A cotter pin is a reusable wedge or a pin that passes through a hole in a fastening element, either a bolt or a clevis pin, and secures it in place. They’re primarily employed as locking pins to secure fastening parts in high-stress load applications in which fasteners are subjected to significant stress and vibration.

Source: Wikimedia Commons

However, due to their simplistic design, they’re also used for fastening parts in very light applications that don’t imply high-stress loads and excessive vibration. Split pins aren’t the only type of cotter pin on the market; currently, there are a dozen standard cotter pin types available.

In this article, we’ll discuss some of the most common types of cotter pins and their applications, as well as a quick reference guide for different pins.

Different Types of Cotter Pins

Cotter pins come in a variety of shapes and sizes. Here are some of the most common types of cotter pins:

• Prong Cotter Pin — Prong cotter pins are more commonly known as split pins, which consist of a narrow U-shaped piece of metal with a circular top. These provide a secure bond in applications involving vibration and rotation, as they’re inserted into pre-drilled holes, and their tines are bent in the opposite direction, preventing their spontaneous removal. A variation with a bent longer prong is called a hammerlock pin.
• Hairpin Cotter Pin — Also known as R-clips of hitch pin clips, are reusable and easily removable in less demanding applications. They can be installed and removed without tools, which makes them ineffective in high-vibration and rotation applications, as those may cause accidental disengagement.
• Rue Ring Locking Cotter Pin — Rue ring cotters are among the safest self-locking cotters, as they enclose the shaft and lock themselves in place. They’re additionally tightened by a manually-triggered secondary lock which helps reduce further vibration. They’re perfect for light- to medium-load applications in both vibrating and rotating conditions.
• Bow-Tie Locking Cotter Pin — Bow-tie locking pin is a self-locking variation of an R-clip, which is locked in a zero-tension state when its pin leg is pushed into the drilled mating hole. It significantly lowers the chance of falling out, which makes bow-tie pins perfect for rotating and vibrating applications.
• SLIC Pins — This is an interesting type of cotter pin since it’s classified as a cotter pin but resembles and acts as a clevis pin. SLIC stands for Self-Locking Implanted Cotter Pin, since it acts as a clevis and a cotter pin all in one and functions as a quick-locking pin wherever such functionality is deemed necessary.
• Tapered Pins — Tapered pins are cylindrical or wedge-shaped pins that are typically used to prevent axial movement along the shaft. Their tapered nature means they’re compressed as they’re driven into the hole, causing friction and preventing them from working loose.

Here’s a quick reference guide for several types of cotter pins:

	Bow-Tie	Rue Ring	Circle	Split Ring
Common Names	Double Bridge Pin, Bow Tie	Ring	Cotter Ring	Circle Ring
Applicable Standards	-	-	-	-
Fabrication	The wire formation Part requires more specialized tooling and heat treatment for spring tempering.	The wire formation Part requires more specialized tooling and heat treatment for spring tempering.	The wire formation Part requires more specialized tooling and heat treatment for spring tempering.	The wire formation Part requires more specialized tooling and heat treatment for spring tempering.
How to Identify	Nominal wire diameter × nominal length	Nominal wire diameter × nominal length	Wire diameter × outside diameter	Wire diameter × outside diameter
Common uses	Extra loop provides additional security. The pin can be manually installed or removed.	Heavier duty versions use a heavier gauge wire. When installed, the ring encircles the mating pin locking itself in place.	Lighter duty. Used in applications where protection from pong ends is required.	Heavier duty, for use as a handle for removal, rather than a locking device.
Comments	More secure than a standard bridge pin.	A straight prong can be bent over to provide a nearly immovable lock feature.	Uses a round wire cross-section.	Manufactured from heavier duty half-round wider, which enhances threading through the hole of the mating part.

 

Cotter Pin Applications

Source: Reid Supply

There’s a vast application of cotter pins in various types of machinery, especially in aerospace, automotive, and maritime industries. In a vast majority of said applications, these metal fasteners are used to secure pairing components from disjoining or disengaging.

Depending on the application and stress loads, cotter pins also come in a variety of materials, including zinc-plated high-carbon steel, mild steel, and stainless steel, though they can be made from softer metals as well.

Summary

There are more than a dozen types of cotter pins, but most industrial applications rely almost exclusively on the aforementioned types, with a few notable exceptions in special applications.

If you want to learn more about cotter pins, or any other types of fasteners, visit Reid Supply, one of the largest North American parts and components suppliers, with an extensive catalog of parts, and a download library filled with professional literature.