What is the cause of the fever of three-phase motor bearings?

Rolling bearings performance depends not only on the manufacturing precision of the bearing, and precision and matched with the shaft and hole size, the shape position tolerances and surface roughness, selection of fit too tight (i.e., the public profit), assembly will make the bearing clearance becomes too small, there are even close to zero. This bearing rotation is not flexible, operation will be fever, and even burn bearings. But if the bearing inner ring and shaft with a loose, or bearing outer ring with the end cover with loose, then at run time, bearing inner ring and shaft, or bearing outer ring and end cover will occur relative movement, friction heat, resulting in bearing overheating.