Quick Answer: What Is a Press Fit?
A press fit (also called an interference fit) is a fastening method where a shaft is machined slightly larger than its mating hole, creating a permanent joint through elastic deformation when the parts are forced together. The interference generates contact pressure that provides the holding force. Press fits follow ISO 286 tolerance classes and suit bearing installations, gear-to-shaft assemblies, dowel pins and bushings.
This calculator uses Lamé's thick-wall cylinder equations to compute: diametral interference (mm or inches), interface pressure (MPa or psi), required assembly force (N or lbf), and von Mises equivalent stress in the hub to verify it stays below yield strength. Inputs include shaft and hub dimensions, material properties (Young's modulus, Poisson's ratio, yield strength), and ISO 286 fit class.
Typical press fit interference values: Light press (H7/p6): 0.001–0.025 mm | Medium press (H7/r6): 0.015–0.040 mm | Heavy press (H7/s6): 0.025–0.060 mm for a 25 mm nominal diameter.
How Press Fits Work
A press fit holds two parts together with friction. The shaft is slightly bigger than the hole. When you push them together, the hub stretches and the shaft compresses. This creates contact pressure that locks the parts in place.
Interference
Interference is the size difference between the shaft and the hole. More interference means more grip. Too much interference can crack the hub. This calculator shows you the safe range.
Interface Pressure
Lame's equation finds the contact pressure between the shaft and hub. It uses the wall thickness, diameters, and material stiffness of both parts. Stiffer materials create higher pressure for the same interference.
Assembly Force
The force needed to push the shaft into the hub depends on three things: interface pressure, contact area, and friction. A longer press length or higher friction needs more force. Lubrication lowers the friction and makes assembly easier.
Pro tip: Heat the hub or cool the shaft before assembly. A 200°F temperature change on aluminum adds about 0.002 inches of clearance per inch of diameter.
Hoop Stress and Safety Factor
Hoop stress is the tension in the hub wall. It is highest at the bore surface. The safety factor compares this stress to the material yield strength. Keep the safety factor above 2.0 for most applications.