The Common Faults and Routine Maintenance of Rolling Mill Bearings

1. Common faults and cause analysis
Rolling mill bearings may be damaged due to various reasons during operation, such as improper assembly, poor lubrication, intrusion of moisture and foreign matter, corrosion and overload, etc., may cause premature damage to the bearing. Even if the installation, lubrication and maintenance are normal, after a period of operation, the bearing will suffer fatigue peeling and wear and fail to work normally. In short, the reasons for the failure of rolling mill bearings are very complex, and the common failures and causes are now analyzed as follows.
1.1 Fatigue spalling
The inner and outer raceways and rolling element surfaces of rolling bearings bear both load and relative rolling. Due to the alternating load, cracks are formed at a certain depth below the surface (the maximum shear stress), and then they expand to the contact surface to cause the surface layer to spall pits. Finally, it develops to large flaking, this phenomenon is fatigue flaking. Fatigue spalling will increase shock load, vibration and noise during operation. Normally, fatigue spalling is often the main reason for the failure of rolling bearings.
1.2 Wear
Due to the intrusion of dust and foreign matter, the relative movement of raceways and rolling elements will cause surface wear. Poor lubrication will also aggravate wear. As a result of wear, the bearing clearance will increase, the surface roughness will increase, and the bearing operation accuracy will be reduced, as well as vibration. And the noise also increases accordingly. Therefore, to a certain extent, the amount of wear limits the life of the bearing.
1.3 Plastic deformation
When the bearing is subjected to excessive impact load or static load, or additional load caused by thermal deformation, or intrusion of foreign matter with high hardness, dents or scratches will be formed on the surface of the raceway. This will cause the bearing to produce severe vibration and noise during operation, and once there is an indentation, the impact load caused by the indentation will further cause the nearby surface to peel off.
2. Daily maintenance
2.1 Inspection of bearings during operation
In accordance with the maintenance procedures, the bearings are regularly listened to, observed and measured. Listening is to use a listening rod to face the axial end cover of the bearing and the radial housing of the bearing seat, and the other end to listen to whether the bearing is impacted or mechanically rubbed during operation. Observation is to observe the operating environment, installation location, vibration deviation, lubrication, etc., to see if there is any bad working condition. The measurement is to use a thermometer and a vibrometer to check the bearing seat regularly. Use these methods to periodically check the bearing to determine whether the bearing is in normal condition.
2.2 Inspection of the bearing at rest
Check the tightening of related parts to confirm whether the parts are in the correct position and whether there is any looseness or abnormality. In addition, the bearing lubrication is tested to detect whether there is foreign matter in the lubricating oil. When the diameter of the foreign matter is smaller than the minimum working clearance of the bearing, the foreign matter can be stirred with the lubricating oil through the clearance. Friction occurs on the working surface of the rolling element. When the diameter of the foreign body is larger than the working gap of the bearing, the foreign body is crushed or crushed into thin dust to adhere to the bearing surface or embedded in it. If the crushed foreign debris is not embedded in the working surface, but enters the lubricating oil, it will worsen the bearing wear. As a result, fatigue spalling of the bearing occurs. Causes the temperature of the working surface to rise. The lubricating oil then loses its effect. If the bearing fails, the foreign matter is crushed or embedded in the rolling elements, which quickly destroys the oil film, causing the bearing to be damaged by working and running in a dry friction state.
2.3 Inspection after disassembly of the bearing
After the roller is replaced, a cleaning agent can be used to clean the bearing, dry it with compressed air, and then inspect the roller, cage and raceway. If the rollers and raceways appear dents, pitting, etc., they should be replaced according to the specific conditions. At the same time, replace the necessary bearing seals to prevent poor seals from affecting the life of the bearings. If the bearing is not well sealed and the emulsion enters the bearing, it will cause water pollution to the lubricating oil or grease, resulting in a reduction in the fatigue strength of the bearing material and easy cracking. At the same time, the seal is not good, and the iron oxide scale during steel rolling enters the bearing to destroy the lubrication condition, causing abrasive wear on the surface of the bearing element.
2.4 Bearing installation
2.4.1 Press-in installation
Press-in installation generally uses a press, but also can use bolts and nuts. When it is necessary to use a hand hammer to install, it is necessary to pad the inner ring with a pad. This practice has repeatedly become the cause of damage to the bearing installation operation, so it is limited to the case of small interference and cannot be used for large or medium-sized bearings. For non-separable bearings such as deep groove ball bearings, the inner ring and outer ring need to be installed with an interference amount, so it is necessary to pad the work with the inner ring and the outer ring at the same time with a screw or oil pressure. The outer ring of self-aligning ball bearing is easy to tilt, even if it is not an interference fit, it is best to install it with a pad. For separable bearings such as cylindrical roller bearings and tapered roller bearings, the inner ring and outer ring can be installed on the shaft and housing respectively. When combining the installed inner ring and outer ring, the key is stability. Close the two firmly so that the center of the two does not deviate. Forcing it in will cause the raceway surface to jam.
2.4.2 Hot loading
Large bearings require a lot of force to press in, so it is difficult to press in. Therefore, the bearing is heated in oil to expand it, and then mounted on the shaft. This hot mounting method is widely used in the installation of large bearings. Using this method can make the bearing avoid unnecessary external force and complete the installation work in a short time. The heating temperature of the bearing is based on the size of the bearing and the required interference. Heat the bearing to about 100℃~120℃. In order to prevent the bearing from directly touching the bottom of the oil tank, it is best to consider placing the bearing on a metal mesh table, or hoisting the bearing, and heating the bearing to 20℃ higher than the required temperature~ 30°C, so that the inner ring does not become cold during operation and it is difficult to install. After installation, the bearing cools down and the width direction shrinks, so it must be tightened with a shaft nut or other appropriate method to prevent a gap between the inner ring and the bearing shoulder.
2.5 Inspection after the bearing is installed
After the bearing is installed, in order to check whether the installation is correct, it is necessary to perform a running check, which can be rotated by hand to confirm whether the rotation is smooth. The inspection items include improper operation caused by foreign objects, scars, and indentation, uneven rotation torque caused by poor installation, poor processing of the mounting seat, excessive torque caused by too small clearance, installation error, and seal friction Wait. If there is no abnormality, it can start to run. Check whether there is vibration, noise, whether the rotating parts are in contact, etc., and enter the power operation after confirming that there is no abnormality. The operation check starts from the no-load low speed, and slowly increases to the rated conditions for operation. The items to be checked during the test run are: whether there is any abnormal noise, bearing temperature shift, lubricant leakage and discoloration, etc. If an abnormality occurs during the trial operation, the operation should be stopped immediately, the mechanical equipment should be inspected, and the bearing should be removed for inspection if necessary.