Every specification table on this site lists a precision row, normally P0 P6 P5 P4 P2. Those are ISO tolerance classes, and they control dimensional and running accuracy, not load capacity or material quality.
What The Classes Control
The class sets limits on bore and outside diameter deviation, on width variation, and on running accuracy: radial runout, axial runout and raceway to face squareness. P0 is normal class, and each step from P6 to P2 tightens those limits.
Rough Equivalences
| ISO | ABEC | Typical use |
|---|---|---|
| P0 normal | ABEC 1 | General industrial, motors, gearboxes |
| P6 | ABEC 3 | Better running accuracy, quieter motors |
| P5 | ABEC 5 | Machine tool secondary spindles, pumps |
| P4 | ABEC 7 | Machine tool main spindles |
| P2 | ABEC 9 | Very high accuracy grinding spindles |
The tables are approximate. ISO and ABEC do not control identical parameters, so a direct swap should be confirmed against the actual tolerance values.
When A Higher Class Is Worth Paying For
Precision class buys running accuracy. If the machine has a rotating tool or workpiece and holds a tolerance, it matters. If the bearing supports a fan, a conveyor pulley or a pump impeller, it usually does not, and the money is better spent on sealing and lubrication.
Precision Is Wasted On A Loose Fit
A P4 bearing in a housing machined to a general tolerance runs no more accurately than a P0 one. Shaft and housing tolerances have to be tightened in step with the bearing class, or the improvement disappears in the fit.
