The figure illustrates one of several related pin-puller mechanisms that harness the recovery characteristics of a shape-memory alloy (SMA). Like some other SMA-actuated mechanisms described previously in NASA Tech Briefs, this pin puller is suitable for use in place of a pyrotechnically actuated pin puller and can be operated under remote control to release a door and/or deploy a stowed object, for example. Unlike pyrotechnically actuated pin pullers, this and other SMA-actuated pin pullers can be reused.
The SMA component in this pin puller serves as a trigger to release potential energy stored in the driver spring. This spring biases the pin toward retraction (downward in the figure). However, under normal (nonactuation) conditions, the pin is set in the extended (uppermost) position and the ball-detent latch prevents retraction.
A component made of an SMA undergoes a pronounced deformation to a "remembered" shape when its temperature rises through a transition value, causing a transformation in its metallurgical structure from a martensitic to an austenitic phase. In this case, the SMA component is a wire, made of a nickel/titanium alloy denoted by the trade "Nitinol," that has been stretched. To initiate retraction of the pin, electric current is made to flow along the wire, thereby heating the wire above its transition temperature and causing it to shrink to its "remembered" (unstretched) length. The shrinkage of the wire pulls the latch piece upward, against the downward bias of the reset spring. As a result, the balls are no longer forced to protrude through the detent holes. Thus, the balls no longer block the retraction of the pin.
Later, the mechanism can be reset by pulling on the pin to extend it. Provided that the SMA wire has cooled enough to recover its stretched length, the mechanism will remain reset because the reset spring biases the latch piece into its lowest position, where it forces the balls to protrude through the detent holes.
This work was done by Michael Bokaie of TiNi Alloy Co. for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Mechanics category, or circle no. 167 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7-3
21000 Brookpark Road
Refer to LEW-16511.