9 - Bearings

Abstract

A bearing is a machine element that supports a part, such as a shaft, that rotates, slides, or oscillates in or on it. There are two broad classifications of bearings, plain and rolling element. Plain bearings are based on sliding motion made possible through the use of a lubricant. Antifriction bearings are based on rolling motion made possible by balls or other types of rollers. Bearings can be classified based on the types of movement, and there are three major classifications of both plain and rolling element bearings: radial, thrust, and guide. Radial bearings support loads that act radially and at right angles to the shaft center line. Thrust bearings support or resist loads that act axially. Guide bearings support and align members having sliding or reciprocating motion. All plain bearings are also referred to as fluid-film bearings. Plain bearings are available in a wide variety of types or styles and may be self-contained units or built into a machine assembly. Plain bearings are dependent on maintaining an adequate lubricant film to prevent the bearing and shaft surfaces from coming into contact, which is necessary to prevent premature bearing failure. Rolling element bearings also need a lubricant to meet or exceed their rated life. In the absence of high temperatures, however, excellent performance can be obtained with a very small quantity of lubricant. Excess lubricant causes excessive heating, which accelerates lubricant deterioration.