Optimized tooth root strength by controlled shot peening

摘要

Beside modified materials and adapted heat treatments, mechanical surface treatments are a good alternative to improve the performance of the gear wheels. To increase the tooth root strength shot peening is the most common method. This study focuses on the improvement of the tooth root strength of case hardened steels with optimized shot peening parameters. Boundary conditions for this process optimization are the possibility to shot peen different gear wheel sizes in a non-mass production. Shot peening treatments with different peening parameters were performed on case hardened gear wheels with an air pressure peening device from Wheelabrator type Module8 ST6. The resulting skin layer states were investigated. To this surface roughness, hardness and residual stress depth distributions were measured. Additionally the tooth root strengths were determined. Therefor fatigue tests were performed and the nominal stress values σ_(F lim) were calculated. The resulting optimum peening setup was executed with different case hardending steels. Based on these results an arrangement of Almen strip holders was defined to control the shot peening process under production conditions, so that the process stability is ensured. By applying optimized shot peening parameters, the tooth root strength of case hardened gears can be improved significantly. An increase up to 65% of the nominal stress values σ_(F lim) compared to the non-peened condition were reached. It is finally necessary to highlight, that the ideal performance can only be ensured with an optimized shot peening process, a well-known material quality and a controlled heat treatment. Only if these three requirements (material, heat treatment and shot peening) are balanced the quality of the generated skin layer state can be ensured, thus the maximum nominal stress values σ_(F lim) can be achieved.