When bearings rotate under load, material flakes from the surfaces of inner and outer rings or rolling elements by fatigue arising from repeated contact stress (ref. A 152).
This phenomenon is called flaking.
The total number of bearing rotations until flaking occurs is regarded as the bearing "(fatigue) service life".
"(Fatigue) service life" differs greatly depending upon bearing structures, dimensions, materials, and processing methods.
Since this phenomenon results from fatigue distribution in bearing materials themselves, differences in bearing service life should be statistically considered.
When a group of identical bearings are rotated under the same conditions, the total number of revolutions until 90 % of the bearings are left without flaking (i.e. a service life of 90 % reliability) is defined as the basic rating life. In operation at a constant speed, the basic rating life can be expressed in terms of time.
In actual operation, a bearing fails not only because of fatigue, but other factors as well, such as wear, seizure, creeping, fretting, brinelling, cracking etc (ref. A 152, 16. Examples of bearing failures).
These bearing failures can be minimized by selecting the proper mounting method and lubricant, as well as the bearing most suitable for the application.