Tolerances for seats on hollow shafts
When a bearing is mounted on a hollow shaft using an interference fit, the shaft experiences more elastic deformation than a solid shaft. As a result, the effectiveness of the fit is less than for the same size solid shaft. The effectiveness of an interference fit on a hollow shaft depends on certain diameter ratios (fig. 1):
the diameter ratio of the hollow shaft ci = di / d
For diameter ratios ci ≤ 0,5 the reduction of effectiveness is negligible.
the diameter ratio of the bearing inner ring ce = d / de
When the average outside diameter of the inner ring de is not known, the diameter ratio can be estimated from
ce diameter ratio of the bearing inner ring d bearing bore diameter [mm] D bearing outside diameter [mm] k adjustment factor
- 0,25 for self-aligning ball bearings in the 22 and 23 series
- 0,25 for cylindrical roller bearings
- 0,3 for other bearings
- Determine the mean probable interference for the tolerance selected for a seat on a solid shaft, ΔS (→ Tolerances and resultant fits).
- Determine the required increase of interference for the seat on the hollow shaft from diagram 1 based on the diameter ratios ci and ce.
- Calculate the required mean probable interference for the seat on the hollow shaft and select the tolerance class accordingly.
A 6208 deep groove ball bearing with d = 40 mm and D = 80 mm is to be mounted on a hollow shaft with a diameter ratio ci = 0,8. What is the appropriate tolerance class for the shaft seat?
The bearing is subjected to normal loads, and a tolerance class k5 is appropriate for a seat on a solid shaft.
The diameter ratio of the bearing inner ring is
The mean probable interference on a solid shaft is
ΔS = (22 + 5) / 2 = 13,5 μm (→ table 1, k5 for a 40 mm shaft diameter)
The increase in interference for the seat on the hollow shaft is
ΔH/ΔS = 1,7 (→ diagram 1, ci = 0,8, ce = 0,77)
The requisite interference for the seat on the hollow shaft is
ΔH = 1,7 x 13,5 = 23 μm
- The appropriate tolerance class for the seat on the hollow shaft is m6 (→ table 1, mean probable interference, (33 + 13) / 2 = 23 μm)