P4 corresponds to ABEC-7 and halves the running accuracy of P5 — around 2 µm radial runout on a 30 mm bore. It is the class used for grinding spindles, high-speed machining centres, measuring instruments and any axis where the workpiece tolerance is a few micrometres.
At P4 the bearing is measured on a special stand under a defined load, and the runout figure assumes the outer ring is supported as it will be in the housing. In service the mounting error, the preload and the thermal growth all add to the bearing's own 2 µm. A spindle that measures 2 µm on the bench but 6 µm hot has a thermal problem, not a bearing problem.
P4 spindle bearings are almost always used in pairs — back-to-back (DB) for stiffness, face-to-face (DF) for misalignment tolerance, or in a tandem set for one-directional thrust. The pair is matched for preload and the mounting direction is marked. Replacing one bearing of a pair with a single new P4 breaks the match and usually produces worse runout than before the change.
| Also written | Class 4, ABEC-7, DIN P4 |
|---|---|
| Typical radial runout | Around 2 µm for a 30 mm bore |
| Price relative to P0 | 300–450% |
| Housing requirement | Ground bore, IT5, square shoulder |
| Pairing | Normally matched and preloaded |
| Typical use | Grinding spindles, machining centres, metrology |
| Temperature control | Often required — spindle growth is in the error budget |
| Criterion | P5 | P4 | P2 |
|---|---|---|---|
| Radial runout (30 mm) | ~4 µm | ~2 µm | ~1 µm |
| Housing | IT5-IT6 | IT5 ground | IT4-IT5, temperature controlled |
| Pairing | Common | Standard | Standard, matched to preload |
| Price | 180–260% | 300–450% | 600–900% |
| Typical use | Milling spindle | Grinding spindle, metrology | Ultra-precision, aerospace |
Only if the machine's accuracy requirement allows it. On a grinding spindle the loss of 2 µm of runout typically shows directly on the workpiece surface.
Yes, in a matched pair. Fitting one new bearing against a worn one loses the preload match and usually increases vibration.
Because the 2 µm is the bearing alone, on a measuring stand. Housing squareness, fit, preload and thermal growth add to it in the machine.
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