Frames for truss systems designed to support stadium lighting, sound equipment, and even bleacher seating systems are often composed of extruded aluminum tubing and solid or tubular couplers. Considerable liability surrounds the use of these systems for public events, and the interface between these components becomes crucial to the integrity of the system. The fastener and its interface with the truss system becomes extremely critical as they are often subjected to shock, vibration, and high shear values when equipment is mounted to the trusses, or while audience members are seated during an event.

A top manufacturer of truss systems consulted SPIROL about their needs for an existing design that was comprised of one tubular structural member and one solid male member that slid inside the tubular extrusion and was cross-pinned with two high carbon steel Slotted Pins to hold the coupler to the frame. During installation, the Slotted Pin was difficult to install and would often damage the expensive, yet relatively soft, aluminum frame. Once the hole was deformed, the manufacturer noted instances where the pin would begin to walk out of the hole over time, particularly in permanent installations. This was also noted in mobile systems if subjected to extreme loading. 

In some instances, the company noticed that the Slotted Pin had stress cracks located 180 degrees from the slot. In extreme weather conditions, the high carbon steel pin was also prone to corrosion. With a lack of options, the manufacturer had investigated implementation of an expensive machined Solid Pin with a secondary retention feature of a Cotter Pin. SPIROL was asked to evaluate the application and ultimately recommend the most robust and cost-effective solution for this incredibly demanding application.

SPIROL’s engineering team determined that 420 stainless steel coiled pins could act as a high-performance alternative to the Slotted Pins being used. Since the Coiled Pin does not need to be aligned in the shear plane like a Slotted Pin, this eliminates the potential of failure due to pin orientation in the hole. Since Coiled Pins remain flexible in the application after installation, they are able to absorb dynamic loading rather than transferring it into the host—which can cause deformation. This reduction in deformation increased the life of the structure, and reduced the potential of the holes opening up to the extent that the pin would eventually fall out. Since a 420 stainless steel Coiled Pin is heat treated, its shear value exceeds that of a machined low carbon steel Solid Pin. 

Complexity of the assembly was kept to one component rather than two, as proposed with the Solid Pin and Cotter Pin solution; thus saving time during set-up and tear down as well as the task of managing multiple components. By using 420 series stainless steel, the manufacturer would also achieve the necessary corrosion resistance, which would ultimately yield more reliable performance over the life of the structure. 

An added bonus to the Coiled Pin solution was that no adjustment to the hole size would be required to transition from the Slotted Pin to the Coiled Pin. This benefit extended beyond the manufacturer to their customers that already had truss systems in service throughout the country. Implementation of the Coiled Pin ultimately met all of the manufacturer’s performance and commercial requirements and significantly increased the integrity of the assembly.

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SPIROL Coiled Spring Pins