Cause analysis of common bearing quality faults
Cause analysis of common bearing quality faults
2022-05-19
Troubleshooting of bearing sealing fault
Troubleshooting of bearing sealing fault
2022-05-21

Troubleshooting and maintenance of bearing damage

From the surface bearing damage

  • It is a comprehensive technology to judge and predict the reliability and service performance of the bearing through the fault and deterioration of the bearing and the performance state parameters, identify and judge the position, cause and risk degree of the abnormal situation, and determine the repair and improvement methods. 
  • Therefore, the fault diagnosis technology of bearing is not a simple fault detection (or monitoring) technology, nor is it a simple point inspection and instrumentation. Correctly, quickly and effectively collect signals, process data, identify and judge manually or automatically. 
  • In order to determine the scientific repair cycle, necessary tests and estimates of fatigue and wear life should also be carried out according to the mechanism of various faults.
  • Bearing fatigue failure is a surface form of failure, which is mainly manifested in the initiation, propagation and fracture process of fatigue crack, and the long-term impact of load failure under alternation. Cracks are in two ways:
  • From the surface, this is the role of the external load caused by the plastic deformation of the working surface and the periodic change of the contact stress of the strain hardened surface during the rolling contact process of the bearing, and finally the formation of the surface crack between the two from the developing inner surface of the small crack. Due to the lubricant on the working surface and the open force, the crack is severely hit on the wall, forcing the crack to move forward;
  • From the surface layer, under repeated pressure, the contact of the surface will initially produce cracks at a certain depth at the place where the surface is exposed, and from the angle along the surface direction to a certain depth, from behind the surface of the contact surface, and beyond the surface, and finally form corrosion stripping, leaving a horse pit.
  • From the bearing surface, or from the cracks in the surface layer, these two objectives (carburization, quenching and other surface heat treatment of parts). If there are uneven hardness, organization, and non-uniform internal stress and other disadvantages, the contact stress is generally affected by the opposite underground cracks.
  •  If the surface quality of parts is poor, there is a defect (oxidation, decarburization), poor friction or lubrication, and from the surface cracks. The methods to control the sound of raceway are: select low-noise bearings, that is, bearings with small waviness, and carefully select the service conditions. Raceway noise often affects the noise of the whole machinery. Reducing raceway noise can reduce the noise of the whole machinery.
  • Before the bearing is delivered for routine inspection, its oil seal package can be removed, and it can be immersed in washing gasoline or acid free kerosene containing 3% engine oil. Turn its outer ring and carefully wash the junction of cage, rolling element and raceway.
  •  It is required to thoroughly clean the original oil traces left in any hidden parts. After the bearing is cleaned, the attached cleaning oil shall be drenched and dried on the workbench lined with clean neutral lining paper, or dried slowly in the far-infrared oven. The bearing ring, cage or any rolling element shall not be rotated during the whole process from washing oil drying to air drying or drying.
  • The bearings that have been dried or dried shall be dripped with lubricating oil as required and delivered for inspection immediately. At this time, there is no oil film protection on the outer surface of the bearing, and it is not allowed to directly touch the skin of hands during the whole inspection process. 
  • The operator shall wear clean gloves or apply antirust agent on his hands, but the antirust agent on his hands must be avoided from mixing into the rolling surface of the bearing, otherwise the measurement results will be deviated.
  • After the inspection, the bearing shall be immediately delivered for use or treated with anti rust inventory. The whole inspection process shall also be completed quickly to avoid corrosion caused by too long. Note that using pure gasoline without engine oil for cleaning will also cause corrosion.
  • Before inspection, the bearing must be placed on a metal plate in the inspection site together with the gauge block, standard parts and measuring instruments used for inspection, so as to homogenize the temperature and room temperature of these objects quickly.
  • Bearing temperature is too high: during the operation of the mechanism, the part where the bearing is installed is allowed to have a certain temperature. When touching the shell of the mechanism with your hand, it should be normal not to feel hot. Otherwise, it indicates that the bearing temperature is too high.
  • The reasons for high bearing temperature include: the quality of lubricating oil does not meet the requirements or deteriorates, and the viscosity of lubricating oil is too high; The mechanism assembly is too tight (insufficient clearance); The bearing assembly is too tight; The bearing race rotates on the shaft or in the shell; Excessive load; Broken bearing cage or rolling element, etc.
  • Bearing noise: the rolling bearing is allowed to make slight running noise during operation. If the noise is too loud or there is abnormal noise or impact sound, it indicates that the bearing is faulty.
  • The causes of rolling bearing noise are complex:  the mating surfaces of the inner and outer rings of the bearing are worn. Due to this kind of wear, the matching relationship between the bearing and the shell, the bearing and the shaft is damaged, resulting in the axis deviation from the correct position and abnormal noise when the shaft moves at high speed. 
  • When the bearing is tired, the metal on its surface will peel off, which will also increase the radial clearance of the bearing and produce abnormal noise.   Insufficient bearing lubrication, dry friction and bearing breakage will produce abnormal sound. 
  • After the bearing is worn and loose, the cage is loose and damaged, which will also produce abnormal noise and damage to the bearing. During the disassembly and inspection of rolling bearing, the fault and damage cause of bearing can be judged according to the damage of bearing.Troubleshooting and maintenance of bearing damage
  • Metal spalling on the raceway surface: the bearing rolling body and the inner and outer raceway surfaces are subjected to periodic pulsating load, resulting in periodic contact stress. When the number of stress cycles reaches a certain value, fatigue spalling will occur on the working surface of rolling element or inner and outer race raceway.Troubleshooting and maintenance of bearing damage
  •  If the load of the bearing is too large, it will aggravate this fatigue. In addition, improper bearing installation and shaft bending will also produce raceway spalling. The fatigue spalling of bearing raceway will reduce the operation accuracy of shaft and cause vibration and noise of mechanism.Troubleshooting and maintenance of bearing damage
  • Bearing burn: the burned bearing has a return fire color on its raceway and rolling element. The causes of burns are generally insufficient lubrication, unqualified or deteriorated lubricating oil quality, and too tight bearing assembly.Troubleshooting and maintenance of bearing damage
  • Plastic deformation: uneven pits appear on the contact surface between the raceway and the roller of the bearing, indicating that the bearing has plastic deformation. Troubleshooting and maintenance of bearing damage
  • The reason is that under the action of large static load or impact load, the local stress on the working surface of the bearing exceeds the yield limit of the material, which generally occurs on the bearing rotating at low speed.
  • Bearing race crack: the crack of bearing race may be caused by too tight bearing fit, loose bearing or inner ring, deformation of bearing containing parts, poor surface processing of installed bearing, etc.Troubleshooting and maintenance of bearing damage
  • Broken cage: the reason is insufficient lubrication, broken rolling element, seat ring skew, etc.Troubleshooting and maintenance of bearing damage
  • The metal of the cage adheres to the rolling element, which may be due to the rolling element being stuck in the cage or insufficient lubrication.Troubleshooting and maintenance of bearing damage

During the operation of the bearing damage

  • Serious wear of race raceway: foreign matters may fall into the race, insufficient lubricating oil or inappropriate lubricating oil brand. The causes of bearing fracture include defect and overload. When the external load exceeds the material strength limit, the part fracture is called overload fracture. 
  • During the operation of the bearing, defects such as overheating tissue and local burns will also occur, resulting in defect fracture.Troubleshooting and maintenance of bearing damage
  • During the operation of the bearing, sometimes foreign hard particles or hard foreign matters or wear debris on the metal surface will be squeezed between the bearing surfaces, which will cause furrow like abrasion of the bearing. Troubleshooting and maintenance of bearing damage
  • These hard foreign matters enter the interior of the bearing through the lubricating medium. With the continuous operation of the bearing, local friction generates heat, which is easy to cause local deformation of the friction surface and friction micro welding, or even local melting, and then bite.Troubleshooting and maintenance of bearing damage
  • Generally speaking, the bearing itself does not produce noise. What we call noise is actually the sound effect of vibration between the bearing and the surrounding structure directly or indirectly.Troubleshooting and maintenance of bearing damage
  • Excitation caused by the change of the number of loaded rolling elements. When a radial load is loaded on a bearing, the number of loaded rolling elements will change slightly during operation, that is: 2-3-2-3 This causes an offset in the direction of the load. The resulting vibration is inevitable, but can be reduced by axial preloading, which is applied to all rolling elements (not applicable to cylindrical roller bearings in bearings).Troubleshooting and maintenance of bearing damage
  • When the waviness of components is closely matched between the bearing ring and the bearing seat or transmission shaft, the bearing ring may be deformed by matching with the shape of adjacent components. If deformation occurs, vibration may occur during operation. Therefore, it is important to machine the bearing pedestal and drive shaft to the required tolerances.Troubleshooting and maintenance of bearing damage
  • Local damage due to incorrect operation or installation, a small part of the bearing raceway and rolling element may be damaged. During operation, rolling over damaged bearing parts will produce specific vibration frequencies. Troubleshooting and maintenance of bearing damage
  • Vibration frequency analysis can identify damaged bearing components. Vibration behavior in applications in many applications, the stiffness of sliding bearings is the same as that of surrounding structures. Because of this feature, as long as the bearing (including preload and clearance) and its configuration in the application are correctly selected, it is possible to reduce the vibration in the application.Troubleshooting and maintenance of bearing damage
Troubleshooting and maintenance of bearing damage

Then check the vibration of the bearing damage

  • Let’s briefly introduce the rolling sound of the bearing first. The size and sound quality of the rolling sound should be checked by the sound detector. The problem will be accurately judged with advanced tools. Even if the bearing has slight peeling and other damage, it will emit abnormal sound and irregular sound, which can be distinguished by the sound detector.Troubleshooting and maintenance of bearing damage
  • Then check the vibration of the bearing to see if the damage of the bearing is very sensitive. Whether peeling, indentation, corrosion, crack, wear, etc. will be reflected in the bearing vibration measurement. Therefore, the vibration can be measured by using a special bearing vibration measuring instrument (frequency analyzer, etc.), and the specific situation of abnormality can not be inferred by frequency division. Troubleshooting and maintenance of bearing damage
  • However, the values measured by each machine are different due to the service conditions of the bearing or the installation position of the sensor, so it is necessary to analyze and compare the measured values of each machine in advance to determine the judgment standard.Troubleshooting and maintenance of bearing damage
  • Mechanical damage
  • In severe cases, metal peeling and large-area random scratches occur on the contact surface; In general, bearing mechanical damage refers to different levels of groove marks on the alloy surface of bearing bush. Contact surface damage and ablation exist at the same time. The main reason for bearing mechanical damage is that the bearing surface is difficult to form oil film or the oil film is seriously damaged.Troubleshooting and maintenance of bearing damage
  • Bearing cavitation
  • Plastic deformation and cold work hardening occur in the outer layer, and the sliding bearing is subjected to repeated action of cylinder pressure (impact load). With the continuous loss of the ability of the wear debris, the surface of the wear debris layer is gradually deformed, and then the wear debris layer is gradually formed. Troubleshooting and maintenance of bearing damage
  • Generally, when the bearing bush is pitted, the pit first appears, and then the pit gradually expands and causes the cracking of the alloy layer interface. Troubleshooting and maintenance of bearing damage
  • The crack extends along the parallel direction of the interface until it peels off. The main reason for the cavitation of sliding bearing is that the sudden change of the cross section of structural elements such as oil groove and oil hole causes the strong disorder of oil flow. Troubleshooting and maintenance of bearing damage
  • Bubbles are formed in the vacuum area of oil flow disorder, and then cavitation occurs due to the collapse of bubbles due to the increase of pressure. Cavitation usually occurs in the high load area of the bearing, such as the lower bearing bush of the crankshaft main bearing.Troubleshooting and maintenance of bearing damage
  • Seals are extremely important for bearings. If there is a problem in the sealing part, it will have a great impact on the bearing. Seals with irregular shape, lack of meat, micro cracks and other defects are not allowed.
  • The bearing tightness test is to assess the assembly and working reliability of the sealed parts of the oil cylinder and its connections after assembly. Such as the sealing and connection of the cylinder port of the oil cylinder, the dynamic and static seals and sealing conditions on the oil cylinder piston (between the rod cavity and the non rod cavity), etc.
  • The pressure of bearing tightness test is very large, especially for the pressure test without rod cavity. If there is no tooling to bear, the pressure test by rod cavity and the great pressure acting on the piston rod through the piston can not be borne only by the connection of oil cylinder port. Therefore, this kind of pressure test is usually carried out after carefully assembling the oil cylinder before entering the commissioning stage of the final assembly machine.
  • A. Improper assembly method
  • If the interference between bearing and shaft diameter or bearing seat hole is small, press in method is often used for assembly. The simple method is to use a copper rod and a hand hammer to knock the bearing race with interference fit symmetrically in a certain order to make the bearing press in smoothly. In addition, soft metal casing can also be driven by hand hammer or press.
  •  If the operation is improper, the seat ring will be deformed and cracked, or the hammer will hit the seat ring with non interference fit, which will cause indentation on the raceway and rolling element or indirect damage to the bearing.
  • B. Improper temperature control during assembly
  • During assembly, if the interference between the bearing and the shaft diameter is large, the hot assembly method is generally adopted. Put the bearing into the oil barrel containing engine oil, and heat the outside of the oil barrel with hot water or flame. 
  • The process requires that the heated oil temperature be controlled at 80 ℃ ~ 90 ℃, generally not more than 100 ℃, and mostly not more than 120 ℃. After the bearing is heated, it is quickly taken out and sleeved on the journal. If the heating temperature is too high due to improper temperature control, the bearing will be tempered and the hardness will be reduced, and the bearing is easy to wear, peel off and even crack during operation.
  • C. Improper adjustment of assembly time gap
  • The clearance of rolling bearing is divided into radial clearance and axial clearance. Its function is to ensure the normal operation and lubrication of rolling element and compensate thermal elongation.
  • For bearings with adjustable clearance, because there is a positive proportional relationship between axial clearance and radial clearance, the required radial clearance can be obtained as long as the axial clearance is adjusted. However, they are generally used in pairs (i.e. installed at both ends or one end of the shaft). Therefore, it is only necessary to adjust the axial clearance of one bearing. The axial clearance is generally adjusted with gaskets, and some can also be adjusted with screws or thrust rings.
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