Cotter pins are simple metal fasteners with a split design used to secure and lock various components together by passing through a hole and spreading their ends apart. These are regarded as essential fasteners and play a vital role in various mechanical and industrial applications. They are designed to secure components in place, prevent unintended movement, and ensure the integrity of critical connections.

This blog will throw light on Cotter Pin, their working, types, parts and applications. This topic in mechanical engineering is important for your upcoming examinations like SSC JE ME and RRB JE Mechanical Engineering.

What is a Cotter Pin?

A cotter pin, also known as a split pin or cotter key in the USA, is a wedge-shaped metal fastener inserted through aligned holes in components, securing them temporarily by bending its ends to prevent dislodgement. These pins come in various designs and sizes, typically used to prevent axial motion along a shaft by generating friction through compression as they enter the holes. This ensures the secure fastening of components in engineering and mechanical systems, especially where movement or mechanical vibration might cause loosening.

Fig 1: Cotter pin

The modern cotter pin features a flattened inner surface that runs along its length, giving it the appearance of a split cylinder. This design ensures that the pin remains securely in place as both edges of the pin flex, creating a tight fit. However, it's important to note that flex fatigue can occur when removing the pin, which necessitates discarding and replacing it.

Cotter pins are typically made from soft metals, making them easy to install and remove, but they should not be used in situations where shear forces are anticipated. These pins can be constructed from materials such as aluminium, mild steel, stainless steel, bronze, or brass.

Types of Cotter Pins

Cotter pins come in various types, and each is used for a specific application and load conditions. Split cotter pins are the most commonly used, with two tines that are bent to lock the component. It is important to use the right cotter pin as per the operation needs and strength. Various types of cotter pins with a detailed explanation are provided below

• Split cotter pins 
• R-pins or R-clips
• Standard cotter pin
• Spring cotter pin
• Barrel cotter pin
• Dipped cotter pin
• Straight cotter pin
• Cotter pin with a key

1) Split Cotter Pins:

Split cotter pins are a popular type, crafted from a semicircular wire bent over to create bulbous heads. Typically installed in pre-drilled parts and secured with a deformable tine, they serve as brakes for clevis pins. They are usually single-use fasteners, available in various designs based on wire bend and connection.

Fig 2: Split cotter pin

2) R-pins or R-clips:

Also known as hairpin pins, R-pins feature a straight section bent to form a ring or loop on the head, balanced with a bent section. The curved part clamps to the outside of the shaft, while the straight section enters the shaft through a hole secured in place. An accessible ring on the head allows easy removal.

3) Standard Cotter Pin:

Standard cotter pins may have curved or straight designs, providing a secure grip when connecting parts manually. They feature two different ends—one for connecting with another pin and the other for attaching to an object.

Fig 3: Standard cotter pin

4) Spring Cotter Pin:

Spring cotter pins incorporate a spring to generate tension for enhanced gripping power. They are used in conditions where applying pressure on objects, such as hoses and pipes prone to rotation or movement, is challenging. These pins come with either curved or straight "T" shaped ends, chosen based on installation requirements.

Fig 4: Spring cotter pin

5) Barrel Cotter Pin:

Barrel cotter pins resemble cylinders with rounded sides and feature two different ends—one short and one long. They find utility in threaded joints, such as connecting hoses or pipes.

6) Dipped Cotter Pin:

Dipped cotter pins have a flat-headed end with a hole on one side and a "T" shape on the other. They are often used to secure dust caps to mandrels around engine shafts, creating airtight seals when in contact with other components.

7) Straight Cotter Pins:

Straight cotter pins appear cylindrical with no rounded edges and feature straight ends. Their design is ideal for securing frequently moving objects, preventing unintended detachment.

Fig 5: Straight Collar pins

8) Cotter Pin with a Key:

Cotter pins with key-like ends provide added security. They often feature additional keys for increased stability, especially in applications like securing holes in wood where significant force is applied, and the pin is removed before disassembly.

Comparison of Types of Cotter Pins

Cotter pins vary in design, strength and application, which makes their comparison essential for selecting the right type. The Cotter pins with keys ensure tight locking but are bulky. Choosing the right type depends on load, environment, and frequency of disassembly. Candidates can refer to the table provided below for more information.

What is a Clevis Pin?

Clevis fasteners consist of a clevis pin and clevis, featuring holes at the prong ends for pin insertion; these pins secure tangs within the clevis gap and enable rotational movement, with various types of shackles available alongside clevis and pins for different applications, such as straight and twist shackles, where clevis pins, distinct from bolts, are preferred for their safety and stability in withstanding lateral stresses.

Fig 6: Clevis Pin

Disadvantages of Cotter Pins

• They are single-use; once bent, they cannot be reused and must be replaced.
• Cotter pins are not suitable for high-load applications. 
• If they are not installed properly, they can loosen over time. 
• Depending on the material, they may corrode in harsh environments

Applications of Cotter Pins

The various uses of Cotter pins are:

1. Securing bicycle pedal cranks.
2. Locking castle nuts on axles.
3. Fastening hitch pins in trailers.
4. Holding nuts and bolts in machinery.
5. Creating moving parts for DIY projects.
6. Locking clevis pins in place.
7. Ensuring safety in heavy machinery.
8. Joining two or more components together.
9. Making pinwheels for gardens.

This article provided insights into Cotter pins. We suggest you appear in the SSC JE Mechanical mock tests and SSC JE ME Previous Years Papers. Also, get enrolled in the AE/JE Mechanical coaching to get a firm grip on the subject.

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