The common points of frequency modulation and amplitude modulation of gear vibration signal are: ① the carrier frequency is equal; ② the sideband frequency is equal; ③ the sideband is symmetrical to the carrier frequency.
In the actual gear system, the AM effect and FM effect always exist at the same time, so the side frequency component in the spectrum is the superposition of the two kinds of modulation. Although the side frequency generated by either of the two modulation is symmetrical to the carrier frequency, when they are superposed, because the side frequency components have different phases, they are vector sum. After superposition, the amplitude of some side frequencies increases and some decreases, which destroys the original symmetry.
The side frequency is unstable. The relative phase relationship between amplitude modulation and frequency modulation will be affected by random factors, so under the same modulation index, the shape of side band will change, but its overall level will not change. Therefore, in gear fault diagnosis, the information obtained only by monitoring a few side frequencies is often not comprehensive, and the diagnosis conclusion based on it is sometimes unreliable.