A shaft key is a metal piece used for fastening a mating machine component to a rotating shaft. It's typically employed to prevent the relative rotation between two parts, allowing for torque transmission between the two.

Read through the rest of this article to learn more about the types of shaft keys, how they work, the most popular materials used, and the various advantages and disadvantages of shaft keys.

How Does a Shaft Key Work?

For the shaft key to operate properly, the spinning components (such as pulleys, couplings, or other mating parts) and the rotating shaft must both have a key seat and a keyway. One portion of the key fits into the pre-cut shaft keyway, while the remaining portion of the key fits into the hub keyway.

This entire mechanism is referred to as a "keyed joint," and its purpose is to enable relative axial movement and power transmission between two rotating machine elements. Keyed joints are crucial components that ensure that a relative connection transmits power, load, and rotation adequately and without slipping, per design requirements.

How Many Different Types of Shaft Keys Are There?

Source: Engineers Edge

Shaft keys come in various specifications and tolerances, with parallel, rectangular, and tapered shaft keys being the most popular choices among machinists and mechanical engineers.

Parallel Keys

Parallel keys are divided into rectangular keys and square keys, both with uniform thickness and width throughout. They're mostly used in applications where a gear, pulley, or other mating component needs to slide alongside a shaft since they virtually have no taper.

Parallel keys are typically used to transmit unidirectional torques with low beginning loads and at locations where periodic clearance or hub movement is required — if not, the hub is additionally secure with set screws.

Sunk Keys

Sunk keys are sunk into the shaft for half of their thickness and countersunk into the mating part, with their measurements taken from the side of the key. Sunk keys come in various shapes, including the Woodruff, Gib, Feather, Parallel, Square, and Rectangular sunk keys.

Gib Head Keys

These tapered, notched machine keys are used in keyed shafts for power transmission to securely fasten the gears and pulleys to the shaft. The key's head lessens the impacts of a hammering during installation and guards against shaft damage. The keyway depth controls the axial position in the shaft or the component. A tight fit with a protruding gib head actually indicates adequate connection.

Feather Keys

Feather keys are constructed in a way that allows them to attach either the shaft or the hub, thus permitting the relative axial movement of the keyed joint. For example, a double-headed key is mounted to the mating part, and the assembly is then pushed onto the shaft. On the other hand, if the key is mounted into the shaft, a closed mating groove is cut into the surface of the shaft, preventing the axial movement of the key, securing it in the shaft, but permitting the axial movement of the mating part.

Spline Keys

Splines or spline keys are most commonly used in the automotive industry and are usually machined into the drive shaft around the shaft diameter. They're intended to mesh with the groove in a mating piece for toque transfer during the keyed joint's positive drive and axial movement. According to the published ISO, DIN, and ANSI standards, spline keys often have four, six, 10, or 16 partitions (individual keys or grooves).

Circular Keys

Round keys have a circular cross-section, allowing them to slide into holes drilled partially in the shaft and the hub. In addition, they're quite easily manufactured since their keyways may be drilled and reamed after the mating pieces have been joined. Typically, round keys are seen to be the best suitable for low-power drives.

Woodruff Keys

Woodruff keys, also known as half-moon keys, are semicircular shaft keys that can be used on shafts that have a diameter smaller than 2 and 1/2 inches. The semicircular pocket for the keyway is set in the shaft, and the proportions of the pockets are established with the aid of a woodruff keyseat cutter.

After being inserted into the shaft, the flat end of the key is left sticking out, ready for interfacing with the mating part's keyway. These stainless-steel keys are mostly used to increase the shaft's concentricity for high-speed operations, usually in machine tools.

Saddle Keys

Saddle keys are fitted into the hub and not the shaft. They either rest on a flat surface or around the shaft's perimeter, with the power transmission achieved through friction between the shaft and the key. There are two types of saddle keys, both of which are best suitable for light loads:

• Flat saddle keys, also known as tapered keys, have a flat bottom and a tapered top, allowing the key to press against the shaft's flat face when pushed into the tapered hub keyway.
• Hollow saddle keys have a tapered top and a curved bottom, which allow them to sit flush against the shaft's circumference. The shaft's curved circumferential surface is pushed down as the key is inserted into the tapered hub keyway.

Tangent Keys

These keys are typically employed with large machinery. A pair of tangent keys is kept at right angles to one another and is tangent to the surface of the shaft. The tangent keys might be rectangular, square, or wedge-shaped.

What Type of Metals Are Used to Produce Shaft Keys?

Shaft keys can be manufactured of various materials, with high-carbon steel being the key material in their production. They can also be made of stainless steel or any other material required by the assembly design. These keys come in a variety of sizes and forms as well. The two most popular shapes for shaft keys are rectangular or tapered.

How to Cut a Keyway in a Shaft

Source: PICRYL

The hub of a gear, sprocket, pulley and other rotating power devices are furnished with keyways, which are slots that can accommodate keys. On key seating machines, the slot in the hub is carved using milling, shaping, broaching, slotting, and planning.

It's important to know that machining a keyway into the shaft will reduce its torsional strength due to stress concentrations at the keyway corners and a decrease in the shaft's cross-sectional area.

Summary

A shaft key is a fitting in a shaft designed to secure an operating part fastened in or to the shaft. There are nearly a dozen different types of shaft keys, and while some are derived from others, they all share a common function. You can view more machining parts from Raid Supply here, if you want to learn more.