Oil-impregnated sintered bearings are widely used in various products. Friction reduction in them is still a large target for the related industries. In those bearings, lubricating oil exudes from the porous bearing body and lubricates the shaft and bearing surfaces. However, the amount of oil in those sliding areas is often insufficient leading to an unsatisfactory friction. Oil wettability of the shaft surfaces was found to have a large effect on the friction of those bearings. Low oil-wettable shaft could retain a larger amount of oil in the bearing clearances and indicated lower friction than highly wettable shaft. This is because a large contact-angle hysteresis between the oil and the low-wettable shaft surface allows the retention of large oil droplets in the bearing clearances. The control of oil-wettability of the shaft surface could be an effective means of reducing friction for oil-impregnated sintered bearings.
Part of the book: Wettability and Interfacial Phenomena
This chapter describes two characteristic phenomena of metal wear that are usually not often considered but are related to the basic aspects of wear. The first is a mild-to-severe wear transition caused by the wear itself. Convex sliding pairs are usually accompanied by rolling sliding motion, but rolling sliding motion sometimes produces a peculiar wear profile, leading to high contact pressure. When the contact pressure exceeds a certain value that depends on the material, the wear mode changes to severe wear. This is a common wear transition for convex sliding pairs, but it can also occur for other pairs. The second is the similar appearance of wear tracks on various friction pairs. Rubbing metal under relatively severe conditions creates streaked wear tracks. We found the width and depth of these streaks, that is, wear track profiles are similar regardless of the sliding conditions and the material, which leads to similar appearance of the wear tracks. This suggests the existence of a general mechanism for producing wear tracks.
Part of the book: Tribology in Materials and Manufacturing