Correlation of Axial Fatigue Data for Wrought Steel Samples with Stroker Durability Data for Surface Densified Races

摘要

One way clutch (OWC) races used in modern automatic transmissions are routinely subjected to considerable loads and are expected to meet requirements of high durability in tensile as well as contact fatigue. Races of today are made with high density powder metal with added surface densification through a process such as Densiform~®, having displaced conventional materials such as wrought steel. Surface densification methods offer the superior advantage of providing full to nearly full density in component areas targeted by high stress while maintaining lower density in regions unaffected by stress. However, the same advantage in turn creates new challenges in determining the fatigue performance of components made with such materials. The very local nature of the surface densification process renders the development of samples for standardized testing impossible, in turn, complicating the development of new products. A need thus exists for a technique that allows for the easy selection of a surface densified material without resorting to complicated component durability testing. An approach is proposed in this paper that seeks to address this shortcoming in material selection by using samples of wrought steel with chemistry identical to that of typical surface densified sintered steel for fatigue testing. A surface densified one-way clutch outer race used in an automatic transmission is selected as the candidate part for the current development. The outer race has areas of stress concentration that result in tensile stress under maximum torque. Finite Element Analysis (FEA) of the outer race cam is performed to establish maximum principal stresses at different torque levels. Component durability testing in a stroker machine at the different torques is subsequently performed in order to generate a stress life (S-N) curve. For comparison, standard tensile unnotched samples are also made using equivalent wrought steel. The unnotched samples are tested in axial fatigue to generate a stress life curve for the wrought steel. A correlation is then made between the two types of materials using the fatigue curves.