1. the reason of assembly
（1） the deformation of the oil seal was caused by pressure loading.
（2）During the assembly process, the oil seal passes through the thread or the spline, resulting in the scratches at the lip and mouth.
（3） because of the low roughness of the oil seal position and the overloading force, the spring failure or the lip and mouth turn over.
2.the reason of skeleton
Due to the material and manufacturing problems of the oil seal spring, the performance of the oil seal is invalidated. The failure form of the spring is: the spring is too tight or too loose, when the spring is too loose, the spring force is small, and it can not achieve the sealing effect. When the spring is too tight, the spring pressure is large, and the misalignment between the oil seal and the rotating shaft results in the abnormal wear of the oil seal, and finally the oil leakage occurs.
3.the cause of the working medium
The working medium is oil. The choice of oil seal material should consider its compatibility with oil. The compatibility between the two parts is poor. It will cause swelling, hardening, softening, cracking and so on. Finally, oil leakage will occur.
4.cause of wear and tear of lip and mouth
（1）cleaning degree: casting molding sand, machining iron chip, oil liquid impurity and so on, a large amount of oil seal lip parts, abnormal wear when working.
（2） the oil seal position is different from the axis of the rotating shaft, and the oil seal is worn off after loading, and the lip and mouth are abnormally worn.
The oil seal lip mouth grease lubrication, dry friction occurs, resulting in abnormal wear.
5.the cause of the axis
(1) the surface roughness and hardness of the rotary shaft are unreasonable. The roughness of the automobile industry is generally less than 0.8 at present, and the location with the oil seal will be high frequency quenching, and the hardness is no less than HRC55.
The rotary shaft surface defects, such as corrosion, scratches, directional marks etc..
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