Fatigue load limit Pu

The fatigue load limit Pu is the load below which, under ideal conditions — full separation of the contact surfaces by the lubricant film and utmost cleanliness — no fatigue theoretically occurs in the raceways. It transfers the endurance-limit concept of classical fatigue theory to rolling contacts.

Pu is a key input of the ISO 281 modified rating life: via the life modification factor a_ISO, the ratio of Pu to the actual load, the lubrication condition and the contamination level determine the extended life. Lightly loaded, clean, well-lubricated bearings can far exceed their basic L10 life; contaminated or poorly lubricated bearings never realize this potential.