A cotter joint (also known as a socket and spigot joint) is a mechanical component used to connect two rods or shafts together so they can transmit axial tensile or compressive forces. It’s a simple and commonly used joint in mechanical engineering. 

The cotter joint has three components: 

1. A socket or a hole in one rod or shaft. One of the rods or shafts has a hole or socket drilled through it near its end. Typically, the hole is rectangular or square-shaped and is sized to accommodate the pin or cotter. 

2. A pin or cotter. The pin or cotter is a cylindrical or tapered metal component inserted through the hole in the first rod or shaft — extending beyond the hole on both sides. 

3. A wedge. A wedge is driven into the gap between the pin’s or cotter’s ends to secure it in place. Usually, the wedge is tapered and driven perpendicular to the joint’s axis.

When the cotter joint is assembled, the pin or cotter is inserted through the hole in the first rod or shaft. The wedge is then driven into the gap, causing the pin or cotter to expand and tightly grip the walls of the hole. This creates a secure connection between the two rods or shafts. 

Applications
Historically, cotter joints were used to join connecting rods in steam engines and pumps used to drain mines. They’re relatively simple to manufacture and assemble, providing a reliable, rigid joint that can withstand considerable force. Cotters are also not prone to loosening in reciprocating machinery.

Today, cotter joints are often used to connect piston rods to crossheads in reciprocating engines, join levers to shafts, or secure various other types of connections that require axial-load transmission. 

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 become unbalanced and may work loose under the combined vibration and centrifugal force.

Installation
Here are a few tips for the effective use of a cotter joint: 

• Consider the size and load: Ensure the cotter joint components — including the hole, pin or cotter, and wedge — are appropriately sized and matched. The dimensions should be based on the load requirements and the size of the rods or shafts being joined. For example, the cotter should be designed (and installed) to handle the expected forces, vibrations, and temperature variations.

• Keep the surfaces clean: Before assembly, make sure the surfaces of the hole, pin, and wedge are clean and free from dirt, debris, or burrs. Smooth surfaces ensure a better fit and minimize the risk of binding or damage during installation. Depending on the application, apply a small amount of lubrication on the pin and wedge to prevent excessive friction during assembly.

• Proper insertion: Align the holes in the rods or shafts accurately before inserting the pin, and ensure the pin extends evenly on both sides of the joint. Avoid forcing or hammering the pin. Misalignment can cause stress concentration, reducing the joint’s strength. Place the wedge in the gap between the ends of the pin or cotter and use a suitable tool (such as a hammer or mallet) to drive the wedge into place gently. Ensure that the wedge is driven perpendicular to the joint’s axis and fully seated.

• Double-check the fit: Verify that the pin or cotter is securely in place and tightly grips the walls of the hole. There should be no significant movement or play in the joint. If necessary, adjust the wedge or recheck that the assembly is secure.