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, and Tangent, Round, and Spline keys.

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 6.5 in (170 mm) 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.

The keyseat in a shaft for a parallel key

A sprocket with an internal parallel keyway

Cross-section of a parallel keyed joint

Woodruff keys are semicircular, such that, when installed, they leave a protruding flat end. The keyway in the shaft is a semi-circular pocket; the mating part is a longitudinal slot. They are used to improve the concentricity of the shaft and the mating part, which is critical for high speed operation. The main advantage of the Woodruff key is that it eliminates milling a keyway near shaft shoulders, which already have stress concentrations.

Common applications include machine tools, automotive applications, snowblowers and marine propellers.

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