Shaft Key: Definition, Type, and Application

Shaft keys—sometimes called sunk keys—transmit torque between shafts and shaft-mounted components such as gears or pulleys.

They come in a wide range of types and styles, including parallel, Woodruff, taper, Gib-head and perpendicular versions. But some, such as the feather key, are all but forgotten. Perhaps engineers trying to create durable, cost-effective designs should consider them.

What is Shaft key?

A Shaft key is a piece of metal used to connect a rotating machine element to the shaft. A shaft key prevents a relative rotation between the two parts, and may enable torque transmission to occur. For a shaft key to function properly, both the shaft and rotating elements (gear, pulley, and coupling) must have a keyway and a keyseat.

One part of the Shaft key fits into a precut slot in the shaft known as a key seat and the other part fits into a slot in the hub called a keyway. The whole system is called a keyed joint. A keyed joint may allow relative axial movement between the parts.

Shaft Keys are made of varied types of materials and also come in different shapes and sizes. The most common key shapes are rectangular or tapered and are typically made of steel.

A keyway is a slot (groove or pocket) in a hub of a pulley, gear, sprocket, or other rotating power devices to receive a key, and these slots are commonly cut on key-seating machines or by broaching, milling, planing, shaping, and slotting.

Keyed joints are an important part of mechanical power transmission elements shaft and couplings, where it ensures the connection transmits the load, power & rotation without slipping and within the accuracy requirement of the design.

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 the usual proportions of the width of the key and thickness of the key and the diameter of the shaft or diameter of the hole in the hub.

Install them in the shaft to transmit gear from the shaft to the shaft and other components. Machine keys are often the same material and are harder or tougher than the shafts or other components with which they are used. To use a key as part of a sacrifice, choose a soft key that will turn off when exposed to excessive force, so as not to damage expensive equipment.

2. Square Sunk key

In a Square sunk key, there has simply a difference between a rectangular sunk key and a square sunk key in width and thickness are equal.

They are normally specified for shafts with 0.25 to 1.0-in. Diameter but larger square keys are available for shafts up to 6.5 inches. Diameter. Designers use them when they want as much depth as possible with rectangular keys.

3. Parallel Sunk key

The parallel sunk key might be of rectangular or square part regular in width with thickness all over. It might be noted that a parallel key is a taper-less with uses where a pulley, gear, or other mating part is required to slide beside the shaft.

Parallel keys are commonly used in mechanical and automotive engineering because of their inefficiency, availability, and ease of installation.

They are mostly used to transmit only unidirectional torques subjected to lighter initial loads, or in applications that require periodic clearance or sliding of the hub, as the drive component may become free due to multi-axial torches, While the major loosens and gets trapped inside. Machine, the reason for failure.

4. Gib head key

Gib keys, or gib head keys, are tapered and notched machine keys that are used on power transmission keyed shafts to hold pulleys and gears tightly on the shaft. The head of the key serves as a concussion point for hammering without damage to the shaft of the key.

The depth of the slot in both the shaft and the component controls the axial position, but cannot be predetermined. The deeper either slot, the forward key enters axially.

Excessive key slot or keyway depth allows the slide of the Gib-head taper to move so far into the slot that it makes contact with the component face. If this happens, users cannot rely on the key to the taper to munch loudly and should expect failures.

5. Feather key

Feather Keys are a rectangular key connecting the keyways of a shaft and a hub of a gear, pulley, etc., fastened in one keyway and free to slide in the other so that the hub can drive or be driven by the shaft at various positions along with it.

They are drive keys that transmit torque from a shaft to a driven component. They are square keys but with radiused ends. The mating slot in the shaft is machined with an end mill that matches the radius of the shaft key design. Once installed the key is contained in the machined pocket and cannot move axially.

The feather key is a parallel key, which is square and rectangular, with a radius apart and length tolerances at both ends. The mating shaft key slot is mechanized with an end-mill cutting tool to the same length as the wing key. This oval shape holds the key in the shaft and prevents axial motion. Broken keyways in mounted components are similar to traditional keys used for parallel keys, so no changes to mounted components are necessary.

The feather keys have several advantages over traditional keys:

• The key in the radius end component keyway of the main funnel simplifies assembly. And wing keys are easily inserted into the shaft. The radius helps align the key in the shaft slot.
• Feather keys are fully contained. It does not set-up to hold the key in position and the wing keys cannot move out of the assembly.
• An installed wing key cannot run axially. This lets workers subsequently adjust the position of the mounted components along the shaft.
• Feather keys may not fall out of the shaft and may damage nearby machinery. (Loose key in the working machinery is a major cause of damage and injury.)
• There is no possibility of lost keys and associated downtime.
• Disassembly and reforms with a feather key are trouble-free.
• The feather keys are undone to the exact length. There is no need for cutting, measuring, filing, or fitting.

6. Woodruff key

The woodruff key is a shaft key made in the form of a segment of a disk and used with shafts not more than 2¹/₂ inches in diameter.

These semicircular keys when set in place leave a flat end protruding. The keyway is a semi-circular pocket set into the shaft. These pockets can be set by a cutter to set the dimensions. Woodruff Keys are used to improve the concentricity of the shaft for high-speed operation.

The main advantage is eliminating making a keyway near areas that have stress. The materials of Woodruff keys like key steel and stainless steel can be ordered online but they can be found made of other materials such as Aluminum.

MORE: What is a Woodruff key?

Saddle Key

A saddle key is a key for securing a member to a machine shaft that fits into a keyway in the secured member and is concave to grip the shaft by friction compared to a flat key, or sunk key.

Compared to sunk keys, saddle keys are not sunk into the shaft and hub instead they are only sunk into the hub. They either sit on a flat or the circumference of the shaft. Power transmission is achieved through friction between the shaft and the key. The saddle keys are of the following two types:

A flat saddle key is a taper key that fits into the keyway in the hub and lies flat on the shaft. It is likely to round the shaft under load. Therefore, it is used for comparatively light loads.

A hollow saddle key is a taper key that fits into a keyway in the hub and is shaped under the key to fit the curved surface of the shaft. Since hollow saddle keys are gripped by friction, they are suitable for light loads. It is commonly used as a temporary fastener in fixing and installing eccentrics, cams, etc.

Tangent key

Tangent keys are used in heavy-duty equipment. These keys are actually a pair. These pair of keys are placed right angles to each other and tangent to the surface of the shaft, each key will withstand the torsion in one direction only.

The keyway is similar to a parallel key except that it exits the tangent from the outer shaft to the inner shaft. What happens on the side of the heels of each key-way form against which the key sits, and transfers compressive force.

This latter point means that for the reversible motion of the shaft, another key is needed along the tangent in the opposite direction. Typically, this will be offset by 90 ° or 180 ° on the shaft. Keys can be wedge, rectangular, or square-shaped, but rectangular double-taper keys are used exclusively.

Round key

The round key is derived from the key and adding it is what makes the algorithm a block cipher rather than just a permutation. When encrypting a lot of data with a single key, key setup is done once. When brute-forcing a key, the key setup must be done for every key tried.

The round keys are circular in section and fit partially in the shaft and partially drilled holes in the hub. They have the advantage that after assembling the mating parts their key drills can be performed again. Round keys are generally considered best suited for low-power drives.

Splines keys

Occasionally, the keys are formed inseparably with the shaft that fits into the keyway fitted to the hub. Such shafts are known as splendid shafts. These shafts usually have four, six, ten, or sixteen partitions. Peeled shafts are relatively stronger than single keyway shafts.

Spoiled shafts are used when the force to be transmitted is proportional to the size of the shaft as in automobile transmission and sliding gear transmission. By using split shafts, we obtain axial movement as well as a positive drive.

Material Selection for Shaft key

Shaft keys (and machine keys) can be made from a variety of materials. Generally, shaft keys are built from carbon steel or stainless steel but can be made from brass, copper, aluminum, monel, and even nylon, all with material grades. AISI 316 stainless is also prepared for marine applications.

The shaft key material must be chosen based on the operating environment. A designer must very carefully consider the loads that will be applied to the material and what kind of environment will be in the service. Carbon steel is used more broadly than any other material. Carbon steel provides very good strength through heat treatment.

It can be replaced to provide a high degree of strength or wear resistance. Austenitic (300 Series) stainless steel shaft key material should be used where the material will be exposed to a highly corrosive environment. High-strength materials are required and martensitic stainless-steel shaft key material should be used when exposed to mildly corrosive environments.

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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