High Vacuum Undulated Sliding Bearings

Abstract

Normal sliding bearings operating in vacuum and in the dry sliding regime tend to experience high friction and seize readily. This behavior has been attributed to increased adhesion between the sliding surfaces. In this paper, the authors report on an investigation on the feasibility of using undulated surfaces to trap wear particles and thus lower friction and wear in sliding bearings operating in high vacuum without lubricants. Experiments were conducted using a bearing tester in high vacuum of 6.5 × 10⁻⁶ Pa (5 × 10⁻⁸ torr). The results show that the use of an undulated surface on the shaft lowers friction and provides a longer life. Under similar experimental conditions, smooth bearings exhibit a rapid increase in frictional force and quickly fail due to seizure. In addition, the friction coefficient obtained in vacuum using an undulated surface or a bearing with a large diametral clearance in vacuum is the same as that in air. This is contrary to findings in the literature which indicate that the friction coefficient of rubbing surfaces in dry sliding is higher in vacuum than in air. This study shows that the primary cause for the increase in friction coefficient in a vacuum of less than 6.5 × 10⁻⁶ Pa (5 × 10⁻⁸ torr) is not increased adhesion between the surfaces, but is instead an increase in the severity of plowing by wear particles generated at the sliding interface.