Bearing Standards Hub

Every bearing number, tolerance class and clearance code traces back to a published standard. This hub explains how those standards fit together, how to read a bearing designation, and what the precision and clearance codes mean in practice — for buyers, engineers and newcomers alike.

The standards families

Bearings are made interchangeable across the world because their dimensions, tolerances and clearances are fixed by international standards rather than by each manufacturer. A 6205 from one brand fits the same shaft and housing as a 6205 from another. The main bodies are:

How a bearing is sized — boundary dimensions

A rolling bearing is defined first by three external (boundary) dimensions, almost always written in millimetres as d × D × B:

ISO 15 fixes a general plan of boundary dimensions: for any given bore, a small set of standard outer diameters and widths is allowed. These are grouped into dimension series — combinations of a diameter series (how large the OD is) and a width series (how wide the bearing is) for the same bore. This is why, for a 25 mm shaft, you can get a light 6205, a medium 6305 or a heavy 6405: same bore, progressively larger section and load capacity.

Reading a bearing number

Most metric bearing designations follow the same logic: an optional prefix, a basic number (type + dimension series + bore), and one or more suffixes. The exact suffix letters vary slightly between SKF, FAG, NSK, NTN and others, but the basic number is common.

Step 1 — Type (first character/s of the basic number)

Conventions vary slightly by manufacturer; the above is the common ISO-based scheme.

Step 2 — Dimension (size) series

The digit(s) after the type code give the dimension series — the relative cross-section. For the same bore, a higher series means a larger OD, wider section and higher load capacity. Common diameter series in increasing size: 8, 9, 0, 1, 2, 3, 4. So the 6200 series is lighter than the 6300, which is lighter than the 6400.

Step 3 — Bore code (the last two digits)

Worked examples

Precision & tolerance classes (ISO 492 / ABEC)

Tolerance class controls how accurately the bearing is made — its dimensional accuracy and running accuracy (run-out). ISO 492 defines the classes; the American ABEC scale (ABMA Std 20) is the common cross-reference. Watch the direction: in the ISO scale Normal is the loosest and P2 the tightest, while in the ABEC scale a higher number means tighter — so the two scales run in opposite directions.

Internal radial clearance (C2–C5)

Internal clearance is the small internal gap between the rolling elements and the raceways before mounting. It matters because fitting onto a shaft (interference fit) and the temperature difference between the inner and outer rings in service both reduce that gap. Too little operating clearance pre-loads the bearing and makes it run hot; too much causes noise and uneven load.

Suffix decoder (common codes)

Suffixes describe seals, cages, clearance and precision. These differ between makers — always confirm against the manufacturer's catalogue — but the most common are:

Key standards at a glance

Glossary

This page is reference guidance harmonised with the ISO series. For design-critical work always confirm values against the manufacturer's current catalogue and the published standard.