1.There are some impurity material in the grease.
2.Lacking of lubrication. The oil level is too low and improper storage may leads to oil leakage.
3.The bearing clearance is too small or too large.
4.The bearing is mixed with impurities such as sand or carbon. Grinding the bearing.
5.The bearing is mixed with water, acid or paint and other dirt, which will corrode the bearing.
6.The bearing is flattened by the bracket hole.
7.The underside of bearing block is uneven.
8.There are impurities in the inside bore of bearing.