Abstract:
A preload reducing fastener adapted to be fastened into a system in which includes a release system. The fastener may have an integral collar which is adapted to change shape, as in the case of a shape memory alloy. The application of heat to fastener may deform the collar such that the fastener may move axially in the load direction, thus reducing the preload previously held by the fastener. A preload reducing device including a preload reducing fastener may include an outer case adapted to center and capture a preload reducing fastener. The case may center the fastener in a desired location in a preloaded system, and may also capture the fastener after release of the system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/386,568 to Baghdasarian, filed Sep. 27, 2010. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a preload releasing fastener which may be used in systems which release such items as antennas, solar arrays, positioning mechanisms, and other devices. 
     2. Description of Related Art 
     A release apparatus may be used to release a captured member which constrains the deployment of a spacecraft element, such as a solar array and/or reflectors, in the stowed position. In many prior art devices, the spacecraft element was restrained with a wire or a holddown rod system which was released using a pyrotechnic device. Typically, the pyrotechnic device would fire an anvil against a base, with the wire or rod to be cut and released. Although useful in many applications, these devices imparted high shock loads into the units which they were to release, as well as the spacecraft itself. One aspect of the shock imparted into the system involves the shock due to the pyrotechnic device, and another aspect of the shock involves the sudden release of a tensioned system, and the motions involved therein. In some systems, the tensioned system release allows for quick motions of the release system, which are then constrained by motion limiters, and shocks are felt as the moving parts hit mechanical stops. 
     A design that avoided the shocks associated with pyrotechnic release devices was the separation spool device, which used a fused element to release a captured member. U.S. Pat. No. 6,133,818, to Baghdasarian, discusses a release apparatus wherein two piece split spool with an annulus is used to capture a capture member larger in diameter than the annulus of the spool. The two pieces of the split spool are held together with a wire that is wrapped around the spool. 
     However, other methods of reducing the shocks associated with releases of hold downs can be used. 
     SUMMARY 
     A preload reducing fastener adapted to be fastened into a system in which includes a release system. The fastener may have an integral collar which is adapted to change shape, as in the case of a shape memory alloy. The application of heat to fastener may deform the collar such that the fastener may move axially in the load direction, thus reducing the preload previously held by the fastener. 
     A preload reducing device including a preload reducing fastener may include an outer case adapted to center and capture a preload reducing fastener. The case may center the fastener in a desired location in a preloaded system, and may also capture the fastener after release of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a release device and system according to some embodiments of the present invention. 
         FIG. 2  is a side view of a release device in its tensioned position according to some embodiments of the present invention. 
         FIG. 3  is a side view of a release device in its relaxed position according to some embodiments of the present invention. 
         FIG. 4  is a top view of a preload releasing fastener according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In some embodiments of the present invention, as seen in  FIG. 1 , an item to be deployed  102  is seen held down to a surface  101 . In some aspects, the item to be deployed  102  may be a solar array panel, an antenna, or other item. In some aspects, the surface  101  may be a spacecraft sidewall. When used during spacecraft launch, a tensioned release device may allow for sufficient holding force of a deployable object, and be used in enough positions, such that the item to be deployed is able to withstand launch loads without damage. A variety of hold-down locations may be used to not only provide strength but to alter, or lower, the frequency of the non-deployed (stowed) system during launch. 
     In some aspects, the item  102  may need to be deployed after launch in order to perform its mission tasks. For example, a solar array panel, which may have been part of a stowed array of panels, may need to be deployed in order to become part of a deployed array of panels in order to provide electrical power to a spacecraft&#39;s payload and electronics. However, in order to have successfully withstood launch loads, the item  102  may have needed to have been held in place during launch with a pre-loaded fastening system. If the preloaded fastening system used a rod in tension, for example, this rod may be cut using a pyrotechnic device adapted to accelerate n knife-like implement into and through the rod. The tension in the rod may, upon cutting of the rod, result in a shock due to the sudden de-tensioning of the rod. In some aspects of the present invention, the rod is de-tensioned as discussed below. 
     In some embodiments of the present invention, a cutter mounting bracket  103  acts as an apparatus housing and is mounted to a sidewall  101  using fasteners  104 . The cutter mounting bracket  103  may have a base with holes adapted to route the fasteners  104  through the base and into threaded receivers mounted within the sidewall  101 . A pyrotechnic cutter  109  may be adapted to fit within the cutter mounting bracket  103 . A pyrotechnic squib  110  may be used to power a knife-like cutter  111  through a rod  105  used to retain a deployable item  102 . The pyrotechnic cutter  109  may be a removable item such that the cutter mounting bracket may be re-used with a new cutter  109 , which may be replaced after system testing, for example. 
     In order to impart a desired preload into the rod  105 , the system is designed such that the rod  105  can be preloaded via the torque imparted by the tightening of a nut  106 . In some embodiments other preload inducing means may be used. The rod  105  is fastened on a first end into a preload reducing fastener  100 . The preload reducing fastener  100  is adapted to receive the threaded end of the first end of the rod  105  in an internally threaded hole. The nut  106  is placed on the second end of the rod  105  and the rod may be loaded into axial tension by the turning of the nut  106 . A flat  107  on the end of the rod  105  may be used to prevent the rod  105  from turning within the preload reducing fastener  100  during the tightening of the nut  106 . In some embodiments, the rod  105  is inserted into the preload reducing fastener  100  a prescribed number of turns, and then is held at that depth within the preload reducing fastener  100  with a tool on the flat  107  while the nut  106  is tightened. In some embodiments, the nut  106  is tightened to a particular torque. In some embodiments, the nut  106  is tightened a prescribed number of turns. 
     In some embodiments of the present invention, the preload reducing fastener  100  is adapted to provide sufficient resistance and strength to maintain the desired preload imparted into the rod  105 , and also to release some of, or all of, the preload in the rod. The preload reducing fastener  100  may have a collar  135  which is adapted to have a first configuration at a first temperature, such as room temperature, and a second configuration at a second temperature, such as an elevated temperature. A heater  112  may be attached to the bottom of the preload reducing fastener  100 . The heater  112  may be a resistance heater and may be supplied electrical current via wires  113 . In some embodiments, other heating means may be used. 
     In some embodiments of the present invention, as seen if  FIGS. 2 and 3 , the preload reducing functionality allows for axial shift of the threaded interface of the preload reducing fastener  100 . As seen in  FIG. 2 , the collar  135  of the preload reducing fastener  100  may contact the surface  130  of the cutter mounting bracket  103  at contact region  131 . The contact region  131  may be a circle around the outside of the collar  135 . The height  132  of the threaded interface in this tensioned configuration may be the height used when the system is tensioned to the desired preload. 
     In some embodiments of the present invention, as seen in  FIG. 3 , the collar  135  is seen to have “flattened.” The height  133  of the threaded interface is now larger, representing an axial shift in the position of the preload reducing fastener. 
     In some embodiments, the collar  135  of the preload reducing fastener  100 , or the entire preload reducing fastener  100 , may be made of a shape memory alloy. The shape memory alloy may be set such that the fastener  100  retains a first tensioned configuration at a first temperature, and takes a second relaxed configuration at a second temperature, which may be a higher temperature imparted to the fastener  100  by the heater  112 . In some embodiments, the material may be nitinol. A superelastic material, such as Nitinol (NiTi) may exhibit elasticity over large levels of strain. The increased flexibility obtained in these materials by undergoing large elastic strain can improve their performance. At low levels of stress, the material exits in an austenite phase. Upon further loading, the material undergoes a stress-induced transformation from the austenite phase to a martensite phase. The material behaves as linear elastic in both austenite and martensite phases, however, the modulus of elasticity in the two phases is different. During the stress-induced transformation from austenite to martensite there is very little change in stress, but a large increase in strain. Beyond the transition region in the martensite phase ultimately results in permanent unrecoverable set in the material, while unloading for cases that do not reach the transformation state follow the elastic modulus. 
     By heating the collar of the fastener such that there is an axial shift in the position of the fastener, the preload in the rod  105  may be reduced or eliminated. In some aspects, this lower (or zero) preload may result in a lower set of shock loads into the system when the cutter is used to cut the rod  105 . 
     In some embodiments of the present invention, as seen in  FIG. 4 , the preload reducing fastener  100  may be adapted to withstand rotary loads while mounted in its mounting bracket. The axial portion  120  of the fastener  100  may have flats  121  along its sides such that instead of the axial portion being round, and inserted through a round hole, the axial portion instead has flats which prevent the fastener  100  from rotating when rotating loads are placed on it via its threaded interface  122 . Thus, when the rod  105  is tensioned via the nut  106 , the fastener  100  may be held firmly in place with regard to rotary displacement. An advantage of embodiments of the present invention is that in the case wherein the fastener is rotationally constrained, a single piece, the preload reducing fastener  100 , may be used as an anchor for a tensioned rod, as a preload release device, and as rotational constraint for tightening of the rod. Further, when the preload reducing fastener is constrained by the housing, and mounted below the housing but between the housing and the sidewall, the fastener is inherently constrained after the cutting of the rod. 
     In some embodiments, an item to be deployed is stowed using a preloaded rod. The rod is fastened on one end using a preload reducing fastener. After launch, a heater may be used to reduce the preload in the rod by heating the preload reducing fastener such that its collar relaxes and the fastener has an axial shift in position. The cutter may then be fired. The item may then be deployed. 
     As evident from the above description, a wide variety of embodiments may be configured from the description given herein and additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is, therefore, not limited to the specific details and illustrative examples shown and described. Accordingly, departures from such details may be made without departing from the spirit or scope of the applicant&#39;s general invention.