Fault diagnosis of gear by cepstrum analysis and envelope analysis

Gear cepstrum analysis

The frequency spectrum of gear vibration is usually represented by meshing frequency and harmonic sideband, which is generated by the frequency modulation of gear shaft. Under normal operation, they remain unchanged. When the gear fails, the number and amplitude of the sidebands change. As mentioned above, when the gear tooth cracks, the fault gear will produce a local modulation every revolution. Because of the complex structure of the gearbox, a variety of modulation phenomena may exist at the same time, and each modulation phenomenon will produce different series of equal interval period spectrum. Because they are related to modulation sources, these sidebands contain rich fault diagnosis information. According to the concept of time-frequency conversion using FFT, the spectrum analysis results can be converted to a new analysis domain again using FFT technology. In this way, the so-called cepstrum analysis is formed. Cepstrum has the ability to detect and separate periodic components in the spectrum, which will simplify the edge spectrum lines of the original spectrum into a single spectrum line on the cepstrum, so that the complex periodic components in the spectrum become clear and easy to distinguish, which is conducive to fault diagnosis. The disadvantage of this method is that the amplitude of cepstrum is not sensitive to the development of crack length, and it is not easy to locate fault.

Gear envelope analysis

Envelope analysis is to extract the low frequency signal attached to the high frequency signal. From the time domain, it is to take the envelope trace of the time domain waveform. Envelope analysis is often used in the fault diagnosis of rotating machinery such as gears and bearings. When the bearing parts of rotating machinery have pitting, peeling and other damage faults, the periodic impulse impact force will be generated along with the operation of the equipment, which will arouse the inherent vibration of each stage of the equipment. The high-frequency natural vibration caused by impact is selected as the research object, which is separated from the signal by filtering, and then the envelope signal corresponding to the periodic pulse impact force is extracted by envelope detection. From its strength and frequency, the damage degree and position of parts can be judged. This technology is called envelope demodulation, also known as early fault detection method. It is an effective means to judge the damage type fault of equipment parts.

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