The individual life of a rolling bearing is expressed as the number of revolutions or the number of operating hours at a given speed that the bearing is capable of enduring before the first sign of metal fatigue (spalling) occurs on a raceway of the inner or outer ring or a rolling element.

However, under controlled laboratory conditions, seemingly identical bearings operating under identical conditions have different individual endurance lives. A clearer definition of the term “bearing life” is therefore essential to calculate bearing size. All information presented by KMR with regard to load ratings is based on the life that 90% of a sufficiently large group of apparently identical bearings can be expected to attain or exceed.

Due to the statistical nature of bearing life, it must be pointed out that the observed time to failure of an individual bearing mounted in an application can be related to its rated life only if the failure probability, of that particular bearing, can be determined in relation to the general population of bearings running under similar conditions. For instance, if a bearing failure is observed in a bearing fan application counting a total of two hundred mounted bearings working under similar conditions, this represent a failure probability of just 0,5%, thus a reliability for the mounted application of 99,5%.