Ball Bearing Assembly & Fitting Guide

Machined Bearings

The operating characteristics of a system can be drastically affected by variations in the handling and mounting of ball bearings. Substandard performance can result when a bearing is damaged by excessive force or shock loading during assembly, or by a fit which is too tight or too loose. The possibility of producing a malfunctioning device will be considerably reduced by following a few general guidelines during the design of mating parts, and by observing some basic cautions in the assembly process.

Recommended Fits for Ball Bearings

The chart in this document lists recommended fits for most normal situations. There are four cautions which must be observed:

Thermal Exapansion

1. The effect of differential thermal expansion must be accounted for when establishing shaft or housing sizes. The Table of Recommended Fits assumes stable operating conditions. If thermal gradients are known to be present or dissimilar materials are being used, adjustments must be made to the room temperature fits to achieve proper fit at operating temperature. Approximate thermal coefficients for common material are available from IBSCO Applications Engineering staff.

Interference Fit

2. When miniature and instrument ball bearings are interference fitted (either intentionally or as a result of thermal gradients) the bearing radial play can be estimated to be reduced by an amount equal to 80% of the actual diametrical interference fit. This 80% figure is conservative, but is of good use for design purposes. Depending on the materials involved, this factor will typically range from 50% to 80%. The following is an example of calculating loss of radial play:

Radial Play of Bearing: .0002"
Total Interference Fit: .0003"
Tight 80% of Interference Fit (.0003" x 80%) .00024"
Theoretical Resultant Radial Play of Bearing .00004" Tight

Theoretically, the bearing in this example could be operating with negative radial play. A bearing operated in an excessive negative radial play condition will perform with reduced life. However, the above calculation is for design only, and does not take into account housing material, shaft material, or surface finish of the housing or shaft surfaces. 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 serve to 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 above. If assistance on fits and their effect on bearing performance is required, please consult a member of IBSCO Applications Engineering staff.

The table of recommended fits is based on the use of bearings with a tolerance level of ABEC 5 or better.

Clamping & Abutting

3. If the outer or inner ring face is to be clamped or abutted against a shoulder, care must be taken to make sure that the configuration of the shoulder provides a good mounting surface:

  • The shoulder face must be perpendicular to the bearing mounting seat. The maximum permissible angle of misalignment is recommended to be 1/4°.
  • The corner between the mounting diameter and the face must have an undercut or a fillet radius no larger than that shown on the listing page under the column “Fillet Radius r”.
  • The shoulder diameter must meet the requirements shown on the table of recommended shoulder diameters.

Assembly Technique

4. Assembly technique is extremely critical. After the design is finalized and assembly procedures are being formulated, the bearing Static Capacity - Cor - becomes exceptionally important. It is easy, for instance, to exceed the 3 pound capacity of a SR09 during assembly. After assembly to the shaft, damage can be done by direct pressure as well as by a moment load while the bearing-and-shaft subassembly is being forced into a tight housing. A few simple calculations will underscore this point.

Adequate fixturing should always be provided for handling and assembling precision bearings. This fixturing must be designed so that force is applied only to the inner ring when assembling the bearing to the shaft. When assembling into the housing, force can only be applied to the outer ring. The fixturing must also prevent the transmission of any moment or shock loads through the bearing. Integration of these assembly requirements into the total design effort will prevent many problems and provide savings when production starts. Our engineers are especially eager to help in this process, one of the most important phases of taking a product from design to the marketplace.