Clearance, transition and interference fits: a selection guide

What a fit actually is

A fit describes the relationship between the size of a hole and the size of a shaft that assemble together. It is calculated from the limits, not from the nominal size. Two parts can share the same nominal diameter and still produce a clearance, a transition or an interference depending on where their actual sizes sit within the limits.

The clearance is the hole size minus the shaft size. A positive value is a gap. A negative value is an overlap, which is an interference. The maximum and minimum clearance for a pair of limits define the fit. That range is what the designer should check against the function.

A fit is not a guarantee of function on its own. Roundness, straightness, taper, surface finish, temperature and assembly method all change the result. The size limits are the starting point, not the whole design.

The ISO 286 language

ISO 286 defines the standard tolerance grades, usually called IT grades, and the fundamental deviations for holes and shafts. A fit is written as a hole tolerance followed by a shaft tolerance, for example H7/g6. The capital letter is the hole, the lowercase letter is the shaft and the number is the tolerance grade.

The letter sets the position of the tolerance zone relative to the nominal size. The number sets its width. H is the hole basis, where the lower limit is the nominal size. h is the shaft basis, where the upper limit is the nominal size.

ISO 286-1 gives the bases and the standard tolerances. ISO 286-2 gives the tables of limit deviations. The tables should be read from the standard for the actual size range, not from a remembered value. A fit that is correct at 10 mm may not be correct at 100 mm.

Clearance fits

A clearance fit has a gap at every permitted combination of hole and shaft. The minimum clearance is zero or greater. The parts assemble by hand or with light pressure, and the joint may slide, rotate or float depending on the clearance.

Clearance fits are used for sliding bearings, guide pins, shafts in bushes, covers located by spigots and fasteners that need to pass through a hole. The clearance must be large enough for the assembly and the thermal expansion, and small enough for the location accuracy the function needs.

A running clearance is not the same as a location clearance. A shaft that must rotate needs oil film or a defined bushing clearance. A pin that must locate needs a close fit with a short engagement. Use the function to set the allowance, then choose the tolerance grade.

Transition fits

A transition fit can produce a small clearance or a small interference depending on the actual sizes. It is used where the part must be located accurately but still be assembleable with light pressure or a soft mallet. A dowel in a reamed hole is a common example.

The uncertainty is the point and the risk. A transition fit gives good location when the parts land near the middle of the limits, but the assembly force can vary from batch to batch. If the joint must transmit torque or resist vibration, a transition fit alone may not be enough. Add a retaining feature or specify an interference.

Transition fits are sensitive to the actual size distribution, not just the limits. A supplier whose process runs near one limit will produce a different mix of clearances and interferences from a supplier running near the other. Ask for the process capability, not only the tolerance.

Interference fits

An interference fit has an overlap at every permitted combination. The shaft is larger than the hole before assembly, and the parts are joined by pressing, heating the hole, cooling the shaft or a combination. The resulting contact pressure can transmit torque and resist movement without a key or a pin.

The design has to check three things: the assembly force or the shrink temperature, the stress in the hub at the maximum interference, and the retention at the minimum interference. A fit that is tight enough to hold may also be tight enough to crack a thin hub or distort a bearing race.

Interference fits are common for bearing rings, bushes, valve seats, gear hubs and stator laminations. The surface finish matters because the peaks flatten during assembly, which reduces the effective interference. A turned surface and a ground surface with the same nominal size can behave differently.

A worked illustrative example

Assume a hole with limits of 20.00 to 20.05 mm and a shaft with limits of 19.96 to 19.98 mm. The minimum clearance is 20.00 minus 19.98, or 0.02 mm. The maximum clearance is 20.05 minus 19.96, or 0.09 mm. Because both values are positive, this is a clearance fit at every permitted combination.

Now assume a hole of 20.00 to 20.02 mm and a shaft of 20.03 to 20.05 mm. The minimum interference is 20.03 minus 20.02, or 0.01 mm. The maximum interference is 20.05 minus 20.00, or 0.05 mm. Because the minimum clearance is negative at every combination, this is an interference fit.

A transition fit sits between the two. A hole of 20.00 to 20.03 mm with a shaft of 20.01 to 20.04 mm can produce 0.02 mm of clearance at one combination and 0.04 mm of interference at another. The numbers are illustrative only. The method is the part to reuse.

Temperature, coating and geometry

Temperature changes the fit. If the housing and the shaft are different materials, they expand by different amounts. An aluminium housing with a steel shaft can lose interference at operating temperature, while a plastic housing can creep and lose preload over time. Check the fit at the operating temperature, not only at 20 degrees Celsius.

Plating adds thickness to both surfaces. A tin or zinc coating on a shaft reduces the clearance or increases the interference by roughly twice the coating thickness, depending on which surfaces are coated. Thin coatings can also flake or smear during pressing. State the finish and the thickness on the drawing, and say whether the size limits apply before or after plating.

Geometry matters as much as size. A bore that is round at the ends but oval in the middle, or a shaft with a taper, will not produce the contact the size calculation assumes. Use roundness, straightness or profile controls where the fit depends on full contact. A fit calculation based on perfect cylinders is only a starting point.

Specifying and inspecting a fit

Choose the basis first. A hole-basis system is usually cheaper because one reamer or one bore tool can serve several shaft sizes. A shaft-basis system is useful when the shaft is a standard ground bar or when the same shaft must mate with different holes.

Then choose the fit from the function: clearance for movement, transition for location, interference for retention. Pick the tolerance grade from the process capability and the sensitivity of the assembly. A tight grade on a long bore is harder to hold than the same grade on a short one.

Inspection should measure both parts with the same method and at the same temperature. Use a bore gauge or an internal micrometer for the hole, and a micrometer or a comparator for the shaft. For an interference fit, record the actual sizes, not only pass or fail, so the assembly force can be correlated with the interference.

• State the fit designation and the standard on the drawing.
• State whether limits apply before or after coating.
• Check the fit at the operating temperature.
• Add roundness or straightness where contact is critical.
• Record actual sizes for interference fits.
• Agree the assembly method and the press force range.