Mechanical Joints | Cotter joint
Updated: May 21
What are methods of temporary joints? How is it made?
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While building a structure the assemblies are made using different types of joints, some can be permanent while some can be temporary. Earlier we had looked at a permanent type of joint i.e. a welded joint. The type of joint which is suitable in any instance is determined by the situation and conditions it is being used.
If the requirements are as such to form a rapid assembly and easy maintenance, temporary joining techniques are way to go.
One of the temporary joining methods is a cotter joint. In a cotter joint, two coaxial rods are joined using two components, socket, and spigot. One rod is fitted with a spigot and this assembly is fitted into a socket on one end of another rod. There is a slot in the socket and spigot to accommodate a key. These slots are aligned so that the key can be inserted to lock the two rods together.
Cotter 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.
A cotter can be any object acting as a pin to lock the coaxial rods.
A typical cotter joint assembly is made of three components:
Spigot: A spigot is a solid cylindrical profile, having a slot for the cotter to be inserted. On the neck of the spigot, we can see a collar. This collar allows the spigot to form a clearance at the end of the socket cavity. Also, the collar is useful in aligning the cotter slots in the spigot and socket.
Socket: A socket is a hollow cylindrical profile, having a slot for the cotter to be inserted. The spigot cavity has the dimensions of the spigot to accommodate the spigot through it.
Cotter: It is a wedge-shaped piece of metal acting as a key for the joint. The cotter connects two parts i.e. spigot and socket, which are non-rotating.
Significance of cotter:
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.
Why use a tapered cotter?
There are two benefits of it: 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
The types of cotter joints are distinguished based on the alignment and technique employed by the cotter for interlocking the rods.
Following are the three types of cotter joint to connect two rods by a cotter:
Socket and spigot cotter joint
Two coaxial rods are joined temporarily by means of a socket and spigot. The assembly of the joint is such that, the socket is provided on one end of the rod and the other end of another rod is inserted into the socket. The end of the rod is put into a spigot.
A cotter joint knows 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.
Sleeve and Cotter Joint
This method of cotter joint is used to typically assemble two similar coaxial rods. The assembly includes a sleeve and two cotters with tapered wedge-shaped geometry. There are sections made into the sleeve to make provision for the cotter to be inserted.
The sleeve and cotter joint is the simplest cotter joint. The assembly is simple and is very convenient to build and dismantle. With regards to the load-bearing ability, the joint is rigid due to the sleeve overlapping through both the connecting rods. It can bear both tensile and compressive loads, though it is not suitable to undergo rotation.
The sleeve over the rods prevents the angular misalignment of the rods.
Gib and Cotter joint
A Gib and cotter joint is used to connect rods with rectangular or squared cross-sections. The socket in this joint is a profile with open walls making it a fork-like structure also known as the strap. If the joint is made without the gib, the cotter experiences friction with the strap causes it to deform outwards. In order to prevent this, Gib is used to hold the strap ends together.
The Gib also provides a larger bearing surface for the cotter to slide as the bearing power is increased. It causes the friction to increase, in turn reducing the slack back.
A Gib is like a cotter but with two gib head profiles at its ends. The thickness of the gib and cotter is the same. Sometimes to prevent loosening of the cotter, a small set screw is used through the rod jamming against the cotter.
To prevent this, Gib is used to hold the ends of the strap tdogether. 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 and cotter joints are used for rods of square or rectangular cross-section. The end of one rod fits the end of the other rod which made in the form of a strap. A Gib used along with the cotter to make these joints. Gib is likely a cotter but with two gib heads at its ends. Whit the thickness of the gib and cotter is the same. Sometimes to prevent loosening of the cotter, a small set screw is used through the rod jamming against the cotter.
Let us take a look at what are the benefits and setbacks of using a cotter joint.
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 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 cannot connect cylindrical members that undergo rotation.
A brief history about the invention...
The traces of a cotter joint can be traced to the early 1800s. Farmers in the southern part of Germany used a pin to hook the cart-wagon to the horses. Cotter is the Dutch term for the farmer, hence getting the name cotter joint. Inspired by this various design patents were registered. There is a continuous improvement and update with regards to the cotter assembly.
A cotter joint is a type of temporary joint where two coaxial rods are connected using a spigot and socket assembly.
The rod with the spigot end is inserted in the socket cavity. There is a rectangular slot on the spigot and socket through which the cotter is passed. Cotter acts as a key to block the slippage.
A cotter joint cannot be employed where the connecting rods will be undergoing rotation.
Cotter's joints are mostly found to connect horizontal co-axial rods.
It is easy to assemble and dismantle.
It is used to connect piston rods in a crosshead in a steam engine.
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