What Is a Woodruff Key? | Why Use a Woodruff Key? | Sunk Keys

All About Woodruff Key

What Is a Woodruff Key?

A Woodruff key is a unique type of key that has a semi-circular shape. It is designed to fit into a circular recess in a shaft, which is created using a specialized tool called a Woodruff keyway cutter. The shaft may be either parallel or tapered, depending on how the dimensions for the Woodruff key are determined. The dimensions usually include the depth of the recess from the outer edge of the shaft, in addition to the diameter of the recess itself.

Woodruff keys offer a distinct advantage over other types of keys because they can accommodate any tapers in the mating hub on the assembly. This feature allows them to self-align within their circular recess, making them unsuitable for use as a pawn key, as they would become jammed.

These keys are commonly used in machine tools and various applications such as between the flywheel and crankshaft of a small internal combustion engine. The reliability of the drive largely depends on the fit between the shaft and the conical rugged flywheel hub.

The keyway in the shaft consists of a semi-circular pocket, while the mating part features a longitudinal slot. This configuration improves the concentricity of the shafts and mating parts, which is essential for high-speed operations. The circular section can be cut directly using a circular woodruff cutter, without any relief.

One of the main advantages of Woodruff keys is their ability to eliminate the need for milling near the shaft shoulders. This helps prevent stress concentrations and ensures the overall strength of the shaft is not compromised. Additionally, the precise fit of the key and keyway reduces play, further enhancing the reliability of the key.

Woodruff keys are available in various sizes, ranging from ¼” to 2½” (6 mm to 60 mm) in diameter. However, it’s important to note that they are not suitable for bearing the same weight as long parallel keys.

Benefits of Using a Woodruff Key

Why Use a Woodruff Key?

The use of Woodruff keys offers several advantages. One of the key benefits is the elimination of milling near the shaft shoulders, reducing stress concentrations and enhancing the overall strength of the shaft. This advantage is particularly important for high-speed operations where stress concentrations can have a significant impact.

Furthermore, Woodruff keys have the unique ability to fold themselves into their circular recess to accommodate any tapers in the mating hub on the assembly. This feature ensures a secure fit and prevents the key from becoming dislodged or jammed.

These keys are commonly employed in machine tools and applications such as snowblowers and automotive shafts. They are known for their excellent ability to transmit power effectively between the shaft and the hub, while still allowing for relative axial motion.

Sunk Keys:

Sunk keys, also known as recessed keys, are designed to be partially immersed in the shaft. These keys are measured based on their side, rather than along the centerline of the shaft.

#1. Rectangular


Rectangular keys, also referred to as flat keys, are wider than they are tall. They are commonly used on shafts with diameters of up to approximately 500 mm or 20″. The increased width allows them to transmit higher torques without the need for increased depth, which can weaken the shaft.

#2. Square Keys

Square keys, as their name suggests, have a square cross-section. They are typically used on shafts up to about 25 mm or 1″ in diameter. In cases where a deeper key depth is required, square keys can be used on larger shafts. However, it’s important to consider that deeper key depths can weaken the shaft due to a decrease in the effective cross-sectional area.

#3. Parallel Sinking Keys

Parallel Sinking Keys

Parallel sinking keys can have either a rectangular or square cross-section without a taper. These keys are readily available and cost-effective. They are also relatively easy to install, although they may require additional measures such as set screws to ensure they remain securely in place. These keys are fitted tightly to the bottom of the shaft keyway and the sides of the key joint, leaving clearances at the top of the hub keyway.

#4. Gib Head Sunk Keys

Gib head sunk keys are designed with a protruding gib head, which makes them easier to remove. These keys are typically rectangular or square and feature a taper on the top surfaces to ensure a tight fit.

#5. Feather Keys

Feather Keys

Feather keys are used to allow relative axial motion between the shaft and the hub. They come in various types, including three main wing keys, double-headed keys, peg feather keys, and feather keys. This design enables power transmission with parallel opposite faces, as well as allowing for sliding motion.

Advantages of Woodruff Key:

Here are the key advantages of using Woodruff keys:

  • Easily adjustable
  • Capable of accommodating any taper in the keyway
  • Provides a more precise fit, reducing keyway play and stress concentrations
  • Eliminates milling near the shaft shoulders, preventing stress concentrations
  • Particularly beneficial for high-speed operations
  • Enhances reliability and stability of the shaft and key joint

Frequently Asked Questions (FAQ)

Woodruff Key

A Woodruff key, also known as a half-moon key, is a semi-circular machine shaft key that prevents gears, hubs, or other components from moving independently of a rotating shaft or spindle.

What Is a Woodruff Key?

A Woodruff key is a type of shaft key that has the shape of a segment of a disk. It is used with shafts that are not more than 2½ inches in diameter.

Woodruff Keyway

A Woodruff keyway is a circular recess in the shaft that is machined to fit a Woodruff key. These keyways are commonly used in machine tools and automobile shafts, ranging from ¼” to 2½” (6 mm to 60 mm) in diameter.

Square Keys

Square keys have a square or rectangular cross-section. They are widely used in various applications and are selected based on the standardized dimensions for rectangular keys.

Shaft Key Types

There are various types of shaft keys, including saddle keys, sunk keys, parallel keys, tapered keys, square keys, flat keys, gib head keys, and feather keys.

Key Woodruff

The Woodruff key was invented by William N. Woodruff of Hartford, Connecticut. In 1888, he was awarded the John Scott Medal by the Franklin Institute for his invention.

Wood Ruff Key

Woodruff keys are semi-circular discs that are used to connect machine parts to rotating shafts and spindles. They prevent these components from moving independently and are commonly installed into recessed key seats.

Taper Sunk Key

A taper sunk key is a type of sunk key that has a tapered head. It is used to reduce the impact of hammering during installation, thus preventing damage to the shaft.

Woodruff Key Is Used for

Woodruff keys are used to connect machine parts to rotating shafts and spindles, preventing independent movement. They are commonly employed in various applications, including machine tools and automotive shafts.

Crankshaft Key

A crankshaft keyway is a slot or groove machined into the crankshaft. It provides space for a key to be inserted, allowing a pulley or sprocket to lock onto the round bar. The entire system is referred to as a keyed joint.

Woodruff Key Vs Square Key

While square and rectangular keys are typically side-fitting with top clearance and retained in the shaft, a Woodruff key is a half-moon shaped key that is inserted into a curved slot in the shaft.

Woodruff Key Shape

Woodruff keys have a semi-circular shape that leaves a protruding tab when installed. The keyway in the shaft is a semi-circular pocket, while the mating part features a longitudinal slot. This design improves concentricity and is crucial for high-speed operations.

Crank Key

A crank key is another term for a crankshaft Woodruff key. It is used to secure a gear or pulley to the crankshaft, preventing independent movement and ensuring proper engine timing.

Camshaft Key

A camshaft key is used to secure a camshaft to a shaft in an engine. It is typically a rectangular or square metal piece that fits into a keyway or slot in both the camshaft and shaft, preventing independent movement.

Woodruff Keys Manufacturer

Standard Horse Nail Company, founded by Charles Merrick in 1872, is a manufacturer of Woodruff keys. The company initially operated in Fallston, Pennsylvania, before relocating to New Brighton after a significant fire.

Square Sunk Key

A square sunk key is a rectangular sunk key with equal width and thickness. It is sometimes referred to as a square sunk key to differentiate it from other rectangular sunk keys.

Peg Feather Key

A peg or feather key is a type of key that resembles a Woodruff key. It is used to secure rotating components to a shaft, especially in applications with high torque loads. The key has a projecting tab, or “peg,” that fits into a slot in the component.

Related Articles

Back to top button