Press-fit interference calculator
A bushing is held in its housing by interference: the bushing is slightly larger than the bore it is pressed into. Enter the two diameters to get the interference (or clearance), how large it is relative to the diameter, and, for a thin-walled bushing, how much the bore closes after fitting.
Enter the diameters at the same temperature and in the same unit. For a bushing, the "hole" is the housing bore and the "shaft" is the bushing outside diameter. Add the bushing bore to see the wall thickness and the estimated installed bore.
Interference formula and fit types
Diametral interference is the difference between the two diameters: Δ = D_shaft − D_hole. A positive Δ is an interference (press) fit, a negative Δ a clearance fit, and Δ near zero a transition or line-to-line fit. The radial interference is half the diametral value and is what the bushing wall actually has to absorb.
The tool also gives Δ per mille of the hole diameter (‰). Plain bushings are normally pressed with an interference of roughly 0.5 to 2 ‰ of the outside diameter; the exact value comes from the housing tolerance (usually H7) and the bushing outside-diameter tolerance given in each product's size table.
| Δ = D_shaft − D_hole | Fit | Typical use |
|---|---|---|
| Δ > 0 | Interference (press) fit | Bushing in housing; gear or bearing inner ring on shaft |
| Δ ≈ 0 | Transition / line-to-line | Location without heavy pressing; dowels |
| Δ < 0 | Clearance fit | Shaft running in a bushing bore |
What the interference does to a bushing bore
Thin-walled bushings such as wrapped bronze, steel-backed bimetal and PTFE or POM composite take the shape of the housing when pressed. Their bore closes by approximately the diametral interference, which is why these constructions publish the bore tolerance after fitting into an H7 housing, not before. The calculator subtracts the interference from the free-state bore to show the estimated installed bore; the running clearance to the shaft is then set from that value.
Thick-walled cast bronze and hardened steel bushings deform much less, and their installed bore is often finish-machined after pressing in heavy-duty applications. Treat the estimate as an upper bound for closure in that case.
Put the housing bore, its tolerance and the housing material on the enquiry. The interference we recommend depends on all three.
Worked example
A wrapped bronze bushing with a free outside diameter of 30.080 mm is pressed into a housing bore measured at 30.000 mm. Δ = +0.080 mm (80 µm), radial 0.040 mm, 2.67 ‰ of the bore. If the free bore is 25.130 mm, the installed bore is approximately 25.050 mm, and a 25.000 mm shaft then runs with about 0.050 mm clearance.
Measuring for a press fit
A press-fit calculation is only as good as the two diameters that go into it.
- Measure the housing bore at two positions along its depth and two angles; use a bore gauge, not a caliper, for anything under 0.05 mm.
- Measure the bushing outside diameter in the free state; a split wrapped bushing must not be squeezed shut while measuring.
- Measure both at room temperature. Aluminium housings grow about twice as fast as steel with temperature and can lose the fit when hot.
- Record the housing material: the same interference holds far better in steel than in aluminium or cast iron.
Frequently asked questions
What interference should a bushing have?
Enough to keep it from turning or walking under load and temperature, and no more. For plain bushings the outside-diameter tolerance in the product size table combined with an H7 housing gives the intended interference; typical values are 0.5 to 2 ‰ of the outside diameter. For a specific case send the housing bore and material.
Does the calculator account for temperature or material?
No. It returns the geometric interference at the temperature you measured. Thermal expansion and the elasticity of the housing and bushing determine the holding force, which is an engineering assessment rather than a subtraction.
Why is the installed bore only an estimate?
Because how much a bushing closes depends on its wall thickness, material and whether it is split. Thin-walled wrapped and composite bushings close by close to the full interference; thick cast bronze and steel bushings close less.
Can I use it for a shaft in a bearing?
Yes. Enter the bearing bore as the hole and the shaft diameter as the shaft; a negative result is the running clearance.
Not sure about the fit? Send the housing.
Housing bore, tolerance and material, plus the shaft diameter: our engineers return the bushing outside diameter, bore tolerance and running clearance with the quotation.