In an interference (press or shrink) fit the shaft is deliberately made larger than the hole it enters — the diametral interference is the difference between shaft and hole diameters, the quantity ISO 286 limits combine into and the one machinists measure. Standard hole-basis classes ladder up in tightness: H7/k6 and H7/n6 are transition fits that can assemble with clearance at one end of tolerance, H7/p6 is the classic light press fit, H7/s6 a medium drive fit for permanent joints, and H7/u6 a heavy force or shrink fit for maximum holding power.
The mechanics run through Lamé thick-cylinder theory: interference divided by the joint’s elastic compliance gives the interface pressure p, and friction turns pressure into holding capability — transmissible torque T = μ·p·π·d²·L/2 and axial force F = μ·p·π·d·L. A 50 mm steel joint at 50 µm of interference develops 75 MPa at the interface and holds about 1,414 N·m. Tolerance stack sets the design discipline: capacity is guaranteed only at the minimum interference, while hub hoop stress and press force must be checked at the maximum, where the joint is tightest.
Assembly can be force or heat. An arbor press must overcome the friction force; alternatively, heating the hub by ΔT = δ/(α·d) expands its bore by exactly the interference so the parts slip together with zero force — about 87 °C for that 50 mm joint, with real shop practice heating further to leave sliding clearance, or cooling the shaft with dry ice or liquid nitrogen instead.