The development of rolling bearing deterioration is not a linear process, but an exponential one. In different stages of deterioration, fault information appears in different frequency bands, so different fault detection methods are used. At present, it is generally accepted in industry that the deterioration history of rolling bearings can be divided into 4 stages. Stage 1 – bearing failure early At this stage, the bearing is the first to form microscopic cracks or lattice dislocations on the subsurface, while the bearing surface is invisible to cracks or tiny spalling. In the low frequency band of vibration signal, no more obvious impact signal will be formed, and the fault signal cannot be picked up by traditional acceleration sensor, but the microscopic cracks or lattice dislocations on the subsurface will generate acoustic emission signal or stress wave signal. Therefore, at this stage of the bearing fault characteristics are mainly reflected in the ultrasonic frequency band, can be picked up by acoustic emission sensors or resonance-based acceleration sensors, the main performance of the measured signal peak or energy value becomes large. Stage 2 – bearing failure development period At this stage, the microscopic deterioration of the bearing begins to expand from the subsurface to the surface and produces damage points such as cracks or micro spalling on the contact surface of the bearing. When the bearing component surface contact with these damage points, it will form a certain frequency of the impact pulse, according to the Fourier transform can be seen, the short-time impact signal in the frequency domain is a broadband signal, so this impact signal will certainly excite the bearing parts of the high-frequency inherent frequency resonance, so that its vibration to strengthen, through the acceleration sensor will be able to pick up this part of the signal, and then use the envelope demodulation technology can The fault characteristic frequency of the bearing can be observed, and at the end of the second stage, the multiplication of the fault characteristic frequency can be observed. At this stage, the fault characteristic frequency of the bearing is temporarily submerged in the higher noise of the lower frequency band, so that no clear fault characteristic frequency is observed in the fault characteristic frequency band. Stage 3 – bearing failure rapid development period At this stage, with the accelerated development of bearing damage, the impact of the damage point on the bearing contact surface becomes more and more intense, and the multiples of the bearing failure characteristic frequency in the resonance frequency band are more and more, and the energy magnitude of its periodic impact is enough to be directly observed through the power spectrum of the vibration signal, at this time the bearing failure characteristic frequency can be clearly seen directly on the power spectrum of the vibration signal. And its multiplication frequency has more and more tendency. Stage 4 – bearing failure end At this stage, the rolling bearing has almost reached the end of life, the damage point can be observed by the naked eye, the bearing movement noise becomes particularly large, the temperature rises rapidly. At this time the direct power spectrum can not only clearly see the bearing failure characteristic frequency and its multiplier, if the damage point alternately into the load area, but also in the failure characteristic frequency next to see the obvious modulation side frequency. At the end of the 4th stage, the spectrum becomes not very clear, in the power spectrum will form a protruding “thatch pile”, in addition, the energy of high frequency vibration at this time may not rise but fall, if you find that the amount of high frequency monitoring began to decline, not the surface bearing condition becomes better, but that the bearing has almost reached the end of life.  

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