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# Fits for hollow shafts

If bearings are to be mounted with an interference fit on a hollow shaft, to achieve the same surface pressure between the inner ring and shaft seat, it is generally necessary to use a tighter interference fit than would be used for a solid shaft. The following diameter ratios are important when deciding on the fit to be used:

and

Diameter ratios, ci, that are &lt; 0,5 do not have an appreciable effect on the fit.
If the average outside diameter of the inner ring, i.e. the average diameter between the shoulder and raceway (diagram 1), is not known, the diameter ratio ce can be estimated with sufficient accuracy using

where
 ci = diameter ratio of the hollow shaft ce = diameter ratio of the inner ring d = outside diameter of the hollow shaft, bearing bore diameter [mm] D = bearing outside diameter [mm] di = inside diameter of the hollow shaft [mm] de = average outside diameter of the inner ring [mm] (diagram 1) k = a factor for the bearing type = 0,25 for self-aligning ball bearings in the 22 and 23 series = 0,25 for cylindrical roller bearings = 0,3 for all other bearings
The requisite interference fit for a bearing mounted on a hollow shaft can be determined based on the mean probable interference for the same bearing on a solid shaft, neglecting plastic deformation (smoothing) of the mating surfaces produced during mounting. The mean probable interference for the bearing on a solid shaft, ΔS, is the mean value of the smallest and largest values of the probable interference listed in table 1, table 2, table 3, table 4, table 5 and table 6. Diagram 1 provides values for the ratio between the mean probable interference of the bearing inner ring on a hollow shaft, ΔH, and on a solid shaft, ΔS, depending on the diameter ratios ci and ce.

## Example

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 requisite interference and what are the appropriate shaft limits?
If the bearing is to be mounted on a solid steel shaft and subjected to normal loads, a tolerance class k5 is recommended. From table 4, for a 40 mm shaft diameter, the mean probable interference ΔS = (22 + 5) / 2 = 13,5 μm. For ci = 0,8 and

so that from diagram 1 the ratio ΔH / ΔS = 1,7. Therefore, the requisite interference for the hollow shaft ΔH = 1,7 x 13,5 = 23 μm. Consequently, tolerance class m6 is selected for the hollow shaft as this gives a similar fit as k5 for a solid shaft.