Key (engineering)

In mechanical engineering, a key is a machine element used to connect a rotating machine element to a shaft. The key prevents relative rotation between the two parts and may enable torque transmission. For a key to function, the shaft and rotating machine element must have a keyway and a keyseat, which is a slot and pocket in which the key fits. The whole system is called a keyed joint. A keyed joint may allow relative axial movement between the parts. Commonly keyed components include gears, pulleys, couplings, and washers. There are five main types of keys: sunk, saddle, tangent, round, and spline. Types of sunk keys: rectangular, square, parallel sunk, gib-head, feather, and Woodruff. Parallel keys are the most widely used. They have a square or rectangular cross-section. Square keys are used for smaller shafts and rectangular faced keys are used for shaft diameters over or when the wall thickness of the mating hub is an issue. Set screws often accompany parallel keys to lock the mating parts into place. The keyway is a longitudinal slot in both the shaft and mating part. File:Passfedernut.jpg|The keyseat in a shaft for a parallel key File:Sprocket parallel keyway.png|A sprocket with an internal parallel keyway File:Parallel keys.png|Cross-section of a parallel keyed joint W = d/4 H = 2d/3 where W is the key width H is the key height d is the shaft diameter Woodruff keys are semicircular, fitting partly into a circular segment keyway with the remainder fitting into a longitudinal slot keyway in the mating part. The circular segment can be cut directly by plunge cutting with a circular Woodruff cutter without any reliefs. The main advantage of the Woodruff key is the elimination of milling near shaft shoulders, where stress concentrations, and concentricity would be affected. The latter is particularly important for high speed operation. The more exact fit of the key and keyway also reduces play, and stress concentrations in, and improves the reliability of the key.