What is a Cotter Joint?
A cotter joint, also known as a socket and spigot joint, is a method of temporarily joining two coaxial rods. One rod is fitted with a spigot, which fits inside a socket on one end of the other rod. Slots in the socket and the spigot align so that a cotter can be inserted to lock the two rods together.
Cotter’s joints are used to support axial loads between the two rods, tensile or compressive. Although a cotter joint will resist the rotation of one rod relative to the other, it should not be used to join rotating shafts. This is because the cotter will not be balanced and may work loose under the combination of vibration and centrifugal force.
The Cotter joint assembly consists of three components:
- Spigot: It is the male part of the joint, it has a rectangular slot for passing the cotter through it. The spigot has a collar that rests against the socket end.
- Socket: It is the female part of the joint, it also has a rectangular slot for passing the cotter through it. It has a circular hole in which the spigot fits.
- Cotter: It is a wedge-shaped piece of metal that actually connects two parts that are non-rotating.
Cotter is a flat wedge-shaped metal piece, used to connect two rods that transmit the force but without rotation. The force may be axial and of tensile or compressive nature. Cotter is fitted in the tapered slot and remains in its position because of wedge action. This happens because of taper.
Generally, the value of taper on cotter is 1 in 48 to 1 in 24 and it provides mainly two benefits: First, this tapper is easy to remove and makes disassembling of joint a simple process. Second, the tapper also ensures the tightness of the joints and prevents the parts from loosening.
Types of Cotter Joint
Following are the three types of cotter joints to connect two rods by a cotter:
- Socket and spigot cotter joint
- Sleeve and cotter joint
- Gib and cotter joint
1. Socket and spigot cotter joint
A cotter joint known as a socket and spigot joint, is a method of temporarily joining two coaxial rods, one end of the rod provided with a socket type and the other end of the other rod inserted into the socket. The end of the rod goes into a socket also called a spigot.
A rectangular hole is built into the socket and spigot. A cotter is tightly operating through a hole to make a temporary connection between the two rods. The load is usually acting axially, but it changes its direction. And so, the cotter joint should design to carry both tensile and compressive loads. Compressive load is taken by the caller on the spigot.
2. Sleeve and Cotter Joint
A sleeve and cotter joint is a type of joint used to typically connect two similar coaxial cylindrical rods. It includes a sleeve and two wedge-shaped tapered cotters. Appropriate slots are cut in the sleeve and in the cylindrical rods.
The cotter is mounted in these slots. The taper of the cotter is usually 1 in 24. It may note that the taper sides of the two cottars must face each other. The clearance is so adjusted that when the coaters are driven in two rods, they come close to each other and thus tighten the joint.
The sleeve and cotter joint is the simplest cotter joint in existence. It is quite rigid and can take both tensile and compressive loads. it can be easily assembled and dismantled. The joint can also be used to connect similar pipes and tubes.
Rectangular rods and dissimilar cylindrical rods cannot connect using this joint. A sleeve and cotter joint does not allow angular misalignment between its cylindrical members. It cannot connect cylindrical members which undergo rotation.
3. Gib and Cotter joint
A Gib and cotter joint is commonly use at the strap end or the big end of a connecting rod. In some cases, when the cotter alone i.e without gib is run. The friction between its ends and the inside slot in the strap causes the edges of the strap to open outward.
To prevent this, Gib is used that hold the ends of the strap together. In addition, the gator provides a larger bearing surface for the cotter to slide as the holding power increases. Thus, the friction of the cotter reduces the tendency to slack back. Gib also enables parallel holes to use.
Gib and cotter joints are used for rods of square or rectangular cross-sections. The end of one rod fits the end of the other rod which is made in the form of a strap. A gib is used along with the cotter to make these joints. Gib is likely a cotter but with two gib heads at its ends. The thickness of the gib and cotter is the same.
- When one gib is used, the cotter with one side tapered is provided and the gib is always on the outside.
- When two gibs are used, the cotter with both sides tapered is provided.
- Sometimes to prevent loosening of the cotter, a small set screw is used through the rod jamming against the cotter.
Application of Cotter Joint
Following are the cotter joint applications:
- It is used to connect piston rods in a crosshead in a steam engine.
- It is used to connect the piston rod with its extension.
- It is also used in bicycles to connect pedals to sprocket wheels.
- Foundation Bolt
- The joint between the slide spindle and the fork of the valve mechanism
- Cotter and dowel arrangement to join the two halves of a flywheel.
- Use for joining a tail rod with the piston rod of a wet air pump.
- It is used to connect two rods of equal diameters subjected to axial forces.
- The cotter joint is used to connect the piston rod’s extension with the connecting rod in the crosshead in an automobile engine.
- Cotter joints were historically used to connect connecting rods in steam engines and pumps for draining mines.
Advantages of Cotter joints
- Cotter joints can be easily made, and the parts always occupy exactly the same relative positions after reassembly
- The joints can be used to connect similar pipes and tubes.
- Assembling and dismantling of cotter joints parts are quick and easy and don’t require special efforts.
- cotter joints are simple and easy to manufacture and obtain in markets.
- It is quite rigid and can take both tensile and compressive loads.
Disadvantages of cotter joints
- Rectangular rods and dissimilar cylindrical rods cannot be connected using this joint.
- The sleeve and cotter joints that cannot connect cylindrical members undergo rotation.
Failure of Cotter joint
When the cotter joint tightened in the socket and spigot slot, it was exposed to shear stresses. Then if the cotter is loose, the cotter will bent. The various modes of failure in cotter joint are discussed below:
- Failure of the rods in tension
- Failure of the spigot in tension across the weakest section
- Failure of the rod or cotter in crushing
- Failure of the socket in tension across the slot
- Failure of cotter in shear
- Failure of the socket collar in crushing
- Failure of socket end in shearing
- Failure of rod end in shear
- Failure of spigot collar in crushing
- Failure of the spigot collar in shearing
- Failure of cotter in bending
In this article we learn about the cotter joint, this is one type of mechanical joint and, also known as a socket and spigot joint, is a method of temporarily joining two coaxial rods. One rod is fitted with a spigot, which fits inside a socket on one end of the other rod. Slots in the socket and the spigot align so that a cotter can be inserted to lock the two rods together.