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Introduction
According to statistics, among the failed parts of the gearbox, the failure proportion of the gear itself accounts for about 60 at most, which shows that the gear transmission is an important part of the machine failure. Therefore, it is of great significance to carry out gearbox fault diagnosis research.
The shafts, gears and bearings in the gearbox vibrate when they work, and if a fault occurs, the energy distribution of the vibration signal changes, and the vibration signal is the carrier of the failure characteristics of the gearbox. The time domain analysis and frequency domain analysis of the vibration signal of the gearbox with fault are carried out, and an effective diagnostic method is proposed according to the characteristics of the fault signal.
1. Typical fault characteristics
Based on vibration theory and experimental research, the vibration characteristics of nine typical faults in the gearbox were analyzed: tooth shape error, uniform gear wear, box resonance, slight shaft bending, broken teeth, shaft unbalance, shaft severe bending, axial channeling, bearing fatigue spalling and pitting corrosion.
Tooth shape error
When the tooth shape error occurs, the spectrum produces a meshing frequency modulation phenomenon in which the meshing frequency and its higher harmonic are the carrier frequency, and the shaft rotation frequency and its doubling frequency where the gear is located are the modulation frequency, and the spectrum produces a small amplitude and sparse side band near the meshing frequency and its doublingThere are few frequency transfer orders on the demodulation spectrum, and the first order is generally the main one. When the tooth shape error is serious, due to the large excitation energy, the gear resonance frequency modulation phenomenon is generated with the natural frequency of each order of the gear as the carrier frequency, and the rotation frequency of the shaft where the gear is located and its doubling frequency as the modulation frequency.
Fig. 1 is the spectrogram and demodulation spectrum of a water mill reducer when the tooth shape is not good, and the meshing frequency of the gear pair is 28175Hz, the conversion frequency is 12At 35Hz, as can be seen from the demodulation spectrum, the first-order transpose frequency is large, while its octave amplitude is small or absent.
The main characteristics of the tooth shape error are:
1) The gear meshing frequency and its harmonic are the carrier frequency, and the shaft rotation frequency where the gear is located and its doubling frequency are the meshing frequency modulation of the modulation frequencyWhen the tooth shape error is serious, due to the large excitation energy, the gear resonance frequency is modulated with the natural frequency of each order of the gear as the carrier frequency, and the rotation frequency of the shaft where the gear is located and its doubling frequency is the modulation frequency
2) The vibration energy (including the RMS value and the kurtosis indication) has increased to a certain extent;
3) The envelope energy (including the RMS value and the kurtosis indication) has increased to a certain extent.
The gears are evenly worn
When the gears are evenly worn, there is no obvious modulation phenomenon because there is no shock vibration signal. When the wear develops to a certain extent, the meshing frequency and the amplitude of each order harmonic increase significantly, and the higher the order, the greater the amplitude of harmonic increase. At the same time, the vibration energy (including the RMS value and the kurtosis index) has a large increase.
Fig. 2 is a spectrogram of the uniform wear of the gear of the reducer of a cement mill, and the meshing frequency (52.) occurs after uniform wear50Hz) and its 2 octaves (1035hz multiplier (156..)3hz multiplier (208.)98Hz multiplier (256.)84Hz), according to the test results, the harmonic amplitude of each order increases significantly, and the amplitude of the high-order increase is large.
The main characteristics of uniform wear of gears are:
1) The amplitude of the gear meshing frequency and its harmonics increases significantly, and the higher the order, the greater the amplitude increase
2) The vibration energy (including the RMS value and the kurtosis index) has a significant increase.
Cabinet resonance
In the case of box resonance, the natural frequency component of the box appears on the spectrum, and the resonance energy is generally very large, while the other frequency components are small or absent.
Figure 3 is the spectrogram of a cement mill when box resonance occurs. As can be seen from the figure, the meshing frequency of the mill large gear is 5250Hz (the natural frequency of the cabinet is 52.)50Hz) vibration energy is very large, the machine was shut down after the test found that the frequency component was transmitted by another cement mill of the same model in the workshop through the foundation, causing the resonance amplitude of the reducer casing to be still large. The main characteristic of the cabinet resonance is that the vibration energy is very high.
Broken teeth
The time domain of the broken tooth is characterized by a large amplitude shock vibration, and the frequency is equal to the rotation frequency of the shaft with the broken tooth. In the frequency domain, there are side bands with an interval of the rotation frequency of the broken tooth shaft near the meshing frequency and its higher harmonicsIn general, the number of side bands is large, the amplitude is large, and the distribution is wider. In the demodulation spectrum, there are often frequency shifts and their higher harmonics, and even more than 10th order. At the same time, due to the large transient impact energy, the natural frequency is often excited, resulting in the natural frequency modulation phenomenon.
Fig. 4 is the time-domain signal, spectrum, refinement spectrum and demodulation spectrum of vibration acceleration when the intermediate shaft of a gearbox is often meshed with a broken gear, and the meshing frequency is 468 in the first order of the broken tooth gearThere is an obvious side frequency band with the above characteristics near 7Hz, and the modulation frequency of the demodulation spectrum is the rotation frequency of the intermediate axis 1107Hz and higher harmonics. Due to the large transient impact energy generated by the broken tooth, the natural frequency of the gear is often excited, resulting in the phenomenon of natural frequency modulation, and a group of side frequency bands with 1082Hz as the central frequency (the natural frequency of a certain order of the broken tooth gear in the free state is 1069Hz) and the intermediate shaft rotation frequency of 1107Hz as the modulation frequency appears in Figure 4(d). The above phenomenon was also observed in another gearbox intermediate shaft with a gear break that was often engaged.
The main characteristics of broken teeth are:
1) The gear meshing frequency and its higher harmonic are used as the carrier frequency, and the shaft rotation frequency where the gear is located and its doubling frequency are the meshing frequency modulation, and the modulation side frequency bandwidth is high, and the demodulation spectrum appears at the rotation frequency of the shaft and multiple higher-order harmonics
2) The gear resonance frequency modulation with the natural frequency of each order of the gear as the carrier frequency The rotation frequency of the shaft where the gear is located and its doubling frequency are the modulation frequency, and the modulation side frequency bandwidth is high, and the demodulation spectrum appears in the rotation frequency of the shaft and multiple higher-order harmonics
3) The vibration energy (including the RMS value and the kurtosis index) has increased significantly
4) The envelope energy (including RMS and kurtosis indexes) increased significantly.
The shaft is slightly bent
When the shaft is slightly bent, it will lead to tooth shape error in gear transmission, forming a meshing frequency modulation phenomenon with the meshing frequency and its doubling frequency as the carrier frequency, and the shaft rotation frequency where the gear is located as the modulation frequency, if there are multiple pairs of gear meshing on the bending shaft, there will be multiple pairs of meshing frequency modulation, but the number of sidebands on the general spectrum is small and sparse, although it has similar sidebands with the tooth shape error, but its axial vibration energy is significantly increased.
The main characteristics of a slight bending of the shaft are:
1) The gear meshing frequency and its harmonic are the carrier frequency, and the rotation frequency of the shaft where the gear is located and its doubling frequency are the meshing frequency modulation of the modulation frequency, the number of modulation edge bands is small and sparse, and only the rotation frequency of the shaft generally appears on the demodulation spectrum
2) If there are multiple pairs of tooth meshing tables on the bending shaft, there will be multiple pairs of meshing frequency modulation;
3) There is a certain increase in vibration energy (including RMS and kurtosis indicators);
4) There is a certain increase in envelope energy (including RMS and kurtosis index).
Coming soon next
Spectrum analysis of 9 types of gearbox faults and application examples (2).