The nominal bearing life can be calculated according to ISO 281. An equivalent load P and the dynamic load capacity C of the bearing are used for the calculation of bearing life L10. The nominal life is calculated in million revolutions as L10 = (C/P)^p with p=3 for ball bearings and p=10/3 for roller bearings. The nominal bearing life is based on subsurface fatigue as failure mode. Other possible failure modes like wear, smearing, plastic deformation or breaking of the cage are not considered.
With a rotation speed n in rpm a bearing life can be calculated in hours instead as L10h = 16666/n*(C/P)^p. The nominal life is for a reliability of 90% or a failure rate of 10% which is indicated by the index ’10’.
The equivalent load is calculated based on radial and axial loads as P = X*Fr + Y*Fa. The factors X and Y are given in ISO 281 or in bearing catalogs dependent on the bearing type. The dynamic load capacity C can be calculated according ISO 281 too and is usually given in the bearing catalogs. The bearing life according ISO 281 is based on the assumption of zero bearing clearance and no misaligment. Therefore some reduction factors are included in the load capacity.
In extension to the nominal life, the modified life includes the influence of reliability, lubrication and cleanliness. The reference rating life according ISO/TS 16281 would take into account bearing clearance and misalignments.