pitting behaviour of different gear oils by using a rolling four ball test
TRANSCRIPT
Pitting behaviour of different gear oils by using a rolling four ball test configuration
Jens Johansson a*
Mark T. Devlin b
Jeffrey M. Guevremont b
Braham Prakash a
a Luleå University of Technology, Sweden
b Afton Chemical Corporation, Richmond, USA
10th November 2011
Outline
• Motivation for this work
• Experimental
• Results– Effects of oil properties and additive chemistry on pitting life– Pitting mechanisms in rolling four ball tests– Pit formation vs oil parameters
• Conclusion
• Future Work
Engine output
Motivation – Energy Losses
Accessories(10%)
Transmission (6%)
Axle(6%)
Braking(14%)
Coast and Idle(16%)
At Wheels(48%)
Possible optimization, without sacrificing durability?
Data taken from:W.J. Bartz, Gear oil influences on efficiency of gear and fuel economy of cars, Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering. 214 (2000) 189-196.
Motivation – Energy Losses
Possible optimization, without sacrificing durability?
Effects of oil properties and additive chemistry on pitting life
Pitting mechanisms in rolling four ball tests
Gear oil samples and their salient properties
Oil Kinematic Viscosity @100C
(cSt)
High Shear
Viscosity @125C (mPa*s)
Boundary Friction
Coef@130C on
steel
EHD Friction Coef @125C,
1m/s 50N, 4%SR
EHD Film Thickness
@100C & 1 m/s (nm)
R 14.42 6,28 0.110 0.030 106
S 14.36 6,65 0.111 0.013 85
T 13.94 6,47 0.158 0.029 105
U 13.88 6,52 0.158 0.013 89
V 9.53 4,55 0.112 0.027 78
W 9.56 4,61 0.113 0.012 63
X 9.37 4,60 0.162 0.027 76
Y 9.43 4,62 0.148 0.012 66
API GL-5
• Ranking of pitting life
• Fundamental mechanisms• Test parameters:
– 4.3 kN (5.1 Gpa)
– 4000 rpm
9000 stress cycles per minute
– 12.7 mm, AISI 52100 bearing steel
– 120 °C bulk oil temperature
– 15 tests/oil– Additional stopped test
Rolling four ball test machine
Test parameters from:H. Hamaguchi, H. Tanaka, T. Bartels, Test Method for Evaluating Gear Fatigue Life of 4-Stroke motorcycle engine oils, SETC2009, Meeting
Name: Small Engine Technology Conference and Exhibition Penang, Malaysia, 2009.
Pitting results from rolling four ball tests
Oil
Kinematic Viscosity
High Shear
Viscosity
Boundary
Friction Coef
EHD Friction
Coef
EHD Film Thickness
R 14.42 6,28 0.110 0.030 106
S 14.36 6,65 0.111 0.013 85
T 13.94 6,47 0.158 0.029 105
U 13.88 6,52 0.158 0.013 89
V 9.53 4,55 0.112 0.027 78
W 9.56 4,61 0.113 0.012 63
X 9.37 4,60 0.162 0.027 76
Y 9.43 4,62 0.148 0.012 66
Multiple linear regression
Statistical analysis
Multiple linear regression
• Model for pitting life:
• Test variables coded between -1 and 1 through
• Significance level of regression coefficients, found by partial F-tests*
• Low significance due to scatter
εββββ +++++= kk xxxy ...22110
2/)(2/)(
lowhigh
highlow
xxxxx
x−+−
=
* D.C. Montgomery, Design and Analysis of Experiments, 7th ed., Wiley, Hoboken, N.J, 2009.
y = Expected pitting life
x = Test variables (EHD friction etc.)
= Regression coefficients
= Random error
βε
Regression analysis – Oil properties
Oil
Kinematic Viscosity
@120C (cSt)
Boundary Friction
Coef@130C
EHD Friction
Coef@125C
EHD Film Thickness
@100C
R 9.01 0.110 0.030 106
S 9.52 0.111 0.013 85
T 8.77 0.158 0.029 105
U 9.22 0.158 0.013 89
V 6.21 0.112 0.027 78
W 6.54 0.113 0.012 63
X 6.13 0.162 0.027 76
Y 6.45 0.148 0.012 66
EHD Friction Coef (x1)Kinematic Viscosity (x2)EHD Film Thickness (x3)Boundary Friction Coef (x4)
43215 x0.09x.151x 0.64x1.70-1.7910 ⋅−⋅+⋅−⋅=⋅ −Y
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
=
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
=
0.34 0.73 0.63 8.98
)(x)(x)(x)(x
4
3
2
1
0βF 95.305.0 >F
Revolutions until failure:
Significance level 5%
Summary:Low EHD friction(Low kinematic viscosity)(High film forming capability )(Low boundary friction)
SEM/EDX
Analysis of Tribofilm – Elemental Composition
SEM Image
EDX of Tribofilm
Map outside tribofilm
Map of tribofilm
~1µm
X-rayElectron Beam
Regression analysis – Elemental Composition3 samples, each oil
Revolutions until failure:
Weight Concentration (%) C O P S Fe
Oil R, test 6 (EOT) 5,33 5,89 1,71 0,06 86,02
543215 x14.92-x0.31x24.70x41.81-x2.86-1.7810 ⋅⋅+⋅+⋅⋅=⋅ −Y
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
=
⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢
⎣
⎡
=
0.11310.01781.38080.50860.0227
)(x)(x)(x)(x)(x
5
4
3
2
1
0βF
Weight Concentration(%)C (x1)O (x2)P (x3)S (x4)Fe (x5)
Summary:+ Phosphorus
95.305.0 >F
Significance level 5%
5325 x8.877-x25.85x36.47-2.4810 ⋅⋅+⋅=⋅ −Y
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡=
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡=
8.464.205.94
)(x)(x)(x
5
3
2
0βF
Pitting mechanisms in rolling four ball tests-Analysis of the test
Rolling track of the top ball
• Plastically deformed rolling track– “No” mass loss
– Calculated “Wear volume”, 1.8 mg
• Dark etching band
100000 revolutions
Weight reduction
28,9918
28,9923
28,9928
28,9933
28,9938
28,9943
0 20000 40000 60000 80000 100000 120000
Number of revolutions
Wei
ght (
g)
Measured weight
Tribofilm formation
Oil R, 50000 revolutions Oil R, 100000 revolutions
(Oil R) Revolutions C Fe P S O
50000 13,56 68,24 2,44 0,03 15,24
100000 14,16 68,08 2,37 0,02 14,79
142270 15,94 68,02 2,08 0,07 13,15
Oil R, 142270 revolutions (EOT)
Initial film thickness
6.1025.004.0
'min ===Rahλ
mμ0,04 thicknessFilm
mμ0,025 Ra
• Mixed lubrication• Additive
depletion?• Anti wear
Surface deterioration
• Surface damages– Micropitting
-> Differs between the oils
-> Initiation sites
Rolling direction
Oil R, 100000 revolutions Oil S, 150000 revolutions
Changes in near surface materialHardness
“Each stress cycle lowers the activation energy for thermally activated processes”*
*H. Swahn, P. G. Becker, and O. Vingsbo, ”MARTENSITE DECAY DURING ROLLING CONTACT FATIGUE IN BALL BEARINGS.”, Metall Trans A, vol. 7, num. 8, ss. 1099-1110, 1976.
0
100
200
300
400
500
600
700
800
900
1000
0 0,5 1 1,5 2
Depth (mm)
Har
dnes
s (H
V)
Wear scarNew sample
• Dark etching band– Martensite decay
Changes in near surface material
Comparison of 3 oils (EOT)
• Etch band hardness– Temperature
– Hydrostatic pressure
• Etch band depth– Surface shear stress
Etch band hardness (EOT)
400
500
600
700
800
900
0 50000 100000 150000 200000 250000 300000 350000 400000 450000
Revolutions
Har
dnes
s (H
V) Oil R
Oil S
Oil WOil
Results Four ball: Average depth
R 93137 0,21
S 306640 0,21
W 355642 0,2
OilResults Four ball:
R 93137
S 306640
W 355642
Crack initiation
• Surface or sub-surface initiated? – Surface initiated
Pit formation
Pitting life, rolling four ball vs. oil properties
+ Low, EHD friction?
+ (Low, boundary friction?)– Lower contact temperature
-> Decreased martensite decay
– Reduced micropitting
->Fewer initiation sites
+ (High, film forming capability?)– Reduced micropitting
->Fewer initiation sites
+ Phosphorus?– % phosphorus in the tribofilms
controls the films’ ability to prevent wear *
+ (Low viscosity?)
*Devlin MT, Turner TL, Milner J, Hewette C, Sheets R, Ryan H, Jao T-C. Wear prevention by phosphorus species that form thin tribofilms. Proceedings AITC –AIT
2006 International Conference on Tribology, Parma, Italy, September 20–22 2006.
Conclusion
• 8 GL-5 gear oils have been investigated• Results showed clear distinction on pitting life
using rolling four ball
• Rolling four ball pits are surface initiated
• Important oil functions in rolling four ball:- Contact temperature reduction
- Surface protection
Future Work
Future studies will aim at investigating the following aspects of rolling four ball:•Crack propagation- from surface to dark etching band
•Surface degradation with the different oils
•Effects of additives, additive depletion
•Effect of load in rolling four ball
•Correlation of rolling four ball tests to actual gear test rig
Acknowledgements
• The authors would like to thank:– Swedish Energy Agency
– Volvo CE
– Scania
– Afton Chemical
Thank you for your attention!