BEARING

in machine construction, a connector (usually a support) that permits the connected members to rotate or to move in a straight line relative to one another. Often one of the members is fixed, and the bearing acts as a support for the moving member. Rolling bearings (Left) Roller (cylindrical) bearing, (right) ball bearing Most bearings support rotating shafts against either transverse (radial) or thrust (axial) loads. To minimize friction, the contacting surfaces in a bearing may be partially or completely separated by a film of liquid (usually oil) or gas; these are sliding bearings, and the part of the shaft that turns in the bearing is the journal. The surfaces in a bearing may be separated also by balls or rollers; these are known as rolling bearings. In the illustration, the inner race turns with the shaft. Under certain combinations of load, speed, fluid viscosity, and bearing geometry, a fluid film forms and separates the contacting surfaces in a sliding bearing; this is known as a hydrodynamic film. An oil film can also be developed with a separate pumping unit that supplies pressurized oil to the bearing; this is known as a hydrostatic film. Because shaft speed is required for the development of a hydrodynamic film, the starting friction in these bearings is higher than in ball or roller bearings. To minimize friction when metal-to-metal contact occurs, low-friction-bearing materials have been developed; among these are bronze alloys and babbitt metal. See also ball bearing; roller bearing.