The Difference Between Coiled Spring Pins and Slotted Spring Pins
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WHITE PAPER The Difference Between Coiled Spring Pins and Slotted Spring Pins by Adolf Valasek, Applications Engineer SPIROL Czech Republic What is a Spring Pin? A Spring Pin, also often referred to as Tension Pin or Roll Pin, is a mechanical fastener used for securing two or more parts of an assembly together. Spring Pins are hollow, tube-like parts designed to be larger than the hole and are made from various types of spring steel or other materials. The primary difference of a Spring Pin compared to a classic Solid Pin is that its radial stiffness is significantly lower than the host material. Solid Pins can have a smooth, uninterrupted surface (such as dowels) or they may be designed with retention features such as grooves, knurls or barbs. Solid Pins are commonly retained by displacing/deforming the host material. However, in cases when the pin is precision ground and the hole is precision reamed, the fit of a Ground Solid Pin can be very tightly controlled between a slight amount of clearance and a small amount of interference which will not result in the displacement of material from either the pin or the host. However, the precision grinding and reaming of both the pin and the hole makes this pinning option one of the most costly. Spring Pins, on the other hand, can absorb a much larger hole tolerance than rigid Solid Pins. Thanks to their flexibility, not only are Spring Pins less expensive to manufacture, but the hole preparation is also less tedious and less expensive. The comparatively lower stiffness also brings another benefit – Spring Pins do not damage the host hole (when properly selected) - so much so, that they can be serviced without the need of reworking the parts. There are two basic types of Spring Pins – Slotted Spring Pins and Coiled Spring Pins. Coiled Spring Pin Slotted Spring Pin Slotted Spring Pins A Slotted Spring Pin is characterised by its C-shape cross section and a slot running parallel to its axis. While their precise origin is unknown, they have been appearing as a concept in mechanical devices throughout modern history. Over time through various industry standards, they have developed into what is commonly used today in many different types of applications. There are several active industry standards, but four (4) prevail and account for the majority of Slotted Pin demand: ASME B18.8.2, ASME B18.8.4M, ISO 8752 and ISO 13337. Coiled Spring Pins A Coiled Spring Pin is recognised by its 2¼ coils cross section. The history of the Coiled Pin is much clearer. It was invented in 1948 by Herman Koehl, SPIROL’s founding father, specifically as a solution to an application with harsh vibrations and dynamic loads - a rotor of a jet engine. Unlike other traditional fasteners such as nuts and bolts which tend to loosen under severe vibration, or rigid Solid Pins which transmit the dynamic loads to the hole wall and compromise retention of the pin, Mr. Koehl designed a pin that was strong enough to resist the forces generated during use, yet also flexible enough to absorb the forces so as to preserve the integrity of the hole material and keep the assembly intact. The Coiled Pin is available in three different duties for the optimal combination of strength and flexibility to accommodate different types of applications and host materials. The most common industry standards for Coiled Pins are ISO 8750, ISO 8748, ISO 8751 and ASME B18.8.2 and ASME B18.8.3M. These standards do not differ a lot between each other and are considered to be virtually equival

Physical Features & Differences Cross Section The main difference between a Coiled Spring Pin and Slotted Spring Pin is in the cross section as shown in Figure 1. While the Slotted Pin is a C-shaped pin usually rolled from a thicker strip of material, the Coiled Pin is rolled from a thinner strip to achieve its typical 2¼ coils cross section for the same nominal diameter. The key implication of this difference is the pin’s flexibility. A Slotted Pin can only flex so much before it closes its gap and effectively becomes a stiff, solid tube. The slot allows the Slotted Pin to absorb...

Square Ends Due to the different manufacturing methods, Slotted Pins can often have ends not perfectly perpendicular to their axis due to the manufacturing method. This can present an obstruction in automatic feeding where pins stacked atop each other can grab the neighbouring parts and cause a jam. This can also be caused by small “nibs” of material sometimes present at the end of the pin resulting from the manufacturing process (Figure 5). Coiled Pins have square ends that enable them to self-align with the installation punch/quill. This ensures the pin remains straight and that uniform...

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