Woodruff keys are semi-circular discs used to connect machine parts to rotating shafts and spindles. They serve to stop gears or other machine parts from moving independently from the rotating shaft and are often installed into the recessed key seats cut into the said shafts.

Once installed, the gear or other machining component with the proper keyway is fitted or pressed onto the shaft until the Woodruff key interfaces with the component's keyway. Despite preventing relative movement, Woodruff keys serve as a fail-safe that prevents the shearing of expensive parts exposed to extreme stresses.

Woodruff keys are available in various materials and key stock, ranging from high-carbon steel to stainless steel. They're fitted into the woodruff keyseat, which is a half-moon slot cut into the rotating shaft, with a keyway or a Woodruff cutter.

The long, flat face of the key projects above the host shaft once it has been inserted into the keyseat, and the mating component with a pre-cut keyway is placed onto the host shaft. The fastener key, interfacing with the keyway of the mating part, prevents the mating component from moving independently from the spinning shaft and vice versa.

The Basics: What Is a Woodruff Key?

Before we dive into the basics, we have to mention that woodruff keys aren’t exactly a novelty but well-known machining and mechanical fasteners. They were actually invented in the 19th century by William N. Woodruff of Hartford, Connecticut. In 1888, Woodruff was awarded the John Scott Medal by the Franklin Institute for his invention.

Woodruff’s original design hasn’t really changed from its inception, which only attests to the effectiveness of the shaft key in various machine assemblies, such as crankshafts, pulleys, and flywheel drive shafts.

The Different Types of Woodruff Keys

There's a wide woodruff key assortment available on the market, available in both metric and imperial measurements and fashioned of various materials. Full radius and flat bottom half-moon keys are the two most popular varieties, and although they’re durable and suitable for various applications, both types essentially function the same.

Full-Radius Woodruff Keys

These are the standard woodruff keys that are full half-circle in size and are designed to fully interface with the shaft’s key seat. The key seat has to be precisely cut according to the key size to accommodate the key and prevent any interference between the key height and the depth of the host shaft’s key seat.

Flat-Bottom Woodruff Keys

These are very similar to the full-radius keys, but their bottom section is actually ground flat instead of being rounded. This allows easier placement, as it prevents the key from moving within the key seat while also reducing any potential for interference at the seat’s base. This allows the Woodruff key to fully engage with the key seat while also resolving any potential clearance issues and concerns associated with tall woodruff keys.

Hi-Pro Woodruff Keys

High-profile woodruff keys, commonly referred to as Hi-Pro keys, are a type of full-radius woodruff key that have added feet at the protruding segment of the key. These feet prevent the Hi-Pro key from wobbling inside the rounded key seat because they extend over the face of the host shaft.

What Are Woodruff Keys Made Of?

Woodruff keys are mostly made of alloy steel, though high-carbon and stainless steel Woodruff keys are the most commonly found keys in the market. They can be made of softer metals, such as copper and tin alloys, or even ceramics if certain applications demand them. These keys also come in a variety of sizes and shapes.

Shafts, on the other hand, are almost always made of high-carbon steel, but some applications might require other materials. However, mismatching materials of different hardness will most definitely ensure the damage to some parts.

Which Industries Use Woodruff Keys?

Woodruff keys are used in a wide range of industries — for example, all industries that rely on electromotors or turbines for different applications rely on the use of woodruff keys. They’re mostly used in machine tools and automotive and marine industries, since they’re needed for car shafts ranging from 1/4-inch to 2 1/2-inch in diameter. It’s still important to note that woodruff keys aren’t capable of carrying the same load as the longer, parallel keys.

What Are the Other Types of Keys?

Woodruff keys and other types of shaft keys come in a variety of specifications and tolerances, and while they perform the same function, some are more popular than others.

Rectangular/Square Keys

These are best known as parallel keys, as both the rectangular and square variants offer a constant thickness and width throughout the length of the piece. Since they essentially have no taper, they’re typically utilized in applications in which a mating component has to slide alongside the shaft. Parallel keys are frequently employed to transfer unidirectional torques with low starting loads.

Feather Keys

The design of feather keys enables them to connect to either the shaft or the hub, allowing for the relative axial movement of the keyed joint. For instance, the mating portion is attached using a double-headed key, and the assembly is then forced onto the shaft.

In contrast, when the key is placed into the shaft, a closed mating groove is carved into the surface of the shaft, impeding the key's axial movement and holding it in place while allowing the axial movement of the mating portion.

Gib-Head Keys

Gib-head keys are used in power transmissions to firmly bind the gears and pulleys to the shaft. The head of the key protects against shaft breakage and absorbs some of the force from a hammering installation, and in all actuality, a tight fit with a protruding gib head indicates a good connection.

Summary

Woodruff keys are non-threaded fastening components that interlock the rotating shaft or spindle with the mating component, preventing their independent motion. 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.