The frequency components of gear failure are very complex, which are all broadband signals. According to the introduction of some data, the relationship between the common faults of gears and the characteristic frequency, its harmonic and side band is summarized as follows:

① The failure caused by the uniform wear of the gear teeth, the excessive clearance of the gear side and the excessive load of the gear will increase the meshing frequency FM and its higher harmonics 2fm, 3FM The amplitude of frequency component does not produce sideband. Among them, the amplitude increment of high-order harmonic of meshing frequency is larger when wear is serious, and the amplitude of second-order harmonic even exceeds the amplitude of fundamental wave of meshing frequency when wear is serious.

② The non-uniform distribution faults such as gear eccentricity, periodic change of pitch and load fluctuation will produce amplitude modulation and frequency modulation, thus forming side band on both sides of meshing frequency and its harmonics. The interval frequency of side band is the rotational speed frequency of defective gear. In general, the gear eccentricity only appears the lower sideband (differential frequency) fm-n Fe (n = 1,2,3,…) , the upper sideband is rarely seen.

③ The local faults of concentrated defects, such as broken teeth, peeling off of tooth surface and cracks, will cause periodic impact pulse, and also produce amplitude modulation and frequency modulation. For example, if there is one broken tooth or two peeling off parts of the pinion, each revolution of the pinion will produce one or two obvious periodic impact. In this case, the meshing frequency is modulated by the pulse frequency, and a series of sidebands are formed on both sides of the meshing frequency and its harmonics. The sidebands are characterized by a large number of sidebands, wide range, uniform distribution and relatively flat. In addition, severe local tooth breakage will also lead to the increase of the rotating frequency and its harmonic amplitude. The main characteristic of the broken gear is that the amplitude of the rotation frequency and the meshing frequency of the gear increases obviously.

④ Pitting, scratches (i.e. slight gluing) and other defects with uniform distribution will also produce periodic impact pulse, amplitude modulation and frequency modulation. However, it is different from the local faults such as broken teeth in that the order of the sideband distributed on both sides of the meshing frequency and its harmonics is small and concentrated, and the sideband is characterized by high and narrow amplitude variation and large fluctuation. However, with the development and deterioration of the fault, the graph will also change.

⑤ The fracture or crack of the tooth will produce an impact when entering the meshing, which may arouse the natural frequency (natural frequency) of the gear. However, because the natural frequency of the gear is generally 1 ~ 10kHz high frequency, this high frequency component has been basically attenuated when it is transferred to the gearbox shell, and in most cases, only the meshing frequency and the modulated side frequency can be measured.