How Does a Spring Pin Work?

Also known as a roll pin, a spring pin is a type of fastener that’s used to join two or more machine components. They are called “spring pins” because they contain a spring that expands upon insertion. What’s perplexing about spring pins, however, is their size. They are wider than the hole in which they are inserted, leaving many people to wonder how they work exactly.

The Mechanics of Spring Pin

Even though they are wider than the hole in which they are inserted, spring pins work because they deform and then expand. They feature at least one chamber, allowing them to enter the hold. And once inside, spring pins will expand, causing them to compress against the inside walls of the hole. This compression will continue to happen until the spring pin achieves the same diameter as the hole.

Coiled Spring Pins

The most common type of spring pin is coiled. Also known as a spiral spring pin, it features a coiled spring that expands when inserted into a hole. They are made by forming strips of metal into a spiral shape. Upon insertion, a coiled spring pin will begin to expand to create a strong and secure environment for the joined machine components.

There are three universally recognized and accepted standards in which coiled spring pins are made, including the following:

  1. ISO 8750
  2. ISO 8748
  3. ISO 8751

Slotted Spring Pins

Also known as C pins, slotted spring pins consist of rolled strips of steel, iron, aluminum or other metals that expand upon insertion. They rely on the same principle of compression as coiled spring pins, but they feature simpler design. Rather than using a coiled spring, they use strips of metal that expand and compress inside of the hole in which they are inserted.

Spring Pin vs Split Pin: What’s the Difference?

Although they are both used to to join the components, spring pins aren’t the same as split pins. A split pin is simply a metal fastener with two long pieces of metal that are deformed upon insertion. As the split pin bends, it secures the components together. Split pins, however, typically do not revert back to their original shape — not without human intervention, at least.

Spring pins have been around for more than a half-century. The world’s first coiled spring was invented by German aviation mechanic and pilot
Herman Koehl in 1948. Since then, they’ve become a common mechanical faster used in heavy machinery, automobiles, airplanes and even household furniture.

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