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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, and couplings. There are three 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.
  • Woodruff keys are semicircular shaped, such that, when installed, leave a protruding tab. The keyway in the shaft is a semi-circular pocket, the mating part, 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.

  • The tapered key is tapered only on the side that engages the hub. The keyway in the hub has a taper that matches that of the tapered key. Some taper keys have a gib, or tab, for easier removal during disassembly. The purpose of the taper is to secure the key itself, as well as, to firmly engage the shaft to the hub without the need for a set screw. The problem with taper keys is that they can cause the center of the shaft rotation to be slightly off of the mating part.[3] It is different from a tapered shaft lock in that tapered keys have a matching taper on the keyway, while tapered shaft locks do not.
  • A “Scotch key” or “Dutch key” also provides a keyway not by milling but by drilling axially into the part and the shaft, so that a round key can be used. If the key is tapered, it is referred to as a “Dutch pin” and is driven in, and generally cut off flush with the end of the shaft. A Hirth joint is similar to a Spline joint but with the teeth on the butt of the shaft instead of on the surface.

Source: Wikipedia