Woodruff Key
A semi-circular pocket-fitting metal key used to lock a rotating gear or pulley to a shaft and maintain alignment.
A Woodruff key is a semi-circular piece of metal that fits into a matching pocket-like keyway machined into a shaft, such as the snout of an engine crankshaft. The flat top of the key projects above the shaft's surface, sliding into a straight slot on the mating gear, pulley, or sprocket. This mechanical lock prevents the gear from spinning on the shaft, ensuring positive power transfer and timing.
The key's semi-circular design allows it to rock slightly in its pocket, making it self-aligning as the matching gear is slid onto the shaft. Woodruff keys are widely used to align and lock timing sprockets, distributor drive gears, and harmonic balancers. Because they are designed to be the weakest link in the connection, they are engineered to shear (snap) if the driven component jams, protecting more expensive shafts and gears from twisting.
During engine assembly, the Woodruff key must be checked for tight fit inside its slot. If the key is loose or worn, it can rock and damage the keyway, leading to timing shifts. A new key should always be used during an engine rebuild, and it must be tapped flat into the pocket before sliding on the pulley.