GlossaryKind of term: Manufacturing termInterference fit
What is an interference fit?
An interference fit is a fit in which the shaft is larger than the hole at every permitted size, so the joint is held by that interference.
The condition holds at the limits: even the smallest permitted shaft is larger than the largest permitted hole. At ⌀20 H7/s6 the interference lies between 0.014 and 0.048 mm, at H7/p6 between 0.001 and 0.035 mm. The parts are joined by pressing them together, or by heating the hub or cooling the shaft; that second form is called a shrink fit. The joint holds a bush, pin, bearing or hub in place and can carry torque or axial load without a key or adhesive.
What does an interference fit depend on?
What an interference fit can carry depends on the actual interference, the diameter and length of the joint, the wall thickness of the hub, the materials and their stiffness, the friction between the surfaces and the operating temperature. During joining the roughness peaks are partly flattened, so the effective interference is smaller than the measured sizes suggest. After assembly the hub expands and a bush or bearing ring closes in at the bore, which can take a tightly toleranced inside diameter out of tolerance.
What does the standard say about an interference fit?
The tolerance classes come from ISO 286-1 and ISO 286-2; whether a combination is an interference fit follows from the limit deviations at the nominal size. ISO 286 does not calculate whether the joint is strong enough. Separate calculation methods exist for that, such as DIN 7190-1 for cylindrical interference fits, which also account for the flattening of roughness and for temperature.
How does an interference fit work in practice?
An interference fit needs two tight sizes that belong together, for example a reamed or bored H7 hole and a finish-turned or ground shaft at p6 or s6, with suitable roughness on both surfaces. If you assemble yourself, you add pressing or heating, alignment and a check after assembly, and sometimes a finishing operation on the bore. If you deliver loose parts, assembly sits with the customer and your share is the two sizes and their measurement.
What is an interference fit often confused with?
An interference fit is not a transition fit: at H7/k6 a pair can end up with clearance or interference, depending on where both sizes fall. Pressing and shrinking are not different fits but different ways of assembling. And an interference fit is not a bonded joint or one secured with retaining compound: the interference holds the part, not a compound between the surfaces.
Frequently asked questions
What is the difference between a press fit and a shrink fit?
The same interference, assembled differently. A press fit forces the shaft into the hole. A shrink fit heats the hub or cools the shaft, so the parts go together with little force and lock as the temperatures even out.
Should I price the assembly of an interference fit?
If you join the parts yourself, yes: a pressing or heating step, alignment and a check after assembly, sometimes with a finishing operation on the bore. If you deliver loose parts, you price the two sizes and their measurement.
Why can a bush be off size after pressing it in?
Because the interference compresses the bush and its bore gets smaller. How much depends on the interference, wall thickness and materials. If the bore is tightly toleranced, it is often reamed or bored again after pressing.