Design for rolling contact fatigue

The use of powder metallurgy (P/M) materials is rapidly expanding to include the manufacture of structural parts such as gears and camshaft lobes. Components such as these are highly loaded under rolling contact conditions. However, when selecting materials, especially P/M materials, for use involving rolling contact conditions, the lack of reliable test data and the lack of reliable methods to predict contact fatigue life have proven to be major obstacles. to insure that P/M materials have the ability to compete with wrought steels, costly and time consuming tests are carries out on finished parts and additional safety factors are applied in order to avoid failure under service conditions. The paper gives an overview of the state of the art in rolling contact fatigue design of gears. A summary of gear design parameters and their effect on rolling contact fatigue conditions is given. Surface and subsurfaces stress distribution has been studied under various contact conditions such as elliptical vs. line contact, variation of gear design parameters, sliding, and lubrication. Failure modes and there relationship to material properties and contact conditions are discussed. Several rigs to test materials under rolling contact are discussed and compared in regard to their importance for gear applications. In addition, actual test results on P/M materials under constant and variable loadings are presented. the effect of sliding, density, chemical composition, sintering conditions, and surface treatment, which are major factors on contact fatigue life, are discussed. Advanced P/M technologies such as warm compacting, sinter hardening, and the use of special powder mixtures may be beneficial for rolling contact applications. Improved fatigue life may justify higher production costs correlated with those technologies. Finally, examples of the transformation of test results into actual fear design are given.