When two parts mate (like a shaft in a hole), the fit describes how loosely or tightly they go together, and it’s created by the tolerances on each part. There are three families.
| Fit | Result | Example use |
|---|---|---|
| Clearance | Always a gap — slides freely | Rotating shafts, sliding parts |
| Transition | May be slightly loose or tight | Locating parts that must be removable |
| Interference (press) | Always tight — must be pressed | Permanent joints, bearing races |
How tolerances create fits
Each part has an allowed size range. If the hole is always larger than the shaft, you get clearance; if the shaft is always larger, interference; overlap of the ranges gives a transition fit. Standard systems (like ISO hole-basis fits) define these with code letters and numbers so designers specify a fit consistently. Match the fit to function, and don’t over-specify — tighter fits cost more to machine.
Frequently asked questions
What’s a clearance fit? One with a guaranteed gap, so parts slide or rotate freely.
What’s an interference fit? A tight fit that must be pressed together and holds by friction.
How do tolerances set the fit? The overlap (or gap) between the hole and shaft size ranges determines it.