01 Selection and installation of sensors

 

　　In order to accurately obtain the gear vibration signal, if the gear meshing frequency or natural frequency exceeds 1000 Hz, an acceleration sensor should generally be used. For spur gears, the sensor should be installed radially. For gears bearing axial loads, the sensor should be installed axially. The closer the measuring point is to the bearing, the better.

 

　　02 Analyze the frequency structure to determine the gear state

 

　　Using simple vibration diagnosis to determine the gear status on site, the most effective and feasible method is to analyze the frequency change of the gear vibration signal, which is mainly analyzed from two aspects:

 

　　1) Identify gear failure from changes in meshing frequency

 

　　From the change of gear meshing frequency to judge the gear state, there are two main reasons: one is to look at the increase and decrease of meshing frequency amplitude; the second is to look at the distribution of meshing frequency harmonics.

 

　　The change of meshing frequency reflects the change of tooth profile caused by the change of gear meshing load. In addition, the uniform wear of the gear teeth causes the tooth profile deviation, and also generates harmonic signals of the meshing frequency. The change in meshing frequency caused by gear wear is shown in Figure 1.

 

　　Fm——meshing frequency; 2fm, 3fm——higher harmonics of meshing frequency;

 

　　Solid line (——): initial frequency amplitude; dotted line (……): frequency increase caused by wear

 

　　The error of the tooth profile causes significant distortion of the meshing frequency component. This kind of distortion is particularly manifested in the changes in the amplitude of the harmonic components, the amplitude of the higher harmonics is greater than the increase in the amplitude of the meshing frequency. Therefore, to identify the gear state from the meshing frequency, it is necessary to pay special attention to the change of the harmonics, at least to look at the first 3 harmonic frequencies (ie fm, 2fm and 3fm) to judge.

 

　　2) Analyze sideband characteristics to identify gear failures

 

　　Gears produce modulation during meshing operation, so a sideband structure is formed on the frequency spectrum of gear vibration signals.

 

　　Gear meshing frequency is modulated by the fault frequency due to gear failure or machining accuracy error, resulting in an infinite number of sidebands. At this time, the meshing frequency is the carrier frequency, and the rotating frequency and its multiplier of the faulty gear are the modulation frequencies. They produce side frequency components on both sides of the meshing frequency.

 

　　Gear failure is closely related to the sideband structure. Gear failure will cause an increase in vibration energy. This situation is usually reflected in the change of the side frequency component. Therefore, the existence and change of sidebands are valuable information reflecting the state of gears. Sideband analysis and identification are important and effective methods for on-site diagnosis of gear mechanisms.

 

　　There are two situations in the formation of amplitude modulation sidebands. One is to reflect the local defects of the gear, such as pitting on the gear pitch line, and the modulation results produce a wider sideband; the other is the gear defect showing a wider range Distributed defects, such as uniform wear of gears, when the sidebands appear steep and narrow. The time-domain waveform and spectrum structure in these two cases are shown in Figure 2 a and b respectively.

 

　　A) Partial defects in gears; b) Defects in gear distribution

 

　　When the speed is not constant, sidebands formed by frequency modulation will occur. When the FM phenomenon is not serious, only two or three pairs of sidebands can be considered. When frequency modulation and amplitude modulation exist at the same time, the sidebands often appear asymmetrical.

 

　　The sideband information contained in the gear vibration signal not only implies the existence of a certain fault in the gear, but also has a very valuable feature, that is, in most cases, the fault can be found from the interval between the two spectral lines of the sideband Source, determine which gear the fault is on. Because in the frequency spectrum of the modulated signal, the spectral line is centered on the meshing frequency fm and symmetrically distributed with the rotating frequency fz of the faulty gear as the interval, a pair of side frequencies can be expressed as fm±fz. If there are several pairs of side frequencies, they can be expressed as mfm±nfz (m, n=1, 2, 3...).

 

　　However, if two gears meshing with each other have the same number of teeth (at this time, the gear transmission only changes the direction and does not change the speed, such as the transmission gear pair used by Roots blower), then the rotation frequency of the two gears is the same, so it cannot be based on fz to identify which gear is the fault.