Accuracy and Surface Finish of Shafts and Housings
If the accuracy of a shaft or housing does not meet the specification, the bearings will not perform to their full capability. For example, inaccuracy in the squareness of the shaft shoulder may cause misalignment of the bearing inner and outer rings, which may reduce the bearing fatigue life by adding an edge load in addition to the normal load. Cage fracture and seizure sometimes occur for this same reason. Housings should be rigid in order to provide firm bearing support. Highly rigid housings are advantageous also from the standpoint of noise, load distribution, and other factors.
For normal operating conditions, a turned finish or smooth bored finish is sufficient for the fitting surface; however, a ground finish is necessary for applications where vibration and noise must be low or where heavy loads are applied. In cases where two or more bearings are mounted in one single-piece housing, the fitting surfaces of the housing bore should be designed so both bearing seats may be finished together with one operation such as in-line boring. In the case of split housings, care must be taken in the fabrication of the housing so the outer ring will not become deformed during installation. The accuracy and surface finish of shafts and housings for normal operating conditions are listed in the following tables.
| Item |
Bearing Class |
Shaft |
Housing Bore |
| Tolerance for Out-of-roundness |
Normal,Class 6
Class 5,Class 4 |
 ~ 
 ~
|
~ 
~
|
| Tolerance for Cylindricality |
Normal,Class 6
Class 5,Class 4 |
~
~
|
~
~
|
| Tolerance for Shoulder Runout |
Normal,Class 6
Class 5,Class 4 |
|
- 
|
Roughness of Fitting Surfaces,
R max |
Small Bearings
Large Bearings |
0.8
1.6 |
1.6
3.2 |
Remarks
This table is for general recommendation using the radius measuring method, the basic tolerance (IT) class should be selected in accordance with the bearing precision class. For the values correspoinding to the IT classes, click here. When an outer ring is mounted on the housing bore with interference or when a thin cross-section bearing is mounted on the shaft and housing, the accuracy of the shaft and housing should be checked carefully since it affects the bearing raceway directly.
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