Press fit chart
Common interference (press) fits for assembling parts that must stay locked together without fasteners. The shaft is made slightly larger than the hole, so pressing them together creates a permanent or semi-permanent joint held by friction.
ISO interference fits (hole basis)
| Fit | Type | Typical use |
|---|---|---|
| H7/p6 | Light press | Locating fit for thin sections, easily pressed, parts stay rigid |
| H7/r6 | Medium interference | Medium drive fits for general assemblies |
| H7/s6 | Medium drive | Permanent or semi-permanent joints, shrink fits on light sections |
| H7/t6 | Heavy drive | Heavier interference for steel on steel |
| H7/u6 | Force fit | High interference for stressed parts or where shrink fitting is impractical |
Inch interference classes (ANSI)
| Class | Name | Typical use |
|---|---|---|
| FN1 | Light drive | Light assemblies, easily pressed |
| FN2 | Medium drive | Steel parts of ordinary section, the most common press fit |
| FN3 | Heavy drive | Heavier steel parts |
| FN4 | Force fit | Parts that can be highly stressed |
| FN5 | Heavy force | Very high interference for the heaviest duty |
Interference is the amount the shaft exceeds the hole, and it grows with diameter. Heating the hub or cooling the shaft (shrink fitting) eases assembly of tight fits. Always check that the resulting hoop stress stays within the material strength, and that the fit transmits the needed torque or thrust.
Fitting a bearing rather than a solid shaft?
Rolling bearings use their own recommended fits — see the Bearing Fits Chart for shaft and housing tolerances.
How a press fit holds
When a shaft larger than its hole is forced in, the hole stretches and the shaft compresses. The material pushes back, creating contact pressure and friction that lock the parts together. More interference means more holding force, but also more stress and a harder press, so the fit balances grip against the risk of cracking the hub.
Press fitting vs shrink fitting
A press fit pushes parts together cold with an arbor or hydraulic press. A shrink fit heats the outer part, or cools the inner, so it expands enough to slip on, then grips as temperatures equalize. Shrink fitting suits large interferences that would be hard or damaging to press cold.
FAQ
What is the most common press fit?
For inch work, the FN2 medium drive fit; for metric work, H7/s6. Both suit ordinary steel parts and give a firm, permanent joint.
How much interference does a press fit need?
It depends on diameter, material, and the load to transmit, typically a few thousandths of an inch. The interference grows with shaft size and is set by the chosen fit class.
Can a press fit be taken apart?
Light fits can be pressed back out, often with surface damage. Heavy and force fits are effectively permanent and usually need heating or machining to separate.