Ball bearing

A ball bearing is a common type of rolling-element bearing, a kind of bearing.

The term ball bearing to mechanical engineers usually means a bearing assembly which uses spherical bearing balls as the rolling elements. To laypeople the term often means an individual ball for a bearing assembly. The remainder of this entry uses the term ball for the individual component and "ball bearing" or just "bearing" for the assembly.

Ball bearings typically support both axial and radial loads and can tolerate some misalignment of the inner and outer races. Also, balls are relatively easy to make cheaply compared to other kinds of rolling elements. Ball bearings tend to have lower load capacity for their size than other kinds of rolling-element bearings due to the smaller contact area that spherical shapes provide.

Although Leonardo da Vinci has been credited with the discovery of the principle behind the mechanics of ball bearings the first patent was taken out by Philip Vaughn, a Welsh carriage-maker, in 1791.
There are several common designs of ball bearings, each offering various tradeoffs.

A radial ball bearing uses axially symmetric inner and outer races that are shaped so a radial load passes radially through the bearing. Most radial designs also support modest axial loads.
An angular contact ball bearing uses axially asymmetric races. An angular load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact bearings allow 'combined loads' (loading in both the radial and axial directions) and the contact angle of the bearing should be matched to the relative proportions of each. The larger the contact angle (typically in the range 10 to 45 degrees), the higher the axial load supported, but the lower the radial load. In high speed applications, such as turbines, jet engines, dentistry equipment, the centrifugal forces generated by the balls will change the contact angle at the inner and outer race. Ceramics such as silicon nitride are now regularly used in such applications due to its low density (40% of steel - and so significantly reduced centrifugal force), its ability to function in high temperature environments, and the fact that it tends to wear in a similar way to bearing steel (rather than cracking or shattering like glass or porcelain).

An axial ball bearing uses side-by-side races. An axial load is transmitted directly through the bearing, while a radial load is poorly-supported, tends to separate the races, and anything other than a small radial load is likely to damage the bearing.
A deep-groove radial bearing is one in which the race dimensions are close to the dimensions of the balls that run in it. Deep-groove bearings have higher load ratings for their size than shallow-groove bearings, but are also less tolerant of misalignment of the inner and outer races. A misaligned shallow-groove bearing may support a larger load than a similar deep-groove bearing with similar misalignment.
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