H7/p6 fit: calculating the exact interference
On the drawing of a pinion mounted on a gearbox shaft, the joint is specified as a ⌀40 H7/p6 interference fit. The fitter has to work out the minimum interference, to make sure the pinion won’t slip under rated torque, and the maximum interference, to avoid bursting the hub when it is pressed on.
Calculating the interference of an ISO H7/p6 fit
| Nominal size ⌀40 mm | H7 hole (mm) | p6 shaft (mm) | Resulting interference (mm) |
|---|---|---|---|
| Upper limit | 40.025 | 40.059 | Max interference = 40.059 - 40.000 = 0.059 |
| Lower limit | 40.000 | 40.043 | Min interference = 40.043 - 40.025 = 0.018 |
All values are given for the reference environmental conditions of 20 °C.
Maximum and minimum interference
Maximum interference occurs when the shaft is at its upper limit (p6 max) and the hole at its lower limit (H7 min). Max interference = 40.059 - 40.000 = 0.059 mm (59 µm). Minimum interference occurs when the shaft is at its lower limit (p6 min) and the hole at its upper limit (H7 max). Min interference = 40.043 - 40.025 = 0.018 mm (18 µm). To go back over clearance fits, see converting ISO 286 fits into numerical limits.
Press-fit forces and thermal expansion
An interference of 59 microns calls for a large press force, or heating the pinion to 150 °C before fitting it. Checking the allowable stresses in the hub material is mandatory.
Example: checking parts before pressing them together
A hole measured at 40.012 mm and a shaft measured at 40.050 mm are checked. 1. Actual interference: 40.050 - 40.012 = 0.038 mm. 2. Allowed range: [0.018 mm; 0.059 mm]. Verdict: the actual interference (38 µm) lies within the range. The assembly is approved.
Shop-floor pitfalls with interference fits
The first pitfall is pressing the parts together dry, without anti-seize lubricant. The second is the distortion of the hole after pressing. The third comes from measuring the shaft while it is still warm. To automate your fit calculations, use CoteCote’s automatic ISO 286 detection.