key

A key is a small metal block, usually steel, that sits in matching grooves cut into a shaft and a mating part (typically a pulley, gear, or coupling) to prevent relative rotation. When the shaft spins, the key transfers torque directly to the hub without slipping. The key absorbs shear stress across its flat faces and relies on friction and the walls of the groove to hold everything in place.

Keys come in several standard forms. The most common is the square key, a simple rectangular bar, often with a slight taper on top. The parallel key (also called flat key) has no taper and sits flush in the groove. The Woodruff key is semicircular in profile, seated in a curved recess on the shaft, and preferred where components must slide axially along the shaft. Tapered keys are used in heavy machinery where wedging action helps grip the hub tightly.

Keys are sized proportionally to the shaft diameter. A 12 mm shaft typically takes a 4 mm x 4 mm square key, while a 25 mm shaft uses a 8 mm x 7 mm key. Length varies by application, from 15 mm in tight spaces to over 100 mm in industrial drives. Material is almost always medium-carbon steel, hardened to resist wear in the grooves. The key itself must be softer than the shaft or hub; if it is harder, it will score the grooves rather than wear evenly.

The grooves (or keyways) are cut into the shaft and hub using a broach tool or end mill. Tight tolerances are critical: too loose and the key rattles or shears under load; too tight and installation damages the key or hub. Standard tolerance classes exist in manufacturing specifications to ensure parts fit correctly.

Keys fail when torque spikes exceed what the key can absorb in shear, causing it to fracture or deform. A sheared key signals either shaft overload or a seized coupling downstream. Unlike a bolt or bearing, a key is a one-time fail-safe: it gives way before the shaft bends. Replacement requires dismounting the hub, which is why keys are sometimes paired with set screws or flanges to add insurance. In high-vibration environments, the key can slowly migrate out of the grooves if the fit is loose, leading to gradual loss of drive.