Half-moon keys, also known as woodruff keys, are semi-circular discs that are used to couple machine components with rotating shafts and spindles. They're usually fitted into the recessed key seats cut into the rotating shafts and serve to prevent gears or other machine components from moving independently from the rotating shaft.

The recessed key seat is cut into the shaft with a woodruff key cutter, and once fitted, gear or some other machining component with the appropriate keyway is fitted onto the shaft until the half-moon key interfaces with the component's keyway.

In addition to coupling various components, half-moon keys also act as a fail-safe that prevents shearing of pricey parts exposed to excessive loads. The "half-moon" profile allows woodruff keys to sit deeper in the shaft, preventing the possibility of the key rolling out of the pre-cut groove.

How Does a Half Moon Key Work?

Woodruff keys come in a variety of materials and keystock, ranging from high-carbon steel to stainless steel keys, which are appropriate for different applications. They're fitted into the woodruff keyseat, which is a half-moon slot cut into the rotating shaft.

After they're fitted into the keyseat, the long, flat face of the key protrudes above the host shaft. A mating component with a pre-cut keyway — a cut-through slot that interfaces with the protruding section of the woodruff key — is then pressed onto the host shaft, and the key interfaces with the keyway.

When adequately installed, the fastener key prevents the mating component from moving independently from the rotating shaft.

What Are the Advantages of Half Moon Keys?

The primary advantage of woodruff keys is that they eliminate the necessity for milling around the shaft shoulders, which reduces stress concentrations at the shaft's end. This makes them perfect for high-speed rotating assemblies, as it effectively reduces the possibilities of damage.

Half-moon keys also provide a more precise fit, which effectively reduces any play in the key seat, reducing vibrations and significantly reducing the possibilities of damage in high-speed or high-load applications.

Additionally, due to their semi-circular form, they're easier to remove during disassembly, and they're able to move during assembly, thus adjusting to any taper present in either the mating hub or gear or the shaft slightly.

What Are the Different Types of Half Moon Keys?

There's a wide woodruff key assortment available on the market, either measured in metric or imperial measurement units and made of different materials. And measuring units and material types are categories in their own right. But there are also different types of woodruff keys themselves.

The two most common types of half-moon keys are the full radius and flat bottom. Both types effectively serve the same function, but are suitable for different applications. In most cases, they come in standardized sizes, but custom sizes are also available.

• Full-radius half-moon keys — Full-radius half-moon keys are a full half-circle in size, which fully interface with the shaft's key seat. They require a precisely cut seat groove to prevent any interference between the key height and the depths of the mating part's keyway.
• Flat-bottom woodruff keys — They have a similar profile as the full-radius keys, but the bottom section is actually flat instead of being rounded. The flat bottom lessens the possibility of interference at the seat's base, enabling the woodruff key to fully engage with the groove and resolving any potential clearance concerns brought on by an excessively tall key.
• Hi-Pro woodruff keys — High-profile woodruff keys, commonly referred to as "Hi-Pro" keys, are a type of woodruff key that has a full-radius profile with added feet. The feet prevent the key from wobbling in its rounded seat when it is inserted because they extend over the face of the host shaft.

Summary

Woodruff keys are non-threaded fastening components that interlock the rotating shaft or spindle with the mating component, preventing them from moving independently of one another. If you're interested, you can learn more about half-moon keys here. At Reid Supply, we can help you find the part number and an appropriate woodruff key for your application, as well as provide answers to some of the most frequently asked questions (FAQs) about woodruff keys.