What Are Machine Keys,
Also Known as Shaft Keys?

Machine keys are used in power transmission to transmit torque between a shaft and a shaft mounted component such as a gear, pulley, or coupling. Both the shaft and the component must be machined with a keyway of the appropriate size and style for the key to be used.

Machine keys are sometimes called shaft keys, or sunk keys, and are available in a variety of types and include Parallel Keys, Flat Keys, Woodruff Keys, Square Keys, Rectangular Keys, Plain Taper Keys, Gib-Head Taper Keys, Perpendicular Pins and the Feather Key.

Machine Keys have a wide range of applications in our daily lives. Machine Keys hold machinery parts in place or align components. Machine Keys are used in power transmissions, motors, speed reducers, variators, automobiles, motorcycles, and industrial machines.

Each machine key has few attributes such as width, height, length, finish, and tolerance. It is important to understand the tolerance attribute. There are two type of tolerance. An oversize tolerance machine key provides a slightly tighter fit and an undersize tolerance machine key provides a slightly looser fit. Tolerance is specified in standards such as DIN6885 and JIS B 1301. Machine keys with metric (mm) measurement are almost always made as close tolerance undersized, whereas machine keys with imperial measurement (inch) tend to be made in both oversize and undersize tolerance sizes.

Parallel Machine Keys

Parallel keys are the most widely used. They have a square or rectangular cross-section. Parallel keys include Square Keys, Rectangular Keys. The shaft is usually key slotted for the full length for the shaft or slotted from the end of the shaft and along the length of the shaft for a distance greater than the length of the key to be used. Parallel keys can be installed by lining up the keyslots in the shaft and the component and then pressing the parallel key between the slots. There are three shapes of parallel keys depending on the ends of the keys. The end of a key can be either square or radius or round. The type of ends are referred to as “Forms”. Form A refers to a round end and Form B refers to a square end. For a Form A key, both ends are round, sometimes called the double round key. Form B key has both ends square. Form AB is a key with one end round and one end square, sometimes called the single round key.

Woodruff Keys

A woodruff key is a half moon shaped key that is inserted into a curved slot in the shaft. Woodruff keys are machined metal fasteners used to transfer torque from a shaft to gears where tight tolerances and functionality are vital. They are precisely a fail-safe device to ensure that the key breaks before the gears do, cutting replacement costs down significantly. Woodruff keys are used in a variety of applications including, lawn and garden equipment, automobiles, trucks, pumps, electric motors, agricultural equipment and other heavy equipment. They are used more often than machine keys in automated assemblies, such as automotive, because they are easier to feed and maintain tight tolerances. No matter the application, the woodruff key’s job is the same, to transfer the torque by locking the gears into the crank shaft.

Machine Key Standards

There are two sets of standards in the world – one for the imperial (inch) and another for metric (mm). DIN 6885 and JIS B 1301 deals with metric keys and ANSI B17.1 and BS 46 deals with imperial machine keys. These standards define the material, dimensions, tolerance and finish of the keys. There is a great similarity between DIN 6885 and JIS B 1301-1996. DIN 6885 is more popular in Europe and JIS in Asia.

• ANSI B17.1 By American National Standard Institute
• DIN6885 from German
• JIS B 1301-1996 from Japan
• BS 46 from England
• GB 1567-2003 from China

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