IDB-SHC-030
Power transmission · interference fits · keys · splines
Shaft-hub connections
Getting torque from a shaft into a hub — interference fits, keys, splines, tapers and clamp hubs — with the torque each carries, the trade-offs, and how to size them.
Abstract
A shaft-hub connection transfers torque (and sometimes axial load) between a rotating shaft and a gear, pulley, sprocket or coupling. The choice — interference fit, key, spline, taper, set screw or clamp hub — trades torque capacity against centring accuracy, ease of (dis)assembly, shaft damage and cost.
Section 1 compares the methods. Section 2 covers interference (press/shrink) fits and their torque. Section 3 covers keys and keyways. Section 4 covers splines. Section 5 covers tapers, clamp hubs, keyless bushings and set screws. Section 6 is a selection checklist.
1.Comparing the methods
| Method | Torque | Centring | (Dis)assembly | Shaft damage | Notes |
|---|---|---|---|---|---|
| Interference (press/shrink) fit | high | excellent | hard (press/heat) | none | Clean, no stress raiser; needs force or heat |
| Parallel key + keyway | moderate–high | good | easy | keyway (stress raiser) | Cheap, standard, the default |
| Spline | very high | excellent | easy (axial slide) | none extra | Many teeth share load; costly to cut |
| Taper fit (+ nut) | high | excellent | moderate | none | Self-centring, precise, removable |
| Clamp / split hub | moderate | good | very easy | none | Repositionable; no machining of shaft |
| Keyless locking bush / shrink disc | very high | good | moderate | none | High torque without a keyway |
| Set screw (on flat) | low | poor | very easy | mars shaft | Light duty only; use a flat |
1.1Terms
2.Interference (press/shrink) fits
The shaft is made slightly larger than the bore; assembly creates a contact pressure p (from the interference δ and the Lamé thick-cylinder equations, with hub geometry and both materials' E and ν). Friction at that pressure carries the load:
- Torque
T = µ · p · π · D² · L / 2 - Axial push-out force
F = µ · p · π · D · L
where D is the fit diameter, L the engagement length and µ the friction coefficient (≈0.1–0.15 dry steel; lower if oiled during pressing). Use the Press-fit interference tool to get p, T and F from δ.
- Specify with ISO 286 fits: light
H7/p6, mediumH7/s6, heavyH7/u6. Bigger interference → more torque, but also more hub hoop stress (check the hub doesn't yield) and more assembly force. - Shrink/expansion fit (heat the hub or chill the shaft) reaches the same interference with little assembly force and full holding capacitybest for large or high-interference fits.
- Surface finish matters: rough surfaces flatten on assembly, so the effective interference is less than the measured onea fit specced from nominal sizes over-predicts torque on a coarse finish.
3.Keys and keyways
A parallel (square/rectangular) key in matching shaft and hub keyways is the standard, cheap method. Size it so it doesn't shear or crush, and respect the keyway as a fatigue stress raiser in the shaft.
- Shear: key width × length resists
F = 2T/Dacross the key's shear plane. - Bearing/crush: the key side bearing on the keyway wall is usually the limit; check it too.
- Standard sizes (DIN 6885, key b × h by shaft Ø):
| Shaft Ø (mm) | Key b × h | Shaft Ø (mm) | Key b × h | |
|---|---|---|---|---|
| 10–12 | 4 × 4 | 30–38 | 10 × 8 | |
| 12–17 | 5 × 5 | 38–44 | 12 × 8 | |
| 17–22 | 6 × 6 | 44–50 | 14 × 9 | |
| 22–30 | 8 × 7 | 50–58 | 16 × 10 |
The keyway cut in the shaft raises local stress (Kt ≈ 2–3) — a fatigue concern on reversing/heavily-loaded shafts. Use generous keyway-end radii, or a keyless method, where fatigue governs.
4.Splines
Splines are many keys cut integral to the shaft, so the load shares across all teeth — far higher torque and better centring than a single key, with no separate part to shear.
- Straight-sided (older, simple) vs involute (stronger, self-centring, easier to manufacture to gauge)involute is the modern default.
- They allow axial sliding under load (e.g. driveshafts, gear shifts) while transmitting torque.
- Costlier to produce (broaching/hobbing), so reserved for high-torque or sliding applications.
5.Tapers, clamp hubs and keyless bushings
- Taper fit (+ retaining nut): a tapered shaft/bore self-centres precisely and locks by friction; removable. Common on tool spindles and crankshaft pulleys.
- Clamp / split hub: a slit hub squeezed by a screw grips the shaft by frictionno shaft machining, infinitely repositionable; moderate torque. Great for prototypes and adjustable setups.
- Keyless locking bushings / shrink discs: tapered rings draw up to clamp hub-to-shaft at very high torque with no keyway and no fatigue notchused where keys would fatigue.
- Set screws: light duty only; always seat on a flat (or use a dog-point into a hole) so they don't burr the shaft and slip. Two screws at 90° resist better than one.
6.Selection checklist
- Torquesteady value, plus reversing or shock peaks (these drive keyless/spline choices).
- Centring / runoutinterference, taper and splines centre best; set screws worst.
- Axial behaviourdoes the hub need to slide (spline) or stay located (fit/key/nut)?
- (Dis)assemblyservice access and frequency: clamp/keyless for easy removal, fits for permanent.
- Shaft damageavoid keyways/set-screw marks on fatigue-critical or hardened shafts (use keyless).
- Costkey cheapest; spline and keyless bush most expensive.
- Then: size the method (interference via the press-fit tool; key by shear + bearing; spline/keyless from supplier torque ratings) and check shaft fatigue at any keyway.