ISO Fits: Hole & Shaft

What do H7 and g6 on a drawing mean in micrometres, and will the shaft slide, locate or have to be pressed in? Choose a nominal size and the tolerance classes of the hole and the shaft to get their limits from the ISO 286 system, the largest and smallest clearance or interference, and the kind of fit. For transition fits it also estimates how often the parts will interfere.

TolerancesISO 286Fits

Inputs

Input examples: ISO preferred fits (hole basis)

A tolerance class is a letter and a grade. The letter sets where the tolerance band lies relative to the nominal size: capital letters for holes, small letters for shafts; H and h start exactly at the nominal size, letters before H/h give clearance and letters after it give more and more interference. The grade (IT1 to IT18) sets the width of the band: 6 and 7 for precise machined fits, 9 to 11 for looser work. Most designs use the hole basis system (hole H, the shaft letter chooses the fit), because holes are made with fixed-size tools such as reamers. Sizes from just above 0 up to 500 mm, following ISO 286-1.

Nominal size — diameter in mm, up to 500

Hole: letter and grade

Shaft: letter and grade

How Does the ISO Fit System Work?

A shaft that should turn in a bore, or a bearing pressed onto a shaft, needs both parts to be made within a few micrometres. ISO 286 gives every size range a set of standard tolerances IT1 to IT18: the higher the grade, the wider the band, and the larger the part, the wider the band for the same grade. A fundamental deviation, coded by the letter, then places the band above or below the nominal size.

The combination of a hole and a shaft is a fit. In a clearance fit the shaft is always smaller than the hole, so the parts slide or turn. In an interference fit the shaft is always larger and has to be pressed in or shrunk on; friction then holds it. A transition fit lies between: depending on the actual sizes, some pairs have a little clearance and some a little interference. It is used to locate parts accurately while still being able to assemble them by hand or with a soft mallet.

The letters are not arbitrary: H7/g6 is the classic sliding fit, H7/k6 a locating transition fit for bearings and gears on shafts, and H7/p6 or H7/s6 press fits. The clearance or interference in micrometres grows with the size, but the character of the fit stays the same.

H / hband starts at the nominal size

a–gshaft below h: clearance

js, k, m, naround zero: transition

p–ushaft above the hole: interference

Tolerance Zones

H7/g6

hole

shaft

zero line: the nominal size

Result

Choose the size and the classes above

Preferred Fits at This Size

Fit	Use	Hole (µm)	Shaft (µm)	Clearance (µm)

Worked Techniques

Width From the Grade

Look up the size range (25 mm lies in “over 18 up to 30”) and read the standard tolerance for each grade.

Example: H7 at Ø25: EI = 0, ES = 0 + 21 = +21 µm, so the hole is 25.000 to 25.021 mm.

Position From the Letter

For shafts a to h the letter fixes the upper deviation es; for j to u it fixes the lower deviation ei. The other limit follows by adding or subtracting the IT value. Holes mirror the shafts, with a small correction Δ for K to U in the fine grades.

Example: g6 at Ø25: 24.980 to 24.993 mm.

Clearance at the Extremes

The largest hole with the smallest shaft gives the largest clearance; the smallest hole with the largest shaft gives the smallest. A negative value is an interference.

Example: H7/p6 at Ø25: C_max = 21 − 22 = −1, C_min = 0 − 35 = −35: interference of 1 to 35 µm.

Sources & Verification

References

ISO 286-1:2010, Geometrical product specifications (GPS) – ISO code system for tolerances on linear sizes – Part 1: Basis of tolerances, deviations and fits — standard tolerance grades (Table 1), fundamental deviations of shafts and holes and the Δ values (Tables 2 and 3).
ISO 286-2:2010, Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts — the tabulated limits used to check this page.
ANSI B4.2-1978 (R2020), Preferred Metric Limits and Fits — the ten preferred hole-basis fits and their descriptions.