High-reliability reusable lock nuts employing shape memory alloys

Lawrence McDonald Schetky; Richard Goff

doi:10.1117/12.310650

16 June 1998 High-reliability reusable lock nuts employing shape memory alloys

Lawrence McDonald Schetky, Richard Goff

Proceedings Volume 3326, Smart Structures and Materials 1998: Industrial and Commercial Applications of Smart Structures Technologies; (1998) https://doi.org/10.1117/12.310650
Event: 5th Annual International Symposium on Smart Structures and Materials, 1998, San Diego, CA, United States

Abstract

Prevailing torque lock nuts are divided into classes which relate to their temperature of use, and for temperatures up to 250 degrees F the most commonly employed locking device is a nylon insert. Although manufacturers of these devices claim that they are reusable, the Air Force requires that once removed, this type of lock nut must be replaced. An approach to a lock nut which has installation and removal torque's which conform to MIL specifications and can be reused many times is based on a castleated nut whose fingers are confined by a shape memory alloy ring. When installed on a male threaded component, the ring is forced open against the shape memory ring restraint, producing the frictional force inhibiting nut loosening from vibration and shock. Pseudoelastic nickel titanium rings were shown to perform well, but the temperature range for pseudoelasticity is limited to about 40°C. MIL specifications require that the locking characteristics be uniform from -51 to +71°C. Linear superelastic behavior can be generated in a NiTi alloy by cold work, and in this form strains up to 4% are fully recoverable over a wider temperature range: from -100 to +200°C. The square cross section rings employed on the smaller nut sizes, e.g. 1/4", 5/16" and 3/8" require precise dimensional control creating a difficult machining problem since the NiTi family of alloys are notoriously difficult to machine. An alternative titanium based alloy, developed for medical devices and contains no nickel, and was successfully substituted. Nut design and locking torque characteristics of nuts subjected to vibration and shock will be described.

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Lawrence McDonald Schetky and Richard Goff "High-reliability reusable lock nuts employing shape memory alloys", Proc. SPIE 3326, Smart Structures and Materials 1998: Industrial and Commercial Applications of Smart Structures Technologies, (16 June 1998); https://doi.org/10.1117/12.310650

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KEYWORDS

Shape memory alloys

Tolerancing

Temperature metrology

Nickel

Titanium

Resistance

Manufacturing

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