For radial rolling bearings, tolerances are set by ISO 492. It defines tolerance classes Normal, 6, 5, 4 and 2, where Normal is the widest and class 2 is the tightest.
In the US you will often see ABEC classes, from ABEC 1 to ABEC 9. These come from US industry standards and line up roughly with the ISO classes for ball bearings.
How the classes compare
The table below is the common rough match. Exact limits differ by size and feature, so always check the standard itself.
- ISO Normal is close to ABEC 1 and suits most general machinery, motors and pumps.
- ISO class 6 is close to ABEC 3 and suits quieter or faster-running machines.
- ISO class 5 is close to ABEC 5 and suits machine tool spindles and precision equipment.
- ISO class 4 is close to ABEC 7 and is used in high-speed precision spindles.
- ISO class 2 is close to ABEC 9 and is used in instruments and very high-precision spindles.
What the tolerance class controls
The class sets limits on bore and outside diameter, ring width and running accuracy. Running accuracy covers radial runout and face runout. A tighter class means a more accurate and usually quieter bearing.
The class does not set the load rating, the steel, the internal clearance or the noise level. Those need separate requirements in your specification.
Other standards you will meet
- ISO 15 sets boundary dimensions for radial bearings, so a 6205 fits the same space from any maker.
- ISO 281 explains how to calculate dynamic load ratings and rating life.
- ISO 76 covers static load ratings.
- ISO 5753-1 sets radial internal clearance groups, such as C2, CN, C3 and C4.
- ISO 683-17 covers steels for rolling bearings.
- Tapered roller bearings have their own tolerance tables in ISO 492, and inch-size tapered bearings often follow separate US industry classes.
How to specify it
Most industrial and automotive replacement bearings are Normal class. Do not ask for a tighter class unless your machine needs it, because it slows production and raises cost.
If you replace a bearing in an existing machine, read the class and clearance from the original part number suffix or the machine maker's manual. Copying the bore and width alone is not enough.
Write the class, clearance group, seal type, grease and noise limit into your purchase order. Then ask for an inspection report that measures them on a sample from each lot. A Rajkot bearing supplier should be able to state which class it can hold consistently, and show its measuring equipment.
Next step
We check that your supplier can measure what your specification asks for, then have lots tested before loading. Book a free sourcing review to go through your bearing specification.