3D friction map
Friction in Elasto Hydrodynamically
Lubricated 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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Elasto Hydrodynamic Lubrication (EHL)
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Elasto Hydrodynamic Lubrication (EHL)
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Project aims
Conformal and non conformal contacts
• Improve the understanding of friction in EHL
• Find a good way to map out system performance
Lubricant
• Improve efficiency of machine components working
in EHL
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Large contact area
Small contact area
Low contact pressure
High contact pressure
Typical contact pressure in rolling element
bearings, 1-4 GPa!
Viscosity increase several orders of magnitude
30-120 cars on one coin
Mineral oil
Ester oil
water
coin
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Example of EHL contact
Example of EHL contact
Viscosity
Entrainment speed
Dry contact
Pressure-viscosity behaviour
Hertz pressure
…
w
Film pressure
Not
to scale!
w´
w
u2
inlet
u2
outlet
u1
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10
outlet
h(x)
hmin
inlet
hc
hc
u1
EHL shear profile
u1=u2
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EHL shear profile
u2
u2
u1
u1
u1<u2
No sliding
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sliding
Limiting shear stress
Friction versus Slide to Roll Ratio (μ-slip)
μ
Shear stress
Non-Linear
Limiting shear stress
Thermal
Linear
Shear strain rate
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SRR
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Lubrication regimes
Stribeck curve
μ
Full film lubrication
Mixed lubrication
Hydrodynamic
EHL
Boundary lubrication
Boundary
Full film
Mixed
Film thickness
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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
Application example
Entrainment velocity
Effective radius
Gear tooth load
• The surfaces are deformed
• Thin lubricant films <1μm (10-1000 nm)
addendum
dedendum
• Examples: ball bearing, gears, cam followers
Pitch point
Slide to roll ratio (SRR)
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Action line
coordinate
Experimental setup
Wedeven Associates Machine (WAM) no. 11
Entrainment speed
Ue =
u1 + u2
2
0.34-9.6 m/s
w´
u2
outlet
u1
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20
hmin
h(x)
hc
inlet
Experimental setup
Experimental setup
Slide to roll ratio (SRR)
Load
u1 + u2
Ue
0.0002-0.49
w=200 N
w´
w´
w
u2
u2
outlet
inlet
hc
u1
21
outlet
h(x)
hmin
h(x)
hc
inlet
Dry contact
Maximum Hertzian pressure
1.7 GPa
Mean Hertzian pressure
1.13 GPa
u1
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hmin
SRR =
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
Schematic 2D map of EHL friction regimes
T
SRR
M
NL
L
Entrainment speed
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3D map – EHL friction regimes
Test parameters
Mixed
• Temperatures – 40 and 90°C
• Viscosities – low and high
Thermal
• Base oil type – mineral and ester
• EP additive content – 0 and 2 %
• Surface roughness – rough and smooth
Linear
Non-Linear
• 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
Low viscosity
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Difference in friction coefficient, low – high viscosity
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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–
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–
40
%
–
1D Simulation model
%
Principle
Lubricant
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1D Simulation model
Effect of DLC coating on oil film temperature increase
Schematics
Steel
Coating/Steel
Steel
Lubricant
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Application example
Influence of several parameters on coefficient of friction
Entrainment velocity
Effective radius
Gear tooth load
addendum
dedendum
1. Different results depending on surface roughness, temperature etc
2. Given comparable viscosities
Pitch point
Slide to roll ratio (SRR)
Licentiate thesis:
www.ltu.se/lib search ”Marcus Björling”
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Action line
coordinate
Where to operate?
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Friction power MW/m²
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In conclusion
Future work
• Correlate WAM tests with tests with actual gears
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Power re-circulating test rig
Future work
• Correlate WAM tests with tests with actual gears
• Model the circular contact
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Model vs experiment
Future work
• Correlate WAM tests with tests with actual gears
• Model the circular EHL contact
• Model gear contact friction
Model
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Experiment
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Thank you for your attention!
Questions?
Acknowledgements:
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•
•
•
Volvo Construction Equipment
Scania
Vicura AB
•
•
Statoil Lubricants
IonBond
•
Swedish Foundation for Strategic Research (SFF, ProViking)
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
Symmetric behaviour
μ
Dry contact
Hertz pressure
Film pressure
w´
u2
hmin
hc
u1
+
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SRR
outlet
h(x)
inlet
x
+
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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 densitypressure-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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Oil parameters
Specimen parameters
Description
•
“Smooth” ball and disc
–
–
–
–
•
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52100 (AISI) Bearing steel
HRc:60
Ball surface roughness (Ra): 30 nm
Disc surface roughness (Ra): 80 nm
SL324
SL326
SL211
SL212
Type
Ester
Mineral
Mineral
Mineral
Additive
None
None
None
EP
Kinematic visc @40° C, cSt
103.7
109.3
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
“Rough” ball and disc
Density@ 15° C, kg/m³
928
885
872
872
–
–
–
–
Viscosity index
157
99
104
104
9310 (AISI) Gear steel
HRc:63
Ball surface roughness (Ra): 200 nm
Disc surface roughness (Ra): 220 nm
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30.8
30.7
Diamond Like Carbon (DLC) in EHL
Wedeven Associates Machine (WAM) no. 11
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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Test combinations
–
Reference case
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–
Change in coefficient of friction with DLC coating
Reference case
Evans, R.; Cogdell, J. & Richter, G.
Traction of Lubricated Rolling Contacts between
Thin-Film Coatings and Steel
Tribology Transactions, 2009, 52(1), 106-113
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Xu, H.
Development of a generalized mechanical efficiency prediction methodology for gear pairs
Graduate School of The Ohio State University, 2005
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Possible explanations for friction reduction
Possible explanations for friction reduction
•
Boundary slip
Boundary slip
u2
Evans, R.; Cogdell, J. & Richter, G.
Traction of Lubricated Rolling Contacts between Thin-Film Coatings and Steel
Tribology Transactions, 2009, 52(1), 106-113
Choo, J.; Glovnea, R.; Forrest, A. & Spikes, H.
A low friction bearing based on liquid slip at the wall
Journal of Tribology, 2007, 129(3), 611-620
u1
u1<u2
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sliding
Possible explanations for friction reduction
Possible explanations for friction reduction
Boundary slip
Boundary slip - Limitations
u2
Profile for uncoated surface
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
u1
u1<u2
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sliding
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1D Simulation model
Possible explanations for friction reduction
•
Principle
Thermal effects
– Decrease in limiting shear stress
– Decrease in shear resistance
Lubricant
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Steel
1D Simulation model
1D Simulation model
Schematics
Material parameters
Coating/Steel
Steel
Steel
Lubricant
Density [kg/m³]
Thermal conductivity [W/mK]
Heat Capacity [J/kgK]
Coating/Steel
DLC
2500
2
1000
52100(AISI)
7810
46.6
475
Wojciechowski, K.; Zybala, R. & Mania, R.
Application of DLC layers in 3-omega thermal conductivity method
Journal of Achievements in Materials and Manufacturing Engineering, 2009, 37(2), 512-517
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Kim, J.; Yang, H.-S.; Jun, Y. & Kim, K.
Interfacial effect on thermal conductivity of diamond-like carbon films
80 Journal of Mechanical Science and Technology, 2010, 24(7), 1511-1514
Steel
1D Simulation model
1D Simulation model
Film thickness
Boundary conditions
Heat source
Steel
Coating/Steel
• Hamrock & Dowson central film thickness
• Hsu and Lee thermal correction
Hamrock, B.
Fundamentals of fluid film lubrication
McGraw-Hill, 1994
Hsu, C.-H. & Lee, R.-T.
An Efficient Algorithm for Thermal Elastohydrodynamic Lubrication Under
Rolling/Sliding Line Contacts
81 Journal of tribology, 1994, 116(4), 762-769
Steel
Bulk temperature
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Continuity
Bulk temperature
1D Simulation model
Data points for simulations
Heat source
Heat source, Q
Q = μPh
∂P
SU e
+ εT h
∂t
hcen
Measured
friction
coefficient
Circular
pressure
SRR
andHertzian
entrainment
speed
Compression
of lubricant
Hamrock
&
Dowson
Hsu & Lee
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Effect of DLC coating on oil film temperature increase
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