Press Fit Calculator — Force, Pressure & Thermal Assembly

Force an oversized shaft into a hub and both parts strain: the shaft compresses, the hub stretches, and the elastic push between them — the contact pressure — is what grips. The Lamé thick-cylinder equations turn an interference into that pressure, and pressure into everything you care about: assembly force = p × π × d × L × µ, torque capacity from the same friction, and the hoop stress trying to split the hub. At Ø25 with 35 µm of interference (the top of H7/p6) and a 50 mm steel hub, the pressure is around 100 N/mm² and the press force roughly 30 kN — three tonnes, which is why this is a hydraulic press job and not a mallet job.

The hub outside diameter matters more than intuition suggests: a thin-walled hub simply stretches and gives the pressure away. The same interference in a hub whose OD is only 1.3× the bore generates a fraction of the grip — and correspondingly less hoop stress — while a massive hub converts nearly all of it to pressure. This is the wall-ratio term (do² + d²)/(do² − d²) in the working, and it is the first thing to check when a press fit design will not hold torque on paper.

Thermal assembly is often the better factory answer: heat the hub until its bore grows past the interference plus a working clearance and it drops on with no force at all — no scored surfaces, no axial stress, full grip on cooling. The temperature follows from the expansion tool’s formula in reverse: ΔT = (δ + clearance) ÷ (α × d). For the Ø25 steel example, about 175 °C above ambient does it — oven or induction territory, well below tempering temperatures for most steels. Aluminium hubs need barely half the rise. The calculator reports this per your materials.