94-03 Bearing Tolerances

The objective of this brief is to provide a general outline of bearing accuracy and tolerances.

Measures of Bearing Accuracy

Bearing accuracy refers to three aspects - external dimensional accuracy, machining accuracy, and running accuracy.

Dimensional Accuracy

Dimensional accuracy is a measure of the error in the bearing's external dimensions - for example, bore diameter (d), outer diameter (D), inner ring width (B), and outer ring width (C). The difference between an actual bearing dimension and the nominal or target value is called the dimensional deviation. The most commonly used measures are the single plane mean bore and outer -diameter deviations (Admp and ADmp), and the inner and outer ring width deviations (ABs and ACs). These are governed by standardized tolerances. Dimensional accuracy is important for determining shaft and housing fits, for example.

Machining Accuracy

Machining (or form) accuracy measures the variation in the above dimensions when a series of measurements is taken on a single bearing and compared to one other. The most common measures are single radial plane bore and outside diameter variation (Vdp and VDp), mean single plane bore and outside diameter variation (Vdmp and VDmp), and inner and outer ring width variation (VBs and VCs). Variation refers to the difference between the largest and smallest measurements in a series, while mean variation refers to the average difference between consecutive measurements. Machining accuracy reflects the precision of the manufacturing process and is an important consideration when recommending tolerances for shafts and housings.

Running Accuracy

Running accuracy (or runout) is a measure of the degree of eccentricity (for radial runout) and "squareness" (for bore and O.D. with side) of the bearing. Inner and outer ring radial runout (Kia and Kea) are the measures most often used. A bearing with a high degree of radial runout, for example, will produce greater vibration especially at high speeds.

ISO/AFBMA/JIS Tolerance Classes

Allowable error limitations for the three areas of bearing accuracy have been internationally standardized for many years as tolerance classes. Each tolerance class specifies a group of limits for all the measures of accuracy (varying in proportion to the bearing size). The most recognized standards are compared in the table below (note that each column represents a set of equivalent classes). For the IS0, JIS, and DiN standards, bearings with standard accuracy are classified as class O. This is then followed by class 6. From there on, decreasing class numbers denote progressively improving accuracy.

Common International Standards