In the mechanical engineering field, shaft keys are made up of keys and keyways. These keys can be machined into different forms depending on the type of key joints required for the application.
Keyways are machined surfaces on a shaft in the dimension of the key. They are fixed together to ensure no pulling apart of components and prevent relative movement between shafts.
Well, in this reading, we’ll explore what keys and keyways are, their applications, parts, diagrams, materials, types, and how they work. We’ll also explore their advantages and disadvantages.
Let’s get started!
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What are Keys and Keyways (shaft keys)?
A key is a machine element that is used to join a rotating machine element to a shaft. For a key to function, there must be a keyway on the shaft and a keyseat on the rotating machine element. The key is then tightly fitted between the keyway and the keyseat; hence, the mechanism is called a shaft key or key joint.
The primary function of keys and keyways is to make up keyed joints to secure the hub and shaft so that relative movement between a power-transmitting shaft and an attached component won’t occur.
A keyed joint often allows relative axial movement between the components. This key prevents relative rotation between the two parts and may enable torque transmission.
Applications
A keyed joint is used as an important part of mechanical power transmission elements, shafts, and couplings. It ensures efficient transmission of the load, power, and rotation without slipping.
A good example of a key and keyway application is gear drives, pulleys, or sprockets tightly connected with a key to the power-transmitting shaft.
Widely in the mechanical field, keyed joints are a convenient method of creating a secure connection between shafts and sprockets. Although it is used as an alternative on other shaft applications where the load needs to be pulled or pushed. This is often found in agricultural applications, machine tools, industrial equipment, etc.
Milling, slotter, and shaper machines are the common machine tools used to create keyways and keyseats. Although there special machine used for making keyseats.
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Components of Keys and Keyways
Since there are various types of keyed joints, key and keyway components will be different depending on the type and additional features. However, the major components of keyed joints include a key, a keyseat, and a keyway.
- Key – the key is usually made of steel machined to a rectangular, square, or circular shape. It’s inserted between the shaft and the hub of the component in an axial direction to prevent relative movement.
- Keyseat – this is the technical name of a recess area in the shaft. It’s often performed on a shaper machine.
- Keyway – a keyway is also a recess in the hub that receives the key and securely locks the mechanism. The keyseat and the keyway shares half portion of the key. This will be further explained in the working principle section.
Diagram
Types of Keys
Below are the various types of shaft keys used in a keyed joint:
1. Sunk Keys
Sunk keys are types of keys that are designed so that half the thickness of the rod is sunk into the shaft and the remaining half will be for the keyseat. For the attainment of this keyed joint, accuracy is very important.
For instance, if a key is 8mm, the shaft keyway will be machined at a depth of 4.1 mm, the same as the keyseat. Sunk keys are of different types, which include
Rectangular/square keys
These types of keys are sometimes called flat keys, as they are wider than their height. They are often used on shafts of about 500mm or 20 in diameter.
The remaining key width allows greater transmission torque without increasing the depth. An increased depth means a weaker shaft due to a reduction in effective shaft cross-sectional area.
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Square keys
A square key has a square cross-section and is used on a shaft of about 25mm or 1”. They are used on larger shafts when deeper key depth is required compared to rectangular keys.
However, an increase in depth means a weaker shaft because of the reduction in effective shaft cross-sectional area. A square or rectangular key may have a taper of 1 in 100 along the length of the key.
Parallel sunk keys
Parallel sunk keys are either rectangular or square sections but do not have tapers. The keys are cheap and readily available for use. It’s one of the easiest to install, but it needs to be held with a set of screws to the hub. This is because the vibration or rotational direction reversal often pushes out the key.
The key is generally tightly fitted to the bottom of the shaft keyway and the sides of the keyed joint. They leave a clearance at the top of the hub keyway.
Gib head sunk key
A gib head sunk key is either rectangular or square in shape with a taper on the top surface to get a tight fit. It’s employed so that it can be easier to remove.
Feather keys
A feather key has three types: double-headed, peg feather, and feather key. They are attached to either the shaft or the hub to cause a relative axial movement. Thus, it enables power transmission between the shaft and hub with its parallel opposite faces. Hence allowing it to slide.
Woodruff keys
A woodruff key is a semi-circular disc fit into a circular recess of a shaft, which can only be machined by a woodruff keyway cutter. It’s mostly used in machine tools and some automobile shafts from ¼” to 2½” (6mm to 60mm) in diameter.
The strength a Woodruff key will offer is extremely different from the long parallel keys; this means they cannot carry the same load.
2. Saddle Keys:
Unlike sunk keys, saddle keys are not sunk into shafts; instead, they are sunk into the hub only. They either sit on a flat or circumference of the shaft, while the power transmission is achieved through the friction between the shaft and the key.
Saddle keys are subdivided into two: flat saddle and hollow saddle keys. They are used only for light loads to avoid slipping along the shaft.
Flat saddle key
Flat types of saddle keys are tapered at the top and flat at the bottom as they fit into a tapered hub keyway, pushing down on the flat face of the shaft.
Hollow saddle key
A hollow saddle key is tapered at the top and curved at the bottom edge. The key is fit into a tapered hub keyway and pushed down on the curved circumferential surface of the shaft.
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Tangent Keys:
These types of keys are sometimes called tangential keys. They are fitted as a pair at a right angle where each key withstands torsion in one direction only. They are often used in large, heavy-duty equipment.
The pair of keys is placed at a right angle to each other and tangent to the surface of the shaft. This design will allow each key to withstand the torsion in one direction.
A later point that indicates a reversible motion of the shaft shows another key is required along the tangent in the opposite direction. This is often offset by 90 or 180 degrees.
Round/circular Keys
Round keys are circular in section and inserted into holes partly drilled in a shaft and hub. These types of keys are easy to manufacture, as the keyways may be drilled or reamed after the mating parts are assembled. Round keys are suitable for low-power drives.
Splines Keys
These types of keys are formed with a shaft known as a splendid shaft that tightly fits into the keyway, which is fitted to the hub. These shafts can be designed to have four, six, ten, or sixteen partitions.
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How Do Keys and Keyways (Shaft Key) Work?
The working of key and keyway is less complex and can be easily understood. Since there are various types of keys and keyway setups, the workings of some types will be completely different.
Well, the shaft keys are made with metal used to connect a rotating machine element to the shaft. This helps to prevent a relative rotation between the two machine parts as torque transmission occurs.
Keyway and keyseat must be available on both the shaft and the rotating elements like gears, pulleys, and couplings. One part of the shaft key must fit into a precut slot, which is the keyseat, and the other part fits into a slot in the hub known as the keyway.
We can also say keyways are slots (grooves or pockets) machined in a hub of a pulley, sprocket, gear, or other rotating device. These slots are cut on machine tools like broaching, milling, planing, shaping, and slotting.
The keyed joint is the final product of a shaft key where it has been mounted on the device where it ensures the connection transmits the load, power, and rotation. Accuracy is very important during the design to avoid slippage.
Advantages and disadvantages of keys and keyways (keyed joints)
Advantages:
- Their production cost is drastically low.
- Offers medium to high torque transmission.
- Easy to mount and dismount.
- Can be reused or fixed on other components of the same type.
- It’s a standard method (ISO, BS, DIN, & ANSI).
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Disadvantages:
- It cannot be used for alternating directional loads and shocks.
- Too many securely fixed keyed joints might become difficult to dismantle.
- Causes shaft imbalance.
- They introduce stress points due to the notch effect and reduce shaft strength.
- Possible axial displacement of hub unless locked by an extra component such as a circlip or setscrew.
- It is difficult to calculate and combine the load-carrying and the tolerance stack analysis.
Conclusion
Keys and keyways are vital mechanical elements used to connect rotating machine elements like gears or pulleys to shafts. They ensure torque transmission without relative motion between the shaft and the component.
Different types of keys—such as square keys, flat keys, Woodruff keys, and splines—are chosen based on the application, torque requirements, and ease of assembly. Proper keyway design and fit are critical to the durability, safety, and efficiency of rotating equipment.
FAQs on Keys and Keyways
What is a key in mechanical engineering?
A key is a machine element inserted between a rotating shaft and a component (like a gear or pulley) to prevent relative motion and transmit torque.
What is a keyway?
A keyway is the slot or groove cut in both the shaft and the mating component to accommodate the key.
What are the common types of keys?
- Square key
- Flat (rectangular) key
- Woodruff key
- Gib head key
- Spline (multiple keyways)
What is the purpose of a Woodruff key?
A Woodruff key provides a better fit in curved keyways and is often used in high-speed or precision applications.
How do keys transmit torque?
The key fits tightly in the keyway and locks the shaft and hub together so they rotate as one unit, transmitting torque through shear resistance.
What causes key or keyway failure?
- Improper fit or installation
- Overloading
- Fatigue from cyclic stresses
- Poor material selection
How are keyways manufactured?
Keyways are commonly produced by broaching, milling, shaping, or EDM (Electrical Discharge Machining).