Design Nomenclature / Systems & Spec
Fit class
Also called fit, ISO fit, H7/g6, running and sliding fit, RC fit
A standardised pairing of hole and shaft tolerance zones — H7/g6, RC4 — producing a known relationship without designing limits from first principles. Three families: clearance, where a gap is guaranteed; interference, where overlap is guaranteed; and transition, where either may result depending on where the parts land in their bands.
In practice
Called out by code as though the code were self-explanatory. H7/g6 means nothing until it is resolved against the size — the same class yields different numbers at 10 mm and 100 mm.
Not to be confused with
- Clearance fitA fit in which the hole exceeds the shaft across the whole of both tolerance ranges, so the parts always assemble with a gap — guaranteed by the limits rather than by luck. The size of that gap is the design decision: a running fit needs room for lubricant and thermal growth, a location fit wants the least gap that still slides.
- Interference fitA specified fit where the shaft exceeds the hole across the whole of both tolerance ranges, so the two must be forced, heated or chilled together. The metal deforms elastically and grips permanently — this is how bearings are retained. It is a number on a drawing, not a feel, and a wrong number destroys both parts on assembly.
- AllowanceThe intended difference between mating parts: the tightest condition the limits permit, computed from the largest shaft against the smallest hole. It is chosen deliberately, before tolerances are assigned. A positive allowance guarantees clearance; a negative one guarantees interference.