Shaft Key: Definition, Type, and Application

Shaft keys, often referred to as sunk keys, play a crucial role in mechanical systems by allowing torque to be transferred between a shaft and attached components like gears or pulleys.

Over the years, various forms of shaft keys have been developed parallel keys, Woodruff keys, taper keys, Gib head keys, and even perpendicular types. Interestingly, some variations, such as the feather key, seem to have faded into obscurity.

This raises a practical question: are we overlooking simple, durable, and cost effective solutions by not giving these lesser known keys a second look? It might be worth it for today’s engineers to revisit some of these almost forgotten options when designing reliable mechanical assemblies.

What is Shaft key?

A shaft key is a simple yet essential component in mechanical engineering, serving as the link between a rotating machine element such as a gear, pulley, or coupling and the shaft itself.

Its primary function is to lock these parts together, so they rotate as a unit, preventing any slippage between them. This firm connection makes it possible to transmit torque from the shaft to the attached component efficiently.

For a shaft key to work as intended, both the shaft and the mating element need to be specially prepared. The shaft features a slot called the keyseat, while the mating hub (whether that’s a gear, pulley, or similar device) has a corresponding slot referred to as the keyway.

When the key is placed so it bridges both the keyseat and the keyway, the result is known as a keyed joint. Interestingly, while this arrangement prevents rotational movement between the two parts, it may still permit a certain amount of axial movement along the shaft, depending on the design.

Shaft keys are manufactured from a range of materials, with steel being the most prevalent due to its strength and durability. These keys come in various shapes and sizes to suit different applications, but the rectangular and tapered forms are by far the most common choices in industry.

The process of cutting the keyway in the hub is critical for proper fit and function. Typically, keyways are created as grooves or pockets using specialized machinery such as key-seating machines.

Other methods like broaching, milling, planing, shaping, and slotting are also employed, depending on the requirements of the job and the equipment available.

Keyed joints play a vital role in mechanical power transmission. Their primary task is to ensure that the shaft and its attached components like couplings or gears rotate together, reliably transmitting load and power.

When designed and manufactured correctly, keyed joints help maintain the accuracy of the system, ensuring smooth, slip free operation and meeting the necessary performance standards.

Types of Shaft Keys

There are five main types of shaft keys: sunk, saddle, tangent, round, and spline

• Sunk Keys
• Rectangular & square keys.
• Parallel keys.
• Gib head keys.
• Feather key
• Woodruff key.
• Saddle keys
• Tangent keys
• Round/Circular keys
• Splines keys

Sunk Keys

A sunk key is a key that fits into keyways in both the shaft and the secured member in machinery.

Sunk keys are sunk into the shaft for half its thickness, where the measurement is taken at the side of the key.

There are six main types of Sunk keys:

1. Rectangular sunk key
2. Square sunk key
3. Parallel sunk key
4. Gib head key
5. Feather key
6. Woodruff key

1. Rectangular sunk key

Rectangular sunk keys are commonly designed with specific proportions relating their width and thickness to the diameter of the shaft or the hole in the hub.

These keys are fitted into the shaft to facilitate the transmission of torque between the shaft and other machine components, such as gears.

In most cases, machine keys are manufactured from the same material as the shaft, but they are usually made to be either harder or tougher than the components they work with. This ensures durability and reliable performance under load.

However, there are situations where it is intentional to use a softer key. The idea here is straightforward: if excessive force or overload occurs, the key will deform or shear before causing any damage to more valuable machine parts.

Essentially, the key acts as a sacrificial element, protecting the more critical and expensive equipment from harm.

2. Square Sunk key

A square sunk key differs from a rectangular sunk key primarily in that its width and thickness are equal.

Typically, these square keys are used for shafts ranging from 0.25 to 1.0 inch in diameter, although larger versions are available for shafts with diameters up to 6.5 inches.

Designers often choose square sunk keys when they require maximum possible depth something that rectangular keys may not always provide.

3. Parallel Sunk key

A parallel sunk key is typically manufactured in either a rectangular or square shape, maintaining consistent width and thickness throughout its length.

Notably, this type of key does not have any taper, which allows components such as pulleys, gears, or other mating parts to slide smoothly along the shaft when needed.

In fields like mechanical and automotive engineering, parallel keys are widely favored primarily because they are easy to install, readily available, and, frankly, quite cost effective. Despite being so common, it’s important to recognize their limitations.

Parallel keys are best suited for situations involving lighter, one way (unidirectional) torque loads or applications where it’s helpful to periodically remove or reposition the hub. This is especially relevant when regular adjustments or maintenance are anticipated.

However, parallel keys aren’t ideal for handling complex, multi directional torques or heavy starting loads. Over time, such conditions can cause the driving component to loosen, and if the fit becomes slack, the key might end up jamming within the machine often leading to failure.

So, while their convenience is hard to beat, it’s important to weigh their practical limits during design and application.

4. Gib head key

Gib keys, sometimes called gib head keys, are specialized machine keys characterized by their tapered and notched design. They play an essential role in securing pulleys and gears onto power transmission shafts, ensuring these components remain firmly in place during operation.

The distinct head of the key isn’t just for show it provides a practical surface to strike with a hammer when fitting or removing the key, which helps avoid damaging either the shaft or the key itself.

When it comes to positioning, the depth of the slot cut into both the shaft and the attached component determines where the key sits along the shaft’s axis.

However, it’s important to note that this axial position isn’t fixed in advance; it depends directly on how deep each slot is. The deeper the slots, the farther the key can slide in.

There’s a potential issue if the keyway or slot is made too deep. In that case, the tapered portion of the gib head key can travel so far into the slot that it ends up pressing against the face of the component.

When this happens, the key’s ability to function as intended is compromised, and the mechanical connection becomes unreliable users should anticipate possible failures if this situation isn’t addressed.

5. Feather key

Feather keys serve as a rectangular connector between the keyways of a shaft and a hub such as those found in gears or pulleys. What sets them apart is that they’re secured in one keyway, while the other end is free to slide.

This design allows the hub to move to different positions along the shaft, making it possible for the shaft to either drive the hub or be driven by it as needed.

These keys are specifically made to transmit torque between a shaft and whatever component it’s attached to. Visually, they look like square keys with rounded (radiused) ends.

To install one, a matching slot is milled into the shaft using an end mill that creates a radius matching the shape of the feather key. Once the feather key is in place, it fits snugly in its pocket and can’t shift along the shaft’s axis.

Structurally, a feather key is considered a type of parallel key. It can be either square or rectangular in cross-section, with radiused ends and precise length tolerances.

The slot cut into the shaft matches the feather key’s length, creating an oval-like pocket that holds the key firmly in place and keeps it from sliding axially.

Interestingly, the keyways in mounted components (like gears or pulleys) remain unchanged from those used for regular parallel keys. So, if you’re switching to feather keys, there’s no need to alter your existing components they’ll fit just fine.

The feather keys have several advantages over traditional keys:

• The rounded end of the keyway in the main funnel makes assembly much simpler. Because of this radius, wing keys slide easily into the shaft, and the key is guided right into the slot without much fuss.
• Feather keys are designed to fit entirely within their housing, so there’s no need for extra steps to keep them in place. Plus, once everything is assembled, the wing keys stay put and can’t work themselves loose.
• Once you’ve installed a wing key, it won’t move along the shaft by itself. This actually makes it easier for technicians to adjust the position of attached components later on, whenever that’s needed.
• Feather keys are secure and unlikely to fall out during use, which matters because a loose key can cause real harm either by damaging machinery or even posing a risk to operators.
• Since feather keys are locked in place, there’s little risk of losing them or having to deal with costly downtime just to replace a missing part.
• When it comes to taking things apart or putting them back together, feather keys make the whole process straightforward and hassle free.
• Finally, feather keys come pre made to the exact length required, so there’s no need to spend time cutting, measuring, filing, or fitting. They’re ready to use straight out of the box.

6. Woodruff key

A woodruff key is a specialized type of shaft key, shaped like a small segment of a disk. You’ll typically find it used in shafts with diameters up to 2½ inches.

The design is quite practical: once installed, the semicircular key sits in a matching pocket milled into the shaft, with a flat end sticking out slightly. Creating that pocket, or keyway, usually involves a cutter designed to achieve just the right dimensions.

One of the main reasons engineers reach for woodruff keys is their ability to enhance the concentricity of the shaft, especially in high speed applications where precision matters.

They also offer a distinct advantage when it comes to stress distribution. By allowing you to avoid cutting a keyway near high stress areas, woodruff keys help maintain the overall strength of the shaft.

As for materials, key steel and stainless steel are most commonly used, and both are easy to source online. However, woodruff keys can also be made from other materials aluminum, for instance, is sometimes chosen depending on the requirements of the job.

MORE: What is a Woodruff key?

Saddle Key

A saddle key is a type of key used to secure a component onto a machine shaft. What makes it distinct is how it fits: rather than sitting deep inside both the shaft and the attached part (like a sunk key does), a saddle key only fits into the keyway of the hub. Its surface is concave, allowing it to grip the shaft through friction, not by being embedded into the shaft itself.

Unlike sunk keys, which are set into matching grooves in both the shaft and the hub, saddle keys simply rest on the shaft either on a flat surface or along the shaft’s curve.

The actual transmission of power comes from the friction between the key and the shaft, rather than a rigid mechanical interlock. There are two main types of saddle keys to be aware of:

Flat Saddle Key:

This type is basically a tapered key that slides into a slot in the hub and lies flush against the shaft. Because it isn’t anchored into the shaft itself, there’s a risk it could slip and round off the shaft if too much force is applied. That’s why you’ll typically find flat saddle keys used in setups where the load isn’t too heavy.

Hollow Saddle Key:

A hollow saddle key, on the other hand, is also tapered but has a curved underside that matches the contour of the shaft. Like the flat version, it fits into the keyway in the hub, and its grip depends entirely on friction.

Since this connection isn’t the most robust, hollow saddle keys are mostly used for light duty applications. You’ll often see them serving as temporary fasteners for instance, when installing or adjusting parts like eccentrics or cams.

Tangent key

Tangent keys play an important role in heavy duty machinery. Unlike a single key, these are used in pairs. Each key is set at a right angle to the other, and both are positioned so they’re tangent to the shaft’s surface. The design allows each key to handle torsional forces, but only in one direction.

The shape of the keyway for a tangent key is somewhat similar to that of a parallel key. However, the keyway for a tangent key is cut at a tangent from the outer surface of the shaft toward its center.

When you look at the heel of each keyway the surface where the key sits that’s where the compressive force gets transferred as the shaft turns.

A key point to note here is that if you expect the shaft to reverse direction, you’ll need a second key, placed tangent to the shaft but facing the opposite way. Typically, these keys are spaced either 90 degrees or 180 degrees apart on the shaft.

Tangent keys come in different shapes wedge, rectangular, or square but in most cases, engineers rely on double-taper rectangular keys for this application.

Round key

The round key plays a crucial role in transforming a simple permutation into a true block cipher, as its derivation from the original key and subsequent addition to the process distinguishes the algorithm’s security framework.

In practical terms, when large volumes of data are encrypted using a single key, the key setup is only required once at the outset. However, in scenarios such as brute force attacks where an adversary systematically tries different keys the key setup must be repeated for each new candidate key.

Structurally, round keys are circular in cross section, designed to fit partly into the shaft and partly into holes that have been specifically drilled in the hub.

One notable benefit of this configuration is that, after assembling the corresponding components, it is still possible to perform additional key drilling operations if needed.

Typically, round keys are regarded as most effective for applications involving low power drives, where their unique mechanical properties offer practical advantages.

Splines keys

Sometimes, the keys are actually integrated into the shaft itself, forming a single, inseparable unit that fits into the keyway of the hub. These are typically called splined shafts.

You’ll usually find these shafts with several divisions commonly four, six, ten, or even sixteen splines. Compared to shafts that only have a single keyway, splined shafts generally offer greater strength.

Splined shafts find their place in situations where the amount of force that needs to be transmitted is closely tied to the shaft’s size think of automotive transmissions or sliding gear mechanisms.

By using splined shafts, you’re not just getting reliable power transmission; you also gain the advantage of allowing axial movement along the shaft while still ensuring a solid, positive drive connection.

Material Selection for Shaft key

When it comes to making shaft keys (sometimes called machine keys), there’s actually quite a range of materials to choose from. The classics are carbon steel and stainless steel, mostly because of their strength and reliability.

But depending on the specific job, you might see keys made from brass, copper, aluminum, monel, or even nylon, each with their own material grades. For applications out at sea, AISI 316 stainless steel is a popular choice, thanks to its extra resistance to corrosion.

Selecting the right material isn’t just about picking the toughest option available. You really have to think about the environment where the key will be used.

For instance, it’s crucial to consider both the types of loads the key will need to handle and the conditions it’ll face during operation. In practice, carbon steel is kind of the go to for most situations it offers a solid balance of strength and can be made even tougher with the right heat treatment.

Of course, if you’re dealing with situations where wear resistance or extra strength is critical, there are options to tweak the material for that purpose.

For really corrosive environments, austenitic stainless steels (the 300 Series) are usually the best bet they hold up well where other metals might fail. On the other hand, if you only need a material that’s tough and just mildly resistant to corrosion, martensitic stainless steel is a practical pick.

Benefits and limitations of keyed joints

There are various advantages and disadvantages of using shaft keys so granular detail of the overall design must be taken into account to evaluate the suitability of the keyed joint.

Advantages of shaft key and Keyed joints

• Cheaper construction cost
• Well-standardized (ISO, BS, DIN, and ANSI)
• Medium to high torque transmission
• Easy to mount and dismantle, hence easily reusable

Disadvantages of shaft key and Keyed joints

• Not suitable for alternating directional loads and shaking
• Possible axial displacement of the hub unless locked by additional components such as set screws or circles
• Over time the joint can be very difficult to eliminate.
• Introduces key stress points due to the sludge effect and reducing shaft strength
• Introduces shaft imbalance
• There is difficulty in calculating and combining load carrying and tolerance stack analysis, so the keyed joints have higher dimensions
• To transmit axial force, it requires a stop lock

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