Between P6 and P5 the runout difference is about 2 µm on a 30 mm bore, and the price difference is roughly 40–60%. Whether that is worth paying depends entirely on what the machine does with the last two micrometres — and on whether the housing is accurate enough to deliver them.
| Criterion | P6 | P5 |
|---|---|---|
| Radial runout (30 mm bore) | ~6 µm | ~4 µm |
| Bore tolerance | Tighter than P0 | Tighter than P6 |
| Raceway parallelism | Standard | Tightened |
| Housing requirement | IT7 | IT5–IT6 |
| Preload | Optional | Normally specified |
| Price relative to P0 | 130–160% | 180–260% |
| Typical use | Quiet motor, gearbox | Machine tool spindle |
A spindle's total error is the sum of bearing runout, housing bore error, shoulder squareness, shaft roundness and thermal growth. If the housing contributes 8 µm and the shaft 5 µm, a bearing at 4 µm and a bearing at 6 µm produce assemblies that measure 17 µm and 19 µm — the class change is 10% of the total and invisible in the workpiece. If the housing is ground to 2 µm, the same change is 40% of the budget and clearly matters.
No. Load ratings depend on geometry and material. P5 changes accuracy, not strength.
You can, but you will not obtain P5 accuracy. The housing sets a floor on the assembly runout that is usually higher than the difference between P6 and P5.
Speed alone does not require it, but high-speed spindles almost always need it for vibration reasons, and they are normally supplied as matched pairs with defined preload.
P4 (ABEC-7), at roughly 2 µm runout, then P2 at about 1 µm. Each step roughly doubles the price.
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