Design Procedure for Knuckle Joint & Cotter Joint [Formulas & PDF]

Written by Mohammed SHAFI

A Knuckle Joint is used for the application of the Tie rod joint of jib crane or Tension link in the structure of the bridge. In this article, I am going to present a detailed explanation of the design procedure for Knuckle Joint and Cotter Joint.

Let's see the definition of Knuckle Joint...

Knuckle Joint Definition:

When there is a requirement of an angular moment or a small amount of flexibility, the Knuckle joint is used.  A knuckle joint is used to connect two rods that are under a tensile load.

Knuckle Joint Diagram:

The Diagram of Knuckle Joint is shown below.

Knuckle Joint Diagram

Knuckle Joint Assembly or Parts of Knuckle Joint:

The Assembly of Knuckle Joint consists of 3 parts. They are

  • Fork or Double-eye
  • Single eye
  • Knuckle pin
  • Coller
Knuckle Joint Assembly

There is a single eye at one end of the rod and Double-eye on the end of another rod which is also called Fork End.

Both, Single eye and Double-eye (Fork End) are connected employing a Knuckle pin inserted through the eye.

One end of the knuckle pin has a head section and the other end is tapered along with a hole on its surface for the insertion of Taper pin after assembly as shown in the above figure.

The cross-section at the ends of the rod are of octagonal and is designed for the purpose of Gripping.

The pin is used to hold one eye end of a rod and the fork end of the other rod firmly. Now, the Coller is placed exactly on the surface of the rod having a hole such that the Taper pin can be inserted into it.

Thus the assembly of Knuckle Joint can be done.

Lets see the drawing of Knuckle joint in Orthographic Representation.

Knuckle Joint Drawing:

The Drawing of Knuckle joint in the form of 2D sketch i.e. Front View, Top View, and Side View is shown below.

Knuckle Joint drawing
Knuckle Joint Front view and Top View

Knuckle Joint Failure:

The failures of Knuckle Joint are as follows.

  • Tensile failure of the flat end bar
  • Crushing of pin against the rod
  • Shear (single shear) failure of the pin

Applications of Knuckle Joint:

The applications of Knuckle Joint is as follows.

  • Knuckle joint is mostly used in the Automobile to connect the propeller shaft when two shafts are not straight to each other.
  • Knuckle joint is used as the Tie rod joint of the jib crane.
  • It is used as the Link of roller chain.
  • It is also used in the Tension link in the bridge structure.

Design Procedure for Knuckle Joint is Made to Calculate:

  • The outside diameter of eye or fork (mm)
  • The thickness of each eye.
  • The enlarged diameter of each rod by an empirical relationship.
  • Diameters of the pin by shear considerations.
  • Bending considerations.

Specifications for Knuckle Joint:

The assembly diagram of knuckle joint is as shown in fig.

The dimension of knuckle joint are:

  • Thickness of fork = t1
  • Thickness of single eye = t
  • Diameter of knuckle pin = dp
  • Outside diameter of double eye = dod
  • Outside diameter of single eye = doe
  • Diameter of rod = d
  • Axial tensile force on rod = P
Knuckle joint Derivation
Derivation of Knuckle joint

4. Design of fork (double eye):

Derivation-knuckle joint
The text was taken from

The detailed explanation for the Design Procedure for Knuckle Joint is shown below in the form of video.

I hope you have understood well with the help of above video. Let's discuss about Cotter Joint too...

Cotter Joint:

Cotter's joint is widely used to connect the piston rod and crosshead of a steam engine, as a joint between the piston rod and the tailor pump rod, foundation bolt, etc.

Cotter Joint Definition: 

Cotter's joint is used to connect two rods subjected to compressive or axial tensile loads. It is not at all suitable to connect rotating shafts which transmit torque.

Cotter Joint Diagram:

The Diagram of Cotter Joint is shown below.

Cotter joint sketch

Cotter Joint Assembly:

The Cotter joint Assembly or the Parts of Cotter Joint are as follows.

Cotter joint parts or assembly

Cotter Joint Parts:

The parts of Cotter joint are

  • Socket
  • Spigot
  • Cotter Pin

The explanation for the parts of Cotter Joint are as follows.


The socket is the female part of the Cotter joint which has a hole at the center such that the spigot can fit into it.

It also has a rectangular slot on its surface such that the cotter can pass through it.


The spigot is the male part of the Cotter joint which has also a rectangular slot on its surface such that the cotter can pass through it.

Cotter Pin:

The Spigot is inserted into the socket such that the rectangular slots of both (spigot and Socket) can meet with each other and then the Cotter pin is inserted into it for fixing it firmly.

Cotter Joint Failure:

The Cotter Joint Failures are as follows.

  • Tensile Failure of Rods
  • Tensile Failure of Spigot
  • Tensile Failure of Socket
  • Shear Failure of Spigot End
  • Shear Failure of Socket End
  • Crushing Failure of Spigot End
  • Crushing Failure of Socket End
  • Shear Failure of Spigot Collar
  • Crushing Failure of Spigot Collar
  • Shear Failure of Cotter
  • Bending Failure of Cotter

Design Procedure for Cotter Joint is Made to Calculate:

  • The diameter of each rod
  • Design of Single eye and double eye.
  • The thickness of the cotter by an empirical relationship
  • The diameter of the spigot on the basis of tensile stress
  • Shear failure
  • Crushing failure of spigot etc.

Applications of Cotter Joint:

The applications of Cotter Joint are as follows.

  • It acts as a joint between the crosshead and the piston rod of steam engine.
  • It acts as a foundation bolt.
  • The cotter joint is used between the railway bogies.
  • The cotter joint acts as a steam engine connecting rod strap end.

Advantages of Cotter Joint:

The Advantages of Cotter Joint are as follows.

  • Cotter Joint can take both tensile as well as compressive forces.
  • The Assembly and disassembly of Cotter Joint are quicker.

The remaining part for the design procedure of a cotter joint is presented in the form of a video which is shown below.

This is the complete explanation of the Design Procedure for Cotter Joint and Knuckle Joint which is shown in a detailed manner. If you have any doubts, feel free to ask from the comments section. Please Share and Like this blog with the whole world so that it can reach to many.

Some FAQs on Cotter Joint and Knuckle Joint:

Where is the knuckle joint used?

The knuckle joint is used to connect two rods under tensile load.

What type of joint is knuckle?

knuckle joints are biaxial.

More Resources:

Overdrive in Automobile
Hotchkiss Drive & Torque Tube Drive

References [External Links]:

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