# Root Cause Failure Analysis of Deep-Groove Ball Bearing Used in a Governor
## 4 Discussion  
### Failure mode and direct failure cause  
The failure mode of the inner ring raceway and steel balls is contact fatigue spalling. The failure modes of the outer ring raceway are wear and contact fatigue spalling. The failure mode of the cage is fatigue fracture. Contact fatigue spalling and wear are the predominant modes of failure in rolling element bearings. They are affected not only by loading conditions but also by material properties, lubricants and surface conditions. Studies have shown that improper mounting, sealing and inadequate lubrication contribute to 90% of bearing failures [35,36]. There are no discoloration and overheating marks, according to the results of the damage appearance and microstructure examination results. Hence, failure of the bearing has nothing to do with insufficient lubrication [37,38]. The microstructures of the inner ring, outer ring raceway, steel balls and cage are normal. The hardness of the inner ring, outer ring raceway and steel balls meet the required specification. Therefore, the failure cause of the bearing is not related to the material quality, and the failure cause of the bearing should be high load. The fact that the spalling damage area along the whole circumferential area adjacent to the centrifugal block shaft of the inner ring raceway can be roughly divided into three parts with different damage degrees indicates a high unbalanced load of the bearing. Moreover, the metal extrusion and accumulation at the border of the raceway near the side edge of the centrifugal block shaft indicates excessive axial load during the operation of the bearing [39]. Therefore, failure of the inner ring raceway should be related to a high unbalanced axial load of the bearing. The damage morphology of the outer ring raceway is an uneven-distribution circumferential strip located at the spring side with shallow wear and spalling and raceway surface metals queezed to the guide surface. These characteristics indicate that a high unbalanced axial load may be the failure cause of the outer ring raceway. One side of the cage is broken into three parts, and the other side of the cage has a fracture. This indicates that the side broken into three parts of the cage is subjected to a higher load than that of the other side, and the fatigue cause of the cage may be the high unbalanced axial load. Multi origins of the cage fatigue fractures indicate that the initial stress of the fracture was high and that the fracture should occur after the failure of the inner ring raceway, outer ring raceway and steel balls. There are two main reasons for the cage fracture: one is misalignment, and the other is an abnormal load acting on the cage in the bearing operation due to misaligned mounting between the inner and outer ring [40,41].Therefore, the high unbalanced axial load should be the cause of fatigue fracture of the cage. The contact stress of the inner ring raceway and outer ring raceway of the deep-groove bearing were calculated by the ROMAX software, and the basic rating life (L\({}_{10}\) life) on the contact fatigue of the deep-groove bearing was also calculated under different axial loads. The calculation results of the contact stress and L\({}_{10}\) life are shown in Table 4. It can be seen that the contact stress increases with the increase in axial load and the L\({}_{10}\) life decreases with the increase in axial load. Under the designed working conditions, the maximum axial load of the bearing is 128 N and has a L\({}_{10}\) life of 2925 h, while the operation time of the failure groove ball bearing is 1440 h, which is smaller than 2925 h. In addition, the metal extrusion and accumulation at the border of the raceway indicate that the bearing was subjected to a large axial load during the failure process. Therefore, premature failure of the deep-groove ball bearing should be related to the high abnormal axial load.