Load-carrying capacity of surface-hardened gears (Influence of surface roughness on surface durability)

Abstract

The influence of material, heat treatment, tooth profile and surface roughness on the surface durability of surface-hardened gears was investigated. First, the effect of surface roughness on rolling/sliding contact fatigue was studied in detail using a 2-roller-type fatigue testing machine. From these results, it is found that the initial surface roughness has a considerable influence on the surface durability of surface-hardened steel, and that the decrease of surface roughness with running does not always have an advantageous effect. Secondly, the load-carrying capacities of case-carburized gears and induction-hardened gears were investigated using a power circulating-type gear testing machine. The phenomenon of grey-staining, which consists of many micro-cracks and -pits, occurs on the ground tooth surface under a heavy load. In these tests, it is found that the grey-staining failure, which causes the degradation of tooth profile and the spalling, is most influenced by the initial tooth surface roughness. Furthermore, the effect of initial surface roughness on the surface durability of surface-hardened gears can be grasped quantitatively.