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A fit describes how a shaft and a hole go together: with a gap, with a slight grip, or with a firm press. The ISO 286 system expresses each fit as a pair of tolerance codes, such as H7/g6. The capital letter is the hole, and the small letter is the shaft.
Why most fits start with H7
In the hole-basis system, the hole is made to an H tolerance, which starts exactly at the nominal size and goes upwards. Holes are harder to make to odd sizes than shafts, so standard reamers and gauges are made for H holes, and the shaft is varied to get the fit you want. H7 is the most common hole grade for precision work.
Three kinds of fit
- Clearance: the shaft is always smaller than the hole. Parts slide or turn. Examples: H7/g6 for sliding parts, H8/f7 for running fits.
- Transition: depending on the actual sizes, there may be a small gap or a small grip. Used for accurate location of parts that can still be taken apart, such as H7/k6.
- Interference: the shaft is always larger than the hole. Parts are pressed together or assembled with heat. Example: H7/p6.
What these fits mean at 40 mm
Scroll the table sideways to see all columns on a small screen.
| Fit | Type | Range at Ø40 mm |
|---|---|---|
| H7/g6 | Clearance | 0.009 to 0.050 mm clearance |
| H7/h6 | Clearance | 0 to 0.041 mm clearance |
| H7/k6 | Transition | 0.023 mm clearance to 0.018 mm interference |
| H7/p6 | Interference | 0.001 to 0.042 mm interference |
Choosing a fit
- Bearings: follow the bearing maker's recommendation for the shaft and housing, which depends on load and rotation.
- Locating pins and dowels: transition or light interference fits.
- Gears and pulleys on keyed shafts: often transition fits, with the key carrying the torque.
- Bushes pressed into housings: interference fits.
