Appliance Design - August 2008 - (Page 35)

JOINING Fig. 1. Standard 60 Deg V-shaped thread form has gap that makes assembly susceptible to vibration-induced loosening. Fig. 2. The preload locking internal thread form uses truncation of the female screw thread to eliminate the gap between the mating threads. be almost impossible. This does not mean that fastener designers have failed to address reusability. Several secondary-locking devices have been developed and widely used for many years. These devices are routinely mechanical in nature. Typically, mechanical features such as serrations, protrusions, and tabs are incorporated into washers and are termed lock washers. Beyond washers, there are retaining rings, pins, and multiple fasteners used to lock a fastened joint. These additional components can achieve the desired locking requirement but, as the word additional implies, they cause an increased part count. Increased part count escalates costs and inventory. In the field, the additional components can be difficult to retain during the service procedure. Also, proper reassembly and reuse by service technicians cannot be guaranteed. A reusable, cost-effective fastener is not a dream for the fastener industry. There is an existing screw-thread technology to solve this dilemma, the preload locking internal thread form. This has been a successful solution for many demanding fastening applications for more than 25 years. Perfected and patented by Ace Holmes, the fastener design is a simple modified buttress or truncated female thread. It was proven that the major cause of vibration-induced loosening in the standard 60 Deg, V-shaped thread form is the gap between the male and female threads. To easily assemble the male and female threads, there must be clearances between the mating threads. This clearance creates a gap. The gap between the mating threads produces an area where lateral movement will occur under vibration. (See Fig. 1.) www.applianceDESIGN.com Combined with the shallow flank angle of the V-shaped thread, the threads will begin to progress along the helical angle of the thread and the bolt/screw will lose tension. To maintain bolted joint integrity, the bolt or screw must remain in tension and act as a spring. Once tension is lost in the male fastener, it is not a question of if the fastener will loosen, but when. As mentioned earlier, the preload locking internal thread form uses truncation of the female screw thread to eliminate the gap between the mating threads. (See Fig. 2.) Designed to mate with a standard Class 2A or 3A (metric 6g/6h) male thread, the truncation is created by an additional ramp angle perpendicular to the trailing flank angle of a 60 Deg V thread. The combination of ten- sion on the male fastener, the elimination of the gap, and the steep angle of the ramp style truncation significantly increases resistance to fastened joint loosening. Any improvement in technology must be tested and proven. Product engineers worldwide have been perplexed on how to test bolted joint integrity in an accurate, cost-effective, and timely manner. When warranty costs are rising due to a threaded fastener loosening, engineers rarely have the luxury of full life-cycle testing on an application with new technologies. They need accelerated testing results. Reliable and accurate, accelerated boltedjoint testing is another area not well known by product and reliability engineers, but the fastener industry has a solution for this prob- Fig. 3. Junkers vibration test rig features a load cell and two transverse moving plates that are clamped between the nut and bolt. applianceDESIGN August 2008 35 http://www.appliancedesign.com

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Appliance Design - August 2008 Contents Editorial Shipments/Forecasts News Watch Plastics Switches Noise & Vibration Control Joining New Products Design Marts Association Report: CEA Advertiser’s Index

Appliance Design - August 2008

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