Types of shaft keys and their definition.

What is a shaft key and shaft keyway?

The shaft Key is a small metallic component that plays a major role in connecting the shaft and hub with other rotational component such as gears, bearings, etc. Shaft keys are always parallel to the axial of the shaft. There are several types of shaft keys.

When the hub and shaft are joint with the key, then the whole component is said as a keyed joint.

Basically, the shaft key comes with two main functions, they are

1. To transfer mechanical energy in the form of torque, from the transmission shaft to the rotational component and it can be reversed.
2. It prevents the relative rotation motion and axial motion between the joined rotational component and the transmission shaft. 

There must be a dedicated space for the installation of a key in the Shaft and hub. The space for installing keys in the Shaft and hub is called keyways.

Keyways can be created by any machining process (milling, grinding, turning, etc..)done in the Shaft and hub for the required dimensions.

After installing the key in the Shaft and hub, the whole component is called a keyed joint.

Mostly carbon steel is used in the manufacturing shaft key. It helps in wear resistance and stress tolerance while continuous transmission of torque occurs between shaft and hub to the joined rotational component.

Types of shaft keys

The shaft keys are classified into certain types according to their standard specifications and dimensions. Shaft key types are listed and explained below.

Types of shaft keys:

• Saddle keys
• Sunk keys
• Parallel keys
• Tapered keys
• Square keys
• Flat keys
• Gib head keys
• Feather keys
• Woodruff keys
• Kennedy keys
• Tangent keys
• Round keys
• Barth keys
• Spline keys

Saddle keys

Saddle keys are one of the shaft key types which is installed on the surface of the shaft and not in the keyway on the shaft.

This shaft key installation requires a keyway on the hub and it does not require any keyway on the shaft.

The saddle keys are classified into two types, they are

• Flat saddle keys
• Hollow saddle keys

These two types of shaft keys are classified based on the base surfaces they have.

Flat saddle key

Flat saddle keys have a flat surface at the bottom. To implement these keys, the shaft must be machined for the flat surfaces.

Hollow saddle key

Hollow saddle keys have a curved surface instead of flat surfaces at the bottom.

Both types prevent relative motion. Friction occurs due to power transmission through the shaft key. Flat saddle keys can handle better power transmission compared to hollow saddle keys, and it has much better slip-resistant.

So flat saddle keys are better than hollow saddle keys.

The Advantages of saddle keys are, The manufacturing and implementation cost is low and The implementation process is easy than others.

Saddle keys are mainly used for low-duty work or transmissions. It can not handle much slip-resistant during work is done above than low transmission. These are the disadvantages of saddle keys.

Sunk keys

Sunk keys are the Types of keys which is installed on both the keyways of the shaft and hub. The upper half is placed on the hub keyway and the down half is placed on the shaft keyway. The sunk key also prevents relative motion between the shaft and hub. Shear resistance plays a major role in power transmission. They are used for heavy-duty work and for industrial purposes. This is the main advantage of sunk keys. 

The disadvantages of sunk keys are, The cost will be higher than saddle keys, because of machining keyways on both shaft and hub.

Sunk keys are classified into four types. They are,

• Square key
• Flat key
• Parallel key
• Tapered key
• Gib head key

Square keys

The cross-section of this key has the same dimensions of height and width, and it appears like a cross-section of the square.

So this key is named square keys.

Flat keys

The cross-section of this key does not have the same dimensions of height and width, it looks like a rectangular cross-section. And it also has a flat surface at the bottom.

So this key is named as flat keys.

Parallel keys

This type of sunk key has a uniform height throughout the key at both ends.

So it is named parallel keys.

Tapered keys

Like parallel keys, this sunk key type doesn’t have a uniform height, instead, they are tapered at the upper surface in the ratio of 1:100. So they are called tapered keys. While fitting these tapered keys, they are hammered at another end for tight-fitting.

Gib head keys

The Gib head key is also a tapered key, but it has a special head at another end. The purpose of the gib head is for easy removal. They are easy to remove compared to other keys.

Feather keys

Feather key is a type that comes under the category of square key due to the square cross-section along with curved or radiused on both ends. They are parallel keys that are fitted on either on hub or shaft. They are fitted tightly by using screws on both ends. This key permits axial movement and prevents relative rotational movement between the shaft and the hub. It permits axial movement. Used in gear shifting devices.

• Peg feather key
• Single-headed feather key
• Double-headed feather key

Peg feather key

The Peg feather key has an additional projection at the center point of the key towards upward. This additional projection is called as peg. This projection is fitted to the hub keyway which has an additional gap. 

Single-headed feather key

The single-headed feather key is similar to the gib head key. But like gib head key, wedge hammering is not required to fit the key. Instead, the key is screwed on the hub.

Double-headed feather key

The double-headed feather key has two heads at each end. The middle portion of the key has a gap to fit the hub. 

Woodruff keys

Woodruff key is a type of sunk key. This key is a semicircular disc-shaped key. The thickness of the key is uniform. The shaft keyway is designed with a semicircular gap to fit the key in the Shaft. But the keyway in the hub is machined with a uniform depth like in other keys. While fitting this key, the semicircular part is fitted to the shaft keyway and the remaining upper part is fitted to the hub keyway. The keyway in the shaft is always designed and machined with some extra depth.

Advantages of Woodruff key

• Woodruff key can align itself with the tapper shaft. 
• It has high slip resistance because of the extra depth in the shaft keyway.

Disadvantages of Woodruff key

• Stress concentration is increased.
• The strength of the key is reduced due to the extra depth in the shaft keyway.
• There is no axial movement between the shaft and the hub. 

Kennedy key

Kennedy key is a pair of the square key. This pair of square key is placed apart from each other either at 90 or 120 degrees. The torque is equally transmitted at both square keys. Assembling this key is an easy process. The correct centering of the shaft and hub can be achieved by using this key. Kennedy key is quite similar to the flat key. Like a flat key, in Kennedy key also the failure occurs due to shear stress and compressive stress. This key is used for industrial purposes. It is the main advantage of this key. 

Tangent key

The tangent key is a pair of rectangular keys which is placed at a right angle to each other on the shaft. It is placed on the tangential surface of the shaft, so it is named a tangent key or tangential key. It can transmit a high amount of torque and so it can be used for high duty purposes. To transmit the torque in one direction, a single tangent key is used.

Round key

The round key is a cylindrical-shaped pin-type key. It is also named pin key and Nordberg key. Sometimes it is tapered at the ratio of 1:50 for light-duty purposes and a ratio of 1:200 for heavy-duty purposes. The keyway is easily made by drilling partially in the shaft and hub. The shaft keyway and the hub keyway have a cylindrical gap to fit the key. One-half of the key is fitted on the shaft keyway and the remaining upper part is fitted on the hub keyway. The keyway is machined by partial drilling on the shaft and the hub. The diameter of the key is 1/6 times the shaft diameter. Round keys are used to transmit a high amount of torque. Shear concentration is less while torque transmission.

Barth key

The Barth key is modified from the square key. Barth key has bevel-shaped smooth edges at the upper part. These bevel-shaped smooth edges help the key to fit tightly. Like a square key, the Barth key is also used in many applications.

Spline 

The spline is not like ordinary keys. They are integrated with the shaft while machining. The shaft is machined by a milling machine and the hub is machined by a broaching machine. Splines are three types, they are

• Straight-sided Spline
• Involute Spline
• Serration Spline

Straight-sided Spline

This type of spline shaft has an equally spaced straight-shaped key projection like spur gears.

Involute Spline

This Spline is commonly used. The teeth are involute and equally spaced. They align themselves and are self-centered while torque transmission.

Serration Spline

The serration shaft has “V” shaped teeth and they are closely arranged. These shafts are always in a smaller diameter.