Interference Fit

An interference fit is a mating condition where the shaft or male part is intentionally larger than the hole or bore over the full tolerance range, so assembly requires force, heating/cooling, or pressing; once assembled, the overlap creates contact pressure and friction that resists axial and rotational movement. Typically used for bearings, gears, and bushings where zero clearance and load transfer via friction is required.

In CNC machining, the bore and shaft are programmed to a controlled fit class so manufactured dimensions land in a planned interference window. The assembly method uses an arbor press, hydraulic press, thermal expansion of the housing, or chilled shafts to manage insertion force. Proper interference fit prevents rotating components from fretting, walking out, or spinning under torque, vibration, or axial load. The joint works because elastic deformation generates normal force, increasing friction and retention. On the shop floor, the machinist measures maximum hole size and minimum shaft size against the fit callout to confirm designed interference exists at worst-case tolerance stack. If interference is light, parts are press-fitted; if heavier, thermal methods are used. Quality checks ensure retention force without cracking the hub, galling surfaces, or distorting components. ISO fit systems like H7/p6 define controlled interference conditions.

• ⚠️Undersized bore from tool wear: The hole comes in too small, causing spike in press force and risk of cracking or galling because actual interference exceeds material limits.
• ⚠️Excessive interference selection: Assembly requires extreme force or heat, distorting parts and shifting clearance, leading to residual stress and eventual loosening or failure.
• ⚠️Surface finish or material mismatch: Roughness or incompatible materials cause scuffing during insertion and reduce post-assembly frictional hold.