Fasteners are usually the smallest, least expensive components within an assembly. Even though fasteners usually hold the entire assembly together, they are often overlooked until the end of the design.
As important as fasteners are, Design Engineers usually receive no formal training on the mechanics of fastening and joining. (Not something you want to think about next time you are on an airplane.)
An assembly can be held together in many ways: bolts, rivets, screws and pins, to name a few. The methods fall into two broad categories:
- Methods that take two components or two operational steps to stay in place.
- Methods where the components are self-retaining.
The challenge for the Design Engineer is to choose a method that provides the highest quality joint with integrity over time at the lowest manufacturing cost. For many applications, a self-retaining pin is the winning solution.
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The difficulty is selecting the proper pin with the appropriate strength and flexibility for the application. Technically, when a load is applied, something has to give: the pin, the hole, or an element of the assembly.
A pin that is too rigid causes the hole in which it is retained to elongate and leads to eventual assembly failure. A pin that is too flexible will fatigue under dynamic loading.
Solid Pins
Solid Pins come in many different forms that include:
Dowel pins.
Knurled pins.
Grooved pins.
In general, Solid Pins are strong, and relatively inflexible. They do not absorb shock and dynamic loads, but rather transmit these to the mating components. While there are many applications where a Solid Pin is an effective solution, the dynamic forces in many applications need to be closely examined. For example, there is a common paradigm that
Solid Pins are the best option for heavy duty applications. On the contrary, due to the pin’s inflexibility, oftentimes Solid Pins damage the holes when used in a dynamic loading application, which leads to premature failure.
Additionally, using a softer Solid Pin material reduces host damage, but commensurately reduces the pin’s strength. A heat-treated Spring Pin is often stronger than a Solid Pin and its inherent flexibility maximizes assembly life in dynamic, heavy-duty applications.
Spring Pins
There are two distinct types of Spring Pins:
Slotted spring pins.
Coiled spring pins.
Both types are flexible and accommodate wider hole tolerances than Solid pins.
Slotted Pins
Slotted pins reduce manufacturing costs with their flexibility. However several disadvantages to Slotted Pins limit their applicability in new designs – particularly in those applications with a soft host material, such as aluminum or plastic, which are subject to dynamic loading.
The Slotted Pin is less flexible than the Coiled Pin and it only flexes 180º from the gap. This combination can cause premature assembly failure.
Additionally, Slotted Pins are very difficult to automatically feed and install. The most appropriate applications for Slotted Pins are non-critical assemblies, manufactured out of mild to hardened steel, that are manually assembled.
Coiled Spring Pins
Coiled Pins were produced to address the drawbacks of the Slotted Pins, and to offer the Design Engineer flexibility to tailor the pin’s strength and flexibility to match the application.
“Customizing” the function of the pin ensures that each application has the optimum combination of strength and flexibility. Light duty pins are generally recommended for soft or brittle materials. Medium (or standard) duty pins are recommended for use in mild steel and non-ferrous assemblies. Heavy duty pins should be used in hardened components.
The Coiled Pin is capable of absorbing shock and vibration after insertion, therefore providing prolonged useful life of the assembly, and it is conducive for automatic feeding and installation in high volume situations.
Coiled Pins can be used as hinge pins, alignment pins, stop pins and to fasten multiple components together (e.g.: to pin a gear and shaft). Coiled Pins are not usually recommended as cam followers, or where there is limited length of engagement for the pin. These applications are usually best served by a Solid Pin with retention features.
For more information visit www.SPIROL.com