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78 THE DISTRIBUTOR’S

78 THE DISTRIBUTOR’S LINK SPIROL INTERNATIONAL CORP. 30 Rock Avenue, Danielson, CT 06239 TEL 1-860-774-8571 FAX 1-860-774-2048 EMAIL info@spirol.com WEB www.spirol.com LIGHT DUTY COILED SPRING PIN USAGE IN ASSEMBLIES WITH SOFT MATERIALS by Christie L. Jones, Market Development Manager In recent years, the tendency throughout the industry has been to explore the use of new materials in order to reduce weight and process cost, or to provide solutions where more traditional materials such as steel, machined components or castings would not perform as well. Under those considerations, the use of plastic, aluminium and soft alloys has seen an exponential increase, and nothing seems to indicate that this trend is going to stop. On the contrary, as plastics and moulding processes continue to advance, more and more applications that were traditionally made in steel are being reviewed to overcome the issues that prevented the use of these lighter, softer materials that provide equal or even better performance. We can currently find numerous plastic components being used in demanding situations such as high temperature, high strength and chemical resistant applications. However, although the main component receives significant design attention and consideration, the mating components and the fastening method are habitually neglected until the end of the project. Also, it is routinely assumed that parts and design concepts that previously worked in steel will work with the softer alloys and plastics. When it comes to pinning applications in new materials, it is frequently a situation of going for the cheapest option and what is known in the industry. In many cases this is the (heavy duty) Slotted Pin per ISO 8752 (also known as the roll pin), which has been around for many years, developed from the old DIN 1481 standard. This pin was designed long before the advancements in production techniques and materials made lightweight / high-volume production feasible. The ISO 8752 Slotted Pin can and does, in fact, create significant assembly and quality issues in these new materials. Its thick wall, combined with an out-of-round or horse-shoe shape, does not provide the required flexibility for its use in relatively soft materials - often transferring the load from the pin to the wall of the host assembly, thereby damaging the hole, and resulting in premature assembly failure. In an attempt to reduce the disadvantages associated with the heavy duty Slotted Pin per ISO 8752, a light duty Slotted Spring Pin per ISO 13337 was designed. The main difference is the thickness of the pin wall. A thinner wall means increased flexibility, but also reduced shear strength, while other important issues remain unresolved. In fact, the thinner wall in and of itself creates new additional limitations with respect to fatigue resistance. Light duty (ISO 13337) Slotted Spring Pins, similar to the more common heavy duty Slotted Pin (per ISO 8752), present a horse-shoe shape cross-section which will, in many instances, create skiving during insertion into the hole. Coupled with that, the expanded diameter of the pin in relation to the size of the hole has the effect of creating a pin with a very wide slot. The result is a pin with an insertion force, and subsequent radial pressure against the wall of the hole, that will still be too high for most soft material applications, especially where holes are close to the edge of the component. TECHNICAL ARTICLE CONTINUED ON PAGE 178