The accuracy of cylindrical bearing seats on shafts and in housing bores, of seats for thrust bearing washers and of the support surfaces (abutments for bearings provided by shaft and housing shoulders etc.) should correspond to the accuracy of the bearings used. In the following, guideline values for the dimensional, form and running accuracy are provided. These should be followed when machining the seats and abutments.

Dimensional tolerances
For bearings made to Normal tolerances, the dimensional accuracy of cylindrical seats on the shaft should be at least to grade 6 and in the housing at least to grade 7. Where adapter or withdrawal sleeves are used, wider diameter tolerances (grades 9 or 10) can be permitted than for bearing seats, see table 1.

The numerical values of standard tolerance grades IT to ISO 286-1:1988 can be found in table 2.

For bearings with higher accuracy, correspondingly better grades should be used.

Tolerances for cylindrical form
The cylindricity tolerances as defined in ISO 1101:2004 should be 1 to 2 IT grades better than the prescribed dimensional tolerance, depending on requirements. For example, if a bearing shaft seat has been machined to tolerance m6, then the accuracy of form should be to IT5 or IT4. The tolerance value t₁ for cylindricity is obtained for an assumed shaft diameter of 150 mm from t₁ = IT5/2 = 18/2 = 9 μm. However, the tolerance t₁ is for a radius, hence 2t₁ applies for the shaft diameter. table 3 provides guideline values for the cylindrical form tolerance and the total runout tolerance for the different bearing tolerance classes.

When bearings are to be mounted on adapter or withdrawal sleeves, the cylindricity of the sleeve seat should be IT5/2 (for h9) or IT7/2 (for h10), see table 1.

Tolerances for perpendicularity
Abutments for bearing rings should have a rectangularity tolerance as defined in ISO 1101:2004, which is better by at least one IT grade than the diameter tolerance of the associated cylindrical seat. For thrust bearing washer seats, the tolerance for perpendicularity should not exceed the values of IT5. Guideline values for the tolerance for rectangularity and for the total axial runout can be found in table 3.

Tolerances for tapered journal seats
When a bearing is mounted directly onto a tapered shaft seat, the seat diameter tolerance can be wider than in the case of cylindrical seats. Fig shows a grade 9 diameter tolerance, while the form tolerance stipulations are the same as for a cylindrical shaft seat. SKF recommendations for tapered shaft seats for rolling bearings are as follows.

shows a grade 9 diameter tolerance, while the form tolerance stipulations are the same as for a cylindrical shaft seat. SKF recommendations for tapered shaft seats for rolling bearings are as follows.

The permissible deviation of the taper incline is a ± tolerance in accordance with IT7/2 based on the bearing width B (fig). The value can be determined by

Δ_k = IT7/2B

The permissible range of dispersion (variation of the taper incline)thus becomes

V_k = 1/k ± IT7/2B

where

The straightness tolerance is IT5/2, based on the diameter d and is defined as:
"In each axial plane through the tapered surface of the shaft, the tolerance zone is limited by two parallel lines a distance "t" apart."

The radial deviation from circularity is IT5/2, based on the diameter d and is defined as:
"In each radial plane along the tapered surface of the shaft, the tolerance zone is limited by two concentric circles a distance "t" apart." When particularly stringent running accuracy requirements are stipulated, IT4/2 is to apply instead.

The best way to check that the taper is within the recommended tolerances is to measure with special taper gauges, based on two saddles. More practical methods, but less accurate, are to use ring gauges, taper gauges or sine bars.