Rolling Contact Fatigue Testing, Bearing Life Prediction, and Steel Properties

Few rolling bearing users realize that the fundamentals of bearing life predictions are based on experimental data derived under conditions very different from the ones generally used in RCF testing today. The basic life formulas derived in the 1940s largely were based on tests run on steel with very different characteristics than the materials used to produce bearing components today. Still, the fundamental life calculations are based on the (C/P)(n) concept, which was experimentally derived by Lundberg and Palmgren (Lundberg, G. and Palmgren, A., "Dynamic Capacity of Rolling Bearings," Acta Polytech. Scand., Mech. Eng. Ser., Vol. 1, No. 3, 1947), even if the basic equation has been expanded and adjusted to reflect the lives recorded for modern bearings and to incorporate the concept of a fatigue limit (Ioannides, E., Bergling, G., and Gabelli, A., " An Analytical Formulation for the Life of Rolling Bearings," Acta Polytech. Scand., Mech. Eng. Ser., Vol. 137, 1999, pp. 9-12, 21-24). In an attempt to better understand the premature and unpredictable failures that sometimes occur in certain industrial applications today, the test procedures used by Lundberg and Palmgren have been revisited, and data have been derived that might contribute to an understanding of the short lives sometimes experienced in certain industrial bearing applications. Based on this test procedure, a better understanding of the development of micro-crack associated plastic deformations at non-metallic inclusions also has been gained. The propensity of different non-metallic inclusions to drive the formation and growth of micro-cracks has been studied in detail, and this knowledge has been used to develop steel that is less prone to butterfly development and micro-crack growth under very high contact stress conditions. Today's bearing life prediction models presume that the Palmgren-Miner rule of accumulated damage is globally applicable. Experimental data questioning this assumption are presented.