Influence of the metalworking fluid on the micropitting wear of gears

Boundary layers can protect tooth flanks from metal-to-metal contacts. The characteristics of boundary layers and the resulting frictional and wear behavior are determined by the processes of manufacturing and running-in. This paper focuses on the influence of the metalworking fluid on the boundary layer formation of gear surfaces and the resulting wear behavior of the gears in terms of micropitting. Therefore, test gears were manufactured with different metalworking fluids consisting of mineral oil, ester oil and additives. The test gears were measured regarding their boundary layer properties, e.g. microstructure, reaction layers and residual stresses, which correlated with the choice of metalworking fluids in the respective grinding processes. In the subsequent tests of the wear behavior, repeatable differences in the degree of micropitting were observed. Especially the use of ester oil or chemically active sulfur additives in the grinding oils led to better tribological performances of the gears in the later application. Carry-over effects of the grinding oil from manufacturing into application as well as the need of a certain degree of mechanical and chemical energy to improve the precondition process of the gears are possible explanations for the observations made in this study.