Life ratings for ball and roller bearings

Since the beginning of the twentieth century, bearing manufacturers and users have sought to predict the fatigue endurance of ball and roller bearings. The first generally accepted method to predict bearing life and rate bearings accordingly was published in 1947 by Lundberg and Palmgren (LP). This method was adopted in the United States as the basis for a Standard in 1956, and it remains the basis for the current ISO Standard. The LP theory, however, pertains only to bearings manufactured from AISI 52100 through-hardened steel, and it permits ratings for bearings loaded solely by the Hertz stresses associated with simple applied loading.

To accommodate the various steels from which bearings are manufactured and the effect of lubrication on fatigue life, the life calculated according to the LP method was adjusted by life modification factors; this multiplicative life factor concept was inserted into the ISO Standard during the 1970s. Even with these life factors, fatigue lives experienced in many bearing applications substantially exceeded the predictions.

Life predicted according to their method could ideally be infinite if the fatigue limit stress is not exceeded. Moreover, as opposed to the LP theory, which relies on Hertz stresses alone, the IH method permitted consideration of the totality of stresses acting on the rolling contact members. The IH method has now been further developed into an even more generalized Stress-Life method. This method, which is still based on the LP formulation, employs a single stress-life factor to modify the LP predicted fatigue life. In the fatigue life prediction for a bearing application, the stress-life factor integrates the effect on life of all applied, induced and material residual stresses together with empirically determined fatigue limit stresses. This paper details the historical development of bearing life ratings culminating in the detailed methods of application of the stress-life method.