Study on cracks and process improvement for case hardened gear shaft straightening

Deformation of the gear shaft in transmission is inevitable after case hardened heat treatment. Straightening is the normal way to ensure the straightness requirement. During the straightening process, cracks often appear and result in scrapped parts. Based on the situation of high rejection rate of straightening crack of the internal input shaft of a dual-clutch automobile transmission, this paper makes an in-depth analyses of the pressure straightening process theory, the crack generation mechanism, the working process of the automatic straightening machine and the crack detection system, etc., and establishes the relationship between the ram increment, the total displacement and the crack rate during multi-step progressive straightening, proposes a way to quickly judge the straightening performance of the parts. Finally, after optimizing the pressure position by finite element analysis, optimizing the straightening procedure by adjusting ram increment and adopting vector straightening method, optimizing the parts martial properties by heat treatment improvement, the straightening crack rate of internal input shaft was reduced from 1.95 % to 0.14 %.