keyed

A keyed connection uses a rectangular metal wedge, called a key, fitted into matching grooves on a shaft and hub to transmit torque and prevent relative rotation. The groove in the shaft is called a keyseat; the groove in the hub or wheel is called a keyway. When the key sits fully in both grooves, the two parts move as one unit, with the key bearing the shear load between them.

Keys come in several standard profiles. The most common is the square key, a simple rectangular bar that typically has a depth equal to one quarter of the shaft diameter. Parallel keys have flat top and bottom surfaces and slightly tapered sides; they slide in from the end of the shaft. Woodruff keys are semicircular and sit in a rounded keyseat, offering better concentricity for high-speed applications. Gib-head keys have a protruding head for removal by hammer or puller, useful in disassembly-heavy work.

The keyed joint transfers power through friction and direct contact between the key flanks and the walls of both grooves. Load capacity depends on key length, material strength, and the pressure acting perpendicular to the key. A short key on a shaft that carries shock loads can shear; an undersized keyseat can deform under cyclic stress. Conversely, an over-designed key joint becomes a rigid connection that may mask bearing failure elsewhere in the system.

Common failure modes and selection

Keys fail by shear when torque exceeds their cross-sectional strength, by crushing if the material in the keyseat or keyway yields under concentrated pressure, or by fretting wear if relative motion occurs despite the key. The key material is almost always steel, hardened to resist deformation; the shaft and hub materials vary widely. Proper fit requires minimal clearance; a loose key rattle causes wear and noise without transmitting full torque.

Keyed assemblies are preferred over pins, press fits, or splines in applications where disassembly is routine and concentricity is not critical, such as pulleys, sprockets, and gears on general machinery. On precision spindles or high-speed shafts, splines or shrink fits often replace keys because keys create stress concentrations at the bottom of the keyseat, raising fatigue risk. The term keyed also applies to the finished product: a keyed shaft, a keyed hub, or a keyed coupling.