
• Statoil Lubricants• IonBond
• Swedish Foundation for Strategic Research (SFF, ProViking)
Questions?
Acknowledgements:
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Thank you for your attention!
Questions?
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Test procedure
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Test procedure
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Example of EHL contact
inlet outlet
u1
u2
h(x)
w´
h min
h c
Film pressure
Dry contactHertz pressure
+ +
x
inlet outlet
u1
u2
h(x)
w´
h min
h c
Film pressure
Dry contactHertz pressure
+ +
x
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Symmetric behaviour
μ
SRR
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1D thermal traction model
• Simulates temperature distribution and shear rate in oil film
• Tait equation of state is used for the density-pressure-temperature dependence
• The Doolittle free volume model is used for the viscosity-pressure-temperature dependence
• A Carreau-Yusada model modified by Bair is used for the shear viscosity relationship
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Test settings
• Entrainment speed: 0.34 – 9.6 m/s• Slip: 0.02-49 %
• Load: 200 N• Maximum Hertzian pressure: 1.7 GPa• Mean Hertzian pressure: 1.13 GPa
• Temperatures: 40° and 90° C
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Specimen parameters
• “Smooth” ball and disc– 52100 (AISI) Bearing steel
– HRc:60
– Ball surface roughness (Ra): 30 nm
– Disc surface roughness (Ra): 80 nm
• “Rough” ball and disc– 9310 (AISI) Gear steel
– HRc:63
– Ball surface roughness (Ra): 200 nm
– Disc surface roughness (Ra): 220 nm
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Oil parametersDescription SL324 SL326 SL211 SL212
Type Ester Mineral Mineral Mineral
Additive None None None EP
Kinematic visc @40° C, cSt 103.7 109.3 30.8 30.7
Kinematic visc @100° C, cSt 15.4 11.98 5.3 5.3
Dynamic visc @40° C, mPas 94.5 94.9 27.1 27.1
Dynamic visc @100° C, mPas 13.4 9.97 4.46 4.46
Density@ 15° C, kg/m³ 928 885 872 872
Viscosity index 157 99 104 104
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Diamond Like Carbon (DLC) in EHL
In literature• Focus on wear and boundary friction• Mostly pure sliding contacts
New study• Focus on EHL friction in full film lubrication• Rolling and sliding contacts
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Wedeven Associates Machine (WAM) no. 11
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Test combinations
Reference case
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–
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Change in coefficient of friction with DLC coating
Reference case
Xu, H.Development of a generalized mechanical efficiency prediction methodology for gear pairsGraduate School of The Ohio State University, 2005
Evans, R.; Cogdell, J. & Richter, G.Traction of Lubricated Rolling Contacts betweenThin-Film Coatings and SteelTribology Transactions, 2009, 52(1), 106-113
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–
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Possible explanations for friction reduction
• Boundary slipEvans, R.; Cogdell, J. & Richter, G.Traction of Lubricated Rolling Contacts between Thin-Film Coatings and SteelTribology Transactions, 2009, 52(1), 106-113
Choo, J.; Glovnea, R.; Forrest, A. & Spikes, H.A low friction bearing based on liquid slip at the wallJournal of Tribology, 2007, 129(3), 611-620
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Possible explanations for friction reduction Boundary slip
u2
u1
u1<u2 sliding
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Possible explanations for friction reduction Boundary slip
u2
u1
u1<u2 sliding
Profile for uncoated surface
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Possible explanations for friction reduction Boundary slip - Limitations
RMS surface roughness of about 1 nm Reduction in friction
RMS surface roughness of about 10 nm No reduction in friction
Combined roughness in present test: > 155 nm
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Possible explanations for friction reduction
• Thermal effects– Decrease in limiting shear stress
– Decrease in shear resistance
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1D Simulation modelPrinciple
Lubricant
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1D Simulation modelSchematics
SteelCoating/Steel
Lubricant
Steel
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1D Simulation modelMaterial parameters
SteelCoating/Steel
Steel
DLC 52100(AISI)Density [kg/m³] 2500 7810Thermal conductivity [W/mK] 2 46.6Heat Capacity [J/kgK] 1000 475
Kim, J.; Yang, H.-S.; Jun, Y. & Kim, K.Interfacial effect on thermal conductivity of diamond-like carbon filmsJournal of Mechanical Science and Technology, 2010, 24(7), 1511-1514
Wojciechowski, K.; Zybala, R. & Mania, R.Application of DLC layers in 3-omega thermal conductivity methodJournal of Achievements in Materials and Manufacturing Engineering, 2009, 37(2), 512-517
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1D Simulation modelFilm thickness
SteelCoating/Steel
Steel
• Hamrock & Dowson central film thickness• Hsu and Lee thermal correction
Hsu, C.-H. & Lee, R.-T.An Efficient Algorithm for Thermal Elastohydrodynamic Lubrication Under Rolling/Sliding Line ContactsJournal of tribology, 1994, 116(4), 762-769
Hamrock, B.Fundamentals of fluid film lubricationMcGraw-Hill, 1994
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1D Simulation modelBoundary conditions
Bulk temperature Bulk temperature
Heat source
Continuity
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1D Simulation modelHeat source
Heat source, Q
tPT
hSUPQ h
cen
eh ∂
∂+= εμ
Measured friction coefficientCircular Hertzian pressureSRR and entrainment speedHamrock & Dowson Hsu & LeeCompression of lubricant
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Data points for simulations
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Effect of DLC coating on oil film temperature increase
