In industrial and agricultural production, rolling bearings are often used. Their working state is directly related to the normal use of mechanical equipment and the service life of the whole machine. Although the bearings are selected according to a sufficient degree of safety in the design, there are still a considerable number of rolling bearings that fail before reaching the design life, which affects the normal operation of the equipment.
The main failure modes of rolling bearings are: fatigue pitting, plastic deformation and wear. In addition, there are gluing, cage fracture, outer ring fracture, rolling element crushing, rust and so on. The following will study the causes of bearing failure and find out the corresponding solutions to avoid premature bearing failure and prolong the service life of rolling bearings.
1. Reasons for premature bearing failure
There are many reasons for premature bearing failure, but the most important one is improper use and protection. The most common ones are poor bearing lubrication, pollution, severe overload, improper assembly, shock and impact during transportation or loading and unloading.
2. Poor lubrication
The viscosity of the oil is too low to provide sufficient lubrication, which will cause the race to wear quickly. The discoloration of raceways and steel balls is an initial sign of lubricating oil (grease) loss or excessive temperature causing lubricating failure, followed by severe wear and tear with overheating. The higher the speed, the more heat and wear.
The vast majority of bearings used on machines operate in dusty conditions. Although the sealing device is used, foreign particles still often enter the bearing, causing dents to be rolled on the surface of the race, which in turn expands fatigue pitting and causes failure. At this time, the entire surface of the seat ring will be indented. More serious is the contamination of lubricants. After the lubricant is contaminated, it is difficult to check, and some solid impurities are mixed in the lubricant and become an abrasive. If the contamination is high, the bearing will wear out prematurely.
4. Improper assembly
The angle error between the center line of the bearing hole and the center line of the shaft will cause the rolling elements to deviate from the normal raceway during operation and distribute on the non-circular raceway. The larger the angle error is, the greater the additional moment generated on the bearing, the more “strength”, and the easier it is to cause the bearing to fail prematurely. cause center
The reasons for line misalignment are as follows.
(1) The position of the bearing or shoulder is out of tolerance;
(2) The tolerance between the shaft and the bearing hole is improper;
(3) The centerline of the hole between the bearing seats is not aligned.
At the same time, if the inner and outer races are knocked too hard during installation, the races will be pressed into dents by the rolling elements. The greater the external load, the greater the rolling force. After several blows, flaky fatigue pitting corrosion occurs at the indentation. In addition, raceway indentation generates noise, vibration and additional torque. There is a small interference between the inner and outer rings of the bearing, which must be strictly controlled. If the interference is too large, it will cause assembly difficulties, make the prestress too large, cause heat generation, early damage and other failures, and even directly damage the bearing; if the interference is too small or there is a gap, it will cause loosening and wear, and the inner and outer rings will be the same as the matching parts. Relative rotation occurs, and the matching parts are destroyed.
If the bearing is overloaded, the bearing steel ball raceway will be severely worn, and will be peeled off in a large area and sometimes accompanied by overheating; if the gap between the rolling element of the bearing and the inner and outer raceways is too small, poor lubrication will also cause the bearing to overheat. If the bearing is overheated, it will lock up. In a period of time before locking occurs, excessive wear and high heat will soften the bearing steel, and at the same time cause the volume expansion of each part in the bearing, and the mutual squeezing force will increase until the bearing “locks”. Overheated bearings are often accompanied by smaller gaps between the raceways of the inner and outer rings, the rolling elements and the cage, and the frictional force will also generate stress on the cage, which may break the cage and accelerate the bearing failure.
6. Vibration erosion
When an external load acts on the bearing and does not rotate, the external vibration causes the rolling elements in the bearing to vibrate, resulting in elliptical indentations on the inner and outer raceways of the bearing. This situation often occurs during the transportation of the equipment or the vibration caused by external factors when the equipment is not working, which eventually leads to fatigue pitting corrosion of the raceway. .
If the bearing is exposed to corrosive liquids or gases, red or brown stains will appear on the entire bearing, causing corrosion failure of the bearing.
8. Methods to avoid premature bearing failure
- Good lubrication is one of the important conditions to avoid bearing failure and prolong its life. Good lubrication can not only improve the smoothness and accuracy of motion, but also play a role in absorbing vibration, buffering and reducing wear. The viscosity of the lubricating oil must be carefully selected. If it is too viscous, it will increase the resistance; if it is too thin, it will affect the lubrication effect. Which lubrication method, grease or lubricating oil, depends on the specific working conditions.
- The pollution of bearings and lubricants is unavoidable in some industrial production, especially in the industrial production of building materials industry. Through the actual investigation, it is found that 50% of the failed rolling bearings in the building materials industry are caused by pollution. Usually, a protective cover or a sealing strip is used to prevent the intrusion of dust and dirt, but since the inner ring and the outer ring of the bearing are in relative motion, it is impossible to have a perfect sealing device, so the pollution of the lubricant is inevitable. In order to prolong the service life of the bearing, pay attention to cleanliness when assembling and disassembling the bearing, and ensure that the bearing has a good working environment during use.
- Reasonable bearing assembly is also extremely important to avoid bearing failure and prolong bearing life. The key to the assembly of the bearing is whether the assembly method is appropriate and whether the appropriate clearance and clearance can be guaranteed. Likewise, there are methodological issues when removing bearings. If the disassembly is improper, it will cause indentation on the raceway, and the bearing will be scrapped. When the inner ring of the bearing is assembled and disassembled, the inner ring should be stressed, and the outer ring should be stressed when the outer ring of the bearing is assembled and disassembled. In addition, when the bearing is assembled, the axial clearance is too small, which will increase the additional moment, and the angular displacement between the center line of the bearing and the center line of the bearing hole is too large, which will deteriorate the working state of the bearing. In actual work, these should be avoided.
- Excessive load or excessive initial load will cause premature fatigue of bearing materials. If overloading is the cause of bearing failure, then a bearing with a high load-carrying capacity should be used. In addition, sudden overload during work will cause fatigue pitting corrosion of the bearing, which should be avoided as much as possible. If it cannot be avoided, the actual life of the bearing should be carefully calculated according to the actual situation in order to formulate a bearing maintenance plan.
- In order to avoid bearing failure caused by vibration and corrosion, the vibration source should be eliminated and the bearing should be kept well lubricated. Add vibration isolation pads or separate the foundations of mechanical equipment from each other. Reduce vibrations caused by the environment. Apply a light initial load to the bearing so that the rolling elements of the bearing are basically in close contact with the raceway to improve the anti-vibration capability.
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