How is bearing life defined?

Generally, a rolling bearing cannot rotate for ever. Unless operating conditions are ideal and the fatigue load limit is not reached, sooner or later material fatigue will occur. The period until the first sign of fatigue appears is a function of the number of revolutions performed by the bearing and the magnitude of the load. Fatigue is the result of shear stresses cyclically appearing immediately below the load carrying surface. After a time these stresses cause cracks which gradually extend up to the surface. As the rolling elements pass over the cracks fragments of material break away and this is known as flaking or spalling.
The flaking progressively increases in extent and eventually makes the bearing unserviceable (fig 1, fig 2, fig 3, fig 4)
The life of a rolling bearing is defined as the number of revolutions the bearing can perform before incipient flaking occurs. This does not mean to say that the bearing cannot be used after then. Flaking is a relatively long, drawn-out process and makes its presence known by increasing noise and vibration levels in the bearing. Therefore, as a rule, there is plenty of time to prepare for a change of bearing.
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