Shaft Keys: Types, Materials & Applications- Your Ultimate Guide

What is a shaft key?

The shaft key is a machined element used for connecting the transmission shaft to rotating parts like gear. The function of the key and keyway is to prevent relative motion between shaft and hub. It helps to transmit torque. The shaft key can fix, only if the shaft has a key seat and the hub has a keyway. The Half portion of the shaft key fix into the key seat and the half portion fix into the keyway. This combination is named a keyed joint.

Keyway is the groove cut in a part of a hub, gear, or other rotating machine element and the electrical connector, to ensure their orientation towards each other which is fitted with a key.

Shaft keys are available in various materials and various sizes like rectangular, square, and semi-circular. 

Types of shaft key

Shaft keys are divided into the main 5 categories.

1. Sunk Keys
2. Saddle keys
3. Tangent keys
4. Round/Circular keys
5. Splines keys

Sunk Keys

The sunk key is a type of key in which half-thickness of key fits into the keyway of the hub, and the other half-thickness fits into the shaft. For that, it is necessary to have keyways on the shaft and hub. It is a standard type of key.

The sunk key is suitable for heavy-duty applications because the shear resistance offered by the keyways prevents relative motion. As it fixes into the keyway, so it cannot slip due to heavy loading. It is a type of positive drive key.

The material of the shaft keys is always stronger to transmit the motion. Less strong materials can break when they are imposed by heavy force and torque.

There are six main types of Sunk keys:

• Rectangular keys
• Square keys.
• Parallel keys.
• Gib head keys.
• Feather key
• Woodruff key.

Rectangular sunk key

In most industrial applications, a rectangular sunk key is using. It is more stable for industrial applications. It is also called a flat key. The width to thickness ratio of a key is different. This key is suitable for the shaft diameter up to 500mm.

Square sunk key

Square sunk is commonly suitable for industrial machinery. The width to thickness ratio of the key is the same. This key is suitable for the shaft diameter up to 25mm.

Parallel sunk key

A parallel sunk key is considered a square or rectangular key without having taper. This key has opposite faces that are parallel to each other. During the operation, high rotational speed and vibration may cause getting keys to push out from the hub. Because of that, there is a need to screw the key with the hub.

Gib head key

Gib headed key is rectangular with a taper on one side and backed by the gib head from another side. As it is taper from one side, we can install this key very efficiently in the keyway. 

Feather key

Feather key is a type of parallel key which is rectangular and square. Corners of the feather key are curved. Before installing a feather key, the same size slot is machined with milling in the shaft. So, once the feather key gets installed in the shaft, it is prevented for an axial moment. Feather key can be screwed as well in some of the cases.

Woodruff key

Woodruff key is a flat semi-circular-shaped disk with a uniform thickness, which directly fits into the semi-circular recess of the shaft. The bottom circular portion is placed in the shaft by hammering & the top flat portion is inserted into the keyway of the hub.

This type of key is mainly using for light loads. It is suitable for shafts whose diameter ranges between 6mm to 60 mm.

Saddle keys

All type of keys fits in the shaft and hub. But the saddle key is a little bit different from other keys. It only fixes into the hub. Flat and hollow saddle keys are two different types.

The saddle key works on the principle of friction. The friction prevents relative motion between hub and shaft and causes to rotate hub with the shaft. There is a chance of slippage due to the high load. So these keys are not suitable for heavy-duty applications and high loads.

As in the flat key, the shaft is machined, where the key is going to fix. So there are fewer chances of slipping in flat keys as compare to concave keys or hollow keys.

The manufacturing and fixing cost of the saddle is less as compared to sunk key. Because it requires the only keyway in the hub, not in the shaft. This is the advantage of the saddle key over other keys.

It is only suitable for light loads and low torque. Because high load and torque cause slipping of a key’s surface over the shaft and causes disengagement of the assembly.  

Flat Saddle Key

The flat key is flat from the bottom, which sits on the machined flat portion of the shaft.

Hollow Saddle key

While the hollow saddle key is concave in shape from the bottom, which is similar to the outer diameter of the shaft.

Tangent keys

Tangent keys come in the form of pairs. They are mounted at a right angle to each other along the tangent to the surface of the shaft. These keys can sustain the torsional force acting during the operation in one direction only.

The tangent key is also known as the Kennedy key. It is rectangular or square according to the application. This pair of the key can able to withstand failure due to compressive force and shear force. Because of that, they are suitable for heavy-duty applications.

Round/Circular keys

The round circular key is circular in cross-section, which fixes into the drilled hole by drilling machine partly on shaft and hub. This round key helps to prevent relative motion shaft and hub. It is suitable for light load and smaller power drives.

Splines keys

These spline keys are generally used for the application like there is axial relative motion between shaft and hub present. Splines cut on the shaft with the milling process and the hub with the broaching process. 

1. Involute splines having stub teeth at 30 degrees of pressure angle. It is more popular than straight splines. The cost requires for manufacturing is higher than straight splines.
2. Serration splines are used where the size of the assembly requires to keep small. 

Shaft Key Material

The selection of material for the key depends upon the type of operating environment and load acting. Stainless steel and medium carbon steel are both common materials for the shaft keys. Because these two-material having high yield strength.

Keys can also manufacture aluminum and copper alloys like brass and bronze. SS316, brass, and bronze keys are generally suitable for marine applications, and ss are suitable for food processing equipment.

For a highly corrosive environment, Austenitic Stainless Steel is a good choice due to its strength, and ability to non-corrode. 

Advantages of shaft key and keyed joints.

1. Manufacturing cost is less.
2. They are standardized as per various standards like DIN, IS, BS, ISO, and ANSI
3. Easy to use and reuse for different applications.

Disadvantages of shaft key and keyed joints.

1. These are not suitable for the alternative loads in opposite direction.
2. It causes the shaft to become imbalanced. 
3. It reduces the strength of the shaft due to slots.
4. Fixed and secure keyed joints are difficult to dismantle.