# Root Cause Failure Analysis of Deep-Groove Ball Bearing Used in a Governor
## 4 Discussion
### Failure mode and direct failure cause
According to the above analysis results, the direct cause of the bearing failure should be the high unbalanced axial load, which may be induced by the misalignment of the bearing. Comprehensive analysis of the failure root cause  
Load on the bearing is a pair of balancing forces acting on the inner ring and outer ring, respectively, by the centrifugal block shaft and spring, whose load is applied on the outer ring by the spring housing and spring base. Because the inner ring is fixed on the centrifugal block shaft with good positioning, the high unbalanced axial loads not caused by the centrifugal block shaft. Therefore, the high unbalanced axial load may be caused by the spring, spring housing or spring base. There are two contact marks on the end face of the spring, which match the two marks on the corresponding spring housing, as shown in Figure 15. The spring and the spring housing are not in whole surface contact as required by the design, but in local contact. The centers of the spring and the cap are not on the connecting line between their two marks. This will induce misalignment of the outer ring and inner ring and thereby induce a high unbalanced axial load, as shown in Figure 16.  
The flatness examination result of the cap end face is normal, while the flatness examination result of the spring end face is 0.2, which is greater than 0.1 of the technical requirement. It is also found that the end face of the spring is composed of two planes, with one plane being large and the other plane being small. Therefore, high points are formed by the intersection of the two planes in areas of the above two contact marks. (3) Comparative experiment  
In order to determine the influence of greater flatness and high points in the spring end face on working state of the deep-groove ball bearing, a comparative experiment was carried out on the spring (faulty spring)corresponding to the failure bearing. The faulty spring with a new deep-groove bearing was installed in the governor and assembled on the  
Figure 16: Transmission path of spring force received by outer ring: (**a**) under the normal condition of whole surface contact; (**b**) under the abnormal condition of local contact. test equipment. The comparative experimental parameters were adopted from the normal working conditions of the governor. After testing for 50 h, the damage morphology of the corresponding bearing was acquired by visual observation and stereo light microscope examination and the damage morphology of the test bearing was compared with that of the failure bearing, and that of the bearing corresponded to the normal spring which was normally worked for 1400 h.  
Working tracks of the outer ring raceway of the test bearing and failure bearing were almost the same. Their working tracks intersected with the guide surface, resulting in a certain degree of climbing, metal extrusion and accumulation at the spring side border of the raceway, which originated from the raceway and propagated to the guide surface, as shown in Figure 17a,b. Meanwhile, for the bearing corresponding to the normal spring, there was a certain distance between the working track of the outer ring raceway and the guide surface, and no climbing phenomenon, as shown in Figure 17c. Therefore, it can be concluded that the high axial load on the bearing was induced by high points of the faulty spring end face, and the unbalanced load caused the bearing failure during service. Through the examination of the spring processing, it is found that greater flatness and high points of the spring end face are induced by improper processing and can be prevented by adopting a refined grinding process and adding detection requirements of flatness.