Types of fit

A fit describes the relationship between the size limits of mating holes and shafts. A clearance fit retains clearance at the limiting conditions. An interference fit has overlap and may require force or thermal assembly. A transition fit can result in either clearance or interference depending on the actual limits. Select the fit based on load, movement, lubrication, disassembly, materials and manufacturing process.

Calculating from limit sizes

Let Dmin and Dmax be the hole limits and dmin and dmax the shaft limits. Maximum clearance Cmax = Dmax − dmin; minimum clearance Cmin = Dmin − dmax. If Cmin is at least zero, the fit is clearance; if Cmax is below zero, it is interference; otherwise it is transition. In ISO systems, the letter indicates fundamental deviation and the grade indicates tolerance width. Check the standard and nominal size on the drawing.