Engineering Reference
Miniature & Instrument Ball Bearings
Bearing Fits and the Design of Mating Equipment
Accurate shaft and housing fits are important for proper bearing installation and operation. In ordinary applications, miniature ball bearings are used with a light load, so a light interference fit to a narrow clearance fit is usually sufficient. Light cross section bearings, such as Torque Tube and Thinex bearings, are more sensitive to shaft and housing imperfections and require looser fits than heavier cross section bearings.
A bearing that is fitted too tightly or too loosely, or is damaged during assembly by excessive force or shock loading, may degrade the performance of your assembly. Reduce this possibility by following some general guidelines during the design of mating parts and by observing the following four cautions during the assembly process.
Below the caution statements are recommendations for shaft and housing fits for a number of different applications and a table of recommended shoulder diameters for select NHBB part numbers.
Caution 1
When establishing shaft or housing sizes the effect of differential thermal expansion must be taken into account. If thermal gradients are present or dissimilar materials are used, room temperature fits must be adjusted accordingly. Approximate thermal coefficients for common materials are available from NHBB.
Caution 2
When miniature and instrument bearings are interference fitted (either intentionally or as a result of thermal gradients) the bearing radial play is reduced by an amount equal to approximately 80% of the actual diametral interference fit. Thus:
- Radial play = .0002"
- Tolerance interference fit = .0003T
- 80% of interference fit = (.0003 x .8) = .00024"
- Resultant operational radial play = (-.00004) tight
The bearing will be subject to early failure in most applications because it may already be overloaded. This problem can be solved either by a higher radial play or looser fit.
Caution 3
If the outer ring or inner ring face will be clamped or abutted against a shoulder, make sure the shoulder configuration provides a good mounting surface:
- The shoulder face must be perpendicular to the bearing mounting diameter within .0002 inches per inch.
- The corner between the mounting diameter and the face must have either an undercut or a fillet radius no larger than that shown on the table pages under the column “r”.
- The shoulder diameter must conform to this table .
Caution 4
It is relatively easy to damage a miniature and instrument bearing during assembly simply by exceeding its load capacity. Adequate fixturing must be provided for handling and assembling precision bearings to ensure that:
- When assembling the bearing to the shaft, force is applied only to the inner ring.
- When assembling into the housing, force is applied only to the outer ring.
- Any movement or shock loads which would be transmitted through the bearing are eliminated.
Recommended Fits
Table of Recommended Fits for Typical Applications
Typical Applications | Shaft Fit◊ | Shaft Diameter | Housing Fit | Housing Diameter |
---|---|---|---|---|
Tape guide roller, pulley, cam follower, outer ring rotation | .0000– .0004L | d – .0002 d – .0004 | .0001L – .0003T | D –.0001 D –.0003 |
Drive motor (spring preload) | .0001T– .0003L | d – .0001 d – .0003 | .0000– .0004L | D +.0002 D –.0000 |
Precision synchro or servo | .0000–.0002L* | d – .0001 d – .0003 | .0000– .0002L* | D +.0001 D –.0001 |
Potentiometer | .0001T– .0003L | d – .0001 d – .0003 | .0000– .0004L | D +.0002 D –.0000 |
Encoder spindle | .0000– .0002L* | d – .0001 d – .0003 | .0000– .0002T* | D –.0001 D –.0003 |
*For radial play .0002 to .0005
◊Measurement in inches.
L = Loose fit.
T = Tight fit.
d = Bearing bore as listed.
D = Bearing OD as listed.
+Bearings must be purchased with bore and OD coding.
Example: To use SSR-2 bearing in a potentiometer, the shaft diameter should be .1250 –.0001 to .1250 –.0003 or .1249 to .1247. The housing should be .3750 +.0002 to .3750 –.0000 or .3752 to .3750.
Recommended Shoulder Diameters*
Basic Size | Minimum Shaft Shoulder Diameter | Maximum Housing Shoulder Diameter |
---|---|---|
SSRI-2 | 0.060 | 0.105 |
SSRI-2 1/2 | 0.071 | 0.132 |
SSRI-3 | 0.079 | 0.164 |
SSRI-4 | 0.102 | 0.226 |
SSRI-3332 | 0.114 | 0.168 |
SSRI-5 | 0.122 | 0.284 |
SSRI-418 | 0.148 | 0.226 |
SSRI-518 | 0.153 | 0.284 |
SSRI-618 | 0.153 | 0.347 |
SSR-2 | 0.179 | 0.325 |
SSR-2A | 0.179 | 0.446 |
SSRI-5532 | 0.180 | 0.288 |
SSR-1640X | 0.210 | 0.580 |
SSRI-5632 | 0.210 | 0.288 |
SSRI-6632 | 0.216 | 0.347 |
SSR-3 | 0.244 | 0.446 |
SSR-1650X | 0.250 | 0.580 |
SSR-1950 | 0.250 | 0.700 |
SSR-1960 | 0.290 | 0.700 |
SSRI-614 | 0.272 | 0.352 |
SSRI-814 | 0.284 | 0.466 |
SSR-4 | 0.310 | 0.565 |
SSRI-1214 | 0.322 | 0.678 |
SSR-2270 | 0.325 | 0.810 |
SSR-2280 | 0.370 | 0.810 |
SSR-2690 | 0.420 | 0.950 |
SSRI-8516 | 0.347 | 0.466 |
SSRI-1038 | 0.435 | 0.565 |
SSRI-1438 | 0.451 | 0.799 |
SSR-100 | 0.450 | 0.949 |
SSRI-1212 | 0.560 | 0.690 |
SSRI-1812 | 0.625 | 1.025 |
SSRI-1458 | 0.665 | 0.835 |
SSRI-1634 | 0.790 | 0.960 |
*Measurement in inches.