The fits of the ball bearing on the shaft and in the housing essentially determine the operational behaviour of miniature ball bearings. When selecting fitting tolerances, the following criteria should therefore be considered.

Rotation conditions

Rings with circumferential loading should have a tighter fit than rings with point loading.

Circumferential loading occurs in the case of rotating ring and stationery load or stationery ring and rotating load. Point loading occurs in the case of a stationery ring and stationery load or rating ring and rotating load.

Running accuracy

The same high standard of accuracy and surface quality expected of the bearing must be applied to shaft and housing.

Loading

High loads demand tighter fits.

Temperature

There may be temperature differences between the ball bearing and mating components while the ball bearing is in operation. The dimensional changes caused by differential expansion should be taken into consideration.

In the case of miniature bearings great important is placed on simple mounting and high running accuracy, hence only a close sliding or transition fit is usually possible. Furthermore it must be remembered that irregularities on the shaft or in the housing bore are transferred to the relatively thin-walled bearing rings. In order to improve the fit, it is possible to calibrate the bore and outside diameter into groups (see Calibration of bore & outside diameter section).

The values shown in tables 12 and 13 are only valid for materials with similar thermal expansion coefficients (11 . 10-6 . K-1).

If the expansion coefficient varies or if there are temperature variations between the outer ring and housing or between the outer ring and housing or between the inner ring and shaft, tolerances which ensure the appropriate fit at operating temperatures should be selected.

Shaft tolerances in µm
                             .0001”

Ball bearing bore   Grade ® Tolerance in mm Tolerance in .0001 inch ®   Operating Conditions	P0 0/-8 0/-3	P5 0/-5 0/-2	Calibration		Type of Fit
			0/-2.5 0/-1	-2.5/-5 -1/-2
Low loading Medium speed No vibration	-5/-13 -2/-5	-5/-11 -2/-4	-5/-8 -2/-3	-8/-11 -3/-4	Sliding fit
Low to medium loading Medium speed Low vibration	0/-8 0/-3	0/-6 0/-2.5	0/-3 0/-1.2	-3/-6 -1.2/-2.5	Transition fit
Heavy loading High speed High frequency vibrations	+4/-4 +1.6/-1.6	+4/-2 +1.6/-1	+4/+1 +1.6/+0.4	+1/-2 +0.4/-1	Press fit

Table 12: Shaft tolerances 

Housing tolerances in µm
                                  .0001”

Ball bearing bore   Grade ® Tolerance in mm Tolerance in .0001 inch ®   Operating Conditions	P0 0/-8 0/-3	P5 0/-5 0/-2	Calibration		Type of Fit
			0/-2.5 0/-1	-2.5/-5 -1/-2
Low loading Medium speed No vibration	+5/-3 +2/-1.2	+5/-1 +2/-0.4	+5/+2 +2/-1	+2/-1 +1/-0.4	Sliding fit
Low to medium loading Medium speed Low vibration	0/-8 0/-3	0/-6 0/-2.5	0/-3 0/-1.2	-3/-6 -1.2/-2.5	Transition fit
Heavy loading High speed High frequency vibrations	-4/-12 -1.6/-5	-3/-9 +1.2/-3.5	-3/-6 +1.2/-2.5	-6/-9 -2.5/-3.5	Press fit

Table 13: Housing tolerances

The information on this page applies to steel shafts and housings. The linear thermal expansion coefficient of other materials (e.g. aluminium housing) must be taken into consideration at operating temperatures other than ambient (20^oC).