Most keyed connections lock a hub axially to a shaft — the key transmits torque, and a setscrew or shoulder keeps the hub from sliding. But some machines need the hub to slide along the shaft while still transmitting torque. That's what feather keys (and their heavier-duty cousin, the sliding spline) are for.

A feather key is a parallel key that is either screwed to the shaft or screwed to the hub, but never both. The unattached side rides in a keyway that runs the full length of the sliding travel. Torque transfers through the key's flanks; the hub is free to translate along the shaft axis. Classic uses:

• Sliding gears in manual transmissions (old-style crash gearboxes and PTO shifters) — the gear slides into and out of mesh while the shaft spins.
• Telescoping drive shafts on farm equipment and industrial rollers, where the shaft length must change as machinery flexes or repositions.
• Adjustable pulleys and clutch throwout mechanisms, where a component must move axially under load.

For higher torque or when multiple sliding elements are stacked, engineers move up to sliding splines — typically straight-sided or involute splines that distribute the load across many teeth simultaneously. A driveshaft slip yoke on a rear-wheel-drive car is the everyday example: as the rear axle moves up and down over bumps, the driveshaft's spline allows the shaft to lengthen and shorten while transmitting engine torque to the differential.

Design rule of thumb — feather key length: Size the key for shear and bearing stress like any parallel key, but add axial travel. For a shaft of diameter D transmitting torque T, the required key contact length is roughly:

L ≈ 4T / (D × h × σ_allow)

where h is the key height engaged in the hub and σ_allow is the allowable bearing stress (typically 100 MPa for mild steel keys). Then add the sliding travel distance to get total keyway length in the shaft (or hub, depending on which side the key is fixed to).

Watch out for: Feather keys wear the keyway walls under load because the hub is sliding while transmitting torque — surface hardening the keyway or using a hardened key is common. Also, feather keys can only handle light to moderate axial sliding under load. If the hub must slide while transmitting significant torque continuously (e.g., a driveshaft), use a spline instead — the load distributes across many teeth and the low contact pressure per tooth reduces galling.

Rough guide: feather key for occasional shifts under light load; sliding spline for continuous sliding or high torque.