The Role of Lubrication in Propagation of Contact Fatigue Cracks

Comparative life tests of tapered roller bearings in mineral oils and synthetic fluids, Mil L-7808-F and Mil L-23699, demonstrate a strong influence of lubrication upon bearing life and failure characteristics. The inclusion origin mode of failure propagates very rapidly and takes on the macroscopic appearance of the point surface origin mode when fatigue cracks interact with a lubricant having low viscosity at the operating temperature. Observations indicate that bruises from debris in the lubricant are stress raisers of severity equivalent to nonmetallic inclusions in bearing materials for nucleation of contact fatigue cracking. Surface fatigue by “peeling” or superficial pitting can also act as the nucleus of fatigue cracks which propagate in the same manner as the point surface origin mode. In general, any surface stress concentration in combination with low lubricant film thickness promotes surface origin modes of contact fatigue and low lubricant viscosity at the operating temperature promotes rapid propagation by a hydraulic pressure mechanism.