**Abstract**

 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.

**Keywords:** friction, lubrication, wettability, surface tension, contact angle, surface treatment

## **1. Introduction**

 Oil-impregnated sintered bearings are widely used in various products, especially in motor vehicles and office automation (OA) equipment [1, 2]. For example, they are used in more than 30 parts of automobiles [3]. In these bearings, lubricating oil exuding from the porous bearing body to the sliding surface lubricates the shaft and the bearing. The exudation occurs due to thermal expansion of the oil volume by frictional heat, the "pumping effect" caused by negative pressure in the lubricating oil film [2, 4], and capillary force. Due to such mechanisms, these bearings are expected to have a long service life and relatively low friction without an external oil supply.

 Generally, friction of oil-impregnated sintered bearings is larger than fully lubricated bearings, because they are often under boundary lubrication condition. These bearings are often used in relatively small parts, and their friction loss is a serious issue because of small power consumptions allowed for such parts. Frictional noise is also a problem, especially for OA equipment, which is related to lubrication conditions. To improve the lubrication conditions and reduce the friction of these bearings has been the challenge for many researchers and is still a major problem today.

In this chapter, lubrication conditions of oil-impregnated sintered bearings are roughly explained, and the effects of oil wettability of the shaft surface on improving the lubrication conditions are discussed [5].
