Abstract:
The invention is a separable connector that includes nut and bolt assemblies for joining two structural elements together. The thread geometry is selected such when the nut and bolt assemblies are threadably joined and a tension load applied therebetween, upon release of nut the tension load is converted into rotational energy causing the nut to rotate at high speed acting as a flywheel causing the nut to spin off of the bolt.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to the field of fasteners and, in particular, to a non-pyrotechnic fastener that automatically separates a nut from a bolt upon actuation.  
           [0003]    2. Description of Related Art  
           [0004]    Reliable fasteners that separate upon actuation have many applications. One critical application is on launch vehicles designed to place spacecraft into orbit. Not only must the fasteners reliably secure booster stages together under high loads, they must rapidly separate upon actuation in order to achieve proper timing of stage separation. This is particularly true when several fasteners must be simultaneously separated. Thus pyrotechnically actuated devices are typically used. One example can be found in U.S. Pat. No. 3,926,090 “Separation Nut” by J. W. Bunker. An extreme example is an explosive actuated system that uses a metal coupling to join the segments of the fairing together. A tubular member is positioned next to or within the coupling. Upon ignition, the explosive expands the tubular member, which in turn fractures the coupling. Such a system is disclosed in U.S. Pat. No. 5,443,492 “Payload Housing And Assembly Joint For A Launch Vehicle” by A. L. Chan, et al.  
           [0005]    However, pyrotechnic fasteners and the like, while well proven, can not be tested prior to use, thus must be assembled with great care. This makes them generally expensive to manufacture. Special storage areas must be set aside for any device containing explosives. They are always subject to inadvertent actuation, and, therefore, handled with great care. Additionally, they are particularly subject to ignition by electromagnetic interference (EMI) and thus must be protected by EMI shielding devices, which also raises the cost. One of the most important disadvantages is that upon actuation, most generate significant shock loads, which can damage nearby equipment.  
           [0006]    One approach to eliminate such problems is to use shape memory alloys to actuate the fasteners. Shape memory alloys offer a solution to the problem. There are basically two types of shape memory alloys:  
           [0007]    1. Simple memory alloys where a deformation undergone in an austenitic state is definitively cancelled out during the passage to the austenitic state.  
           [0008]    2. Reversible memory alloys where a deformation undergone in the martensitic state is cancelled out during the passage into the austenitic state, but is reassumed during a subsequent passage to the martensitic state. However, the transformation takes place with a certain hysteresis.  
           [0009]    There are numerous alloys having shape memory characteristics such as Ti—Ni, Au—Cd, In—Zn, Ti—Ni—Cu, Cu—Zn—Al and Cual—Ni and many are commercially available. The theory of shape memory alloys is well established and, therefore, need not be discussed in further detail.  
           [0010]    There are many examples of fasteners making use of a shape memory alloy (SMA). For example, U.S. Pat. No. 5,312,152 “Shape Memory Metal Actuated Separation Device” by W. H. Woebkenberg, Jr., et al. uses a segmented nut that is kept in engagement with a threaded bolt by a retainer. The retainer is held in place by a SMA element. Upon heating of the SMA element, it returns to its un-deformed state and releases the retainer, which in turn releases the nut. U.S. Pat. No. 5,722,709 “Separation Device Using A Shape Memory Alloy Retainer” by B. K. Lortz also uses a segmented nut. However, in this case the nut is retained in contact with the threaded bolt by a SMA collar. Upon heating, it expands to its original shape releasing the segmented nut. Other examples of fasteners using shape memory alloys can be found in U.S. Pat. No. 5,060,888 Temporary Linking Device Especially For An Artificial Satellite Lengthening Piece, And Method To Free Such A Link” by G. Vezain, et al., U.S. Pat. No. 5,129,753 “Shape Memory Wire Latch Mechanism” by K. S. Wesley, et al., U.S. Pat. No. 5,150,770 “Recharge Device, Particularly For Drive Mechanisms For Extending And Withdrawing Operative Members Of A Space Vehicle” by G. Secci and U.S. Pat. No. 5,718,531 “Low Shock Release Device” by E. C. Mutschleer, Jr. All use SMA materials as the primary actuating force. However, when using SMA material as the primary actuating device, precise timing of the release can prove difficult to achieve. In addition, shape memory alloys are sensitive to high temperature environments.  
           [0011]    Another approach is to use of ball latches. U.S. Pat. No. 3,887,150 “Internal Ejector Mechanism” by T. Jakubowski, Jr. U.S. Pat. No. 4,132,147 “Store Retention And Release Mechanism” by A. Contaldo, U.S. Pat. No. 4,350,074 “Mechanical And Electrical Coupling Device Fore Charges, Particularly Military Charges” by J. P. Rouget, et al., U.S. Pat. No. 4,520,711 Loop Retention Device For Hook Operated Bomb Arming Solenoids” by P. R. Robinson, U.S. Pat. No. 5,364,046 “Automatic Compliant Capture And Docking Mechanism for Spacecraft” by M. E. Dobbs, et al., U.S. Pat. No. 5,520,476 “Tie-Down And Release Mechanism For Spacecraft” by G. W. Marks, et al. all disclose the use of ball detent mechanisms to secure components of one type or another together. The main problem with such ball latch fasteners is limited trigger force reduction, which is required for activation with SMA systems. In launch vehicle and spacecraft, which are subjected to very large vibration loads, the satellite must be secured using very high pre-loaded joints. Ball latch systems typically don&#39;t allow for the application of type of pre-loads that can be obtained with a threaded fastener. However, they are very good locking devices.  
           [0012]    In U.S. Pat. No. 5,603,595 “Flywheel Nut Separable Connector And Method” by W. D. Nygren an attempt was made to take advantage of SMA technology to provide actuation initiation for a conventional nut and bolt and to use the high pre-load forces therebetween to provide the primary separation forces, i.e. to rotate the nut to the point of separation. The nut having a high helix angle or lead is essentially a flywheel. It is torqued until the desired pre-load is achieved. Thereafter, the flywheel is latched. The latch is secured by a SMA spring. Upon heating the spring, the latch releases the flywheel and the stored energy in will cause the flywheel to initially rotate at high speed. The strain energy due to the pre-load is dissipated as the nut unwinds, the stored energy in the flywheel continues to cause the nut to rotate until separation occurs. The advantages are numerous; high pre-loaded joints are possible and the need to only heat a small wire spring greatly reduces actuation time. However, this design had problems in that it had a greater parts count than equivalent explosive actuated separation nuts and was somewhat more massive and occupied more volume.  
           [0013]    U.S. patent application Ser. No. 09/610,594 “Connector Assembly” by W. Nygren discloses an improved fastener using a wrap spring. In detail, this invention includes a connecting member including a threaded end with a specific pitch diameter, thread lead angle, and helix angle. A hollow housing having a cylindrical wall with a specific thickness contains a body rotatably mounted therewithin. The body is threadably engagable with the threaded end of the connecting member. A first mechanism releasably restrains the rotatably supported body from rotating until released. The first mechanism includes the cylindrical wall having a plurality of rectangular slots. A plurality of cylindrical rollers are movably mounted in the slots. The body includes a plurality of cylindrical grooves alignable with the slots in the body. A wrap spring is wound about the cylindrical wall of the housing movable from a first position such that the it engages the rollers forcing the rollers into the grooves locking the body to the cylindrical wall of the housing, when the grooves are aligned with the slots to the a second position allowing the rollers to move out of the slots in the body. A second mechanism is included for winding the spring about the cylindrical wall of the housing such that spring is moved from the second relaxed position to the first position. This second mechanism comprises a circular shaped ratchet assembly mounted about the housing. The ratchet assembly includes a first member having flexible pawl springs rigidly attached to the housing and a second member in the form of a ratchet rotatably mounted to the housing and attached to the first end of said spring. A third mechanism is included for releasing the spring when in the first position such that when the third mechanism releases the spring the spring can move to the second position. A fourth mechanism is includes for restraining the threaded end from rotating. A fifth mechanism is included for applying a selected tensile load to the member.  
           [0014]    Thus when the connecting member and the body are engaged to form a connection, and with a tensile load applied to the member causing the connection to be strained, the selected thread geometry causes the tensile load to be resolved as a torque applied to the body sufficient to cause the body to rotate when released allowing the threaded end to translate out of engagement with the rotatably supported body allowing the connection to separate. While providing almost shockless separation, it is somewhat difficult to set, because the ratchet assembly of the second mechanism requires the use of a spanner wrench to wind the spring.  
           [0015]    Thus it is a primary object of the invention to provide a fastener assembly that automatically separates upon actuation.  
           [0016]    It is another primary object of the invention to provide a non-pyrotechnically actuated fastener assembly.  
           [0017]    It is a further object of the invention to provide a fastener assembly that automatically separates upon actuation and absorbs the stored energy produced by the pre-loading of the fastener to reduce shock loads.  
           [0018]    It is a still further object of the invention to provide a fastener assembly that automatically separates upon actuation and is easily re-settable.  
           [0019]    It is a still further object of the invention to provide a fastener assembly that has low-mass, volume and parts count.  
           [0020]    It is another object of the invention to provide an improvement to the prior connector assembly using a wrap spring by simplifying the wrap spring setting mechanism.  
         SUMMARY OF THE INVENTION  
         [0021]    The invention is separable connector assembly for joining two surfaces together. In detail, the invention includes a first fastener half, typically a nut, translationaly mounted to the first surface. The first fastener half includes a threaded end with a selected thread geometry including a selected thread pitch diameter, thread lead angle, and helix angle. Preferably the selected helix angle is between 18 degrees and 45 degrees, the selected thread angle is between 0 degrees and 30 degrees (7 degrees is preferred), and the selected thread lead is between 0.5 thread pitch diameters and 1.5 thread pitch diameters.  
           [0022]    A hollow housing having a cylindrical wall with a specific thickness is mounted to the second surface. A second fastener half, typically a bolt is rotatably mounted within the hollow housing, the second fastener half threadably engagable with the threaded end of the first fastener half. A first mechanism is included for releasably restraining the rotatably supported second fastener half from rotating. It includes a the cylindrical wall having a plurality of rectangular slots. A plurality of cylindrical rollers are movably mounted in the slots, the rollers having a diameter greater than the thickness of the cylindrical wall of the housing. The second fastener half includes a plurality of cylindrical grooves having a depth less than the diameter of the cylindrical rollers, the grooves alignable with the slots in the second fastener half. A coil spring (wrap spring) is wound about the cylindrical wall of the housing movable from a first position such that it engages the rollers forcing the rollers into the grooves locking the body to the cylindrical wall of the housing, when the grooves are aligned with the slots, to a second position allowing the rollers to move out of the slots in the second fastener half.  
           [0023]    A second mechanism is mounted on the housing to wind the spring about the cylindrical wall of the housing such that the spring is moved from the second position to the first position. Preferably, the second mechanism includes a first ring mounted to the housing and a second ring rotatably mounted to the housing coupled to the second end of the wrap spring for moving the wrap spring from the second position to the first position. A locking mechanism is includes for locking the first and second rings together after the second ring has been rotated such that the wrap spring is in the first position. A third mechanism releasably restrains the second end of the wrap spring from moving. Thus when the third mechanism restrains the second end of said spring and the first second ring is rotated, the wrap spring is wound about the housing from the second position to the first position thereof. When the spring reaches the first position, the locking mechanism can inserted locking the two rings together.  
           [0024]    The third mechanism is mounted on the housing and coupled to the second end of the spring, when the spring is in the first position such that when the third mechanism releases the spring the spring can move to the second position. Preferably, the third mechanism includes a latch shaft assembly rotatably mounted within the housing movable from a first position to a second position, the shaft assembly having a latch surface that restrains the second end of the spring when the shaft assembly is in the first position and releases the second end of the spring when the shaft is in the second position. A lever is attached to the shaft for moving the shaft from the first position to the second position. When in the first position, the wrap spring biases the shaft lever to rotate the shaft lever to the second position. A balanced latch lever and spring assembly secures the lever such that the shaft is in the first position. The balanced latch lever is acted upon by a SMA wires. Upon heating by the application of electrical current, the SMA wires returns to its original shortened length rotating the balanced latch lever and so releases the shaft lever releasing the shaft and, of course, the wrap spring.  
           [0025]    A fourth mechanism is mounted in the housing for restraining the threaded end of the first fastener half from rotating. Finally, a fifth mechanism is mounted on the first surface for applying a selected tensile load to the first fastener half.  
           [0026]    Thus when the first fastener half and the second fastener half are engaged to form a connection, and with a tensile load applied to the joined first fastener half causing the connection to be strained, the selected thread geometry causes the tensile load to be resolved as a torque applied to the second fastener half sufficient to cause the second fastener half to rotate when released allowing the threaded end to translate out of engagement with the rotatably supported second fastener half when the mechanism releases the second fastener half allowing the connection to separate.  
           [0027]    Preferably, the rotatably supported second fastener half has a selected mass moment of inertia and the selected thread geometry is such that less than 10 percent of the strain energy stored in the connection between the first fastener half and the rotatably supported second fastener half, not dissipated as heat due to friction, is converted into translational kinetic energy of the first fastener half during separation.  
           [0028]    The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a partial cross-sectional of the connector assembly shown connecting two structural elements together with the first connector half of the connector assembly in cross-section.  
         [0030]    [0030]FIG. 2 is an enlarged cross-sectional view of FIG. 1 taken along the line  2 - 2  illustrating the second part of the connector assembly in the first position (latched).  
         [0031]    [0031]FIG. 2A is an enlarged portion of FIG. 2.  
         [0032]    [0032]FIG. 3A is a is a cross-sectional view of FIG. 2 taken along the line  3 - 3  in FIG. 2 illustrating the second half of the connector assembly, in the first (latched) position.  
         [0033]    [0033]FIG. 3B is a cross-sectional view similar to FIG. 3A illustrating the connector assembly in the actuated (unlatched) condition.  
         [0034]    [0034]FIG. 4A is a is a cross-sectional view of FIG. 2 taken along the line  4 - 4  in FIG. 2 illustrating the release mechanism the latched position.  
         [0035]    [0035]FIG. 4B is a is a cross-sectional view similar to FIG. 4A illustrating the release mechanism in the unlatched position. FIG. 5 is a partial perspective view of the latch mechanism illustrated in FIG. 4A.  
         [0036]    [0036]FIG. 6A is an exploded cross-sectional view of a first portion of the connector assembly shown in FIG. 1.  
         [0037]    [0037]FIG. 6B is an exploded cross-sectional view of a second portion of the connector assembly shown in FIG. 1.  
         [0038]    [0038]FIG. 7 is a cross-sectional view similar to FIG. 1 illustrating the connector assembly in the unlatched or actuated condition.  
         [0039]    [0039]FIG. 7A is an enlarged portion of FIG. 7. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0040]    Referring to FIGS. 1 through 7A, the connector assembly, generally designated by numeral  10 , is used to join two structural elements  12  and  13 , respectively. Structural element  12  includes a hole  14  therethrough, while structure  13  includes a hole  15  therethrough. The connector assembly  10  includes first connector half  10 A mounted on surface  16  of structural element  13  and second connector half  10 B mounted on surface  18  of structural element  12 . Connector half  10 A includes a hollow cylindrical member  20 , having an internal bore  22  and an external flange  24 . The flange  24  includes fastener holes  26  having fasteners  28  engaging threaded holes  30  in the surface  16  of structural element  13 . A first hollow fastener half  32  is slidably mounted in the bore  22 . It includes a circular shaped end  33  with flats  33 A and with internal threads  34  extending through the hole  15  and into hole  14  (see FIG. 6A). The purpose of the flats  33 A will be subsequently discussed. The fastener half  32  also includes a externally threaded end portion  35 ,. A washer  36  is mounted about the threaded end  35  of the first fastener half  32  secured thereto by a threaded nut  38 . A coil spring  40  is mounted about the housing  20  and extends between the flange  24  and washer  36  biasing the first fastener half  32  away from the structure element  13 .  
         [0041]    The second connector half  10 B of the connector assembly  10  includes a housing  59  having circular portion  60  with a longitudinal axis  61  with first and second ends  62  and  64 , respectively. The first end  62  of the housing  59  is adapted to extend into the hole  14  in structural element  12 . A flange  66  is mounted to the circular portion  60  in proximity to the end  62  and which includes and extend portion  67 . A flange  68 , also having an extend portion  69  is mounted to the circular portion  60  in a spaced relationship to the flange  66  forming a space  70 . The housing  60  further includes a central bore  71  having a counter bore  72  and a slot  73  all aligned with the longitudinal axis  61 . The flange  66  includes fastener holes  73  having fasteners  74  engaging threaded holes  75  in the surface  18  of structural element  12  (best seen in FIG. 1). The second end  64  is in the form of a hollow cup shaped member  79  that extends from the flange  68  and includes a plurality of equally spaced slots  80 . As illustrated, there are  6  slots  80 , but depending upon the particular application there could be more or less. The cup shaped member  79  also includes internal threads  81  at its open end. A shaft bearing  82  is mounted within the counter bore  72  by means of a press-fit. A thrust bearing  84  sandwiched between washers  86  and  87  are all mounted on the top surface  88  of the flange  68  within the cup shaped member  79 .  
         [0042]    A second fastener half  100  is rotatably mounted within bore  71  of the housing  59 . The second fastener half  100  includes a bolt portion  102  that is supported in part by bearings  82  and  84  and extends through bore  71  in the housing  59  and having a threaded first end  104  releasably engaged with the threaded end  34  of the first fastener half  32 . The second end  106  includes a cup shaped member  108  with an external surface  112  and a having plurality of equally spaced vertical grooves  114  alignable with the slots  80  in the cup shaped member  79  of the housing  59 . The second fastener half  100  further includes a central post  116  having a screwdriver slot  117 .  
         [0043]    Cylindrical roller pins  118 , having a diameter  119 , are mounted in the slots  80  and are extendable into the grooves  114 . The width  121  of the slots  80  and the maximum depth  122  of the grooves  114  have a geometric relationship with a radius equal to half the diameter  119  of the pins  118 . Thus with the roller pins  118  at the maximum depth within the grooves  114 , the pins are approximately flush with the external surface  112  of the cup shaped member  108 .  
         [0044]    A second shaft bearing  130  is mounted about the post  116 . A cap  132  having threaded end  134  is engaged with the internal threads  81  on the member  79  such that end flange  135  contacts the end of the cup shaped member  79  when the threads are fully engaged. The cap  132  further includes bore  136  with a counter bore  137  in engagement with the bearing  130  and providing support therefore. It also includes a plurality of equally spaced threaded holes  138  in circle having a diameter  139 . Mounted about the member  79  is a wrap spring  140  having first and second ends  142 A and  142 B. A cup shaped member  146 , having a hole  147  therethrough, is movably mounted over the cap  132  and secured to the member  79  of the housing  59  by means of snap ring  148  mounted in mating grooves  149 A and  149 B in the members housing  59  and member  146 , respectively. The first end  142 A of the wrap spring  140  engages a hole  150  in the member  146 . The member  146  further includes a plurality of equally spaced holes  151  in circle having a diameter  139 ′, but with two less holes than the cap  132 . For example  18  treaded holes  138  in member  132  and  16  holes  151  in member  146 . Fasteners  152  join the member  146  to the cap  132 . A hollow cover  154  having an open first end  155  and an end  156 , having a hole  157 , therein covers the housing  59 . It is attached by screw  158  extending through hole  159  the cover  154  and engages threaded hole  160  in the flange  68  thereof.  
         [0045]    A latch shaft assembly  164  is rotatabley mounted in space  70  between the flanges  66  and  68  of the housing  59  on top of a spacer  166 . The latch shaft asembly  164  includes a latching surface  165  that extends through a hole  168  in the flange  68 . It is positioned by means of a shaft assembly  170  that extends through a hole  172  in the flange  66  and a hole  174  in a spacer  166  and threadably engages a threaded hole  175  in the cam shaft assembly  164 . The latching surface  165  is in releasable engagement with the second end  142 B of the wrap spring  140 . The latch shaft assembly  164  also includes an arm  176  having a notch  178  at its free end. A pin  179  mounted in flange  66  acts as a stop preventing clockwise rotation of the cam shaft  164 . A lever arm  180 , made of a non-conductive material, is pivotally mounted at its center to a pin  181 . The pin  181  includes an end  182  that extends through a hole  183  in the flange  66 , a hole  184  in the lever arm  180  and engages a hole  185  in the flange  68 . The first end  186  of the lever arm  180  includes notch  188  having a pin  190  therein that is in releasable engagement with the notch  178  of the cam shaft assembly  164 . The second end  191  includes two sets of holes with each set having two holes  192 A and  192 B and  193 A and  193 B. A spring  194  includes a first end  195  that extends into a hole  195  in the flange  66  a middle portion wound about pin  181  and a second end  197  in engagement with the lever arm  180 . Thus the spring biases the lever arm  180  in a clockwise direction into engagement with the cam shaft assembly  164 . A pin  202  is mounted to the flange  66  having four circular grooves  204 A, B, C, D. A terminal block  206  is also mounted to the flange  66  by means of screws  208 .  
         [0046]    A first SMA wire  210  is connected by its ends  211 A and  211 B to the terminal block  206  and extend about the pin  202  via the notches  204 A and  204 B through hole  192 A and back through hole  192 B around notch  204 B and back to the terminal block  206 . A second SMA wire  213  is connected by its ends  214 A and  214 B to the terminal block  206  and extend about the pin  202  via notches  204 C and  204 D through hole  193 A and back through hole  193 B around notch  204 D and back to the terminal block  206 . The terminal block is connected to an electrical power supply (not shown).  
         [0047]    Thus with the latch shaft assembly  164  is prevented from rotating by the lever arm  180 , which is biased to prevent rotation away from the latch shaft assembly, the latch surface  165  prevents the second end  142 B of the wrap spring  140  from moving. In this position, the first and second connector assembly halves  10 A and  10 B are mounted on the structural elements  12  and  13 . The first fastener half  32  can then be threadably engaged with the second fastener half  100 . To accomplish this the cover  154  is removed. The latch shaft assembly  164  is rotated to the position shown in FIG. 4A such that the lever arm  180  is in engagement therewith. A screw driver (not shown) is inserted through the hole  147  in member  146  and hole  136  in the cap  132  and engaged with the slot  117  in the post  116  of the second fastener half  100 . The screwdriver is used to both connect the second fastener half  100  with the first fastener half  32  and to align the grooves  114  in cup shaped member  108  with the slots  80  in the cup shaped member  79  of the housing  59 . This requires that the first fastener half  32  to be pushed into contact with the second fastener half  100 . A this point, the cap  146  is rotated causing the wrap spring  140  to wind about the cup shaped member  79  forcing the rollers  118  into the grooves  114 , locking the second fastener half place. Thereafter, the tension load on the first and second fastener halves  32  and  100  is applied by torquing the nut  38 . The amount of torque applied can be correlated with the amount of tension load applied to the first and second fastener halves  32  and  100 .  
         [0048]    When an electrical current is applied to the SMA wire  210  and/or SMA wire  213 , they will heat up and return to their high temperature state and shorten. This causes the lever arm  180  to rotate causing it to release the latch shaft assembly  164  to rotate to a position allowing it to disengage from the end  142 B of the wrap spring  140  allowing it to unwind freeing the rollers  118  and allowing them to move out of contact with the grooves  114  in the second fastener half  100 . At this point in time the second fastener half  100  is free to rotate.  
         [0049]    In U.S. Pat. No. 5,603,595 “Flywheel Nut Separable Connector” by W. D. Nygren, Jr., incorporated by reference herein, it is disclosed that by proper selection of the internal threads  34  on the first fastener half  32  and external threads on the threaded first end  104  of the second fastener half  100  and proper tensioning of the connection between the first and second fastener halves, upon release of the second fastener half, it automatically and rapidly unthreads from the first fastener half. This is particularly true if the treads have a thread angle between zero and 30 degrees (preferably 7 degrees), helix angles of between 18 and 45 degrees and second (both half) fastener leads of between 0.5 and 1.5 pitch diameters. The important criteria is that the selected thread geometry generates, under the load, a sufficiently high torque to overcome the rotationally resistive load bearing friction torque of the thrust bearing acting on the second fastener half  10 , and the resistive torque due to thread friction form the thread end engagement of the first and second fastener halves. The theoretical calculations in support of this design are found in the above referenced patent. Once released, the first fastener half  32  being biased away from the first fastener half  100  by spring  40 , it retracts, but is prevented from leaving the member  20  because the end  33  is larger than the bore  22 .  
         [0050]    While the invention has been described with reference to a particular embodiment, it should be understood that the embodiment is merely illustrative as there are numerous variations and modifications which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims.  
         [0051]    Industrial Applicability  
         [0052]    The invention has applicability to fastener manufacturing industry.