# A Fault-Signal-Based Generalizing Remaining Useful Life Prognostics Method for Wheel Hub Bearings
## 2 Fault Factors and Indicators of Wheel Hub Bearings  
### Fault Mechanism of Wheel Hub Bearings  
The wheel hub unit is designed with an inner flange bolted to the drive shaft and an outer flange mounting the entire bearing together, as shown in Figure 1a. The wheel hub bearings are composed with an outer ring, ball, and inner ring, as shown in Figure 1b.  
In the actual operation process, the factors causing faults or even failure of the automobile hub bearing may be various, including: (a) The racking stress of the wheel hub bearings is generated by the applied load from the contact surface of the rolling element with the inner and outer ring raceways. In the maximum racking stress section, an initial defect is produced under the surface of hub bearings. Then, the racking stress evolves to the contact surface, and the surface layer begins to peel off. Eventually, a large peeling of the surface of each component of the bearing appears. (b) Due to the seamless penetration of dust and impurities, the surface of the raceway and the rolling element will cause surface wear, the clearance of the wheel hub bearing will continue to expand, and the surface roughness will increase. Then, the running accuracy of the bearing will decrease, and the corresponding operating efficiency and accuracy of the equipment will be affected. therefore, the RUL of the wheel hub bearings is reduced. (c) The plastic deformation will occur when the wheel hub bearings are directly beaten with iron tools during installation, the wheel hub bearings are subjected to a large static load or impact, an additional load is generated by thermal deformation, or the indentation is generated on the raceway surface due to hardness penetration. The plastic deformation is accompanied by severe vibration and noise. (d) The bearing has poor sealing performance during the installation process or there are corrosive materials, such as moisture, acid, or alkaline in the lubricant, which cause serious chemical corrosion of the components of the hub bearing due to chemical reactions, resulting in a decrease in running accuracy. When the bearing stops running normally, the moisture in the air condenses into water droplets due to the temperature drop, and adheres to the bearing surface, which also causes rust. (e) Serious fractures of the bearing components will occur when the applied load is greater than the maximum stress that the wheel hub bearings can suffer. At the same time, the thermal stress generated by the bearings during operation will also cause serious fractures of the bearing components. When the bearing design, manufacturing process, and assembly process are unreasonable, the ferrule rib and the roller chamfer will be missing. (f) The deformation of the cage will occur when wheel hub bearing installation is not in place, hard foreign matter infiltrates the area, or the material has defects. The friction between the inner and outer rings and the rolling may be intensified during the operation of the deformed cage, accompanied by strong vibration and noise, meaning the wheel hub bearings may be damaged or even fail earlier. All these factors will affect the results of RUL prognostics. To consider these factors together, the fault vibrational signal generated from wheel hub bearings is used in this study.