Clearance and interference

The calculation, in two subtractions

An assembly is judged at its two extremes, never on nominal sizes. Maximum clearance appears when the hole comes out at its largest and the shaft at its smallest: it's the upper deviation of the hole minus the lower deviation of the shaft. Minimum clearance appears the other way round.

If both results are positive, there's always clearance. If both are negative, there's always interference. And if they have opposite signs, the fit is a transition: depending on which parts actually come off the line, some will slide and some will grip.

Why transition fits exist, and why they're useful

A transition fit looks like a design defect. It rarely is. H7/k6 and H7/m6 are made for it: you want precise location with no perceptible play, and the part is held some other way, by a key or a shoulder. It goes together with a mallet, and comes apart the same way.

What you must not do is rely on a transition fit to transmit torque. The parts at the loose end of the range will have no grip at all, and the assembly will spin.

The hole-basis system

In almost every case the hole stays at H and it's the shaft that changes position according to the fit wanted. The reason is economic: a hole is made with a dedicated tool, a reamer or a set boring bar, whereas a shaft is ground to whatever size is needed without changing tooling. Keeping the hole constant limits the tools you have to own.

This is why you'll nearly always meet H7 with something. The shaft-basis system, where the shaft stays at h, is only used in particular cases, typically on drawn bar or on commercial shafting that is already to size.

One example, carried through

A ⌀40 H7/g6. The H7 hole gives 0 to +25 microns. The g6 shaft gives −9 to −25 microns. Minimum clearance is 0 − (−9) = 9 microns, maximum clearance 25 − (−25) = 50 microns. The assembly slides in every case, with a clearance between one and five hundredths: the classic rotating guide.

The same ⌀40 in H7/p6 gives interference between 1 and 42 microns. The parts no longer go together by hand: you need a press, or you expand the hole with heat.

Frequently asked questions

What is the difference between clearance and interference?

There's clearance when the hole is always larger than the shaft: the parts slide. There's interference when the shaft is always larger than the hole: you have to press or heat to assemble them.

What is a transition fit?

A pair where, depending on the parts actually produced within their tolerances, you get either a small clearance or a small interference. H7/k6 and H7/m6 are of this kind.

Which fit for a bearing?

It depends on which ring rotates. A rotating inner ring is mounted tight on the shaft, typically k6 or m6. Always check the manufacturer's recommendation, which takes precedence.

Why the hole-basis system?

Because a hole costs more to adjust than a shaft. So the hole is kept at H and the shaft is varied, which limits the number of drilling and reaming tools you have to own.

For the deviations of a single fit taken on its own, see the ISO 286 calculator.