The correct meshing of the gears requires that the modulus and pressure angle of the two gears are equal respectively. Since the modulus is uniform in design, the main reason is the change of the pressure angle. The specific reasons may be as follows:

1. Xin'an gear, with small contact surface and high noise, will be better after running in for a period of time.

2. The tooth surface is damaged if the time is too long.

3. Gear eating or heat treatment is defective, resulting in insufficient contact fatigue strength.

4. The box body boring error, caused by the parallelism error of the axis line.

5. Lubricating oil is not updated in time.