The deterioration development of rolling bearings is not a linear process, but an exponential process. In different degradation stages, fault information appears in different frequency bands, so different fault detection methods should be adopted. At present, it is generally believed in the industrial field that the deterioration process of rolling bearings can be divided into four stages. Stage 1 – Early stage of bearing failure At this stage, the bearing first forms microscopic cracks or lattice dislocations on the subsurface, but no cracks or tiny peelings can be seen on the bearing surface, and no obvious shock signals will be formed in the low frequency band of the vibration signal. The traditional acceleration sensor cannot pick up the fault signal, but the microscopic cracks or lattice dislocations on the subsurface will generate acoustic emission signals or stress wave signals. Therefore, at this stage, the fault characteristics of the bearing are mainly reflected in the ultrasonic frequency range, which can be picked up by the acoustic emission sensor or the resonance-based acceleration sensor, which is mainly manifested in the increase of the measured signal peak value or energy value. Stage 2 – bearing failure development period At this stage, the microscopic deterioration of the bearing begins to spread from the subsurface to the surface, and damage points such as cracks or tiny spalling occur on the contact surface of the bearing. When the surface of the bearing component comes into contact with these damage points, a shock pulse of a certain frequency will be formed. According to the Fourier transform, the short-term shock signal is a broadband signal in the frequency domain, so this shock signal will inevitably arouse the high-frequency natural frequency of the bearing parts to resonate, so that its vibration will be strengthened. This part of the signal can be picked up by the acceleration sensor, and then the characteristic frequency of the bearing fault can be observed by using the envelope demodulation technology. At the end of the second stage, the multiplier of the fault characteristic frequency can also be observed. At this stage, the fault characteristic frequency of the bearing is temporarily submerged in the higher noise in the low frequency band, so no clear fault characteristic frequency can be observed in the fault characteristic frequency range. Stage 3 – rapid development of bearing failure At this stage, with the accelerated development of bearing damage, the impact of the damage point on the bearing contact surface is becoming more and more intense, and the frequency multiplication of the bearing fault characteristic frequency demodulated in the resonance frequency range is increasing, and the energy of the periodic impact is enough. It can be directly observed through the power spectrum of the vibration signal. At this time, the fault characteristic frequency of the bearing can be clearly seen directly on the power spectrum of the vibration signal, and its frequency multiplication has a tendency to increase. Stage 4 – End of Bearing Failure At this stage, the rolling bearing has almost reached the end of its life, and the damage point can be observed with the naked eye, the noise of the bearing movement becomes particularly loud, and the temperature rises rapidly. At this time, not only the fault characteristic frequency and its multiplier of the bearing can be clearly seen on the direct power spectrum, but also the obvious modulation side frequency can be seen next to the fault characteristic frequency if the damage points alternately enter the load area. At the end of the fourth stage, the spectral lines on the frequency spectrum become not very clear, and a protruding “thatched pile” will be formed on the power spectrum. In addition, the energy of high-frequency vibration may not rise but decrease at this time. If it is found that the high-frequency monitoring value begins to decline, it does not mean that the surface bearing condition is getting better, but that the bearing has reached the end of its life.  

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