A tolerance class is a promise about how close the finished ring is to its nominal size and how round it runs. This page collects the values that decide whether a bearing will fit the housing you have already machined, and how much internal clearance it will start life with.
The first number on an incoming inspection report is the single plane mean bore diameter deviation, written Δdmp. It is measured at several planes and averaged, which is why a ring with a perfectly circular cross-section can still fail: the tolerance is on the mean, and a separate limit covers roundness.
For bearing bores the deviation is always zero or negative. Rings are made at or below nominal, never above, so a shaft ground to k5 or m5 always ends up with the interference the fit table predicts. Values below are in micrometres; one micrometre is one thousandth of a millimetre.
| Bore d (mm) | Normal (P0) | Class 6 | Class 5 | Class 4 | Class 2 |
|---|---|---|---|---|---|
| 10 – 18 | 8 | 7 | 5 | 4 | 2.5 |
| 18 – 30 | 10 | 8 | 6 | 5 | 2.5 |
| 30 – 50 | 12 | 10 | 8 | 6 | 2.5 |
| 50 – 80 | 15 | 12 | 9 | 7 | 4 |
| 80 – 120 | 20 | 15 | 10 | 8 | 5 |
| 120 – 180 | 25 | 18 | 13 | 10 | 7 |
| 180 – 250 | 30 | 22 | 15 | 12 | 8 |
All values are negative: the permitted bore diameter is nominal down to nominal minus the tabulated figure.
| Outside diameter D (mm) | Tolerance (µm) |
|---|---|
| 2.5 – 6 | 8 |
| 6 – 18 | 8 |
| 18 – 30 | 9 |
| 30 – 50 | 11 |
| 50 – 80 | 13 |
| 80 – 120 | 15 |
| 120 – 150 | 18 |
| 150 – 180 | 25 |
| 180 – 250 | 30 |
Sizing tells you the bearing will assemble. Runout tells you how it will behave once it turns. Kia is the radial runout of the inner ring: the total indicator reading when the inner ring is rotated one full turn against the stationary outer ring. It is the single best predictor of vibration and of the noise a motor or fan will make.
| Bore d (mm) | Normal (P0) | Class 6 | Class 5 | Class 4 |
|---|---|---|---|---|
| 10 – 18 | 10 | 6 | 4 | 2.5 |
| 18 – 30 | 13 | 8 | 4 | 3 |
| 30 – 50 | 15 | 9 | 5 | 4 |
| 50 – 80 | 20 | 11 | 6 | 5 |
| 80 – 120 | 25 | 15 | 8 | 5 |
| 120 – 180 | 30 | 18 | 10 | 6 |
ABEC, ISO, DIN and JIS are four names for largely the same idea, and a quotation may arrive using any of them. The mapping below is the one we use when a customer specifies ABEC and our stock is marked to ISO 492 or DIN 620.
Two cautions. First, ABEC applies to ball bearings; the parallel scale for cylindrical, spherical and tapered roller bearings is RBEC, and it has fewer grades. Second, the conversion is reliable at the lower grades and becomes approximate at the top, because the standards do not cover identical parameters - always confirm the parameter that matters to your application rather than the label.
| ABEC / RBEC | ISO 492 | DIN 620 | JIS B 1514 | Typical application |
|---|---|---|---|---|
| ABEC 1 | Normal | P0 | Class 0 | General machinery, electric motors, fans, most industrial duty |
| ABEC 3 | Class 6 | P6 | Class 6 | Pumps, gearboxes and fans where runout must be controlled |
| ABEC 5 | Class 5 | P5 | Class 5 | Machine tool spindles, high speed and low noise duty |
| ABEC 7 | Class 4 | P4 | Class 4 | Precision spindles, measuring and test equipment |
| ABEC 9 | Class 2 | P2 | Class 2 | Ultra precision metrology and high speed spindles |
Radial internal clearance is the total gap you can measure by moving the inner ring radially while the outer ring is held. It is quoted as a range, not a value, because the assembled bearing will end up somewhere inside that range once the fits have tightened it and the running temperature has expanded the rings.
The practical rule: choose clearance from the operating condition, not from the catalogue default. Interference fits on both rings, a hot inner ring, or a heavy preload all consume clearance, and a bearing that starts at the bottom of CN can end up preloaded and destroy itself. That is why C3 is the normal choice for electric motors and anything running hot.
| Bore d (mm) | C2 | CN (Normal) | C3 | C4 | C5 |
|---|---|---|---|---|---|
| 10 – 18 | 0 – 9 | 3 – 18 | 11 – 25 | 18 – 33 | 25 – 45 |
| 18 – 24 | 0 – 10 | 5 – 20 | 13 – 28 | 20 – 36 | 28 – 48 |
| 24 – 30 | 1 – 11 | 5 – 20 | 13 – 28 | 23 – 41 | 30 – 53 |
| 30 – 40 | 1 – 11 | 6 – 20 | 15 – 33 | 28 – 46 | 40 – 64 |
| 40 – 50 | 1 – 11 | 6 – 23 | 18 – 36 | 30 – 51 | 45 – 73 |
| 50 – 65 | 1 – 15 | 8 – 28 | 23 – 43 | 38 – 61 | 55 – 90 |
| 65 – 80 | 1 – 15 | 10 – 30 | 25 – 51 | 46 – 71 | 65 – 105 |
| 80 – 100 | 1 – 18 | 12 – 36 | 30 – 58 | 53 – 84 | 75 – 120 |
| 100 – 120 | 2 – 20 | 15 – 41 | 36 – 66 | 61 – 97 | 90 – 140 |
Deep groove ball bearings only. Cylindrical, self-aligning and spherical roller bearings have their own clearance tables and much wider ranges - ask us for the table for the type you are specifying.
Send the drawing or the model number and we will confirm the tolerance class, clearance and fit for your application.