What is the difference between clearance, transition and interference fits?

How does ISO 286 decide which kind of fit you get?

The class of fit follows from comparing the limits of the two parts: if the largest shaft is no bigger than the smallest hole the fit is clearance, if the smallest shaft is no smaller than the largest hole it is interference, and if the two tolerance zones overlap it is transition.

Each side of the fit is written as a tolerance class, such as H7 for a hole or g6 for a shaft. The letter is the fundamental deviation: it places the tolerance zone relative to the nominal size. Capital letters are holes and lowercase letters are shafts. An H hole starts exactly at the nominal size and grows upward. An h shaft ends exactly at the nominal size and shrinks downward. Shafts from a to g sit below nominal, which leaves a gap against an H hole, while shafts from p onward sit above the top of the hole, which forces an overlap. The letters between them, js, k, m and n, straddle the hole zone and give transition fits.

The number is the standard tolerance grade, the IT grade. It sets the width of the zone, and the width grows with both the grade number and the nominal size. A 7 is wider than a 6, and a 25 mm diameter gets a wider IT7 band than a 10 mm diameter, per ISO 286-1. That is why a fit chart always needs a size column: H7/g6 at one diameter does not give the same clearance as H7/g6 at another.

Most drawings use the hole-basis system, where the hole is always H and the shaft letter selects the fit. The shaft-basis system, with an h shaft and a varying hole, makes sense when one bar-stock shaft carries several parts with different fits.

When should you choose a clearance fit?

Choose a clearance fit whenever the parts must slide, rotate or come apart by hand, and pick the tightest one that still leaves room for oil, heat and misalignment. Clearance that is larger than the function needs only adds play; clearance that is too small makes the assembly seize.

• H11/c11 and H9/d9 suit pivots, hinge pins, linkages and parts that see dirt, paint or large temperature swings, where generous clearance is the point.
• H8/f7 is the usual starting point for a plain bearing running with lubrication at moderate speed.
• H7/g6 fits parts that slide or turn slowly and must still locate well, such as guide pins, spigots and indexing plungers.
• H7/h6 is for parts that are located and then clamped, where the fit only has to let them go together without force.

Temperature is the hidden variable. ISO dimensions are defined at a reference temperature of 20 °C per ISO 1:2022, and a shaft and hub in different materials expand at different rates. An aluminum housing around a steel shaft gains clearance as it warms; a steel housing around an aluminum or bronze bushing loses it. If the assembly runs hot or cold, check the chart values at the operating temperature, not only on the inspection bench.

When does a part need an interference fit?

Use an interference fit when the joint itself must hold the parts together, transmitting torque or axial load, or keeping a bushing or bearing ring from creeping, without separate fasteners. The parts are assembled by pressing, by heating the hub, or by cooling the shaft.

H7/p6 is the light end of the range: it locates rigidly and resists movement, but it is not meant to carry much load through friction alone. H7/s6 and the heavier u-series shafts produce enough grip to transmit torque, and the stress they create in the hub grows with the interference. That makes the hub wall the limiting feature. A thin boss, a die-cast aluminum or zinc housing, or a hub with a cross hole near the bore can crack or relax under a heavy press, so the fit and the wall thickness have to be designed together.

Interference fits also change the parts they join: a pressed bushing closes up in its bore, so if its inside diameter is itself a fit, size it after pressing or dimension it as an assembled size. Rolling bearing seats are chosen from the bearing maker’s tables rather than from the general chart above.

How should a fit be called out on a drawing sent overseas?

Write the ISO 286 tolerance class and the numeric limits together, name the mating part, and say whether the limits apply before or after finishing. An ISO designation reads the same in a shop in Vietnam, Mexico or Ohio, but the numbers remove any doubt about the edition or table used.

• Class plus limits. A callout such as “Ø20 g6” followed by the computed upper and lower limits lets the machinist work from numbers and the reviewer check the intent.
• Tolerance only where parts mate. Leave every other diameter under the general note, per the ISO 2768 and NADCA tolerance guide.
• Gauging method. Say whether a fit bore is accepted with a GO/NO-GO plug gauge or with a measured diameter, and whether a measured report is needed on the first article.
• Finishing sequence. Say whether the fit applies before or after coating, and whether the fit surface is masked.
• Assembly method. For interference fits, say whether the parts will be pressed or shrunk, and whether the supplier or your own line does it.

Fit diameters are among the features most worth flagging in an RFQ, because a turned diameter held to an IT6 band takes more process control than a general dimension. Diameters tighter than a shop’s routine tolerance are reviewed feature by feature; the CNC turning page describes the process. The RFQ package guide lists what else to send, and a complete package can go straight to a quote request.