Ball Bearings: Races, Balls and Cage

A ball bearing has four parts: an outer race, an inner race, the balls that roll between them, and a cage that keeps the balls evenly spaced. Almost all of it is steel, usually 52100 (1.5% chromium, 1% carbon), which can be made very hard and tough by heat treating; 440C stainless is used where rust is a problem. The races are turned from tubing, the balls are cold headed from thick wire, both are hardened, tempered, ground and lapped to an almost mirror finish, and the cage is stamped from thin sheet. Even an inexpensive bearing's balls may be out of round by no more than 25 millionths of an inch; precision bearings allow five millionths.