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!