Question 1

How is the contact pressure of a press fit calculated?

Accepted Answer

Lamé thick-cylinder theory gives p = δ / { d·[ (1/Eo)·((Do²+d²)/(Do²−d²) + νo) + (1/Ei)·((d²+Di²)/(d²−Di²) − νi) ] }, where δ is the diametral interference, d the contact diameter, Do the hub outer diameter and Di the shaft bore (0 for a solid shaft). For a solid shaft the shaft term reduces to (1/Ei)·(1 − νi).

Question 2

What is diametral interference δ?

Accepted Answer

δ is the difference in diameters before assembly: the shaft diameter minus the hole diameter. A 20.02 mm shaft in a 20.00 mm hole has δ = 0.02 mm. The contact pressure is proportional to δ, so doubling the interference doubles the pressure (within the elastic range).

Question 3

How much torque can a press fit transmit?

Accepted Answer

The friction grip gives an axial holding (press-out) force F = π·d·L·p·μ, where L is the contact length and μ the friction coefficient. The transmissible torque is T = F·d/2. Both scale directly with the contact pressure and the friction coefficient.

Question 4

Will the hub yield or crack?

Accepted Answer

The largest stress is the tensile hoop stress at the hub bore, σθ = p·(Do²+d²)/(Do²−d²). Compare it against the hub material's yield strength with a safety factor. A thin hub (Do close to d) raises this stress sharply, so increase the outer diameter or reduce the interference if it is too high.

Question 5

Does this calculator account for surface roughness and temperature?

Accepted Answer

No. It is an elastic, room-temperature estimate. Real machined surfaces lose some interference when their peaks flatten during assembly, and a temperature difference (shrink fitting or service heat) changes the effective fit. Plasticity is also ignored. Treat the numbers as a design starting point and verify critical joints.

Press-FitInterference Calculator

Press-Fit
Interference Calculator