When a certain type of deep groove ball bearing was inspected after long-term use, it was found that the cage was broken, and the raceway was found to be partially peeled after decomposition. This bearing is widely used, but in the course of many years of use, cage fracture and raceway peeling have occurred. What is the reason for this cage fracture? The following China Bearing Network (abbreviation: Huazhou Network) to share with you relevant examples and specific reasons.

Fracture failure characteristics of deep groove ball bearing cage:

The overall color of the faulty bearing is bright, and there is no damage on the inner and outer ring surfaces. The cage pocket where a steel ball is located near the rivet rivet on one side has been cracked and broken, as shown below.

Cracking morphology of bearing cage

After dissecting the outer ring of the bearing, it is found that the bottom of the outer ring raceway has obvious discoloration contact trajectory, the trajectory has become rough, as shown in Figure 3; the inner ring raceway has continued to peel off the entire circumference along the groove bottom, and the contact The trajectory is slightly leaning to the non-typing side of the channel, showing a mild climbing characteristic. As shown in Figure 4, the cracked pocket has been significantly worn and thinned near the fracture and flashed, as shown in Figure 5, remove the rivet and separate the two In the cage, it can be seen that the inner surface of some cage pockets becomes brighter by friction with the steel ball, and the inner surface of the cracked pocket is more obvious as shown in Figure 6 and Figure 7. The surface of the steel ball is complete, and all have different degrees of high temperature discoloration. But there is still a metallic luster, and the appearance of the steel ball where the cracked pocket is located is not abnormal.

Test results: the axial runout of the inner ring and the axial runout of the outer ring obviously exceed the standard regulations, indicating that the internal accuracy has deteriorated;

Spalling part analysis

The spalling part of the inner raceway was observed under a scanning electron microscope, and no material defects such as inclusions were found. There are metal ribs at the spalling source, and the ribs are distributed in an arc shape and gradually expand toward the core of the spalling. 

The energy spectrum analysis of the spalled part found no other elements except the matrix.

Hardness and metallographic structure

According to the visual inspection results after the breakdown of the faulty parts, the inner ring and a steel ball were selected, and the hardness and quenching and tempering structure were tested respectively. The test results showed that the quenching and tempering structure and hardness of the inner sleeve and the steel ball were in compliance with the standard requirements.

Further observation of the metallographic sample of the inner ring showed no high temperature characteristics and abnormal structure.

Failure analysis

It was confirmed by scanning electron microscope analysis that the bearing raceway peeling was fatigue peeling. According to the cracking morphology of the cage pocket and the spalling morphology of the inner ring raceway, it can be judged that the spalling of the inner ring raceway should precede the cracking of the cage pocket. Because when the bearing is working normally, the force on the cage is very small, only the steel ball guides the drag force of the cage, this force will not cause the cage to crack; even if the cage cracks first, its restraint on the steel ball is not obvious Changes, it is impossible to cause the inner ring raceway to peel off.

Conversely, after the raceway of the bearing inner ring is peeled off, the bearing will run abnormally and produce vibration, which will cause the steel ball to run unsteadily when it rotates in the circumferential direction, produce radial runout and circumferential swing, and cause running blockage. The wear of the contact surface between the steel ball and the cage pocket is caused. At the same time, the steel ball also produces a circumferential stretching effect on the cage pocket, which will eventually cause the cage to fatigue fracture at the corners and other weak points. From the physical and chemical test results of the inner ring, the influence of material or cold and hot processing defects can be excluded; the outer ring raceway of the bearing has contact indentation in the entire circumferential direction, indicating that the radial load is too large when the bearing is working. Re-check the bearing installation and matching status, and all meet the technical requirements, which can eliminate the influence of small radial working clearance. Judging from the size test results of 7 steel balls, the diameter group difference is 0.8 u m, which exceeds the standard requirement.

The bearing design analysis software is used to calculate and analyze the contact stress during bearing operation. The calculation results show that, under normal bearing conditions, the contact stress between the steel ball and the inner ring raceway is the largest, the maximum contact stress is 1758MPa, which is less than the maximum allowable contact stress of the bearing steel of 2000MPa; In this case, there is a possibility that the maximum contact stress between the larger steel ball and the inner ring raceway exceeds the maximum allowable contact stress of the bearing steel of 2000 MPa.

Based on the above analysis, it is preliminarily found that the reason for the spalling of the inner ring raceway is that the difference in the diameter group of the steel balls affects the load distribution of a group of steel balls, and the contact stress between the larger steel balls and the inner ring exceeds the maximum material. Contact stress and additional vibration load will cause fatigue peeling of the inner ring raceway after a long time of work. After the raceway of the inner ring of the bearing is peeled off, the operation of the bearing is abnormal, causing a pocket of the cage to crack. Therefore, during the bearing assembly process, the diameter group difference of the steel balls of the bearing should be strictly controlled to prevent the product failure caused by the contact stress change and the additional vibration load caused by the excessive difference in the diameter group difference.