The steering system is an important part of the vehicle safety system. The steering shaft tooth is a core part of the commercial vehicle hydraulic power steering system (HPS), which plays the role of steering torque output in the steering process. The conventional manufacturing process flow of steering shaft gear is: shaft gear forging (normalized) – rough machining – carburizing and quenching – tempering – finishing – flaw detection. With the improvement of steering comfort requirements such as noise reduction, gear grinding process is gradually introduced into some shaft gear parts in the machining process. In this process, grinding cracks are very easy to occur if the grinding process is not properly controlled.
This paper mainly analyzes the causes of grinding cracks in the grinding process of a certain type of steering shaft teeth (20CrMnTi steel), expounds the generation mechanism of grinding cracks, and puts forward preventive measures.
The technological process of a certain type of shaft gear is: shaft gear forging (normalized) – rough machining – carburizing and quenching – tempering – finishing – gear grinding – flaw detection. After grinding, micro grinding cracks with “linear cross” distribution appear at the root of shaft teeth. With the extension of time, the grinding cracks gradually extend to the involute tooth surface, and finally tend to “parallel arrangement” on the tooth surface.
On the one hand, the generation of grinding cracks is related to the heat treatment quality of gears, such as excessive stress after quenching, insufficient tempering, excessive residual austenite and coarse network carbide; On the other hand, it is related to the cold and thermoplastic distortion caused by grinding force and grinding heat and the structural stress caused by the change of material microstructure. In actual working conditions, the relationship between them and grinding cracks is a primary and secondary contradiction. The fundamental to prevent grinding cracks lies in how to reduce the residual stress of the gear, reduce the thermal stress generated in the grinding process, and avoid the structural stress caused by the change of material microstructure caused by local overheating in the grinding process. It needs to be comprehensively considered from the aspects of gear heat treatment, gear grinding and gear design.