B045 BALL BEARING INTERNAL CLEARANCE
Bearing life depends on:
- Operating speed
- Load, or force on the bearings
- Minimum contamination from external solids or acids.
- Keeping the bearing temperature under control
The load on the bearing balls is usually a combination of the external force on the shaft and the internal force produced by the fit or clearance between the inner and outer raceways.
- Ball bearings are built with a small clearance between the balls and the raceway.
- A ball bearing with inside diameter of 50-mm has a normal clearance between the ball and raceway of between 8-Ојm and 28-Ојm (micrometers). When mounted on a shaft, an interference fit between the shaft and bearing inside diameter is usually used. This causes the bearing inner raceway to expand, which reduces or eliminates the original ball clearance.
- The elimination of the ball clearance eliminates any looseness or radial movement of the pump shaft, which maintains the close alignment good for proper operation of the pump seal.
If the pump is operating on high temperature liquids, like 750-deg F. (heat transfer oil) the shaft and inner race of the bearing expand further, putting an additional load on the bearing balls. This additional load on the bearing will significantly reduce its life.
- A possible solution is to use a bearing with greater internal clearance to compensate for the high temperature expansion of the shaft and bearing inner raceway.
- All bearing manufacturers produce a bearing with greater than normal clearance, which is identified as a C3 fit bearing. The same 50-mm bearing will then have an internal clearance of 23-Ојm to 43-Ојm, which will be reduced as the shaft and bearing expand.
- C4 and C5 bearings are also available
CAUTION: Some pump users specify C3 fit as standard Be aware that excessive internal clearance in the bearing could allow excessive radial movement at the shaft seal, reducing seal life and causing leakage.
Try to follow the OEM’s recommendation regarding replacement bearings and internal clearances.