Judging from the picture, the cause of this problem should be that the tension in the early stage of coiling was not enough and the steel coil was not tightened. The coiling tension was adjusted in the later stage (perhaps the inner diameter of the sleeve was also expanded at the same time?) , squeezed by external force, the loose steel coil in the inner ring is squeezed into the open gap of the sleeve, forming an inward convex shape.

The solution is to add a rubber sleeve to the sleeve, and pay attention to the winding tension and not to loosen the winding.