The joining of any two mechanical parts is carried out by two types of joints: Permanent joints, like welding, and Temporary joints, like cotter joints and fasteners. Now, if a part requires faster assembly and easy maintenance, temporary joints are the best choice for manufacturers. For example, joining two shafts over long distances to transmit power is done with temporary joints.
These temporary joints are used to join two coaxial shafts that are subjected to tensile or compressive loads. Let us study these joints, their parts, types, their advantages, disadvantages, and applications in this article.
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.
The cotter used within a cotter joint is typically an elongated wedge, which can be driven into the slots.
Cotter joints are used to support axial loads between the two rods, tensile or compressive. Although a cotter joint will resist 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.
These components were historically used to join connecting rods in steam engines and pumps used to drain mines. Cotter joints were favored because they are relatively easy to design and manufacture, they produce a rigid connection, and they are not prone to working loose in reciprocating machinery. Today, they still find applications in a number of areas, such as anchor bolts. Knuckle joints are often used instead.
A cotter joint is formed by first inserting the spigot end into the socket end. The slots are then aligned before the cotter is driven through the slots in both components. The slots are positions so that as the cotter is driven into position, the spigot is drawn into the socket until a shoulder on the spigot rests securely against the end of the socket. The angle of the cotter is determined by the coefficient of friction and the ease of disassembly required.
Typically, the taper angle is less than the angle of friction, so that no axial force would cause the cotter to push out of the slots. Additional positive locking devices may be used for cotter joints subjected to vibration.
Parts of Cotter Joint
- Spigot
- Socket
- Cotter
#1. 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.
#2. 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.
#3. Cotter.
The term “cotter” refers to a flat, wedge-shaped piece of metal used to join two force-transferring rods that do not rotate. The wedge action keeps the cotter in place when it is inserted into a tapered slot. The value of taper is in the range of ‘1 in 48’ to ‘1 in 24’.
This means that after a length of 48 mm, there will be a 1 mm reduction in cotter size. Generally speaking, the taper on the cotter offers two advantages:
- It offers ease of disassembly, i.e., it is easy to remove the cotter from the taper and disassemble the joint assembly
- It provides rigidity that holds the parts from slipping.
Types of Cotter Joint
- Socket and spigot cotter joint
- Sleeve and cotter joint
- Gib and cotter joint
#1. Socket and Spigot Cotter Joint.
These are types of cotter joints in which one end of the rod is provided with a socket, and another end of the rod is inserted into the socket. The end of the rod goes into the socket known as a spigot. A rectangular hole is formed in the socket with a spigot.
The cotter is then driven across a hole to create a passing connection between the two rods. In this joint, the load is acting axially but reverses its direction. Thus, this joint carries the tensile and compressive loads equally. The collar on the spigot lifts the compressive load.
#2. Sleeve and Cotter Joint.
The sleeve and cotter joint is a type of cotter joint used to connect two coaxial cylindrical rods. It consists of one sleeve and two wedge-shaped tapered cotters. This slot has a cotter mount cut into a sleeve and a cylindrical rod.
In this joint, the taper is approximately 1 in 24, and it may be noted that the taper sides of the two cotters should be facing each other. The clearance is set so that when the cotters are inserted into the two rods, they come close to each other, thus tightening the joint.
Sleeve and cotter joints are simple and rigid enough to carry tensile and compressive forces. They can be easily assembled and dismantled. These are also used to connect two pipes or tubes.
#3. Gib and Cotter Joint.
This type of cotter joint is used where the strap end or larger end of a connecting rod is attached. In some cases, the cotter is driven independently without a gib. Due to this, friction between the ends and the inside slots in the straps causes the sides of the strap to open outwards.
To avoid this, gibs are used, which keep the ends of the strap together. In addition, due to the increased holding power, the gibs provide a more excellent bearing surface for the cotter to slide on. Hence, the movement of the cotter slides backward due to less friction. Gib enables parallel holes to be used as well.
Failure of the Cotter Joint
When you tighten the cotter joint into the socket and spigot slot, the cotter is subjected to shear stress. In this way, the cutter will bend or break at some point when it is on the loose side. The various modes of failure related to the cotter joint are listed below.
- It is due to the rods failing in tension.
- It is due to the spigot failing under tension at the weakest cross-section.
- It is due to the rod or cotter failing under crushing.
- It is due to the socket failing under tension in the slot.
- It is due to the cotter failing in shear.
- It is due to the socket collar failing in crushing stress.
- It is due to the socket end and rod end failing under shear.
- It is due to the spigot collar failing under crushing or shear stress.
- It is due to the cotter failing to bend.
Difference Between Cotter Joint and Knuckle Joint
The main differences between the cotter and knuckle joints are given below:
| Cotter Joint | knuckle Joint |
| A cotter joint is capable of functioning under tensile and compressive load. | Whereas the knuckle joint can work only in tensile load. |
| Cotter joints don’t permit angular movement. | Knuckle joints allow angular movement between rods. |
| With cotter joints there is no issue with respect to bearing failure. | With knuckle joint there are bearing failure issues. |
| A cotter joint has a taper and is provided with clearance. | knuckle joint has no taper and is provided with clearance. |
| Cotter joints are used in cotter foundation bolts, joining two rods with a pipe, and/or for joining piston rods with a c/s head. | Knuckle joints are used in tie bars, links of the bicycle chain, joint for rail shifting mechanisms. |
Advantages of Cotter Joint
Temporary joints have special advantages over permanent joints in any mechanical part assembly or joining process. But let us study the benefits of cotter joints specifically.
- The manufacturing process of the joints is simple.
- After reassembly, the components always occupy the same relative positions, which helps in the correct alignment of the shafts and the joint.
- The process of assembling and disassembling the parts of this joint is simple
- They are conveniently available in industrial markets.
- The joint is sufficiently rigid to support both compressive and tensile loads.
Disadvantages of Cotter Joint
We studied the failure of the joints, which entails a concept of disadvantages. These disadvantages are sometimes the points that distinguish between cotter joints and knuckle joints.
- These joints cannot provide angular momentum
- There is a time-consuming process to provide a taper accurately.
- The joint can sometimes be subjected to rotation, affecting the overall performance of the system.
Applications of Cotter Joint
A cotter joint is used to support the axial load between two coaxial rods, which is the tensile and compressive load. Although a cotter joint will resist the rotations of one rod relative to the other, it should not be used to join a rotating shaft.
- This joint is used between the piston rod and the tail of the pump rod.
- Cotter’s joints are used between the slide spindle and the fork of the valve mechanism.
- Cotter and Dowell arrange to join two parts of a flywheel.
- Foundation bolt: mainly used for fastening foundations and heavy construction machines
- In an automobile engine, the cotter joint is used to connect the extension of the piston rod
- with the connecting rod in the crosshead.
- The Kottar joint has historically been used to connect connecting rods to steam engines and pumps of dumping mines.
- It is used in bicycles to connect the pedal to the sprocket wheel.
- Use a wet air pump to join the tail rod with the piston rod.
- It is used to connect two rods of equal diameter subjected to axial forces.
FAQs.
Q: What is a Sleeve and Cotter joint
This joint is a way of connecting two coaxial cylindrical rods. It consists of one sleeve and two wedge-shaped tapered cotters. The cotter mounts and cut into a sleeve when joining another cylindrical rod.
Q: What is the Gib and Cotter joint?
In some circumstances, the cotter is driven alone, that is, without a gib. Because of this, friction at the ends and inside slots in straps makes the sides of the strap open outward. This can be prevented by using one or more gibs, which hold the ends of the strap together.