Rolling bearings are important parts in rotating machinery. Statistics show that about 30% of mechanical failures in rotating machinery using rolling bearings are caused by rolling bearings. After adopting the state detection and fault diagnosis technology, the accident rate can be reduced by 75%, and the maintenance cost can be reduced by 25% to 50%. 1. Failure forms of rolling bearings (1) Fatigue spalling The inner and outer raceways and rolling elements of rolling bearings enter and exit the load-bearing area alternately. These components are subjected to alternating loads for a long time, and firstly from the maximum alternating shear stress below the contact surface. Fatigue cracks are generated, which then expand to the contact surface to produce point-like spalling on the surface, and gradually develop to large-scale spalling, which is called fatigue spalling. (2) Wear Due to the relative movement of the raceway and the rolling elements and the surface wear caused by dust and foreign matter, poor lubrication will aggravate the wear, resulting in an increase in the bearing clearance, an increase in surface roughness, and a decrease in the bearing running accuracy, which also reduces the machine. The accuracy of motion is manifested in the increase of vibration level and noise. (3) Scratches Due to the uneven force on the contact surface due to the microscopic protrusions or hard particles on the contact surfaces of the inner and outer raceways and rolling elements of the bearing, under the conditions of poor lubrication and high-speed and heavy-load conditions, the heat generated by local friction is caused. Local deformation and friction welding of the contact surface. In severe cases, the surface metal may be partially melted, and the force on the contact surface will tear the local friction welding point from the substrate. (4) Fracture When the load and vibration of the bearing are too large, the defect position of the inner and outer rings will gradually expand and break under the repeated impact of the rolling elements. (5) Corrosion The direct intrusion of moisture or acid and alkaline substances will cause bearing corrosion. When the shaft current passes through the bearing, sparks are caused at the contact point between the raceway and the rolling element, resulting in electrical corrosion, forming washboard-like unevenness on the surface. 2. The failure process of rolling bearings Bearing failure is usually divided into four stages: (1) Stage 1: Ultrasonic frequency vibration stage of the bearing The early failure of the bearing is the abnormal vibration of the ultrasonic frequency in the range of 250kHz to 350kHz. The abnormal frequency gradually decreased and moved to 20kHz to 60kHz. At this time, the small fault of the bearing can be detected by the method of shock envelope and acoustic emission. The maximum value of shock envelope can reach 0.5gE (acceleration envelope, in vibration analysis). an acceleration indicator representing the amplitude). (2) The second stage: the natural frequency vibration stage of the bearing With the operation of the bearing, the rolling surface of the bearing will have slight defects, and the vibration caused by these minor defects will excite the natural frequency (fn) vibration of the bearing components or the bearing support structure. Resonance, the general vibration frequency is 500Hz ~ 2kHz. At the same time, the frequency is also used as the carrier frequency to modulate the fault frequency of the bearing. At first, only the frequency itself can be observed, and later it appears as a side frequency near the natural frequency. If the acceleration envelope method is used to detect, it will be found that the envelope value will rise to about 0.5 ~ 1.OgE. At this point, the bearing can still run safely. (3) The third stage: the bearing defect frequency and its frequency doubling vibration stage With the further expansion of bearing small defects, the bearing defect frequency and its frequency doubling began to appear, and with the further development of bearing wear, more defect frequencies doubled Starting to appear, the number of sidebands around these octaves and the natural frequencies of the bearing components also rises. At this time, the vibration of the bearing is relatively obvious, and it should be considered to replace the bearing as soon as possible. (4) The fourth stage: the stage of random broadband vibration of the bearing is close to complete failure, the life of the bearing is coming to an end, and even the power frequency is also affected by it and rises and produces many frequency multipliers of the power frequency, while the original discrete bearing defect frequency and the natural frequency starts"disappear", replaced by random broadband high frequency"noise vibration", the high-frequency noise vibration and envelope values ​​have decreased, but just before the final failure of the bearing Z, the envelope shock value will increase significantly. 3. Analysis method of vibration characteristics of rolling bearing (1) The advantage of characteristic parameter method is that only a few indicators are used to explain the state of the bearing, and the result analysis is simple and convenient. The characteristic parameters commonly used in the diagnosis of rolling bearings include various time-domain characteristic parameters such as effective value and peak value, and various frequency-domain parameters such as the center of gravity frequency. (3) Frequency spectrum analysis method The vibration of rolling bearing is very rich in frequency components, including both low-frequency components and high-frequency components. Each specific fault corresponds to a specific frequency component, and it is necessary to separate the specific frequency component through appropriate signal processing methods to indicate the existence of a specific fault.
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