Key (engineering)

In mechanical engineering, a key is a machine element that connects a rotating part to a shaft. It stops the two parts turning relative to each other and lets torque pass between them. For a key to work, both the shaft and the rotating part need a keyway and a keyseat, which are a slot and a pocket for the key to sit in, and the whole arrangement is called a keyed joint. A keyed joint can allow the parts to move relative to each other along the axis. Gears, pulleys, couplings and washers are commonly keyed.