Reusable Self Locking Thread Fastener Demonstrates Remarkable Resistance to Breakage and Fatigue in Recent Tests

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Unlike thread locking forms, self locking fasteners with a preload locking internal thread form prevent high logistical, service and warranty costs

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While threaded fastener locking systems have been developed for everyday commercial products, most pose significant drawbacks for OEMs that are addressed by innovative, reusable, self locking threaded fasteners with a preload locking internal thread form.

Although typical "prevailing torque" type fasteners initially appear to be cost-effective solutions to fastener loosening, they have significant hidden costs when reusability is required since they're essentially "single use" fasteners. This can be a costly problem for OEMs. With just-in-time bin replenishment by the fastener distributor, chronic reassembly can prematurely deplete the fasteners on-hand and shut down the assembly line. Besides the costs of field repairs for service and warranty, the process of getting replacement fasteners to remote corners of the world can also be unviable.

While secondary thread locking devices have been developed such as "lock washers," pins, retaining rings and multiple fasteners, these additional components increase part count, which escalates cost and inventory. In the field, additional components may not be on hand to perform service procedures, and their proper reassembly and reuse by service technicians cannot be guaranteed.

Self locking threaded fasteners with a preload locking internal thread form, however, can help solve the dilemma and have done so for 25 years. To address vibration-caused, thread-loosening in the gap between male and female threads, these self locking fasteners use truncation of the female screw thread to eliminate the gap between the mating threads. The truncation is created by an additional ramp angle perpendicular to the trailing flank angle of a 60˚ "V" thread. The combination of tension on the male fastener, the elimination of the gap, and the steep angle of the ramp style truncation significantly increase resistance to fastened joint loosening.

The self locking threaded fasteners have proven themselves in testing and in the field for decades. On the Junkers Vibration Test, a test so aggressive that after 120 seconds the bolt typically fatigues and breakage occurs, the self locking fasteners maintain all but a fraction of the initial preload, while prevailing torque fasteners and secondary thread locking features quickly lose their tension.

One of the self locking threaded fasteners biggest fans is the aerospace industry, specifically NASA. 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. It had to be reusable since most orbit-bound space vehicles are completely assembled and reassembled three times before launch. 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 the preload locking internal thread form can withstand at least 10X sine and random vibration that the Space Shuttle requires without loosening. The tests were repeated 60 times on the same nut and bolt.

Self locking threaded fasteners with a preload locking internal thread form continue to gain fans. This technology is used in a range of applications and industries where reusability is necessary including heavy truck and automotive power trains, deep-hole oil-drilling equipment, high-speed manufacturing equipment, and medical devices.

Visit Spiralock at http://www.spiralock.com call (800) 521-2688; fax (248) 543-1403; or write to Spiralock at Madison Tech Center, PO Box 71629, Madison Heights, MI 48071.

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Kevin Peacock
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