# Other components of gear vibration signal

Sometimes it can be seen that the envelope lines at the upper and lower ends of the amplitude of the total signal in the time domain of the gear are asymmetric, which are quite different from each other. This carrier signal is asymmetric with the zero line, which is often caused by the low harmonic of the gear rotation frequency, called additional pulse, as shown in the right figure. In the figure (a) is the total signal asymmetric to the zero line. By decomposing it, additional pulse signal (b) and original amplitude modulation signal (c) can be obtained. Additional pulses are caused by faults other than gear teeth, such as gear imbalance, misalignment, mechanical loosening, etc. The characteristic frequencies of these faults are all low harmonics of the rotation frequency, which are far less than the meshing frequency. Therefore, the additional pulse has nothing to do with the defect of gear teeth. It is easy to distinguish the additional pulse from the characteristic signal. In the time domain, the additional pulse is directly superimposed on the normal vibration of the gear, rather than in the form of modulation; in the frequency domain, the additional pulse only appears in the low-order harmonic band of the lower frequency rotation frequency, it is impossible to form the sideband on both sides of the meshing frequency.

##### 2) Implied component

Hidden components, also known as ghost lines, are some unknown frequency components and harmonics in the spectrum of new gears, except for rotation frequency, meshing frequency and side frequency. In fact, it is the periodic defect caused by the error of the indexing gear of the gear processing machine transferred to the gear being processed. The research shows that the frequency of the implied component is the meshing frequency of the indexing gear.

It can be judged whether the unknown frequency is the implied component (ghost line) according to the following characteristics of the implied component:

① The implied spectral line is the harmonic of rotation frequency, which always appears near the meshing frequency, as shown in the figure below on the left;

② The load change has little effect on it, as shown in the figure below on the left;

③ With the increase of running in time, the wear makes the defects tend to be uniform, the hidden components and harmonics will gradually decrease, and the meshing frequency and harmonics will gradually increase. The following figure on the right is a typical example. After one month’s operation, the implied component decreases from 123dB to 113dB, and the amplitude of engagement frequency increases from 116dB to 123dB.

##### 3) Rolling bearing signal and cross modulation

When the supporting bearing of gear is rolling bearing, the vibration signal of rolling bearing will be included in the vibration signal of gear. However, under normal circumstances, the vibration energy level of rolling bearing is obviously lower than that of gear, which is generally one order of magnitude smaller. Therefore, in the range of gear vibration frequency (0 ~ 5KHz), the frequency composition of rolling bearing is not obvious.

However, when a serious fault occurs in the rolling bearing, there will be a more obvious characteristic frequency component in the gear vibration frequency band. These components sometimes appear alone, sometimes cross modulation with gear vibration components, and sum frequency and difference frequency components appear. Sum frequency and difference frequency components are not independent. When the basic frequency components that produce them change, they will also change. 