Press fit design guide: interference, tolerances and materials

Press fitting is an interference-fit assembly method where an inner part is forced into a slightly smaller hole. The elastic deformation between the mating parts creates radial contact pressure, holding the joint together by friction alone without separate fasteners.

Shops assemble force fits mechanically using an arbor press for moderate loads or a hydraulic press for high-interference joints. Insertion forces on a press typically range from 500 to over 20,000 pounds. When a joint requires higher interference or you must avoid axial insertion stress, shops use a shrink fit instead. In a shrink fit, the outer part is heated to 150 to 300 °C or the inner part is cooled in liquid nitrogen to about -196 °C so the pieces slip together with clearance and lock as they return to ambient temperature.

Use a press fit when a round part must sit securely in a bore without rotating or slipping axially, and without adding bolts, screws, keyways, or adhesives. Typical applications include:

• Bearing races pressed into housings or onto shafts so the race cannot creep and fret
• Gears mounted on shafts to transmit torque without a keyway or spline
• Rotor cores, commutators, and impellers on motor shafts
• Alignment dowels and cylindrical bushings

Match the fit grade to the load and maintenance needs. Use a light press fit for thin-walled or brittle parts, a drive fit (assembled with a mallet or light press) if the joint must occasionally come apart without damage, and a force fit or shrink fit for permanent joints under heavy axial or rotational loads.

Do not use a transition fit (where the shaft-to-bore difference is roughly -0.001 to +0.001 in) if you need guaranteed retention or torque transfer. Transition fits do not create interference in every mated pair and belong only in locating applications. If an assembly must come apart routinely, use threaded fasteners; if you are joining plastic housings, use snap fits.

Interference controls holding force, but more interference also raises the risk of yielding ductile bores or cracking brittle ones. Calculate the minimum interference needed for your applied load, and use the Lamé thick-walled cylinder equation to keep contact stress below 70% to 80% of the material's yield strength.

Published guidance from manufacturing services such as Xometry groups press fits into general interference ranges (confirm specific capabilities with your shop):

• Light press fit: 0.0005 to 0.001 in (0.013 to 0.025 mm) for thin-walled or brittle parts
• Drive fit: 0.001 to 0.002 in (0.025 to 0.051 mm) for occasional disassembly
• Force fit: 0.003 to 0.005 in (0.076 to 0.127 mm) for continuous heavy loads
• Heavy press fit: above 0.005 in (0.127 mm) on large diameters for permanent structural joints

For permanent joints, select an interference whose axial retention exceeds the maximum pull-out load by a safety factor of 1.5 to 2.5. For joints carrying torque, torque capacity equals the total radial normal force times the shaft radius times the coefficient of friction (0.1 to 0.2 for steel on steel). A 0.002 in steel-on-steel interference fit generates 10,000 to 30,000 psi of contact pressure. For example, 20,000 psi of contact pressure across a 2 in engagement length on a 1 in steel shaft with a 0.15 friction coefficient carries over 300 N·m of torque.

Because retention comes entirely from elastic deformation, press fits depend on strict tolerance control. With proper tolerances, a press-fit assembly can hold a concentricity of 0.001 to 0.003 in (0.025 to 0.076 mm).

Rather than calling out ad hoc dimensional limits on your drawing, specify interference joints using a standard limits-and-fits system such as ASME B4.2 or ISO equivalents. For example, calling out a hole-basis locational interference fit like H7/p6 gives a standard interference of 0.001 to 0.004 in (0.025 to 0.102 mm) depending on diameter. Never substitute one class for another on the shop floor; switching between H7/p6 and H7/r6 shifts interference by 0.0002 to 0.0005 in, which is enough to turn a working joint into a cracked hub or a loose shaft.

Rolling bearings require special care. Bearing races normally take 0.0005 to 0.002 in (0.013 to 0.051 mm) of interference, which also reduces the bearing's internal radial clearance. Give the interference fit to whichever ring rotates relative to the load direction, and give the stationary ring a looser fit. For a rotating shaft under a fixed-direction load, press-fit the inner ring on the shaft and leave the outer ring looser in the housing. Check the bearing manufacturer's fit tables for exact tolerance classes.

How a material responds to a press fit depends on its elastic modulus, yield strength, and ductility. Steel (200 GPa modulus) deforms much less than aluminum (69 GPa modulus) at the same contact pressure, so the two require different interference values to reach the same retention.

• Brittle metals: Cast iron is prone to hoop-stress cracking if interference exceeds 0.003 in (0.076 mm) on bores under 2 in (50.8 mm) in diameter.
• Soft metals: Pure aluminum, copper, and brass with hardness below 70 HRB yield plastically at the bore wall when radial stress passes their compressive yield (about 95 MPa for pure aluminum). Reduce interference, lengthen the engagement, or pick a stiffer alloy.
• Stainless steels: Grades 303, 304, and 316 work-harden and easily gall during pressing unless lubricated. Heat-treated alloy steels with surface hardness above 60 HRC resist galling and tolerate higher contact pressures.
• Engineering plastics: Nylon, PEEK, and Delrin have a low modulus (1 to 4 GPa) and require larger interference than metals. Because plastics creep under sustained stress, they lose retention over time and belong only in low-load, corrosion-sensitive joints.

Always check interference across the full operating temperature range when joining dissimilar metals. Aluminum alloys like 6061-T6 and 7075-T6 expand at about 23 µm/m·°C, roughly twice the rate of steel at 11 µm/m·°C. If a 50 mm steel shaft sits in an aluminum housing, a 100 °C temperature rise causes about 0.06 mm (0.0024 in) of differential expansion, enough to completely lose a light press fit. Conversely, cooling that assembly increases interference and drives hoop stress toward yield.

Both excessive and insufficient interference cause premature failure. Too much interference pushes ductile steel or aluminum beyond its elastic limit, permanently deforming the bore so parts cannot be separated without damage, and causes surface asperities to weld and gall. Too little interference allows micro-slip under cyclic loads; this triggers fretting fatigue, which cuts the endurance limit of steel parts by 30% to 60%.

Follow good shop practices during assembly:

• Lubricate pressed joints: Applying lubricant during axial pressing lowers insertion force and prevents galling without significantly cutting final retention, since the lubricant is squeezed out once seated.
• Keep tooling aligned and surfaces clean: Misaligned press tools introduce bending stress into the shaft and score or distort the bore.
• Press on the tight bearing ring only: When mounting a bearing, apply force through a sleeve directly against the face of the ring that has the interference fit. Pressing on the loose ring routes force through the rolling elements and dents the raceways before the bearing ever runs.