Mechanical Keys: Definitions, Types, Applications and More (With Pictures)

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Two machine parts are joined with the help of machine elements such as keys, key-ways and splines, in such a way that their relative rotary motion is prevented such as a shaft rotating with a hub.

In the shaft and hub joint, a key is used to prevent their relative motion. Key-ways are slots cut on the shaft and hub to accommodate the keys in between them.

Splines are several keyways cut on the periphery of a shaft.

Welcome to ”Engineers Rail– The number one portal for free Engineering Knowledge”.Let’s jump into Mechanical Keys mainstream…

Let’s dive in with the definition first…


Mechanical Keys-

The Key is a device that makes a temporary joint between the shaft and the hub or boss of the pulley and prevents their relative motion.

It fits into the shaft longitudinally parallel to the axis in such a way that half of the thickness enters the shaft and the other half enters into the hub.

The hub could be of gear, wheel, pulley or coupling etc. The axial motion between the shaft and hub can also be prevented by means of the key.

The key transmits torsional loads between the shaft and the hub. The joints made by keys are temporary in nature and can be disassembled easily.

Slots are cut on the shaft and hub to accommodate the key between them. The slot on the shaft is called the “Keyseat” and the slot on the hub is called the “Keyway“.

The key seat in the shaft and keyway in the hub is cut by machines such as slotter, shaper and milling.

The keys are made of mild steel (cold rolled), wrought iron or medium carbon steel.

Keys are generally made in rectangular sections.

mechanical key and keyways

Types Of Mechanical Keys-

Here are some types of key fasteners and they are classified according to shape and size-

  1. Sunk keys.
  2. Saddle keys.
  3. Tangent keys.
  4. Round keys.
  5. Splines.

The Types of key in mechanical engineering are briefly described below:

1. Sunk Keys-

 These keys are used to transmit high torque in shafts. They sink into a shaft and hub equal to half of their thickness.

Types of Sunk Keys-

Some prominent sunk keys are described below-

  1. Rectangular Sunk Key
  2. Square sunk key
  3. Parallel Sunk key
  4. Gib Head key
  5. Feather key
  6. Peg feather key
  7. Woodruff key
types of keys
I. Rectangular Sunk Key-

It has a rectangular cross-section. It contains a taper of 1: 100 throughout its length. The same taper is given at the bottom of the Key-way of the hub.

rectangular sunk key

II. Square Sunk Key-

It is similar to sunk key and transmits torsional load. Its cross-section is made square to provide easy fitting.


III. Parallel Sunk Key-

It is parallel throughout its length, width and thickness i.e. there is no taper on this key. It transmits light torsional loads.

It is used in shafts and pulleys where relative shifting is required. 


IV. Gib Head Key-

This key has a head of a special shape at one end known as Gib Head. The advantage of this head is that the key can be easily inserted and taken out of the shaft and pulley.

Its upper face is tapered like a rectangular sunk key in the ratio of 1: 100. It is used for transmitting medium and variable torsional loads.


V. Feather Key-

It is a parallel sunk key. This type of key is used for transmitting rotary torque as well as axial displacement of pulley and gear such as in drilling machines, spindles and clutches etc.

This key is rigidly fastened either with a shaft or hub. It is fixed in the shaft by means of screws, apart from this feather keys with jib heads on both sides of the key.


VI. Peg Feather Key-

This type of key carries projecting pegs at the middle of the key or its ends. These pegs fit in the hole drilled in the pulley or hub of the gear.

This key prevents the pulley or gear from sliding along the axis of the shaft.


V. Woodruff Key-

This key is a part of a circular disc of uniform thickness in the form of a segment which is less than half of the disc.

It’s one end is flat and another end is circular. The circular end fits in the key seat which is made on the shaft by a special milling process and adjustments.

woodruff key

2. Saddle Keys-

Saddle keys are used to transmit low rotational torque where keys seat are not required to be made in the shaft.

Types of Saddle keys-

 These fastener keys are made in two shapes and they are-

  1. Flat saddle key
  2. Hollow Saddle key
I. Flat Saddle Key-

This key has a surface which rests on the face made the shaft. It can only transmit low rotational loads because at high loads it can slip.

II. Hollow Saddle Key-

This key has a curved seat which rests on the curvature of the periphery of the shaft. The frictional force between the shaft and key prevents the relative motion between the two parts.

It has a taper on the hollow side. It can transmit less load as compared to a flat saddle key.

It is used on cams, eccentrics etc, to make a temporary joint.


3. Tangent Keys-

These keys are used in pairs and fixed in the opposite direction. They work tangentially i.e, one key transmits rotational load in the opposite direction to the other.

It is used at places where high rotational loads are transmitted in opposite directions such as in rolling mills.

4. Round or Pin keys

Keys with circular sections are called round-or-pin keys. These are either straight or tapered. Round keys are used for small loads, having a taper of 1: 200.

Tapered keys transmit high loads. They are fixed in a hub and shaft drilled together with equal depth of curvature.

5. Splines keys

 It carries slots at equal intervals of shafts called splines.

This type of shaft can transmit heavy loads. The power is transmitted through several splines so the load is distributed equally at all places.

Similarly, the hub also carries splines of the size of splines cut on the shaft. The splines of the shaft and hub are matched with each other to shift the hub on the shaft.

This system is mostly used on machine tools, automobiles, sliding gear mechanisms of gearboxes etc.


Advantages of Keys-

The main benefits of keys are:

  • It can be easily fitted and removed.
  • Its cost is low.
  • In case of overloading, it takes up the load and fails to prevent the main parts from damage. Thus it works like a safety device.

Applications of keys-

applications of keys in mechanical engineering are-

  • Transmitting high torque
  • To handle high torsion loads
  • For making temporary joints
  • In Machine tools
  • Automobiles
  • Gear mechanism
  • Heavy duty applications

Difference between key and cotter-

Cotters are fitted across the axis of parts and transmit tensile and compressive loads. The parts joined by cotters have no rotary motion.

Whereas. keys are fitted longitudinally on the shaft along their length and parallel to the axis. They transmit torsional loads.


Wrapping Up-

The session on Mechanical Keys is now complete; I hope you enjoyed yourself and discovered something new. Please let your friends and coworkers know about this crucial information.

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The Importance of Engineering in Multiple Sectors


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