Rolling contact fatigue. Component design and testing for P/M applications

Newly developed sintered steels, combined with new densification processes, make the P/M process a candidate for highly loaded gears or similar components. There is a great need to develop methods which support design engineers with well established and standardized data. Furthermore, the challenge of the global market requires comparable material data and international standards. The absence of this data forces the engineer to use higher safety factors, risk poor reliability, or perform costly tests. The paper describes principle differences in the behavior of porous and non-porous materials under rolling contact conditions. The methods for determining RCF will be critically reviewed in regard to the special needs of P/M materials. Test results on P/M and wrought materials will be discussed. The paper introduces the ZF-RCF test bench, developed by ZF-Friedrichshafen (Germany) and the Gear Research Institute in Munich (Germany). The test principle used fulfills requirements for testing porous materials by offering the following features: Testing of original P/M surfaces. Hydraulic load application, guaranteeing constant load over the test period. Testing under line contact simulating elastohydrodynamic lubrication of gears. Testing variable sliding and loading conditions. Test results obtained with the ZF-RCF test bench have been proven for ranking materials according their pitting behavior. For the first time, variable loading tests give engineers the opportunity to design gears according to the actual loading conditions. Possible applications of this work include high density camshaft lobes, planetary gears, and even sun gears.