Appliance Design - August 2008 - (Page 36)

JOINING Fig. 4. Chart compares the effect of the Junkers test on tension for different fastener assemblies. lem. For many years, the Junkers vibration test has been the benchmark for testing threadedfastener resistance to vibration. Maintaining tension in a screw or bolt is paramount to keeping fasteners from loosening under vibration. Gerhard Junkers’ test is based on this proven theory. The test rig is quite simple. A load cell and two transverse moving plates are clamped between the nut and bolt to be tested. (See Fig. 3.) An eccentric cam mechanism moves the plates at 12.5 Hz for a maximum of 120 seconds. Tension in the bolt or screw is recorded. The Over 65 Adhesive & Dispensing Manufacturers! ISO 9001:2000 Assembly Adhesives & Expert Solutions Silicone Elastomers Thermal Management Solutions UV Curing & Dispensing Equipment Electronic & LED Assembly & Protection Global Reps with Adhesive, Dispensing, Curing & Cost Saving Solutions www.ellsworth.com 800-888-0698 Shop • RFQ • Online Tech Support • Global Locator • MSDS / TDS Engineering Expertise & Award Winning Distribution 36 applianceDESIGN AD08084Ells.indd 1 time of 120 seconds was chosen because the test is so aggressive that typically after 120 seconds the bolt fatigues and breakage occurs. Bolt failure caused by fatigue proves that this test will surpass the rigorous conditions a product will experience in the field over its lifetime. Again, proving a technology is critical to acceptance in any industry. As seen in the Fig. 4 chart, the preload locking internal thread form maintains all but a fraction of the initial preload. When compared to a prevailing torque fastener and a secondary locking feature the results are significant. The aerospace industry in general, and NASA in particular, was an early adopter of this fastening technology. In the early 80s, NASA was searching for a locking screw thread that could not only be implemented into a fastener, but also into a threaded hole. Most importantly, it had to be reusable. Most orbit-bound space vehicles are completely assembled and reassembled three times before being launched into space. This requirement and the extreme operating temperatures eliminated the common approaches to locking fasteners available at the time. Extensive testing by the Goddard Space Flight Center proved that the preload locking internal thread form can withstand at least 10X sine and random vibration that the Space Shuttle requires without loosening. More importantly, the tests were repeated 60 times on the same nut and bolt. Applying the preload locking internal thread form does have a few minor limitations. The thread form is unidirectional. Therefore, the bolt must be assembled into the fastener or threaded hole in a certain direction for the thread form to be effective. This requirement can be overcome by the use of hex flange nut or other unidirectional fasteners. As for threaded holes, proper design of threading tools will insure correct thread orientation. The mechanical locking action of this thread form depends on consistent tension on the male fastener. This requires a hard joint where the materials bolted together will not relax and cause loss of bolt tension during use. Despite those minor limitations, the preload locking internal thread form continues to appear in a wide range of new applications where self-locking features and reusability are desired design characteristics. < For more information, email: KTurowska@spiralock.com www.applianceDESIGN.com August 2008 7/7/08 9:49:15 AM http://www.ellsworth.com http://www.ellsworth.com 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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