The Role of Lubrication in Propagation of Contact Fatigue Cracks

Abstract

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.

Discussion: “An Initial Appraisal of the Contact Fatigue Strength of Electron Beam Melted Bearing Steel” (Widner, R. L., 1972, ASME J. Lubr. Technol., 94, pp. 174–178)

“…The magnitude of improved life appeared to be dependent upon lubricant viscosity with the greater improvement being related to the lower viscosity lubricant. This would be in agreement with [12] where propagation of inclusion origin fatigue spalls was shown to be dependent upon lubricant viscosity and/or film thickness.…”

The formation of fine relaxation cracks along the compressive maximum shear crack

“…A close examination of the past literature on rolling contact fatigue failure (see Styri [2]; Widner and Wolfe [3]; Littmann Widner [4]; Littmann, et al [5]) divulged that the upper, or the lower, fine "tail cracks" of inclined shear cracks present in the subsurface regions of rolling contact fatigue specimens and bearing components bear similar resemblances to the fine "tail cracks" o f the compressive maximum shear cracks reported in paper [1]. In particular, the senses and values of rotation angle 00 of the tail cracks in both cases are quite similar.…”