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Press Fit Calculator

Interference fit calculator: contact pressure, press-in force, torque capacity, hub stress and shrink-fit temperature. Interference comes from verified ISO 286 tables or your own numbers.

Default shown: Ø25 H7/s6 steel shaft in a steel hub (outer Ø50, L = 30 mm). Assumptions: elastic behaviour, plane stress, uniform contact pressure over the engagement length, both parts concentric. Friction μ ≈ 0.05–0.10 for lubricated pressed joints and 0.10–0.20 for dry steel on steel; use the high value to size the press (worst-case press-in force) and the low value when checking torque capacity. Surface smoothing: set Rz values to subtract 0.8·(Rz_shaft + Rz_hub) per DIN 7190 (default 0). Material values are typical handbook numbers; edit them for your alloy and temperature.

How interference comes from the fit

An ISO 286 callout such as Ø25 H7/s6 gives the hole zone H7 (0 to +21 µm at 25 mm) and the shaft zone s6 (+35 to +48 µm). The smallest interference is the smallest shaft minus the largest hole, 35 − 21 = 14 µm, and the largest is the largest shaft minus the smallest hole, 48 µm. Those two numbers are the diametral interference δ at the loose and tight ends of production, and both matter: the minimum decides whether the joint holds, the maximum decides whether the hub survives and how hard the part is to assemble. This calculator reads both from the same verified tables as the ISO 286 fit calculator.

Worked example: Ø25 H7/s6, steel on steel

Solid steel shaft (E = 207 GPa, ν = 0.29), steel hub with outer Ø50 mm, L = 30 mm, μ = 0.15. The Lamé equation gives p = δ / (d · [ (1/E_o)((d_o²+d²)/(d_o²−d²) + ν_o) + (1/E_i)((d²+d_i²)/(d²−d_i²) − ν_i) ]).

• Minimum (δ = 14 µm): p = 43.5 MPa, press-in force F = 15.4 kN, torque T = 192 N·m.
• Maximum (δ = 48 µm): p = 149 MPa, F = 52.7 kN, T = 658 N·m, hub bore hoop stress 248 MPa (von Mises 348 MPa).
• Shrink fit: with 0.025 mm assembly clearance the hub needs ΔT = (0.048 + 0.025) / (11.7×10⁻⁶ × 25) ≈ 250 °C, so it must be at roughly 270 °C.

Note the spread: the same drawing callout can need 15 kN or 53 kN to press together, and can transmit 192 or 658 N·m. Size the press, and check the hub, at the maximum; check holding force and torque at the minimum.

Common press-fit choices: H7/p6, H7/r6, H7/s6

At Ø25 mm the three classic hole-basis choices give: H7/p6 1 to 35 µm (a locational interference fit; the loose end is almost zero, so it positions parts but is not reliable for torque transfer on its own), H7/r6 7 to 41 µm (a medium drive fit for bushings, gears and hubs with light to moderate load) and H7/s6 14 to 48 µm (a permanent press fit for heavy loads on steel). Use H7/p6 where a key or pin carries the torque and the fit only has to locate and prevent rattle.

Heating vs pressing

Pressing is cheap and quick, but it scrapes and burnishes the mating surfaces, can gall, and a misaligned start damages both parts; it also relies on friction so the press-in force is the number to check. Shrink assembly slides the parts together with clearance, keeps full surface contact and does not wear the surfaces, which is why it is used for large or high-torque joints. The cost is temperature control: ΔT above is the bore growth needed, and the hub must stay below the tempering or heat-treatment limit of its material. Cooling the shaft (dry ice is about −78 °C) splits the thermal change between both parts. Aluminium hubs grow about twice as much per degree as steel, so an aluminium hub on a steel shaft needs far less heat but loses interference as it warms in service.

Pitfalls

• Ignoring the maximum. Hub stress and press force occur at maximum interference, holding force at minimum. Check both ends.
• Thin hubs. A hub outer diameter below about 1.5 × d carries high hoop stress; compare the von Mises value with the yield strength of the real heat-treat condition.
• Roughness and finish. A rough surface loses holding force, so specify Rz as well as the fit.
• Differing materials and temperature. Aluminium or bronze hubs on steel shafts change interference in service; the α fields help you check.
• Edge effects. Real pressure peaks near the hub ends and the formula is a first design estimate, not a replacement for FEA or a standard such as DIN 7190 on critical joints.

For choosing the fit class, see ISO 286 fit selection and ISO 286 fits explained; to look up deviations for any size, use the H7 tolerance chart. SuperNX reads ISO 286 fits from NX PMI and checks tolerance chains automatically, so a press-fit callout on your model feeds the stack-up without retyping.