The influence of shot peening on gear teeth micropitting and contact fatigue failure

Abstract

Case-hardened gears present a high contact fatigue load capacity in a compact and light mechanical transmission systems but sometimes, micropitting appears. This damage can lead to early contact fatigue failure called macropitting (NF ISO 10825-1). A previous study has demonstrated that an appropriate shot peening treatment could reduce the micropitting phenomenon via tests on a twin disc machine.The aim of this paper is to study the influence of shot peening on the micropitting of case-hardened gears tooth flank and its development to macropitting. Experimental tests are carried out using a back-to-back gear test rig, an interrupted test method is defined in order to monitor the micropitting initiation and its propagation on the tooth flank. Two variants of carburized spur gear specimens are tested. The reference specimen has a case-hardened ground tooth flanks which is a typical gear application. The other specimen has a case-hardened ground tooth flanks with a shot peened finishing process. The fatigue tests are run until the appearance of macropitting. The performed inspections of specimens indicate that shot peening modifies the surface roughness, the surface topography and increases the compressive residual stress. The experimental results show that micropitting occurs on both shot-peened and non-shot-peened gears after approximately the same running time. However, a difference is observed in the propagation of micropitting on the tooth flanks. In comparison to reference gears, shot peening has postponed the apparition of macropitting by more than 60%. An analytical study is also conducted to validate the prediction of micropitting using the ISO/TR 6336-22 by comparing the results with the occurrence of micropitting on the tested gears.