how can loose fits cause failure in bearings? and in particular affect the outer diameter?i am seeing a failure catalog for bearings, and i read : discolartion and scoring is the result of the outer ring slipping in the housing. why outer ring?
Bearings are matched to shafts and housings to a tolerance of .01mm (ideally). This fit permits creep to distribute wear over time, while providing uniform and stable support for the inner and outer races. Tighter (interference) fits are destructive. Loose fits, which permit chatter and impact, are equally destructive. If you are involved in maintenance you will get your money's worth from the attached link.
The usual condition for the shaft to turn in a bearing is the inner surface of the bearing where the rollers or balls contact the shaft. This location is usually well lubricated and the contact area on the shaft is very minimal. Very little friction is generated and therefore very little heat is generated. If the outer race, surface, spins in the support a very high unlubricated contact surface is present and considerable heat is generated. This is known as spinning a bearing and it is not good and usually the result of the bearing race not fitting tight enough. If the friction fit in the bearing support is less than at the bearing / shaft the bearing will spin and generate heat. The heat will discolor the shaft and the individual bearings will be scored. The bearings out side diameter must be such that a tight fit is achieved when the bearing is installed.
