Cotter Pins 101: Everything You Need to Know

1. What Is a Cotter Pin?

A cotter pin (or cotter key) is a compact fastener designed for axial retention and anti-loosening in rotating shafts, drivetrains, and hydraulic linkages in mechanical systems, serving as either permanent or removable connections.

These wedge-shaped metal pins can slide through aligned holes in parts like shafts or clevis pins, then have their ends bent to lock components in place.

How does a cotter pin work?

When you insert a cotter pin and spread its two legs apart, the resulting friction and mechanical interference prevent the pin from slipping out, even under vibration. This makes them ideal for securing:

• Clevis pins in hitch assemblies
• Wheel axles on trailers
• Machinery linkages

2. Types of Cotter Pins

Cotter pins may appear simple, but engineers have developed specific types of cotter pins to meet different mechanical requirements. The main categories include:

Split Cotter Pins

These straight pins are the workhorse of the family. feature a classic two-prong design with a pre-split shaft. After insertion through aligned holes, the legs are bent outward (typically 45°-90°) to create secure retention. Often sold in cotter pin assortments.

Hairpin Cotter Pins (R-Clips)

These U-shaped cotters look like hairpins. They snap into place without bending and withstand vibration while permitting quick disassembly. Commonly used in linkages of construction equipment.

Locking Cotter Pins

Feature a spring-loaded mechanism for extra security in high-vibration environments. More expensive but reusable.

Stainless Steel Cotter Pins

Constructed from 300-series stainless alloys, corrosion-resistant options are perfect for marine applications, food processing, or outdoor machinery.

Small Cotter Pins

Tiny versions (under 1mm in diameter) for precision instruments, medical-grade material, and electronics.

3. How to Use a Cotter Pin

Follow these steps for proper cotter pin installation:

1. Align the holes in the parts you're securing (like a clevis pin and its mounting point)
2. Insert the cotter pin through the holes - the head should sit flush against the surface
3. Bend the legs:
• For split cotter pins, use pliers to bend each leg 45-90° outward
• For hairpin cotter pins, simply snap the curved end into place
• Trim excess (optional): Cut overlong legs with wire cutters for a cleaner look

Pro Tip: Always bend the legs away from moving parts to prevent snagging.

4. Removing Cotter Pins

When learning how to remove a cotter pin, the key is patience and proper technique. Start by straightening the bent ends with needle-nose pliers. If the pin is stuck, apply penetrating oil (like WD-40) and let it sit for a few minutes. Then, gently wiggle and pull the pin out. Before installing a new cotter pin, always check the hole for any signs of damage.

5. Cotter Pin Sizes and Selection

Cotter pin sizes follow standardized measurements:

• Diameter: Ranges from 0.8mm (small cotter pins) to 13mm
• Length: Typically 10-100mm

Choosing the right cotter pin:

• For general use: Carbon steel cotter pin set
• Harsh environments: Stainless steel cotter pins
• Quick-release needs: Hairpin cotter pin

6. Frequently Asked Questions

Q: Is there a difference between cotters and cotter pins?
A: No, both terms refer to the same fastener, though "cotter key" is an older term still used in some industries.

Q: How do I select cotter pins for a clevis pin?
A: Choose a cotter pin diameter slightly smaller than the clevis pin's cross-hole, and ensure the length allows for proper bending.

Q: Where can I buy a cotter pin assortment?
A: Search "cotter pins near me" for local emergency purchases; hardware stores carry basic sets, while specialized cotter pins are available from JLCMC Industrial Suppliers.