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Increase of Traction Coefficient due to Surface Microtexture

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Abstract

Increasing the traction coefficient of a traction drive system is a key factor in obtaining a smaller, lighter unit and also greater torque capacity. This study focused on the microtexture of the rolling elements, and effect of microtexture was examined with the aim of improving the traction coefficient in the viscous region. Three textures—dimple, transverse, and longitudinal—were examined using a 4-roller tester that enabled tests to be conducted under high pressure and high rolling speed. As a result, it was found that the longitudinal surface texture is the best for improving the traction coefficient. The results obtained with EHL analysis showed that only the surface texture with longitudinal grooves increased the traction coefficient, just as in the tests conducted with the 4-roller tester. The longitudinal surface texture was optimized using the 4-roller tester. The test results made it clear that the groove depth, groove pitch, and also the radius of curvature of the convex portion of the rolling elements are important parameters of the longitudinal grooves for improving the traction coefficient while assuring high durability at the same time. An attempt was then made to increase the traction coefficient of an actual CVT variator by applying the optimized longitudinally grooved microtexture to the traction surfaces. The test results show that the traction coefficient can be increased without sacrificing durability by optimizing the surface microtexture.

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Abbreviations

a :

Semiminor axis of Hertzian contact ellipse

b :

Semimajor axis of Hertzian contact ellipse

E′:

Effective Young’s modulus

G :

Limiting elastic shear modulus

h :

Local film thickness

h a :

Average film thickness

p :

Hydrodynamic pressure

p c :

Asperity contact pressure

P h :

Maximum Hertzian pressure

R x :

Effective radius in x–z plane

R y :

Effective radius in y–z plane

T :

Temperature

t :

Time

U :

(u 1+u 2)/2

u 1, u 2 :

Velocities of surface 1 and surface 2

V :

Surface deformation

α:

Pressure-viscosity coefficient

δ1, δ2 :

Roughness heights for surfaces 1 and 2

η:

Viscosity

ηo :

Viscosity under ambient condition

ρ:

Density of lubricant

ρo :

Density of lubricant under ambient condition

σ:

Composite RMS roughness

τ:

Shear stress

τL :

Limiting shear stress

x :

Rolling direction

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Acknowledgments

The authors would like to thank Professor Tsunamitsu Nakahara and Professor Keiji Kyougoku of the Tokyo Institute of Technology for their helpful guidance in the course of conducting this study.

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Authors and Affiliations

  1. Nissan Motor Co.Ltd., Nissan Research Center, 1-Natsushima-cho, Yokosuka-shi, Kanagawa, 237-8523, Japan

    Toshikazu Nanbu, Yoshiteru Yasuda, Kenshi Ushijima & Jun Watanabe

  2. Eaton Corporation, Innovation Center, 26201 Northwestern Highway, P. O. Box 766, Southfield, MI, 48037, USA

    Dong Zhu

Authors
  1. Toshikazu Nanbu
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  2. Yoshiteru Yasuda
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  3. Kenshi Ushijima
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  4. Jun Watanabe
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  5. Dong Zhu
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Corresponding author

Correspondence to Toshikazu Nanbu.

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Nanbu, T., Yasuda, Y., Ushijima, K. et al. Increase of Traction Coefficient due to Surface Microtexture. Tribol Lett 29, 105–118 (2008). https://doi.org/10.1007/s11249-007-9287-9

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  • DOI: https://doi.org/10.1007/s11249-007-9287-9

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