Machine keys and key stock are demountable machine elements that are used to transmit torque from a shaft to the component. The use of a machine key and key stock prevents movement between the shaft and the component and allows force to be released without causing stress or wear on the machine. Machine keys are usually made of steel or stainless steel and plated with zinc for wear and oxidation resistance.
Keyseating is a machining process in which a machine key's mating internal shapes and forms are cut using a reciprocating, single-point cutting tool—similar to the shaping process. The keyseating cutter is clamped into a holder. Together with a feed bar, it is able to move vertically within a tool guide. Keyseating machines, also called keyseaters, are used primarily to cut internal keyways and other straight-sided shapes and forms in non-production parts.
Other machines used to create keyways include broaching machines, shapers, slotting machines, and milling machines. Broaching is primarily used to cut square-cornered internal keyways. Milling machines can create parallel, tapered, and woodruff keyways.
Variations of machine keys include parallel keys, tapered keys, Woodruff keys, hi-pro keys, and non-rok keys.
- Parallel keys are the most widely used machine key. They have a square or rectangular cross section. Rectangular-faced keys are used for larger shaft diameters and square keys are used for shafts smaller than 6.5 in (170 mm).
- Tapered keys are machined with a taper along the length of the key. Tapered keys are available with a square end or a gib end. Gib head keys are notched and tapered to tightly couple gears or pulleys with a shaft.
- Woodruff keys are semi-circular components that can withstand great strain. They are installed deep in a shaft and firmly embedded. The main advantage of a Woodruff key is in its avoidance of the milling of a keyway near the shaft shoulders, where stress concentration already exists.
- Hi-pro keys are similar to Woodruff keys, but lock into place. They provide superior strength and vibration resistance.
- Non-rok keys are made of the same materials as hi-pro keys, but have shear strengths that are equivalent to those of Woodruff keys.
Choices of key stock include step, special, nonferrous, oversized, and undersized.
- Step key stock is used with power transmission components such as couplings, gears, sheaves, sprockets, and pulleys. Some products are designed for repairing damaged shafts. Others are used to mate different keyways or gears. Typically, step keystock has four dimensions (A, B, C, and D) and is available in a variety of materials and finishes.
- Special key stock is designed for heat treating, tool making, and other engineering applications.
- Nonferrous key stock is made from corrosion-resistant materials and designed for use in food service or marine applications. Special keystock and nonferrous keystock are specified according to height, width, length, material, finish, and quantity.
- Oversized and undersized key stocks are made slightly larger or smaller than standard keys for fitting specific applications.
The key material should be the same strength and hardness as the shaft to prevent key failures. Another important specification to consider is the key geometry and fit. To ensure the key fits properly it should fit tight in the key shaft keyway and have a sliding fit in the hub keyway as this prevents rotation. Every torque reversal causes hammering on the sides of the key, leading to damage.
Proper fittings leave a small clearance between the top of the key and bottom of the shaft keway, and the key length should extend inward from the shaft end (never beyond) to beyond the hub end, by at least the rounded portion of the key.
Machine keys and key stock are used in all types of mechanical devices, from large factory and construction equipment to motors and parts in automobiles and small vehicles. Many shafts such as motors, pumps, and pneumatic systems require a shaft key.
Machine keys and key stock must adhere to certain standards to ensure proper design and functionality. AIA/NAS 20065 discusses machine keys that have a rectangular cross section and two square ends. MS 51935 is a U.S. document discussing machine keys.