Abstract: After about 35 hours of rotation testing on a 40Cr steel rotating shaft, the gear of the rotating shaft showed wear. The causes of gear wear were analyzed using methods such as macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination and hardness testing. The results show that the improper control of the heat treatment process between the rotating shaft gear and the gear shaft sleeve resulted in larger grain size on the surface of the tooth tip. The gear shaft sleeve that matches the rotating shaft gear had a high hardness, causing deformation of the rotating shaft gear and loss of surface material at the tooth tip. The missing material formed abrasive wear, ultimately led to slow peeling of the teeth of the rotating shaft gear.