How to Choose Fasteners that Maximize Assembly Life
By Christie Jones
Market Development Manager
Spirol International Corporation, U.S.A.
Editor's note: The following technical white paper reviews the advantages and disadvantages of common types of pins, and provides guidelines as to when to use each type in certain applications.
DANIELSON, Conn.--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.
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: (1) methods that take two components or two operational steps to stay in place; and (2) 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. 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 come in many different forms. Examples include dowel pins, knurled pins, and 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. Alternatively, a heat-treated spring pin is often stronger than a solid pin, and its inherent flexibility maximizes assembly life in dynamic, heavy-duty applications.
There are two distinct types of spring pins: slotted spring pins and coiled spring pins. Both types share certain characteristics, such as flexibility and their ability to accommodate wider hole tolerances than solid pins.
The pins' flexibility reduces manufacturing costs. However, there are several disadvantages to slotted pins that 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 significantly less flexible than the coiled pin and it flexes 180° 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 invented to address the drawbacks of the slotted pins, and to offer the design engineer the flexibility to tailor the pin's strength and flexibility to match the application. This ability to "customize" 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 the most 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, and 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.
Considering the many different ways to fasten assemblies, it is recommended that engineers take advantage of the application engineering services provided by the manufacturers of engineered fasteners. By partnering with industry experts in fastening and assembly, engineers can be assured that their assembly will be equipped with the most cost-effective solution that provides exceptional performance and preserves the integrity of the application throughout the life of the product.
This white paper can be accessed at the following web address:
Spirol International (www.spirol.com) is a manufacturer of engineered components for fastening and joining. The company manufactures a variety of coiled and slotted spring pins, solid pins, disc springs, alignment dowels and bushings, spacers, and compression limiters. Spirol also produces machined nuts, threaded inserts for plastics, and shims.
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