A press fit (also called an interference fit) joins two parts by making the inner part slightly larger than the hole it goes into, then forcing them together. The resulting elastic deformation creates contact pressure that holds the joint via friction — no bolts, keys, or welds required. Bearings on shafts, dowel pins in housings, and wheels on railroad axles all rely on this principle.

Engineers specify fits using standardized tolerance classes (ISO 286 or ANSI B4.1). Common categories:

• Location fit (H7/p6): Light interference, assembled with an arbor press. Used for dowel pins and locating bushings.
• Medium drive fit (H7/s6): Moderate interference, may need heating or cooling. Used for gears and pulleys on shafts.
• Force fit (H7/u6): Heavy interference, almost always thermally assisted. Used for permanent assemblies like railway wheels.

A shrink fit is the same idea but assembled by heating the outer part (or chilling the inner) so it slips on freely, then locks as temperatures equalize. Heating to 200°C expands a steel ring by roughly 0.0023 mm per mm of diameter — enough to clear a several-thousandths interference on a 50 mm bore.

Quick calculation — interference and contact pressure: For a steel shaft (50 mm dia) in a steel hub with 0.05 mm diametral interference, the contact pressure from Lamé's equation is roughly:

p ≈ E × δ / (2 × d), where δ is diametral interference and d is nominal diameter (simplified for solid shaft, thick hub).

p ≈ 200,000 MPa × 0.05 / (2 × 50) = ~100 MPa contact pressure. With a coefficient of friction of 0.15 and a 40 mm engagement length, the joint resists roughly 94 kN of axial force or 2,350 N·m of torque before slipping.

Real-world example: Railroad wheelsets. A steel wheel is heated to ~250°C and dropped onto a chilled axle with about 0.2 mm interference on a 200 mm journal. As the wheel cools, it contracts onto the axle with hundreds of MPa of contact pressure — strong enough to transmit braking and traction forces for decades without any keyway.

Design rules of thumb:

• Engagement length should be at least 1× to 1.5× the shaft diameter for full torque capacity.
• Hub outer diameter must be at least 2× shaft diameter, or the hoop stress will yield the hub.
• Always check that contact pressure stays below the yield strength of the weaker material.
• Disassembly damages the surface — treat press fits as semi-permanent.