Abstract:
A separable connector assembly includes a connecting member having a threaded end, and a hollow housing having a cylindrical wall that contains a body rotatably mounted therein. This body is threadably engageable with the threaded end of the connecting member. A first mechanism releasably restrains the body from rotating, and a second mechanism is included for winding a spring about the cylindrical wall of the housing. A third mechanism is included for releasing the spring. A fourth mechanism is included for restraining the threaded end from rotating. A fifth mechanism is included for applying a selected tensile load. When the load is applied, a thread geometry of the connecting member causes the load to be resolved as a torque applied to the body sufficient to cause the body to rotate enabling the connecting member to translate out of engagement with the body and subsequently separate from the body.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of Related Art 
     Reliable fasteners that separate upon actuation have many applications. One critical application is on launch vehicles designed to place a 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. 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. 
     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. 
     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: 
     1. Simple memory alloys where a deformation undergone in an austenitic state is definitively cancelled out during the passage to the austenitic state. 
     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. 
     There are numerous alloys having shape memory characteristics such as Ti—Ni, Au—Cd, In—Zn, Ti—Ni—Cu, Cu—Zn—Al and Cu—al—Ni, and many are commercially available. The theory of shape memory alloys is well established and, therefore, need not be discussed in further detail. 
     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. patent applications Ser. 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. 
     Another approach is the use of ball latches. U.S. Pat. No. 3,887,150 “Internal Ejector Mechanism” by T. Jakubowski, Jr., 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., and 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 détente 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 vehicles and spacecrafts, which are subjected to very large vibration loads, the satellite(s) must be secured using very high pre-loaded joints. Ball latch systems typically don&#39;t allow for the application of the type of pre-loads that can be obtained with a threaded fastener. However, they are very good locking devices. 
     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 preload 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 therein tends to cause the flywheel to initially rotate at high speed. The strain energy due to the pre-load is dissipated as the nut unwinds, and 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. 
     Thus it is a primary object of the invention to provide a fastener assembly that automatically separates upon actuation. 
     It is another primary object of the invention to provide a non-pyrotechnically actuated fastener assembly. 
     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. 
     It is a still further object of the invention to provide a fastener assembly that automatically separates upon actuation and is easily re-settable. 
     It is a still further object of the invention to provide a fastener assembly that has low-mass, volume and parts count. 
     SUMMARY OF THE INVENTION 
     The invention is a separable connector assembly for joining two surfaces together. In detail, the invention includes a first fastener half, typically a nut, translationally mounted to a 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. 
     A hollow first housing having a cylindrical wall with a specific thickness is mounted to a second surface. A second fastener half, typically a bolt, is rotatably mounted within the hollow first housing. This second fastener half is generally threadably engagable with the threaded end of the first fastener half. A first mechanism or means is included for releasably restraining the rotatably supported second fastener half from rotating. It includes a cylindrical wall having a plurality of rectangular slots. A plurality of cylindrical rollers is generally movably mounted in the slots. These rollers generally have a diameter greater than the thickness of the cylindrical wall of the first housing. The second fastener half includes a plurality of cylindrical grooves having a depth less than the diameter of the cylindrical rollers. These grooves are generally alignable with the slots in the second fastener half. A coil spring (wrap spring) is wound about the cylindrical wall of the first housing. This coil spring is at least generally movable from a first position to a second position. In the first position, the coil spring generally engages the rollers forcing the rollers into the grooves locking the body to the cylindrical wall of the first housing. In the second position, the coil spring generally allows the rollers to move out of the slots in the second fastener half. 
     A second mechanism or means is mounted on the first housing to wind the spring about the cylindrical wall of the first housing such that the spring is moved from the second position to the first position. Preferably, the second mechanism includes a circular shaped ratchet assembly mounted about the first housing. The ratchet assembly includes a first member having flexible pawl springs rigidly attached to the first housing and a second member in the form of a ratchet rotatably mounted to the first housing and attached to the first end of the spring. A third mechanism releasably restrains the second end of the wrap spring from moving. Thus, when the third mechanism restrains the second end of the spring and when the ratchet member of the ratchet assembly is rotated, the wrap spring is wound about the first housing from the second position to the first position thereof. 
     The third mechanism or means is mounted on the first housing and coupled to the second end of the spring at least when the spring is in the first position. When the third mechanism releases the spring, the spring can move to the second position. Preferably, the third mechanism includes a shaft rotatably mounted within the first housing movable from a first position to a second position, the shaft having a cam surface that restrains the second end of the spring when the shaft is in the first position and releases the second end of the spring when the shaft is in the second position. A shaft 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 SMA wires. Upon heating by the application of electrical current, each of 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. 
     A fourth mechanism or means is provided for restraining the threaded end of the first fastener half from rotating. Preferably, this fourth mechanism or means comprises a hollow second housing mountable to the first surface, wherein this hollow second housing comprises an opening having a cross-section configured for receiving the non-circular cross-sectional portion of the first fastener half. Finally, a fifth mechanism or means is mounted on the first surface for applying a selected tensile load to the first fastener half. Preferably, this fifth mechanism or means comprises the first fastener half having a flange, and a plurality of set screws threadably mounted in the flange and engagable with the second housing. Thus, when the first fastener half is threadably engaged with the second fastener half, the set screws can be adjusted to engage the second housing causing the first and second fastener halves to be strained. 
     In addition, a sixth mechanism or means may be included for releasably securing the shaft of the third mechanism or means in the first location. Preferably, this sixth mechanism or means comprises a lever arm attached to the shaft, a balanced latch lever releasably engaging the lever arm, a biasing means (e.g., a compression spring) to urge the balanced latch lever into engagement with the lever arm, and a seventh mechanism or means for moving the balanced latch lever out of engagement with the lever arm. This seventh mechanism or means preferably comprises a shape memory wire attached between the balanced latch lever and the first housing such that when the wire is heated, the wire shortens causing the balanced latch lever to rotate out of contact with the lever arm. 
     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. This torque is generally 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. 
     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. 
     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 
     FIG. 1 is a partial cross-sectional view of the connector assembly shown connecting two structural elements together with the first part of the connector assembly in cross-section. 
     FIG. 2 is an enlarged cross-sectional view of FIG. 1 illustrating the second part of the connector assembly in cross-section. 
     FIG. 3A is a cross-sectional view of FIG. 2 taken along line  3 — 3  illustrating the connector assembly, particularly the roller pins, in the first (latched) position. 
     FIG. 3B is a cross-sectional view similar to FIG. 3A illustrating the connector assembly in the actuated condition. 
     FIG. 4A is a cross-sectional view of FIG. 2 taken along line  4 — 4  illustrating the connector assembly, particularly the spring biasing the roller pins, in the first non-actuated position. 
     FIG. 4B is a cross-sectional view similar to FIG. 4A illustrating the connector assembly, particularly the spring biasing the roller pins. 
     FIG. 5 is a cross-sectional view of FIG. 2 taken along line  5 — 5  illustrating the retention of the ratchet spring of the ratchet assembly. 
     FIG. 6 is an exploded cross-sectional view of the connector assembly shown in FIG.  1 . 
     FIG. 7 is a cross-sectional view similar to FIG. 1 illustrating the connector assembly in the unlatched or actuated condition. 
     FIG. 8 is cross-sectional view of FIG. 1 taken along line  8 — 8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1-7, the connector assembly, generally designated by numeral  10 , is used to join two structural elements  12  and  13 , respectively, together having a hole  14  therethrough and structure alignment guides  15 A and  15 B. The connector assembly includes first connector half  10 A mounted on surface  16  of structural element  12  and second connector half  10 B mounted on surface  18  of structural element  13 . Connector half  10 A includes a hollow cylindrical member  20 , having an internal square bore  22  and an external flange  24 . The flange  24  includes fastener holes  26  having fasteners  28  engaging thread holes  30  in the surface  16  of structural element  12 . A first hollow fastener half  32  having a square cross-sectional shape is slidably mounted within the bore  22  having an internally threaded nut portion  34 , extending into the hole  14 , and a flange  36 . A spring  38  is mounted between the flange  24  and flange  36  biasing the fastener half  32  away from the surface  16 . However, the first fastener half  32  is restrained within the cylindrical member  20  by a snap ring  40  mounted in a groove  42  on the external surface of the threaded nut portion  34 . A strain gage  44  is mounted within the fastener half  32 . A plurality of threaded screws  50  threadably engage thread holes  52  in the flange  36  and contact the end  54  of the tubular member  20 . The function of the strain gage  44  and screws  50  will be subsequently discussed. 
     The second connector half  10 B of the connector assembly  10  includes a hollow first housing  60  having an open first end  62  and a second end  64 . The external surface  66  in proximity to the second end  64  of the housing  60  includes slots  68 . Retainers  70  include end members  72  in engagement with the slots  68  and include fastener holes  74 . Fasteners  76  extend through the holes  74  and engage threaded fastener holes  78  in the surface  18  of the structural element  13  securing the housing  60  thereto. 
     The housing  60  further includes a first cylindrical bore  80  extending from the first end  62  partially through toward the second end  64  terminating in a flat surface  82 . A smaller diameter second cylindrical bore  84  extends from the flat surface  82  to the second end  64 . A thrust bearing  86  sandwiched between washers  87  and  88  is mounted on the flat surface  82 . Thus, this flat surface  82  of the first cylindrical bore  80  of the housing  60  may be characterized as a load bearing surface of sorts. A shaft bearing  90  is mounted in the bore  84  retained therein by a snap ring  92  mounted in groove  94 . The housing  60  further includes a plurality of equally spaced vertical slots  96  extending through the wall (best seen in FIG.  6 ). 
     A second fastener half  100  is rotatably mounted within the first and second bores  80  and  84  comprising a bolt shaft  102  having a threaded first end  104  releasably engaged with the threaded portion  34  of the first fastener half  32  and a second end  106  coupled to a cup shaped member or head  108 . The head (or cup shaped member)  108  of the second fastener half  100  may be characterized as a mass that is connected with the shaft  102  of the second fastener half  100 . The cup shaped member  108  includes an interior opening  110 , and an external surface  112  including a plurality of equally spaced vertical grooves  114  alignable with the slots  96  in the wall of the housing  60 , and tapered in depth along the circumference (see FIG.  4 B). Cylindrical roller pins  118  are mounted in the slots  96  and are extendable into the grooves  114 . The width  120  of housing wall and maximum depth  122  of the grooves  114  have a geometric relationship with a radius equal to half the diameter  124  of the roller pins  118 . Thus, with the roller pins  118  at the maximum depth within the grooves  114 , the roller pins are approximately flush with the external surface of the housing  60 . 
     The cup shaped member  108  of the second fastener half  100  further includes a circumferential groove  126  in which is mounted a ring shaped spring member  128 . The spring member  128  has an uncompressed diameter greater than the diameter of the bore  80  and includes pins  130  that slidably engage holes  132  in the housing  60 . Thus the spring member  128  generally functions as a biasing mechanism or means for biasing the roller pins  118  outward. A ratchet assembly  134  is mounted on the housing  60  abutting a small flange  135  (best seen in FIG. 2) and held there by a snap ring  136  in a groove  137 . The ratchet assembly  134  includes a ratchet spring  138  having upward directed spring loaded teeth  139  (best seen in FIG. 5) and an inward directed tang  140  that engages a slot  141  on the housing  60 . A rotatable ratchet ring  142  with rigid teeth  144  is mounted between the ratchet spring  138  and the flange  135 . The ratchet ring  142  further includes spanner wrench holes  143  and a pin  145 . The function of the ratchet assembly will be subsequently discussed. 
     A second housing  150  is mounted to the second end  62  of the housing  60  and includes a rotatable cam  152 . A wrap spring  154 , preferably having a square cross-section, is wound about the housing  60  engaging the rollers  118 , has a first end  156  in engagement with the pin  145  of the rotatable ratchet ring  142 , and has a second end  157  in releasable engagement with the cam  152 . With the wrap spring  154  wound tightly about the housing  60 , the rollers  118  are forced in the grooves  114  in the cup shaped member  108  preventing the second fastener half  100  from rotating. When the wrap spring  154  is released it has a larger diameter than the housing and the rollers  118  are thus free to move outward forced thereby by the torque on the second fastener half  100  in combination with the shape of the grooves  114 . The spring member  128  prevents the rollers  118  from bouncing back. Thus the second fastener half  100  is free to rotate. 
     The second housing  150  is attached to housing  60  by means of fasteners  151  that engage threaded holes (not shown) in the second end  64  of the housing  60 . The housing  150  includes a shaft  160  that extends into the cup shaped member  108 . A bearing  162  is mounted within the cup shaped member  108  that also engages the shaft and thus the shaft  160  provides additional rotational support for the second fastener half  100 . The cam  152  is mounted to a shaft  164  rotatably mounted within the housing  150 . A lever arm  170  is mounted to the opposite end  172  of the shaft  164  that includes a notch  174 . A balanced latch lever  176  is rotatably mounted within the housing  150  about a mounting pin  177  having a first end  178  in engagement with the notch  174  of the lever arm  170 . A compression spring  180  is coupled to the balanced latch lever  176  near its second end  181  such that the first end  178  thereof is biased into engagement with the notch  174  of the lever arm  170  biasing the cam  152  into engagement with the second end  157  of the wrap spring  154  such that it remains tightly wound around the housing  60  forcing the rollers  118  into engagement with the grooves  114  in the cup shaped member  108  of the second fastener half  100  preventing it from rotating. This compression spring  180  may thus be characterized as a biasing mechanism or means for urging the balanced latch lever  176  into engagement with the lever arm  170 . 
     A first SMA wire  190  extends through hole  192  in the second end  181  of the latch lever  176  and is attached by its ends  193 A and  193 B to terminals  194 A and  194 B. A second SMA wire  195  extends through a second hole  196  in the second end  181  of the latch lever  176  and is connected by its ends  197 A and  197 B to the terminals  198 A and  198 B. Both sets of terminals  194 A, B and  198 A, B are connected to a source of electrical power (not shown). 
     Thus with the lever arm  170  prevented from rotating by the latch lever  176  which is biased to prevent rotation away from the lever arm  170 , the cam  152  prevents the second end  157  of the wrap spring  154  from moving. The first and second connector assembly halves  10 A and  10 B are 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, a hole  200  is provided in the shaft  160  and a hex socket  202  is provided in the second fastener half  100 . Thus an Alan wrench (not shown) can be inserted through the hole  200  such that it engages the socket  202  and used to thread the second fastener half  100  to the first fastener half  32  and to align the grooves  114  with the rollers  118 . With the first end  156  of the wrap spring  154  in engagement with the pin  145  of the ratchet assembly  134 , the ratchet ring  142  can be rotated with a spanner wrench (not shown) wrapping the spring about the housing  60  causing the rollers  118  to be forced into the grooves  114 , preventing the first fastener half from rotating. Thereafter, the tension load on the first and second fastener halves  32  and  100  can be adjusted by adjusting screws  50  and monitored by monitoring the tension load utilizing strain gage  44 . 
     When an electrical current is applied to the SMA wire(s)  190  and/or  196 , each wire will heat up and return to its unstressed state and shorten. This causes the balanced latch lever  176  to rotate causing it to release the lever arm  170 . This of course will allow the cam  152  to disengage from the end  157  of the wrap spring  154  allowing it to unwind freeing the rollers  118  and allowing them to move out of contact with the grooves  114 . At this point in time the second fastener half  100  is free to rotate. 
     Upon proper selection of the internal threads  58  of the first fastener half  32  and external threads  113  of the second fastener half  100 , and proper tensioning of a connection  101  between the first fastener half  32  and the second fastener half  100 , the second fastener half  100  automatically and rapidly unthreads from the first fastener half  32  upon release of the second fastener half  100 . This is particularly true if the threads 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  86  acting on the second fastener half  100 , and the resistive torque due to thread friction from the thread end engagement of the first fastener half  32  and the second fastener half  100 . The theoretical calculations in support of this design are found in U.S. Pat. No. 5,603,595 “Flywheel Nut Separable Connector” by W. D. Nygren, Jr., incorporated by reference herein. Once released, the first fastener half  32  (being biased away from the second fastener half  100  by spring  38 ) retracts but is prevented from leaving the hollow, cylindrical member  20  by the snap ring  40 . 
     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. 
     Industrial Applicability 
     The invention has applicability to the fastener manufacturing industry.