Bearing tolerances, i.e., dimensional accuracy, running accuracy, etc., are regulated by standards such as ISO and JIS. For dimensional accuracy, these standards specify the tolerances and allowable error limits of the boundary dimensions (inner diameter, outer diameter, width, assembled bearing width, chamfer, and taper) required when installing the bearing on the shaft in the bearing housing. For machining accuracy, the standards provide allowable variation limits on bore, mean bore, outside diameter, mean outside diameter, and raceway width or wall thickness (for thrust bearings). Running accuracy is defined as the allowable limits for bearing runout.
Bearing runout tolerances are included in the standards for inner and
Outdoor ring radial and axial runout; inner ringside runout with bore and outer ring outside surface runout with sid Tolerance and allowable error limitations are established for each tolerance grade or class. For example, JIS standard B 1514 (tolerances for rolling bearings) establishes five tolerance classifications (classes 0, 6, 5, 4, 2). Starting from class 0 (average precision class bearings), the bearing precision becomes progressively more significant as the class number becomes smaller. A comparison of relative tolerance class standards between the JIS B1514 standard classes and other standards is shown in the comparative Table 4.1.
Table 4.2 indicates which standard and tolerance class applies to each bearing type.
Dimensional accuracy is a measure of the outer dimensions of the bearing, such as bore diameter (d), outer diameter (D), inner ring width (B), and outer ring width (C). The difference between the actual bearing size and the nominal value or target value is called dimensional deviation. The most commonly used measurement methods are the average inner diameter and outer diameter deviation of a single plane (Amp and ADmp) and the inner and outer rings (ABs and ACS). Industry-standard tolerances control these values. Dimensional accuracy is essential for determining the fit between the shaft and the bearing housing.
When a series of measurements are taken on a single bearing and compared, the machining (or shape) accuracy measures the dimensional change of the above example. The most commonly used measurement methods are single radial plane hole and outer diameter change (Vdp and VDp), average single plane hole and outer diameter change (Vdmp and VDmp), and inner and outer ring width variation (VBs and VCs). Variation refers to the difference between the most significant and most minor measurement values in a sequence. In contrast, average variation refers to the average difference between consecutive measurements.
Machining accuracy reflects the accuracy of the manufacturing process. It is an important consideration when recommending tolerances for shafts and housings.
Running accuracy (or runout) is a measure of bearing eccentricity (radial runout) and verticality (bore and outer diameter belt side).
Radial runout of inner and outer rings (Kia and Kea) is the most commonly used measure. Operational accuracy is critical to keep excessive vibration and misalignment of the components to a minimum.
ISO/ABMA/JIS tolerance class
For many years, the allowable error limits of the three aspects of bearing accuracy have been internationally standardized as tolerance levels. Each tolerance class specifies a set of limits (which vary in proportion to the bearing size) for all precision measurements. The following table compares the most recognized standards (please note that each column represents a set of equivalent categories). For ISO, JIS, and DiN standards, bearings with standard accuracy are classified as class 0. Then there is level 6. Since then, the reduced number of stages indicates a gradual improvement inaccuracy. The tapered roller bearings of the ABMA or inch series follow a unique but similar tolerance class system.
Table 4.1 Comparison of tolerance classifications of national standard
|Standard||Standard||Tolerance Class||Tolerance Class||Tolerance Class||Tolerance Class||Tolerance Class||Bearing Types|
|Japanese Industrial Standard||JIS B 1514||Class 0 Class 6X||Class 6||Class 5||Class 4||Class 2||All types|
|International Organization for Standardization||ISO 492||Normal class|
|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 series)
|ISO 1224||—||—||Class 5A||Class 4A||—||Precision instrument|
|DIN 620||P0||P6||P5||P4||P2||All types|
|American National Standards Institute (ANSI)|
Anti-Friction Bearing Manufacturers (AFBMA)
|ANSI/AFBMA Std. 201)||ABEC-1 RBEC-1||ABEC-3 RBEC-3||ABEC-5 RBEC-5||ABEC-7||ABEC-9||Radial bearings (Except tapered|
|Class K||Class N||Class C||Class B||Class A||Tapered roller bear-|
ings (Metric series)
|ANSI B 3.19|
AFBMA Std. 19
|Class 4||Class 2||Class 3||Class 0||Class 00||Tapered roller|
bearings (Inch series)
|ANSI/AFBMA Std. 12.1||Class 3P||Class 5P Class 5T||Class 7P Class 7T||Class 9P||Precision instrument|
ball bearings (Metric series)
|ANSI/AFBMA Sts. 12.2||—||Class 3P||Class 5P Class 5T||Class 7P Class 7T||Class 9P||Precision instrument ball bearings|
Notes: 1. JIS B 1514, ISO 492 and 199, and DIN 620 have the same specification level.
The tolerance and allowance of JIS B 1514 are a little different from those of AFBMA standards
Table 4.2 Bearing types and applicable tolerance
|Bearing Type||standard||Applicable tolerance||Applicable tolerance||Applicable tolerance||Applicable tolerance||Applicable tolerance|
|Deep groove ball bearing||ISO 492||class 0||class 6||class 5||class 4||class 2|
|Angular contact ball bearings||ISO 492||class 0||class 6||class 5||class 4||class 2|
|Self-aligning ball bearings||ISO 492||class 0||—||—||—||—|
|Cylindrical roller bearings||ISO 492||class 0||class 6||class 5||class 4||class 2|
|Needle roller bearings||ISO 492||class 0||class 6||class 5||class 4||—|
|Spherical roller bearings||ISO 492||class 0||—||—||—||—|
|metric||ISO 492||class 0,6X||class 6||class 5||class 4||—|
|inch||AFBMA Std. 19||class 4||class 2||class 3||class 0||class 00|
|J series||ANSI/AFBMA Std.19.1||class K||class N||class C||class B||class A|
|Thrust ball bearings||ISO 199||class 0||class 6||class 5||class 4||—|
|Thrust roller bearings||NTN standard||class 0||class 6||class 5||class 4||—|
|Spherical roller thrust bearings||ISO 199||class 0||—||—||—||—|
|Double direction angular contact thrust ball bearings||NTN standard||—||—||class 5||class 4||—|
The following is a list of codes and symbols used in the bearing tolerance standards tables. However, in some cases, the code
or symbol definition has been abbreviated.
d: Nominal bore diameter
d 2: Nominal bore diameter (double direction thrust
D: Nominal outside diameter
B: Nominal inner ring width or nominal center
C: Nominal outer ring width1)
Note 1) For radial bearings (except tapered
roller bearings) this is equivalent to
the nominal bearing width.
T: Nominal bearing width of single row tapered
roller bearing, or nominal height of single
direction thrust bearing
T1: Nominal height of double direction thrust ball
bearing, or nominal effective width of the inner
ring and roller assembly of tapered roller
T2: Nominal height from back face of the housing
washer to back face of the center washer on
double direction thrust ball bearings, or
nominal effective outer ring width of tapered
r: Chamfer dimensions of inner and outer rings
(for tapered roller bearings, the large end of the inner
r1: Chamfer dimensions of the center washer, or
the trim end of the inner and outer ring of angular
contact ball bearing, and the large end of the outer
ring of tapered roller bearing
r2: Chamfer dimensions of the small end of inner and
outer rings of tapered roller bearing