Press Fit & Interference Fit Guide
H7/p6. H7/s6.
H7/p6. H7/s6.
H7/u6.
Interference fits, explained.
Complete reference for press fits, shrink fits, and interference fits per ISO 286. How much interference for which application, how to assemble, and how to calculate the torque capacity of the resulting joint.
01 · Fit classes
ISO 286 shaft-basis fit classes.
Hole is H7 (standard), shaft tolerance determines fit class. Lowercase letter = shaft tolerance zone; number = tolerance grade. Progressing from clearance to interference: g → h → k → n → p → s → u → x.
| Fit | Type | Interference @ Ø25 | Application |
|---|---|---|---|
| H7/g6 | Close clearance | +0.007 / +0.020 mm | Precision sliding, indexing pins |
| H7/h6 | Slide fit | 0 / +0.021 mm | Locating parts with no force |
| H7/k6 | Location fit | -0.002 / +0.019 mm | Light location, hand assembly |
| H7/n6 | Transition fit | -0.015 / +0.006 mm | Location requiring light tap |
| H7/p6 | Light press | -0.022 / -0.001 mm | Bearings in housings, removable bushings |
| H7/s6 | Medium press | -0.035 / -0.014 mm | Gears on shafts, permanent assembly |
| H7/u6 | Heavy press | -0.054 / -0.033 mm | Wheel hubs, coupling shafts, forced fits |
| H7/x6 | Very heavy press | -0.074 / -0.053 mm | Structural joints, permanent fits requiring high torque |
02 · Assembly methods
Three ways to assemble an interference fit.
Mechanical press
Standard for small-to-medium
Hydraulic or arbor press forces parts together at room temperature. Requires controlled alignment, lubrication, and press force monitoring to prevent galling.
- Fits: H7/p6 up to H7/s6
- Force: 1–50 tons typical
- Cost: Low equipment
- Risk: Galling on poor finishes
Shrink fit (heat)
Preferred for larger parts
Heat the outer part (hub) to 150–300 °C to expand. Assembly is effortless while hot. As part cools, interference establishes permanently.
- Fits: H7/s6 through H7/u6
- Temp: 150–300 °C
- Cost: Induction heater or oven
- Risk: Heat damage to heat-treated parts
Cryogenic fit (cold)
For heat-sensitive assemblies
Cool the inner shaft in liquid nitrogen (-196 °C) to contract ~0.5%. Heat-sensitive mating part stays at room temperature. Assembly while shaft is cold.
- Fits: H7/s6 and H7/u6
- Temp: -196 °C
- Cost: LN2 availability
- Risk: Minimal — no heat damage
FAQ
Press fit questions.
Clearance fit: shaft is always smaller than hole — parts slide together with gap. Used for rotating shafts, sliding pins, removable assemblies. Typical: H7/g6 (running fit), H7/h6 (sliding fit). Press fit: shaft is larger than hole at room temperature — parts require force to assemble. Creates permanent or semi-permanent joint relying on friction and elastic deformation. Typical: H7/p6 through H7/u6 depending on how tight.
H7/p6 (light press): 0.001–0.020 mm interference on 10–50 mm diameters. Used for bearings in housings, bushings that may need removal later, light-duty permanent fits. H7/s6 (medium press): 0.020–0.050 mm interference. Standard for gears on shafts, pulleys, heavier bearing fits. Usually requires heating or freezing for assembly. H7/u6 (heavy press / forced fit): 0.030–0.080+ mm. Used for gear couplings, wheel hubs, permanent structural fits requiring high torque transmission.
Press force (F) ≈ π × D × L × μ × P, where D = interface diameter, L = contact length, μ = friction coefficient (0.08–0.15 for steel on steel dry), P = interface pressure from interference. For steel-on-steel H7/s6 fit at 25 mm × 40 mm length, typical press force is 3–8 tons. We can calculate exactly for your specific case in DFM review.
Yes — often preferred for medium/heavy press fits. Heat the hub (or female part) to 150–300 °C to expand; alternatively cool the shaft in liquid nitrogen (-196 °C) to contract. This temporarily creates clearance, allowing assembly without force. After temperature equalizes, the permanent press fit is established. Advantages: no assembly damage, works for large parts, no special press equipment needed.
Diameter tolerance: H7/p6 means hole IT7 (~0.021 mm on 25 mm), shaft IT6 (~0.013 mm on 25 mm). Total variation on interference: ±0.034 mm. Surface finish: Ra 1.6 µm or better on both surfaces — rough surfaces crush and reduce interference. Cylindricity: 0.01 mm or better on both surfaces — ovality reduces contact area. Chamfer the lead-in edge (30° × 1 mm typical) for easier assembly.
Yes but with limitations. Aluminum-on-aluminum press fits can gall (seize) during assembly. Mitigations: use lubricant during assembly (light grease or anti-seize), limit interference (H7/n6 is usually the maximum), or use different aluminum alloys (6061 shaft in 7075 hub) to minimize galling. For high-cycle press fits in aluminum, consider steel bushing inserts. Thermal assembly works well on aluminum — temperature differential of 150 °C produces approximately 0.5% dimensional change.