Keyway

A keyway is a longitudinal slot machined into the bore of a hub — such as a gear, pulley, or coupling — that receives a key (a small rectangular block of metal) to lock the hub to its shaft. The corresponding slot machined into the shaft itself is called a keyseat. Together, keyway + key + keyseat form a keyed joint that transmits torque while preventing the hub from spinning freely on the shaft.

Keyway vs. keyseat

Although the terms are often used interchangeably in casual speech, GD&T Basics and engineering standards draw a clear distinction:

• Keyseat — the slot cut into the shaft (an external cylindrical surface, machined with an end mill).
• Keyway — the slot cut into the hub bore (an internal cylindrical surface, often broached, wire-cut, or milled).

Both are required for the key to sit properly. If only one is present, the key has nothing to press against and the joint fails.

How the joint works

A key — typically rectangular (parallel key) or tapered (taper key) — is pressed into the keyseat. When the hub is slid onto the shaft, the key enters the keyway and fills the gap between shaft and bore. Torque is transmitted through the shear and compressive strength of the key. The hub cannot rotate independently of the shaft, and axial movement is prevented by a set screw, shoulder, or retaining ring.

Common types of key

• Parallel key (square or rectangular) — the most common; sits in both keyway and keyseat and relies on a snug fit.
• Taper key — slightly tapered on one side, driven in with a hammer to create a wedge fit; used where axial positioning matters.
• Woodruff key — a semicircular key that sits in a curved keyseat; self-aligning and common in machine-tool spindles.
• Sunk key — any key fully embedded in the shaft and hub, as opposed to a feather key which is fixed to one part only.

Where keyways appear

Gear drives, pulley systems, coupling assemblies, sprockets, and motor shafts all use keyed joints. The keyway is one of the oldest torque-transmission methods in mechanical engineering and remains standard where cost and simplicity matter more than the precision of splined or interference-fit alternatives.

How to measure a keyway

Keyways are quoted by width × depth (e.g. 8 × 4 mm), and the dimensions are not arbitrary: standards pair each shaft diameter with a key size. An 8 mm-wide keyway belongs on a shaft in roughly the 22–30 mm range; a 4 mm keyway on a much smaller shaft. That is why a keyway that “looks about right” is usually wrong — measure the width with a caliper, check it against the shaft diameter, and the standard size falls out.

On the hub side, an internal keyway is cut all the way through the bore, so you can verify width by measuring the slot at the bore’s face. On the shaft side, the keyseat is open to the air, which makes its width and depth directly checkable.

Common failure: keyway wear

A keyed joint that has run loose develops a distinctive wear pattern: the key and the slot edges become peened and rounded, and the hub develops visible backlash — you can rock it a few degrees on the shaft before it resists. Left alone, the pounding widens the slot until the key shears or the hub spins, and a spun hub is usually scrap. The fix is never “a bigger key”: either restore the original dimensions (weld and re-machine, or cut the keyseat 90° around the shaft) or move to a keyed-and-pinned arrangement. Squeezing in an oversize key masks the problem for a week and destroys the shaft’s remaining accuracy.