The design accuracy of gear pair of fuel oil gear pump is high, and the gear accuracy generally reaches level 5 or higher. The required cylindricity and roughness of the gear shaft diameter are also very high, so as to ensure the minimum critical oil film thickness at the shaft diameter when the gear pump works and eliminate the dry friction at the moving pair. The perpendicularity of the gear end face shall not be greater than 0.005, so as to achieve the best sealing effect with the sliding bearing end face and improve the volumetric efficiency of the gear pump. In order to obtain the high dimensional accuracy and roughness required by the design drawings, in the final process of gear processing, the two shaft diameters of the gear need to be processed on the high-precision cylindrical grinder, and the teeth need to be processed on the forming grinder. However, when grinding obtains high precision, it will produce some negative effects, such as grinding cracks, grinding burns and other defects. These defects have always been the difficulties perplexing product quality, especially delaying the generation of cracks, which seriously reduces the service life of gears and is not allowed to exist.
① The root cause of grinding cracks is that the heat generated on the metal surface in the grinding process can not be taken away by the cooling medium in time, the surface material is re tempered or new quenched martensite is formed, resulting in high tissue stress and thermal stress on the surface and tensile stress.
② The direct cause of the crack is the unreasonable process route and machining parameters, and the large grinding residual stress during finish machining provides the necessary conditions for the crack initiation.
③ Increasing the preliminary heat treatment before chemical heat treatment and aging treatment after grinding can further reduce the grinding residual stress. Controlling grinding feed, grinding wheel speed and workpiece speed can effectively avoid early microcracks.
④ Reducing grinding residual stress is an effective measure to eliminate delayed crack.
According to the process flow, the magnetic flaw detection of a certain type of gear parts was carried out before delivery into the warehouse, and no cracks were found. However, during the assembly process, small cracks were found at the outer circular end face and tooth root of some parts. There are many causes of delay crack. This paper analyzes the causes of delay crack and formulates control measures, which is of great significance to improve the service life of gear parts.