Three kinds of fit
- Clearance fit
- The largest shaft is no larger than the smallest hole, so assembly always leaves clearance. Used where rotation or sliding is needed.
- Transition fit
- Depending on the actual sizes, the assembly may have clearance or interference. Used for accurate location where disassembly is still considered.
- Interference fit
- The smallest shaft is no smaller than the largest hole, so assembly always has interference. Assembled by pressing or shrinking, and used to transmit torque or load.
Shaft letters on an H7 hole basis
With the hole fixed at H7, the character of the fit is set by the shaft letter alone. Moving later in the alphabet raises the shaft zone, shifting the fit from clearance towards interference. The table shows the general tendency, using IT grade 6 (g6, h6 and so on) as the example.
| Combination | Fit | General character |
|---|---|---|
| H7/g6 | Clearance | Small clearance for precise sliding or rotation |
| H7/h6 | Clearance | Location with zero minimum clearance; assembled and removed by hand |
| H7/k6 | Transition | Location assembled with light tapping |
| H7/n6 | Transition | Location biased towards interference; close to a press fit |
| H7/p6 | Interference | Interference fit at the smaller end of the range; minimum interference can be very small in some size ranges, so confirm values in the standard table |
| H7/s6 | Interference | Medium press or shrink fit transmitting torque or load |
| H7/u6 | Interference | Heavy press or shrink fit transmitting high torque |
Size of clearance and interference
The actual ranges of clearance and interference depend on the nominal size range and IT grade and are calculated from the limit deviations in ISO 286-2. The same combination of symbols gives different ranges in different size ranges.
Conditions considered together with the fit
- How often parts are assembled and dismantled, and the assembly method (by hand, press, shrink)
- Torque, load and vibration to be carried
- Service temperature and the difference in thermal expansion between the two materials
- Surface roughness and form tolerances such as roundness and straightness