Comprehensive Guide to Keys and Keyways in Mechanical Engineering

In a mechanical engineering field, keyed joints are made up of keys and keyways. These keys can be machined to different forms depending on the type of key joints required. Keyways are machined surfaces on a shaft in the dimension of the key. They are fixed together to ensure no pulling apart of component and prevent a relative movement between a shaft.

 

Today you’ll get to know the definition, functions, applications, components, diagram, types, and working of keys and keyways. You’ll also get to know the advantages and disadvantages of a keyed joint.

Definition of key and keyway

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 a keyway on the shaft and a keyseat the rotating machine element. The key is then tightly fitted between the keyway and the keyseat, hence the mechanism is called a keyed 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 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 shaft and couplings. It ensures efficient transmission of the load, power, and rotation with 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 push. This is often found in agricultural applications, machine tools, industrial equipment, etc.

Milling, slotter, and shaper machine are the common machine tools used to create keyways and keyseats. Although there special machine use for making keyseats.

Components of keyed joints

Since there are various types of keyed joints, key and keyways components will be different depending on the type and additional feature. However, the major components of keyed joints include a key, keyseat, and the keyway.

• Key – the key is usually made of steel machined to 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 of a key, keyway, keyseat (keyed joint)

Types of keys

Below are the various types of shaft keys used in a keyed joint:

Sunk keys:

Sunk keys are types of keys that are designed so that half-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 the depth of 4.1mm 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.

• Square keys

A square key has a square cross-sectional 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 just as shown in fig below.

• Parallel sunk keys

Parallel sunk keys are either rectangular or square sections but not having 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 push 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 be only machined by a woodruff keyway cutter. It’s mostly used in machine tools and some automobile shafts from ¼” to 2½” (6mm to 60mm) diameter. The strength a woodruff key will offer is extremely different from the long parallel keys, this means they cannot carry the same load.

Saddle keys:

Unlike sunk keys, saddle keys are not sunk into shafts but 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 key are tapered at the top and flat at the bottom as it fits 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.

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 shafts.

Round/circular keys

Round keys are circular in section, 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.

Working principle

The working of key and keyway is less complex and can be easily understood. Since there are various types of keys and keyways setups, the working of some types will be completely different. Well, the video below will teach you how the common keyed joint mechanism works.

Watch the video below:

Advantages and disadvantages of keys and keyways (keyed joints)

Advantages:

Below are the advantages of shaft keyed joints in their various applications:

• 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).

Disadvantages:

Some limitations still occur despite the benefits of keyed joints. Below are the disadvantages of keyed joints in their various applications:

• It’s cannot be used for alternating directional loads and shocks.
• Too much securely fixed keyed joint 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 circlip or set screw.
• It is difficult to calculate and combine the load-carrying and the tolerance stack analysis.

Conclusion

Keyed joints are one of the great mechanical ways of transmitting power. They are used mostly in a motor shaft to achieve their purpose. In this article, we’ve covered the definition, function, applications, components, types, and working of keys and keyways that made up a keyed joint. We’ve also seen their advantages and disadvantages in their various applications.

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