Dimensional, form and running accuracy of bearing seats and abutments

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

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.

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 1 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 2). The value can be determined by Δk = IT7/2B

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

Vk = 1/k ± IT7/2B

where

Vk=the permissible range of dispersion of the taper incline
Δk=the permissible deviation of the taper incline
k=factor for the taper
12 for taper 1:12
30 for taper 1:30
B=bearing width, mm
IT7=the value of the tolerance grade, based on the bearing width, mm
  • 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 tapered gauges, based on two saddles. More practical methods, but less accurate, are to use ring gauges, tapered gauges or sine bars.
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