Written by Dr. Jody Muelaner
Spiral pins are also referred to as coiled spring pins, or simply coiled pins. They are a type of spring pin, normally produced by rolling a sheet of sprung steel into a spiral, to form a pin. They have a chamfer at each end enabling them to be inserted into a hole of a slightly smaller diameter.
When forced into the hole, the sheet material flexes and slides over itself so that it reduces in diameter. Once in position, its tendency to expand, exerts a radial force against the walls of the hole. The resulting friction prevents the pin from easily coming loose 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 normally inserted through a radial hole in the shaft. The protruding pin then acts like a shoulder on the shaft, preventing movement past the pin.
Spring pins may also be used to join or locate components with pins 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 which is stronger and more able to withstand shear loading.
Made from layers of a thinner sheet, spiral pins 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. Typically, slotted pins are used for light duty use or where a more rigid connection is required, for example 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.