H7/p6. H7/s6. H7/u6. Interference fits, explained.

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.