ISO 492 — Tolerance classes and symbols
The six tolerance classes a radial bearing can be ordered in, what each symbol on a tolerance drawing controls, and how the ISO classes line up with the DIN and ABMA names.
What it fixes
ISO 492 covers the accuracy of a finished radial bearing: how far the bore and outside diameter may deviate, how much they may vary around a single plane, how much the ring width may vary, and how far the assembled bearing may run out. It applies to bearings made to the boundary dimensions of ISO 15, ISO 355 and ISO 8443 — that is, to bearings that are already the right size and are now being graded on how accurately they were made.
One ladder, four namesthe same accuracy class as each system writes it
| ISO 492 | DIN 620 | JIS B 1514 | ABMA Std. 20 | Applies to |
|---|---|---|---|---|
| Normal | P0 | Class 0 | ABEC-1 | All radial bearings |
| Class 6 | P6 | Class 6 | ABEC-3 | All radial bearings |
| Class 6X | — | Class 6X | — | Tapered roller bearings only |
| Class 5 | P5 | Class 5 | ABEC-5 | All radial bearings |
| Class 4 | P4 | Class 4 | ABEC-7 | All radial bearings |
| Class 2 | P2 | Class 2 | ABEC-9 | All radial bearings |
The class names are the part worth knowing, because a specification written in DIN numbers and a bearing supplied marked in ABMA numbers can describe the same accuracy. Class 6X is the odd one: it exists only for tapered roller bearings and has no direct equivalent in the other three systems, which is why three cells above are empty rather than filled with the nearest name.
The symbolswhat each one controls, in the order a tolerance table lists them
- Δdmp — mean bore diameter deviation
- How far the average of the bore measurements in a single radial plane sits from nominal. Usually written as an upper deviation of zero and a negative lower one, because a bore that is too large will not hold the shaft.
- ΔDmp — mean outside diameter deviation
- The same for the outside diameter.
- Vdp and VDp — single plane variation
- How much the diameter changes around one radial plane. This is ovality, and it is what a two-point micrometer misses.
- Vdmp and VDmp — mean variation
- How much the mean diameter changes from plane to plane along the ring: taper, rather than ovality.
- ΔBs and ΔCs — single width deviation
- How far an individual inner ring or outer ring width sits from nominal. B is the inner ring, C the outer.
- VBs and VCs — width variation
- How much that width varies around the ring.
- Kia and Kea — radial runout
- Radial runout of the assembled bearing, measured on the inner ring (Kia) or the outer ring (Kea). This is the one that decides how quietly a spindle turns, and it is a property of the assembled bearing, not of either ring on its own.
- Sia and Sea — axial runout
- The same measurement taken in the axial direction, against the ring face.
Why no numbers on this page
Every other standard page in this library carries a table. This one does not, and the reason is the same one that keeps the 61700 series out of the bearing data. The tolerance values are edition-specific: the 2023 edition replaced the 2014 one, revised the symbol system, extended the deviation limits over larger diameters and added stiffness series for tapered roller bearings. Tolerance tables circulating online and in distributor catalogues are a mix of editions, and they disagree cell by cell in a way that is not detectable from the table itself — a wrong tolerance value looks exactly like a right one. Rather than print figures this library cannot check, the page stops at the part that is stable across editions: which classes exist and what the symbols mean. Read the values from the edition the drawing calls up.
Not the same thing as internal clearance
Tolerance class and clearance group are separate suffixes and are chosen for different reasons. The class sets how accurately the bearing was made; the clearance group sets how much room is left inside it for the fit and the heat. A precision bearing is not automatically a tight one, and a C3 suffix says nothing about accuracy.