What Is A Woodruff Key?

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By Charlotte Miller

In the world of machinery and mechanical assemblies, precision and stability are paramount. To achieve this, various components work together, each with its unique role. One such unassuming yet crucial part is the Woodruff key. In this blog, we will explore the Woodruff key, its purpose, design, and how it ensures the efficient and reliable operation of machinery.

What Is A Woodruff Key?

A Woodruff key, also known as a half-moon key, is a small, semicircular metal key that is used to secure two rotating parts, typically a shaft and a hub, in mechanical assemblies. It is named after its inventor, William Woodruff, who patented this design in the 19th century.

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Purpose Of A Woodruff Key

The primary purpose of a Woodruff key is to prevent relative rotational motion between a shaft and a hub while allowing them to transmit torque. By securing these two components together, it ensures that they rotate in unison, a critical requirement in many machines and devices.

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Design And Installation

Woodruff keys are characterized by their unique, half-moon shape, which fits into a semicircular slot machined into the shaft or hub. The keyway on the shaft and hub corresponds to the shape of the key, ensuring a snug fit.

Here’s how a Woodruff key is typically installed:

  1. Machining: A semicircular slot, called a keyway, is machined into both the shaft and the hub, where they meet.
  2. Insertion: The Woodruff key is then inserted into the keyway on the shaft.
  3. Alignment: The hub is aligned with the shaft so that the keyway in the hub matches with the key in the shaft.
  4. Securement: The hub is slid onto the shaft, and the Woodruff key locks them together by filling the voids in the keyway and the key.
  5. Securing: A fastener, such as a nut or bolt, may be used to further secure the assembly in place.

Advantages Of Woodruff Keys

Woodruff keys offer several advantages in mechanical assemblies:

  1. Compact Design: Woodruff keys are small and occupy minimal space, making them suitable for applications with space constraints.
  2. Efficient Torque Transmission: They efficiently transmit torque from the shaft to the hub without slippage.
  3. Alignment: The key’s shape ensures that the shaft and hub are accurately aligned, preventing wobbling or misalignment.
  4. Ease of Replacement: If a key becomes damaged, it can be relatively easy to replace, ensuring the continued functionality of the assembly.

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Woodruff keys are used in a wide range of machinery and equipment, including:

  1. Automotive Engines: Woodruff keys are employed in various engine components, such as the camshaft, crankshaft, and timing gears.
  2. Machine Tools: They secure rotating parts in machinery, ensuring precise and synchronized movement.
  3. Transmissions: In vehicles, Woodruff keys are used in transmissions to connect gears, shafts, and other components.
  4. Agricultural Machinery: Tractors and other agricultural equipment use Woodruff keys in various applications, from power take-offs to drive systems.


While it may appear as a small and unassuming component, the Woodruff key plays a crucial role in maintaining the stability and efficiency of mechanical assemblies. Its unique design and securement method make it an ideal choice for connecting rotating parts and ensuring that they work together harmoniously. In the world of machinery and engineering, it’s often the uncelebrated components like the Woodruff key that quietly enable the smooth operation of countless devices and equipment.


What Is The Difference Between A Parallel Key And A Woodruff Key?

Square and rectangular keys are typically inserted into slots on the side of the shaft with some space above. They’re usually held more firmly in the shaft than in the hub. On the other hand, a Woodruff Key is shaped like a half-moon and fits into a curved slot in the shaft.

What Is The Difference Between A Sunk Key And A Woodruff Key?

A woodruff key is a special type of key used in machinery. It’s shaped like a half-circle and has a consistent thickness throughout. The shaft where it’s inserted has a groove cut into it that matches the shape of the key.

What Is A Woodruff Key On A Crankshaft?

Adding a woodruff key to a crankshaft involves creating a half-circle slot in the shaft using a woodruff cutter tool. The curved part of the woodruff key fits into this slot, while the flat side connects with a slot in the hub, gear, or bushing.

Why Would You Use A Woodruff Key?

Woodruff keys are half-circle-shaped discs used to link machine parts to spinning shafts and spindles. They prevent gears or other machine components from moving separately from the spinning shaft and are typically inserted into specially cut key slots in the shafts.