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
| Standard |
Tolerance
Class |
Bearing
Type |
| JIS B 1514 |
class 0
class 6x |
class 6 |
class 5 |
class 4 |
class 2 |
All types |
| ISO 492 |
normal
class 6x |
class 6 |
class 5 |
class 4 |
class 2 |
Radial bearings |
| ISO 199 |
normal
class |
class 6 |
class 5 |
class 4 |
- |
Thrust ball
bearings |
| ISO 578 |
class 4 |
- |
class 3 |
class 0 |
class 00 |
Tapered roller
bearings (inch type) |
| DIN 620 |
P0 |
P6 |
P5 - |
P4 |
P2 |
All types |
ANSI/AFBMA
Std. 20 |
ABEC-1
RBEC-1 |
ABEC-3
RBEC-3 |
ABEC-5
RBEC-5 |
ABEC-7 |
ABEC-9 |
Radial
bearings (except tapered roller bearings) |
|
|
