Woodruff Key – Functions,Installation, Selection, Types, Advantages, Disadvantages, and Applications

The Woodruff key is a semi-circular disc of uniform thickness. It is a type of sunk key in which the upper part is flat, and the lower part is semi-circular.
It is used in mechanical engineering to securely lock gears, pulleys, or sprockets onto a shaft to transmit torque. 
This key is also known as the half-moon key. This key can tilt and align itself according to the shaft.
In the Woodruff key-keyway system, the keyway has a cross-section similar to that of the key, i.e., a semi-circular disc of uniform thickness.
The woodruff key prevents gears, hubs, or other components from moving independently.
Woodruff keys are commonly found in automotive engines, machine tools, and high-speed applications because they improve concentricity and simplify assembly by preventing the key from shearing and sliding out.
Its shape allows it to sit tightly in the shaft seat, making it easier to line up with the mating part as compared to other keys.

Functions of the woodruff key:-

1) It is a self-aligning key that is used in a tapered shaft.
2) As the depth of the woodruff key and keyway increases, the chance of slipping decreases.
Due to the extra depth in the keyway in the shaft, it provides more transmitting capacity.
3) It is largely used in machine tools.
Also, extra depth in the keyway in the shaft is also a disadvantage because it will decrease the thickness of the shaft and hence will decrease the strength of the shaft.

Why is the Woodruff key used?

Woodruff key is not only used for holding components in place on a shaft but also act as a fail-safe that protects expensive parts by shearing under excessive loads first. 

The crescent profile of the key allows it to sit deeper in the shaft, due to which there is less chance of the key rolling laterally out of the groove.

How is a woodruff key installed and kept in place?

When the woodruff key is installed, the long, flat face of the woodruff shaft key protrudes above the host shaft. 

After that, when a mating hub, gear or other component is mated with the shaft, the protruding section of the key locks the component together.

To install a woodruff key into a keyseat, first press it into a half-moon groove, which is milled into the shaft.

After that, align the top flat portion of the key so that it is parallel with the host shaft before completing the connection.
During the installation, the key may sometimes slip out of the groove when the mating component is pressed on. This causes difficulty in the installation of the key.

The woodruff key is kept in place while installing a mating component by ensuring that the key is tightly fitted in the key seat.
The mating components are made to tight tolerances, which ensures a high-precision fit and function. So, they require a press fit into their seat.
The flat protruding face of the key is lightly tapped with a hammer or millet until the key is firmly seated.
The woodruff will protrude above the mating shaft’s surface and into the keyway to lock the mating hub or gear in place.
If the key still falls or slips out during installation, it is likely too small.

Selection of the Right Woodruff key:-

The required woofruff key is selected based on the type, material, and finish. To do so, it is important to understand the different features of different types of woodruff keys during the design phase of an application.

Types of Woodruff Keys:-

There are mainly 3 types of woodruff keys:-
1) Full Radius Woodruff Keys
2) Flat Bottom Woodruff Keys
3) High Pro Woodruff keys

Let’s discuss these three types of woodruff keys:-

1) Full Radius Woodruff Keys:-
These keys are full half circular in shape and make full contact with the shaft’s keyway. These keys require very high precision designing of the key and keyway, as well as tight tolerance, because a small deviation in size can prevent proper installation of the key.

2) Flat Bottom Woodruff Keys:-
Flat Bottom keys have the same shape as that of full radius woodruff keys, but the bottom of the key is shaved flat. Therefore, they are much easier to adjust in the keyways.

3) High Pro Woodruff keys:-
Higg pro woodruff keys have a full semi-circular shape, similar to that of full radius woodruff keys, but at the top, a flat part is added with a length more than the diameter of the semi-circular shape. This flat part is called the feet, and it prevents the key from rocking in its rounded seat.

Materials Used For Woodruff key:-

Woodruff keys are made up of the following:-
i) Carbon
ii) Stainless steel
iii) Alloy

Strength and Durability:-
* Carbon and Alloy are durable, but they need to be coated to prevent them from corrosion.
* Stainless steel woodruff keys are corrosion-resistant, but they are less strong than carbon woodruff keys.

Coating:-
* Carbon steel and alloy steel are provided with a plain finish.
* Carbon steel keys are zinc-plated.
* Stainless steel keys are passivated with an oxidation process.

Advantages of the woodruff key:-

1) Woodruff keys can be used in high-speed operations.
2) Woodruff keys are easily adjustable.
3) Woodruff keys can tilt in the keyway, which is useful in some cases.
4) Woodruff keys fit with tight tolerance, which results in increased reliability of the key.

Disadvantages of the woodruff key:-

1) Extra depth in the keyway can weaken the shaft.
2) They cannot carry a high load.
3) Woodruff keys are difficult to install.
4) They cannot be used for the transmission of heavy torque.

Applications of the Woodruff Key:-

It is widely used in automotive, crankshafts, water pumps, blowers, and motor shafts.