Abstract:
A separable fastener for holding two objects together with the use of a bolt threaded into the fastener. The necessary inherent axial tension in the bolt/fastener system is accommodated by a relief element which relieves at least some of the stored tension before the nut separates, thereby reducing the physical shock on the assembly. The energy needed to separate the nut and bolt is entirely contained within the system so that pyrotechnic devices are unnecessary for the purpose.

Description:
FIELD OF THE INVENTION 
   A separable threaded fastener for releasing one object from another in a structure, utilizing an actuator which is other than pyrolytic so as to reduce mechanical shock otherwise caused by abruptly expansive gases, and by the abrupt release of axial tensile force in the assembled structure. In addition, the fastener can be reset for a subsequent use without requiring new components. 
   BACKGROUND OF THE INVENTION 
   Especially, although far from exclusively, in the aerospace field objects are held together as part of a structure by means of separable fasteners. Their purpose is to separate the objects from one another on command, meanwhile holding them reliably in the presence of high G loads and strong vibration forces. Such fasteners must themselves be very strong and reliable, and must be able to withstand the same forces as the objects they hold. 
   Classically such fasteners comprise an internally threaded nut held by one of the objects and an externally threaded headed bolt held to the other object, which is threaded into the nut. When the bolt is tightened into the nut, a strong axial tensil preload force is generated in the assembly, which is stored as energy that will be released when the fastener is separated. 
   The nut is provided in the form of a plurality of separate segments, each bearing a fragment of the same thread. Separation of the fastener is caused by releasing the segments from the bolt thread. This must be done quickly. It customarily results in an abrupt release of the preload energy and is therefore objectionable, although necessary. Prior efforts to reduce the peak load of this shock have been made, especially by O&#39;Quinn et al in U.S. Pat. No. 6,352,397, which is incorporated herein by reference for its showing of a related device for reducing the peak load, and to illustrate the disadvantages of pyrolytic separation. 
   In addition to the mechanical shock caused by abrupt release of the axial preload in the installed fastener, a pyrolytic actuator inherently produces a strong mechanical shock because of its abrupt generation of force. Such an abrupt reaction is necessary when very rapid separation is required. Then the need to accommodate for such forces is an accepted disadvantage. Such accommodation can have its price in weight and structure. 
   However, there are many applications in which a slower, but still acceptably rapid, release can be powered with lesser penalty. In such situations, a non-pyrolytic actuator merits consideration. This is because as space systems continue to seek lighter structures, reduction of abrupt forces is a great advantage, and actuation such as proposed by this instant invention can lead to simpler, lighter weight structural assemblies. 
   It is an object of this invention to provide a separable fastener which can utilize an actuator that is not pyrolytic, and which also lowers the peak force exerted by release of the axial preload energy. It has the advantage that all of the energy needed to separate the nut from the bolt is carried by the fastener in a mechanical array. The separation of this occurs at a controllably slower pace than would be caused by an abrupt pyrotechnical reaction. 
   BRIEF DESCRIPTION OF THE INVENTION 
   A fastener according to this invention includes an internally threaded nut that is to receive a threaded bolt. The bolt is retained by one of the objects to be joined. The nut is mounted to the other object to be joined by a housing which itself is attached to said other object. The nut comprises a plurality of separate segments assembled around the bolt to form an interrupted thread. The segments are held in an assembled configuration by a locking ring which is reciprocably mounted in the housing. The above are features of prior art separable fasteners such as the O&#39;Quinn patent. 
   According to this invention a relief element is placed between the segments and the structure associated with the “other” object. In this structure, it is the said housing. Axially drawing down the segments by tightening the bolt will press them against the relief element, enabling the nut to resist rotation and to permitting an axial tensile preload to be established in the bolt. 
   According to this invention, the relief element includes a stator and a rotor. The stator is restrained in the housing against rotation. The rotor is rotatable. The stator and rotor are co-axial. 
   A ramp surface is formed on the stator and on the rotor. They are complementary, and extend arcuately around the axis. Each has a ramp angle such that they form ramp surfaces which substantially abut one another, at a ramp angle. 
   The ramp angle is steeper than a locking angle, so that an axial compressive force will exert a rotational force on the rotor, the stator being keyed to the housing to prevent its rotation. Rotation of the rotor (when it is free to rotate) will result in a reduction of the axial thickness of the relief element, and will thereby relieve the axial tensile preload. 
   All of the force required to release the segments is stored in the fastener at the time it is installed. No further source of energy is required for this purpose. Release of the segments is the principal reason for the use of pyrotechnics in the prior art, made unnecessary with this invention. 
   A release system for this actuator comprises a control ring which controls the radial extension of a group of bearings. In one position the bearings lock the rotor and stator together against rotation, and in another they enable the rotation of the rotor. Rotation of this control ring requires only minimal force that can be provided by one or more small mechanical actuators-linear (solenoid) or rotary electrical motors, for example. 
   Thus, with this invention, peak release loads are greatly reduced, and pyrolytic loads are eliminated entirely. Instead, a gradual force exerted laterally and rotationally balanced is used instead for unlocking the rotor and stator from one another, and energy stored in springs separate the threaded segments from the bolt to release the bolt. 
   The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an axial section, showing the fastener holding objects together; 
       FIG. 2  is a view similar to  FIG. 1  taken at line  2 — 2  in  FIG. 3 ; 
       FIG. 3  is a top view of  FIG. 1 ; 
       FIG. 4  is an exploded view of the fastener; 
       FIG. 5  is a side view of a fastener segment; and 
       FIG. 6  is a top view taken at line  6 — 6  in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The utility of a fastener  20  according to this invention is shown in  FIG. 1 . It forms part of a structural assembly  21 . As an example, one object  22 , exemplified as a plate to which any desired element can be mounted is held to another object  23 , again shown as a plate. These objects are to be held together by fastener  20  until the moment of separation. 
   The fastener is fixed to plate  23  by bolts  24  or by other fastener devices. A bolt  25  with a head  26 , a shank  27  and a thread  28  on the shank is threaded into fastener  20 . Tightening the bolt into a nut to be described holds the objects together. When they are joined, the torque on the bolt exerts through the threads an axial tensile preload force that tightly holds the objects together. The release of the energy stored in the bolt will, unless regulated, exert a strong impact force on the entire structure. 
   Fastener  20  includes a body  35  with a mounting flange  36  and a rising tubular housing  37 . The housing has an internal thread  38  at its upper end. A housing hat  40  includes an upper internal cylinder  41  and a depending skirt  42  with thread  43  engagable with thread  38  to hold the fastener assembled in condition for installation in a structure, ready to receive a threaded bolt. An access port  130  is formed through it for a purpose to be described. 
   Base  35  includes an insert  44  with a central opening  45  to pass a bolt, and a very smooth flat reaction surface  46  (perhaps on an insert  47 ). A rotor  50  has a bottom flat reaction surface  51  which is intended to move in shear motion along reaction surface  46 . 
   The rotor further includes a peripheral sidewall  52  interrupted by a plurality of bearing reliefs  53 . These reliefs are bounded by sloping sides provided for a purpose later to be described. 
   At its upper end, the rotor includes an array of ramp surfaces  55  which extend in a thread-like manner around the central axis  56  of the fastener, at a non-locking angle, generally more than about 14 degrees of slope. 
   A control ring  60 , surrounds the relief element. It includes a plurality of relief ports  61 , identical in number to reliefs  53 , equally spaced. Between relief ports  61 , the inside surface of the control ring is disposed on a cylindrical surface which forms backing surface  62 . The relief ports need not extend all the way through the control ring, although depending on dimensions they might. Often they will have tapered edges for camming. 
   The control ring further includes attach points  63 . One or more, preferably a balanced pair, of actuators  65 , 66  are mounted to the base. They include whatever motive means they employ, and an actuator stem  67 , 68  attached to respective attach points  63 , 64 . Extension or retraction of the stems will rotate the control ring. 
   The motors may be of any type capable of rotating the control ring. A linear motor such as a solenoid will usually be preferred, although rotary types, or even latched spring-loaded plungers are within the scope of this invention. Preferably they will be provided as a pair to provide redundancy and balanced torque and lateral load on the ring. 
   Bearings  70  are placed in respective reliefs in the rotor. The bearings will preferably be roller bearings, but may be ball bearings if desired. The size of relief ports  61  is such as to capture the bearings so they will not come loose, but will permit the bearings to come loose from the rotor. 
   Stator  75  has an upper face  76  around the central axis, a depending skirt  77 , and a downwardly facing ramp surface  78 . Ramp surfaces  78  directly confront and smoothly engage ramp surfaces  55  of the rotor. 
   Stator skirt  77  includes a plurality of windows  79  equal in number to the recesses in the rotor and the relief ports in the control ring. The skirt fits between these two. It will be seen that when the backing surfaces confront the bearings, the bearings will be held in the windows in the stator and in the rotor recesses to bridge them. The rotor and stator are thereby locked together. 
   Because, as will be shown, the stator can never rotate, then while locked to it, the rotor cannot turn. When the control ring enables the bearings to leave the recesses in the rotor, they will cam out of the recesses and while still in the stator windows, enter the relief ports in the control ring. Then relative rotation of the rotor and stator can occur because the bearings then do not bridge the rotor and the stator. Thus the means to keep the fastener assembled, and to enable it to separate, is by appropriately rotating the control ring. 
   A key seat  90  has a bearing surface  91  abutting the top of the stator. It further includes a plurality of axial keys  92  that are fitted in axial splines  95  in the housing at its upper end. This key seat can therefore move axially, but cannot rotate. It further includes coupling keys  94  that extend downwardly and engage in recesses  95  in the top of the stator. Thus, the key seat locks the stator against rotation, but permits axial movement of the stator. 
   The upper face  96  of the key seat includes a tapered expansion face  97  and a bias bearing face  98 . A separation bias spring  99  bears against face  98 . It will preferably be a circular wave spring. 
   A segment locking ring  100  has a lower surface  102  facing spring  99 . It includes an internal cylindrical locking surface  101 . This is the ultimate locking element after the fastener has been installed. 
   A group of three segments  105 , 106 ,  107  is assembled around the central axis. They are all identical, except that each bears a fragment  108  of the same thread. Accordingly these may be considered to have been cut from an internally threaded cylinder, with material removed axially in three equally spaced apart locations. 
   Their bottom end  109  is sloped complementarily to the slope of expansion face  97 . The spacings between the bottom ends are such as to be engaged by keys  110  on the key seat so the relationship between the thread fragments is maintained. Thus a thread on a bolt will smoothly engage all of the segments. 
   Each segment includes an outer locking wall  111  as a fragment of the same cylinder. Thus, when properly assembled, the locking surface  101  in the locking ring  100  will smoothly embrace all of the segments, and will hold them together as a group. 
   At the upper end of each segment there is an upper expansion face  115  which forms a fragment of a truncated cone. These are confronted by an expander  120  which is axially slidable in cylinder  41  in the housing hat. 
   A frusto conical expansion surface  122  is formed on the bottom of the expander  120 . It presses against expansion faces  115 , and tends to separate them radially. A wave spring  123  in the housing hat biases expander  120  against the segments in opposition to the bias force of separation bias spring  99 . When installed, the binding together of the inserts by the locking ring will hold the segments aligned. 
   For a purpose and reason to be described, an access port  130  is formed in the housing hat to admit a tool (not shown) to hold the locking ring down until a bolt is threaded into the nut and tightened. Any suitable tool such as a simple rod or rods will suffice for this purpose. After the bolt is installed, the tool can be removed, because the expansive force against the locking ring will assure that the loaded and locked assembly will remain that way until the system is released. 
   This nut provides the advantage that all energy necessary to the separation process is already present as a mechanical, rather than as a pyrolytic source. The spring  123  will provide all of the necessary separation force. Release requires only modest energy from the motors to rotate control ring. 
   When being assembled, the rotor will be turned to provide the tallest assembly of stator and rotor (the relief element). The control ring will then be turned to press the bearings into the recesses in the rotor. The bearings will therefore bridge the rotor and the stator and lock them together. When ring  60  is rotated so that a window confronts the bearings, the bearings will cam out of the rotor and bridge the stator and control ring instead. Then the rotor can turn. 
   Returning to the installation procedure, with the stator locked, the springs, expanders, and segments are put in place, perhaps with a temporary bolt threaded in, and the relief element set to its longest dimension. Segment locking ring  100  is placed over the segments, in contact with locking walls  111 , and with the separator spring compressing. 
   The unit is now prepared. A tool is placed to hold the locking ring down, and the temporary bolt (if used) can be withdrawn. The nut can now be stored, ready for installation. 
   At the time of installation, the tool remains in place until after the ultimate bolt has been installed. Then it can be removed. 
   The installed nut is stable and ready for its use as a fastener until the motors rotate control ring  60 . The stability of the installed nut is a function of the sliding friction force between the bearing walls of the segments and locking surface  101  in locking ring  100 . This in turn is a function of the applied radial (normal) lock between them. This applied radial load is principally generated by the installed bolt, whose tightened threads on the nut segments exert an outward force resisted by the rigid ring. This generated force must be sufficient to overcome the axial force exerted by bias spring  99 . This is readily attained. 
   When the nut is to be separated, the rotor will be released to rotate and will permit the axial length of the assembled rotor and stator (the relief element) to reduce. This will gradually relieve (although quickly) the axial tensile preload in the fastener. In turn this relieves the radial forces on the segments, and locking ring  100  is freed to move upwardly. Then spring  123  biases the separator downwardly to force the segments radially apart at the top and the bottom. The bolt will now be free from the nut. It will be observed that no external energy was required for separation, except for the motors, and certainly no pyrotechnic. The separation is chemically clean and occurs with a considerably reduced mechanical shock. 
   This invention is not to be limited by the embodiment shown in the drawings and described in the description, which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.