Two mechanical components can be connected together using two types of joints, Permanent joints (welding, etc.) and Temporary joints (cotter joints, fasteners, etc).
However, when the shorter assembly time and/or easier maintenance is preferred for a component, the manufacturer prefers the temporary joint over the permanent joint. For example, an application requiring power to be transmitted across a long distance typically will use a temporary joint to connect the two shafts together.
In terms of temporary joints, there are in fact two classes used to connect two co-axial shafts together, produced with either tensile or compressive load.
What is Cotter Joint?
A cotter joint is a way to connect two coaxial rods temporarily. One rod has a spigot, which goes into a socket on the other rod’s end.
The socket has a slot for a cotter that pinpoints the position of the spigot to socket, connecting the two rods.
This type of joint is also called a socket joint or spigot joint. The cotter joint is used to accommodate axial loads between two rods. This may be axial loads in tension or compression.
This type of joint is designed to prevent the rotation of one rod relative to the other rod. It should not be used to join a shaft that rotates. The spigot does not balance the cotter, so if the two rods are subjected to vibration and centrifugal force, the cotter may work loose, which defeats the purpose for which it was designed.
Parts of Cotter Joint
- Spigot
- Socket
- Cotter
#1. Spigot.
The spigot has a rectangular slot to permit the passage of the cotter. The spigot has a collar that rests on the end of the socket. It is called the male portion of the cotter joint.
#2. Socket.
The socket has a rectangular slot to allow the passage of cotter through. The spigot fits in a circular hole in the socket. It is called the female portion of the cotter joint.
#3. Cotter.
The cotter is a piece of metal in the shape of a flat wedge, and is used to connect together two rods that are transferring forces without rotating. The force being transferred is likely either tensile or compressive. The cotter fits into a tapered slot and is held in place by wedge action of the taper on the cotter.
In general, the taper on the cotter has two benefits: 1) it is easy to remove and the entire joint can be disassembled easily, and 2) the taper helps hold the joints tight and does not allow the parts to become loose.
Types of Cotter Joint
- Socket and spigot cotter joint
- Sleeve and cotter joint
- Gib and cotter joint
#1. Socket and Spigot Cotter Joint.
These cotter joints are designed such that one end of the rod is joined by a socket at the end of the other rod such that it fits into the socket. The end of the rod that fits into the socket is called a spigot. The spigot fits into a rectangular hole in the socket.
For a passing connection between the rods, the cotter is driven thru a hole. The load in this joint is acting axially; however, the load will reverse its direction.
Thus, this joint carries the tensile and compressive loads equally. The collar on the spigot carries the compressive load.
#2. Sleeve and Cotter Joint.
A sleeve and cotter joint is a kind of cotter joint for attaching two coaxial cylindrical rods. A sleeve and two wedge-shaped tapered cotters make up the joint. The cotter is mounted within the sleeve inside of a slot cut into the sleeve and cylindrical rod.
In this joint, the taper is approximately 1 in 24, and it should be noted that the taper sides of the two cotters point toward one another. The clearance should be set to allow the cotters to come close together when inserted into the two rods to tighten the joint.
Sleeve and cotter joints are simple and rigid enough to carry tensile and compressive force. The construction of a sleeve and cotter joint also makes it easy to assemble and dismantle. Sleeve and cotter joints are also used to connect two pipes or tubes.
#3. Gib and Cotter Joint.
When the strap end or other larger end attaches to a connecting rod, this type of cotter joint is used. In some cases, the cotter is driven by itself, without a gib.
In this case, the friction between the ends and the inside slots in the straps causes themselves suquitantly be the cotter joint.
Thus gibs are used, holding together the ends of the strap. Also, due to the compound holding power, the gibs provide an even larger bearing surface for the cotter to slide on.
Thus because of friction decreases, the cotter slides backward. Gib enables parallel holes to be used.
Failure of 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 in bending.
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
- They are simple to make, and the components will stay in the same relative position during reassembly.
- You can quickly join two rods using this style of joint. It may be a pipe or a tube.
- The parts of cotter joints can be assembled and disassembled quickly and easily and do not require extra force.
- They are easy to produce and readily available for purchasing.
- They are quite rigid and can support a tensile load as well as a compressive load.
Disadvantages of Cotter Joint
- You cannot connect uneven tubular or rectangular rods through this joint.
- Sleeve and cotter joints can only be used to connect cylindrical components being rotated.
Applications of Cotter Joint
A coaxial joint holds an axial force between the two rods, which are in tensile and compressive stress. The coaxial joint(s) connect the piston rods with a crosshead in a steam engine. These joints connect the piston rod to its extension.
Additionally, it connects the pedals to the sprocket wheels on a bike. The hinges secured to the fork of what’s called the valve mechanism connect the slide spindle to the coil tubes. The cotter and dowel arrangement connects the two halves of a flywheel.
Primarily, cotter joints connect the tail rod with the piston rod of the wet air pump. They’re also used to connect two rods of equal diameter where axial forces exist.
Cotter joints were used on steam engines and pumps to connect the connecting rod for draining mines.
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 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.