Press fit vs shrink fit: same squeeze, different assembly.

Press fit and shrink fit describe the same interference joint — a shaft made deliberately larger than its bore — but they get the parts together in opposite ways. One uses brute force, the other uses temperature. Pick the wrong method and you crack a hub on the press or spend money heating a part that never needed it. Here is how to choose.

Same joint, two methods

Start with what they share. Both a press fit and a shrink fit are interference fits: the contact pressure, hub stress and holding torque all come from the same diametral interference and the same Lamé thick-cylinder equations. If you specify H7/s6 on a 50 mm interface, the joint is identical whether you press it or shrink it. What changes is only how you overcome the interference during assembly — and that changes force, tooling, surface finish and risk. For the underlying maths of pressure and stress, see the full press fit calculation guide.

Press (cold) fit: assembly by force

A press fit is assembled cold. You put the shaft and hub in an arbor or hydraulic press and push until the shaft is home. The insertion force is:

It is fast, needs no heating equipment, and suits small parts and high-volume lines — bushings, small gears, dowels. The downsides: force scales with size and can reach tens of kilonewtons; the shaft scrapes down surface asperities as it slides in, so you lose some effective interference; and a thin or brittle hub can gall or crack under the axial push.

Shrink (thermal) fit: assembly by temperature

A shrink fit removes the force entirely. Heat the hub so its bore expands past the interference, drop the shaft in with clearance, and let it cool. As temperatures equalise the bore clamps down. The temperature rise needed is:

Because the parts never rub on assembly, a shrink fit keeps more of its designed interference and gives clean, repeatable grip — the reason railway wheels, large gears, turbine discs and bearing races are shrunk on. The trade-offs: you need a heater or oven (or dry ice / liquid nitrogen to cool the shaft instead), the cycle is slower, and overheating past the material's tempering temperature will soften it.

Side-by-side

Worked comparison: gear on a 50 mm shaft

Take a steel gear on a steel shaft: interface d = 50 mm, hub OD dₒ = 80 mm, engagement L = 40 mm, working interference δ = 0.040 mm, E = 210,000 N/mm², μ = 0.12, α = 11.5 × 10⁻⁶/°C. Contact pressure works out to p ≈ 51 N/mm² and holding torque to about 965 N·m — identical for both methods.

Press-fit route: insertion force F = 0.12 × π × 50 × 40 × 51.2 = 38,600 N ≈ 3.9 tonnes. You need a press rated well above that, and roughly 0.6 × (Ra_shaft + Ra_hub) of interference is lost to asperity flattening.

Shrink-fit route: ΔT = 0.040 / (11.5 × 10⁻⁶ × 50) = 70 °C above the shaft, plus about 30 °C of slip clearance — heat the hub to roughly 100 °C over ambient. No press, no scraping, full interference retained.

Same joint strength, completely different shop-floor plan. On a small bushing you would never bother heating; on a 500 mm ring gear you would never dream of pressing. Confirm the numbers for your own geometry with the press-fit calculator, which reports force, stress and shrink temperature together.

Which one should you use?

• Choose a press fit for small parts, high volume, and where insertion force is comfortably within your press — bushings, dowels, small pulleys.
• Choose a shrink fit when force gets impractical, on large or thin-walled hubs prone to galling or cracking, or when you need maximum retained grip — railway wheels, large gears, bearing races, turbine discs.
• Cool the shaft instead of heating the hub when the hub carries heat-sensitive features or coatings, or on thin shafts where cryo shrink is easier than an oven.

A practical middle path many shops use: design the fit as H7/s6, size grip from the minimum interference and stress from the maximum (read the fit bands in our ISO 286 fits and tolerances guide), then decide press vs shrink purely on the insertion force the calculator returns.

Common mistakes

• Thinking one is inherently stronger. Same interference, same holding capacity. Method does not change the physics of grip.
• Pressing a thin hub. If dₒ/d is below about 1.3, cold pressing risks cracking — shrink it instead.
• Overheating on a shrink fit. Exceeding the tempering temperature softens the part. Stay below it and use induction for control.
• Lubricating a press fit. Oil drops μ and can halve holding torque unless the design assumed it.
• Ignoring asperity loss on cold press. Subtract roughly 0.6 × (Ra_shaft + Ra_hub) from your interference for the press route.

Whichever route you take, the interference features have to be inspected exactly as drawn. CadNexa's auto-ballooning tool tags each diameter and tolerance straight off the PDF so the fit-critical dimensions land in your inspection sheet without a transcription error.

Frequently asked questions

What is the difference between a press fit and a shrink fit?

Both use the same interference. A press fit forces the parts together cold; a shrink fit heats the hub (or cools the shaft) so the bore opens, then clamps on cooling. The finished joint is identical; only the assembly method differs.

Is a shrink fit stronger than a press fit?

For the same interference and geometry the grip is the same. A shrink fit can retain slightly more effective interference because it does not scrape surface asperities.

When should you use a shrink fit?

When insertion force is too high, on large or thin-walled hubs at risk of cracking, or when you need maximum retained interference.

What temperature is needed for a shrink fit?

ΔT = δ / (α × d) plus a clearance margin. For 0.04 mm on a 50 mm steel bore, about 70 °C above the shaft.