Generation mechanism of friction anisotropy by surface texturing under boundary lubrication

Continuously variable transmission (CVT) is commonly used in automobile to reduce fuel consumption and emission. Apart from wider gear ratio, reduction of friction loss in a radial direction of the pulley surface is an effective method of improving CVT efficiency. On the other hand, to improve torque transmission efficiency, high frictional force on circumferential direction of the pulley is necessary. Therefore, required frictional characteristics differ depending on sliding directions. To create friction anisotropy on belt-pulley contacts, laser surface texturing was proposed to improve friction characteristic and generates friction anisotropy on the surface. In this study, the relationship between surface texture and friction anisotropy was investigated under boundary lubrication by cylinder-on-disk type sliding test. The sliding test was conducted in four different directions, parallel, thirty degrees, sixty degrees, and perpendicular to the direction of the surface texture. Analysis of real contact area was conducted repeatedly in the sliding directions to evaluate change in interaction of asperities in sliding directions over time. The experimental results were then compared with result of the continuous analysis of real contact area. When a groove-type texture was created by a processing laser, texture pattern changes the sliding distance on the plateaus of the texture, which affects the level of metal contact on the surface, generating friction anisotropy. This hypothesis was supplemented by the continuous analysis of the real contact area, which confirmed that the sliding distance on the plateaus of the texture decreased as the sliding angle increased. Furthermore, clear correlation between sliding distance and friction force was observed, which suggests the possibility that friction anisotropy by surface texturing under boundary lubrication will be predicted by a friction force function.