Tolerances and Fits

Tolerances and Fits

Nothing can be made to an exact size. A tolerance states how much variation is acceptable, and it is what makes mass production possible: any shaft within tolerance fits any hole within tolerance, without hand-fitting.

Terms. The nominal size is the size the designer names, e.g. Ø25. Limits are the largest and smallest permitted sizes. The tolerance is the difference between them. Allowance is the intended clearance or interference between the mating parts.

Three classes of fit:

• Clearance fit — the shaft is always smaller than the hole, so it always assembles and can rotate or slide.
• Interference fit — the shaft is always larger than the hole, so it must be pressed or shrunk in and will not come apart in service.
• Transition fit — the tolerances overlap, so a given pair may end up either slightly clear or slightly tight. Used for accurate location where the part is also removable.

Worked example. A hole 25.00–25.02 with a shaft 24.98–24.99 always leaves 0.01–0.04 mm of clearance: a clearance fit. Widen the shaft to 25.01–25.03 and some pairs are clear while others interfere — a transition fit, and if you expected clearance, some assemblies will not go together.

Core checkpoint: Tighter tolerances cost more. Specify the widest tolerance the function actually allows, not the tightest you can write.