A press fit (or shrink fit) holds two parts together with nothing but interference — the shaft is slightly larger than the hole, and the elastic squeeze creates a contact pressure that transmits torque and thrust through friction alone. No key, no fastener. Get the interference right and it's elegant; get it wrong and you either can't assemble it or it slips.
How it works
Force an oversize shaft into a hole and the shaft is compressed while the hub is stretched. That mutual interference δ produces a radial contact pressure p at the interface. The Press/Shrink Fit tool solves the Lamé thick-wall equations for p from the interference, the materials and the diameters.
From pressure to holding capacity
The pressure acts over the contact area and friction does the rest. The joint can transmit an axial force
F ≈ p·π·D·L·µ and a torque T ≈ F·D/2, where µ is the friction coefficient (~0.1–0.15
for clean steel, higher for shrink fits). More interference or more engagement length means more holding — up to
the stress limit.
The design trade-off
Interference is bounded on both sides. Too little and manufacturing tolerances could leave a loose fit that slips; too much and the hoop stress at the hub bore exceeds yield, or the assembly force becomes impractical. So size the fit from the minimum interference (it must still hold at worst-case tolerances) and check the maximum against hub yield. That's exactly why the ISO 286 fit tables give interference bands — pick one that keeps both extremes safe.
Shrink fits and assembly
For large interferences, don't press cold — heat the hub (or chill the shaft) so it expands
enough to drop on with clearance, then let it return to temperature and grip. The required temperature comes
straight from thermal expansion: ΔT = δ / (α·D). Shrink assembly avoids the
galling that cold-pressing a big interference can cause, and usually gives a higher friction coefficient too.