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Written by G.L. Huyett Marketing Department on 05/06/2021 with 0 comments

Introduction To Woodruff Keys

What Is A 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. Woodruff keys are inserted into a key seat created by a woodruff key cutter or arbor cutter. A gear or other component with a woodruff keyway is then pressed onto the shaft until the woodruff key interfaces with its keyway.

What is a Woodruff Key Used For?

In addition to holding components in place on a shaft, woodruff keys also act as a fail-safe that protects expensive parts by shearing under excessive loads. The crescent profile of the key allows it to sit deeper in the shaft, reducing the likelyhood of the key rolling laterally out of the groove.

How Does a Woodruff Key Work?

When installed, the long, flat face of the woodruff shaft key protrudes above the host shaft. When a mating hub, gear, or other component is pressed onto the shaft, the protruding section of the key locks components together.

Installed Woodruff Key

How to Install a Woodruff Key:

Before we explain how to install a woodruff key, it's worth clarifying some common terms. The woodruff keyseat is the half moon slot cut into the shaft. The woodruff key's curved section is pressed into the keyseat. In contrast, the keyway is the slot cut into the mating part to accommodate the woodruff key's flat side. However, the terms keyway and keyseat are often used interchangeably, and many sources refer to the keyseat as the woodruff keyway or half moon keyway.

When installing a woodruff key into a half moon keyseat, first press it into the semi‑circular groove which is milled into the shaft by a woodruff key cutter. Next, align the top, flat portion of the woodruff key so that it is parallel with the host shaft. During installation, woodruff keys will sometimes slip out of their groove as the mating component is pressed on. This can make woodruff keys difficult to install.

How To Keep A Woodruff Key In Place:

To keep a woodruff key in place while installing a mating component, ensure that it is situated tightly in the key seat. Since woodruff keys are made to tight tolerances, they often require a press fit into their seat. This is done by lightly tapping the flat, protruding face of the key with a hammer or mallet until the key is seated firmly. The woodruff key will still extend above the mating shaft's surface and into the woodruff keyway to lock the mating hub or gear in place. If the key still falls or slips out during component installation, the key is likely too small for the keyway.

Woodruff Key Installation

Selecting The Right Woodruff Key

It is important to understand the different features of a woodruff key during the design phase of an application. Some considerations include key type, key material, and key finish.

What Are The Different Types Of Woodruff Keys?

The two most common types are full radius woodruff keys and flat bottom woodruff keys. Both function the same, but may be appropriate for different applications. It is important to note that both radius and flat bottom woodruff keys are standard and either is shipped at the manufacturer's discretion. If a specific type is required, let G.L. Huyett know!

Full radius woodruff keys are a full half circle and make full contact with the shaft keyway. Full radius woodruff keys require precision of the seat groove as contact can occur between the woodruff key and the bottom of the seat causing interference and the key to protrude from the groove higher than desired.

Flat bottom woodruff keys have the same profile as standard woodruff keys, however, the bottom of the key has been shaved flat. The flat bottom reduces the potential of interference at the bottom of the seat, allowing the woodruff key to seat fully in the groove and alleviating potential clearance issues of excessive key height.

Hi‑Pro keys, also known as high-profile woodruff keys, are a variant of woodruff keys that feature a full radius but have the added feature of feet. When installed, the feet extend over the face of the host shaft, preventing the key from rocking in its rounded seat.

Woodruff and Hi-Pro Key Usage

Woodruff Key Materials

Woodruff keys are crafted in carbon steel, alloy steel, and stainless steel. Carbon steel woodruff keys and alloy steel woodruff keys are both durable, however they must be plated or coated to prevent corrosion. Stainless steel woodruff keys surpass carbon and alloy steel in corrosion resistance but are not as strong.

Woodruff Key Finishes

There are multiple finishes available for woodruff keys. Finishes include plain, clear and yellow zinc, and passivated.

Plain finish is available on carbon steel and alloy steel woodruff keys. This finish is an economical solution for keys that will not be subjected to corrosive elements.

Plain Finish Woodruff Key

Zinc plated woodruff keys are plated in either yellow or clear zinc on carbon steel keys. The zinc plating offers an increased level of corrosion resistance compared to plain parts.

Zinc Yellow Finish Woodruff Key

Passivated finish woodruff keys use an oxidation process suitable for stainless steel parts. This microfilm is added to provide additional corrosion resistance to the already resilient material.


Advantages Of A Woodruff Key Include:

  • Adjusts to any taper present in mating piece or shaft
  • Ideal for high speed rotating assemblies
  • Eliminates milling near shaft shoulders reducing stress concentrations at the shaft end
  • Deeper seat in the shaft reduces risk of rolling out of key seat
  • Provides a more exact fit, reducing play in the key seat
  • Easy removal

Some Disadvantages To Consider Are:

  • Deeper groove depth weakens strength of shaft
  • Can rock during assembly making it difficult to install
  • Carries less load compared to longer machine keys
  • Key seat requires machining with a woodruff key cutter

How Are Woodruff Key Sizes Measured?

Woodruff keys often can be confusing to measure, especially inch woodruff keys. Use the infographic below to learn which measurements are required to select the correct woodruff key sizes for metric woodruff keys, inch woodruff keys, and hi-pro woodruff keys.

How To Measure Woodruff Keys

Looking For More?

For additional information on woodruff keys, machine keys, and machine key stock visit our blog page as well as the Key Stock and Machine Keys FAQ. If you don't see what you are looking for, our experienced and friendly Sales team is ready to help.

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