Question 1

Why do bearings often last much longer than the calculated L10 life?

Accepted Answer

L10 is a conservative, 90th-percentile estimate based on fatigue under ideal conditions. Modern bearing steels (vacuum-degassed, inclusion-free), improved lubricants, and better sealing mean actual median life (L50) is typically 5× the L10 value. With excellent lubrication (κ > 4) and clean conditions, the ISO adjusted life can be 50× the basic L10.

Question 2

How do I determine the equivalent dynamic load P?

Accepted Answer

For combined radial (Fr) and axial (Fa) loads: P = X×Fr + Y×Fa, where X and Y are factors from the bearing manufacturer's catalog. For pure radial load, P = Fr. For pure axial load on thrust bearings, P = Fa. If loads vary with time, use the weighted mean cubic load: Pm = ∛(Σ(Fi³×ni)/Σni) where ni is the fraction of time at each load level.