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 must be used. At present, it is generally believed in the industrial field that the degradation process of rolling bearings can be divided into four stages. 1 Stage 1 – Initial stage of bearing failure At this stage, the bearing first forms microscopic cracks or lattice dislocations on the subsurface, while no cracks or tiny spalling can be seen on the bearing surface. No obvious impact signal will be formed in the low frequency band of the vibration signal, which cannot be achieved with traditional acceleration sensors. Fault signals are picked up, but microscopic cracks on the subsurface or dislocation of the crystal lattice 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 acoustic emission sensors or resonance-based acceleration sensors. The main manifestation is that the measured signal peak or energy value becomes larger. 2 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 micro spalling occur on the contact surface of the bearing. When the surface of the bearing element comes into contact with these damage points, an impact pulse of a certain frequency will be formed. According to Fourier transform, it can be seen that the short-term impact signal is a broadband signal in the frequency domain, so this impact signal will inevitably arouse the bearing. The high-frequency natural frequency of the component resonates, thereby intensifying its vibration. This part of the signal can be picked up by the acceleration sensor, and then the fault characteristic frequency of the bearing can be observed using envelope demodulation technology. At the end of the second stage Doublings 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. 3 Stage 3 – Bearing failure rapid development period At this stage, as the bearing damage accelerates, the impact of the damage point on the bearing contact surface becomes more and more intense. The bearing fault characteristic frequency demodulated in the resonance frequency section has more and more multiplications, and its periodic impact The amount of energy is enough to 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 of increasing. 4 Stage 4 – terminal bearing failure At this stage, the rolling bearing has almost reached the end of its life, the damage points 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 of the bearing and its multiple frequency can be clearly seen on the direct power spectrum, but also obvious modulation side frequencies can be seen next to the fault characteristic frequency if the damage points alternately enter the load zone. At the end of the fourth stage, the spectral lines on the spectrum become less clear, and a protruding “thatch pile” will be formed on the power spectrum. In addition, the energy of high-frequency vibration may not rise but fall at this time. If high-frequency vibration is found, Frequent monitoring volume begins to decrease, which does not mean that the surface bearing condition has improved, but that the bearing is approaching the end of its life.  

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