When the gear has a large period fault, such as motion eccentricity and geometric eccentricity, the frequency spectrum of the simulated gear meshing vibration signal is as shown in the figure. It can be seen from the figure that the amplitude of harmonic component will increase linearly with the increase of large period error amplitude of gear. The sideband frequency centered on the meshing frequency and separated by the rotation frequency is generated by signal modulation, that is, the high-frequency gear meshing frequency is modulated by the rotation frequency of the gear, and increases with the increase of large period error.

When the gear has small periodic errors such as pitting and peeling off, the frequency spectrum of the simulated gear meshing vibration signal is as shown in the figure. When there is small period error in the running process of the gear, the running speed of the gear will change. When the small period error is large, this phenomenon will be more serious. According to the theory of frequency modulation, the frequency spectrum of the running vibration signal of the gear will form the meshing frequency and its higher harmonic and the sideband component distributed around them with the rotation frequency as the interval. Their amplitude will increase with the deterioration of the fault.