In general, the fatigue failure of gear is to produce microcracks at the position of the maximum stress of gear teeth, and then cause crack propagation and fracture.
As shown in the figure, the stress distribution of the pinion at four time points in the whole meshing process is intercepted. It can be seen from the figure that in the process of gear meshing, the contact stress is not uniform on the whole tooth surface of the pinion, and there is stress concentration in the process of gear meshing, and the stress is mainly concentrated in the area between the gear base circle and the tooth root circle; The “edge effect” is caused by the uneven force on the tooth surface. In practical engineering applications, due to the low accuracy requirements of heavy-duty gears, micro roughness will be formed on the meshing surface of gear teeth in the manufacturing process, which leads to the discrete contact point of the meshing surface of gear teeth under load. In the process of gear meshing, the uneven force on the teeth and the manufacturing and installation errors will lead to the instability of the gear transmission, which is also an important reason for the wear of the tooth surface, the occurrence of fatigue microcracks and the tooth surface scuffing.
In conclusion, the number of teeth of the pinion is 26, less than 41, so the diameter of the base circle is larger than that of the root circle. The stress concentration occurs in the tooth between the base circle and the root circle. Due to the existence of stress concentration, fatigue microcracks are most likely to occur. Therefore, the tooth between the base circle and the root circle is defined as the dangerous position of the tooth, It is necessary to study the propagation law of initial crack at dangerous position.