Written by Jody Muelaner
Spiral pins are also referred to as coiled spring pins or, simply, coiled pins. They are a type of spring pin, typically produced by rolling a sheet of sprung steel into a spiral to form a pin. They’re used to provide enhanced strength, flexibility, and fatigue resistance.
Spiral pins have a chamfer at each end, so they can be inserted into a hole of a slightly smaller diameter. Once fitted inside the hole, the sheet material flexes and slides over itself so that it’s reduced in diameter.
After the pin is left in position, it tends to expand, which exerts a radial force against the walls of the hole. The resulting friction prevents the pin from easily coming loose or falling out of the hole.
There are two main types of spring pin: slotted pins and spiral pins. Spring pins are often used to prevent parts from sliding axially along a shaft. In this case, a pin — which is longer than the shaft’s diameter — is usually inserted through a radial hole in the shaft. The protruding part of the pin then acts as a shoulder on the shaft, preventing any movement past it.
Spring pins may also be used to join or locate components — for example, when pins are inserted through concentric holes in the components. This approach can be used on parts of any shape, including shafts and plates.
Slotted pins use a shorter length of thicker sheet material, rolled through less than 360 degrees to leave a slot where the edges of the sheet approach each other. Spiral pins are made from a thinner material, however, the spiral results in walls of at least two layers of this sheet. The increased amount of material results in a pin that’s typically stronger and better able to withstand shear loading.
Spiral pins, which are made from layers of thin sheets, are also more flexible than slotted pins. This reduces stress concentrations and improves fatigue life. Spiral pins are, therefore, better than slotted pins for heavy-duty applications.
Generally, slotted pins are used for light-duty use or where a more rigid connection is required (such as a precise location). A solid pin can withstand higher absolute sheer forces than a spiral pin, although without the benefits of flexibility for fatigue resistance. Examples of solid pins include dowel pins, grooved pins, tapered pins, and cotters.