SBIR-STTR Award

A variety of polymers are used as inserts in nuts to provide resistance to loosening from vibration and shock. Fiber reenforced resins formerly used have generally been replaced with polyamides (nylon) and polyamides. It would be a logistics advantage if these prevailing torque lock nuts could be reused after they have been demated from their bolt or shaft during inspection or repair procedures. Although some manufacturers of the smaller self locking nuts claim that these items can be roused up to 50 times, military procedures require that nuts be discarded after demating; this is particularly true of precision nuts used as retainer and preload devices for bearings. Recently introduced shape memory plastics (SMP) have a dramatic change in modulus when heated above their glass transition temperature (tg), and a deformed part will recover its original shape when heated above this temperature. Shape memory polymers may thus provide the necessary mechanical features to serve as elastomeric locking elements in nuts of these types, while making possible the recovery of the virgin dimensions of the insert upon heat treatment after demating. Evaluation of selected SMPS will be carried out in both their pure form and modified by fillers to establish their torque characteristics and their reusability.

Fastener improvement was identified in the Air Force Advisory Board's study of subsystem and component reliability needs, FACTS. Nylon insert locknuts cannot be reused with confidence and alternative insert materials are required. Phase I studies of polymeric and metallic insert materials demonstrated that shape memory alloys will provide the necessary prevailing and breakaway torque specified in MIL-N-25027-Amendment for 250 degrees F improved locknut, and prior Memry Technologies research provides data which clearly identified the Copper-Aluminum-Nickel system as the best insert candidate. The design of a shape memory alloy insert locknut system requires extremely careful measurement and control of thread dimensions in order to assure that it meets performance goals over the entire range of ANSI thread tolerance. After choosing with ALC personnel a series of locknut sizes best suited to AF field trials and demonstrations, the design and testing of prototype locknuts will be carried out. This will include shape memory alloy optimization and the establishment of dimensional tolerance levels. Industrial applications will be identified for the Air Force locknut design or for a modified design for commercial demonstration.