Machine Design

A few guidelines for fitting miniature bearings into assemblies

The upper curve in the chart shows a single-bearing deflection curve. When two bearings are preloaded together and subjected to an external thrust load, the axial yield rate for the pair drops because of the preload, the interaction of the forces exerted by the external load, and reactions of the two bearings. The lower curve shows that the yield rate for the preloaded pair is essentially linear. Miniature and instrument bearings are typically built to accept light preloads ranging from 0.25 lb to normally not more than 10 lb.

The operating characteristics of a system can be drastically affected by the way its ball bearings are handled and mounted. A bearing that has been damaged due to excessive force or shock loading during assembly, or is fitted too tight or loose may cause the device to perform below expectations. By following a few general guidelines during the design of mating parts and by observing basic precautions in the assembly process, the devices are more likely to perform as expected.

Assemblers should observe these guidelines:

Account for the effect of differential thermal expansion when establishing shaft or housing sizes. The partial table of recommended fits assumes stable operating conditions, so when thermal gradients are present, or dissimilar materials are being used, room temperature fits must be adjusted to attain the proper operating temperature.

When miniature and instrument ball bearings are interference fitted (either intentionally or as a result of thermal gradients) estimate the reduction in bearing radial play by 80% of the actual diametrical interference fit. This 80% figure is conservative, but it is a useful design guideline. Depending on materials involved, this factor will range from 50 to 80%. The following is an example of calculating loss of radial play:

Radial play of bearing = 0.0002 in.
Total interference fit = 0.0003 in. tight
80% of interference fit = 0.0003 (0.80) = 0.00024 in.
Theoretical resultant radial play of bearing = 0.00004 in. tight

Theoretically, this bearing could be operating with negative radial play. A bearing with excessive negative radial play is likely to fail early. However, the above calculation is for design only and does not take into account housing material, shaft material, or surface finish of the housing and shaft. As an example, if the finish of the shaft surface is rough, a part of the interference between the inner ring and shaft will be absorbed by the deformation of the shaft surface. This will reduce the overall interference fit, and thus, the radial play of the bearing will not be reduced as much as is shown in the calculation.

Preloading is an effective means of positioning and controlling stiffness because of the nature of the ball-raceway contact. Under light loads, the ball-raceway contact area is small so the amount of "yield" or "definition" is substantial with respect to the amount of load. As the load increases, the contact area (elliptically shaped) also increases and provides increased stiffness or reduced "yield" per unit of applied load.

This information provided by NMB Bearings, Chatsworth, Calif.



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