Ball and Roller bearings unmounted have internal clearance. This clearance is an actual air gap. As bearings are mounted and pressed onto shafts some of the air gap is removed. As bearings operate the shaft is normally hotter than the housing causing a thermal unbalance which results in more clearance removal.
There are 3 key points to consider when purchasing a bearing:
1. Clearance of the bearing- When heat is involved, the tolerance of the bearing will determine the amount of space the bearing has to expand. You will typically find bearings with C0 clearance. The next larger clearance is a C3, C4, and then C5. Each clearance allows a certain amount of space between the ball bearing and outer race.
2. The material of the bearing: Under too much heat, chrome steel and stainless steel will rust. This limits the amount of heat the metal can take. Your standard chrome steel can handle up to 250ºF. If a bearing exceeds the heat temperature rating, it will fail. Stainless steel can handle more heat. The metal in high temperature bearings is heat treated to be able to withstand higher temperatures.
3. Cage: You will notice that all of the high temperature bearings have no cage (full compliment). This is due to the fact that the bearings expand under heat. If a bearing is encased in a cage, once heated, the bearing will break the cage causing the bearing to fail.