Written by Jody Muelaner
Coiled pins are a type of spring pin. Spring pins are designed to press into a hole that’s slightly smaller than the pin, causing it to contract in size. These types of pins exert a constant pressure on the walls of the hole, providing a secure fastener. Chamfers at the ends allow easy insertion into the hole.
Typically a spring pin will be inserted through two or more components with holes aligned, preventing radial movement. One common application is to pin a collar or wheel onto a shaft to prevent the rotation around the shaft and the axial movement along the shaft.
Alternatively, a pin that’s longer than the shaft diameter may be inserted through a hole in the shaft only, creating a shoulder that prevents the axial motion of the collar. Most spring pins, including coiled pins, are made from sheet steel that’s formed into a cylindrical pin.
Slotted pins are the most common type of spring pin. They use a thicker material that’s formed into a partial cylinder, which does not quite meet or overlap at the ends, leaving a slot. These components are sufficient to hold light-duty applications and can also be useful for a more precise location — although a dowel pin will provide the most precision in aligning holes.
Coiled pins are used in heavy-duty applications, and especially if fatigue or shock loading is a concern. Small coiled pins just a few millimeters in diameter are used in precision mechanisms. Bigger pins are used on the joints of large machinery such as heavy construction or earth-moving equipment (think excavators or bulldozers).
Although coiled pins use a lighter gauge sheet material than a slotted pin, the material is wound around multiple times. The walls of the cylindrical pin are made up of at least two layers of the sheet material. This result: a stronger pin. The more material that’s used, the greater shear force the pin can typically withstand. The multiple layers of thinner material also provide greater flexibility and reduce stress concentrations, adding up to considerably better fatigue life.
While solid pins, such as grooved pins and cotters, can withstand higher shear forces, they’re not always ideal for heavy-duty applications. These types of solid pins are also pressed into holes and remain secured by exerting a constant radial spring force against the walls of the hole. However, they’re less flexible, requiring tighter tolerances.
Coiled pins can provide a strong and resilient fastening, increasing the stress around the hole and reducing fatigue resistance.