Abstract:
A self-retracting extension limiting device. It is designed to be attached between a first object and a second object, where the distance between the first and second objects is variable. The device exerts a restoring force tending to draw the two objects together. A first, relatively weak restoring force is exerted over a range of distance. Then, when the objects have been separated to a defined limit of travel, the device exerts a much greater restoring force. The invention includes overlapping and self-contained elements so that unwanted slack is minimized.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     THIS APPLICATION CLAIMS THE BENEFIT OF AN EARLIER-FILED PROVISIONAL APPLICATION HAVING SERIAL No. 60/902,283, filed Feb. 20, 2007. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of rigging. More specifically, the invention comprises an extendable motion-limiting device that has an integral retracting and aligning feature. 
     2. Description of the Related Art 
     Rigging devices are used to link one object to another. In many applications, this link must be able to accommodate a varying distance between the two objects. One example is the stabilization of a floating dock, where the water level varies over a significant range. A cable securing such a dock must restrain the dock, yet also extend and retract to allow tidal variations. 
       FIG. 1  shows a prior art check cable  14  connected between first block  10  and second block  12 . Throughout this disclosure, the reader should bear in mind that the term “cable” is intended to encompass any tensile member, and could include fiber ropes, synthetic cables, chains, wire ropes, straps, and similar items. In some applications, the cable could even be a rigid rod. 
     The check cable is attached to the blocks by any convenient method. The actual method of attachment is not significant to the present invention. Thus, the attachment means could include tying, embedding, potting, swaging, and many other known approaches. 
     The reader will observe that check cable  14  is slack in  FIG. 1 . It has deflected downward under the influence of gravity. Slack in a stabilizing line often presents a problem. In the case of a floating dock, the slack can become entangled in other structures or moving watercraft. Thus, it is customary to add features which control the slack. 
       FIG. 2  shows a check cable  14  incorporating a spring  16  connected to attachment point  18 . If first block  10  and second block  12  are moved closer together, spring  16  takes the slack out of the cable by creating slack portion  20 . Relative motion between the first and second blocks is thereby governed by two distinct phenomena. As the blocks move further apart from the position shown in  FIG. 2 , spring  16  will expand and slack portion  20  will become more horizontal. When all the slack is removed from slack portion  20  the check cable will draw taut. The check cable is preferably made of a material having a much higher linear extension coefficient than spring  16 . Thus, as the blocks move away from each other, there will be a first range of distance where the force tending to pull the blocks together (a “restoring force”) grows linearly. Then, when the check cable grows taut, the restoring force tending to pull the blocks together will suddenly grow much greater. This combination of force is advantageous in many circumstances. 
     Of course, there are many known devices for producing such a combination of forces.  FIG. 3  shows one such arrangement. The near end of check cable  14  is directly connected to first block  10 . The far end is rolled around retracting drum  22 , which is attached to second block  12 . The retracting drum is biased by a spring so that it tends to “reel in” the free cable. The spring can be a conventional one, so that the restoring force increases as the blocks are moved further apart. On the other hand, a constant force spring can be used in the drum so that the restoring force is constant on the check cable. The end of check cable  14  is solidly connected to the drum so that once a defined extension limit is reached, the check cable draws taut and restricts further motion between the blocks. 
       FIG. 4  shows another variation on this concept. Planar spring  24  is made of a material which can elastically deform to a state where it is completely straight. The dual extension behavior is thereby realized. A relatively weak restoring force is present at first. This builds steadily until the planar spring is completely straightened, at which point the restoring force rises dramatically. 
     A parallel arrangement of two dissimilar materials is shown in  FIG. 5 . Elastic member  26  is an expandable elastic material (such as a piece of natural or synthetic rubber). Check cable  14  is wrapped around elastic member  26 . Though the check cable has the appearance of a helical spring, it does not act as a spring. The assembly expands until check cable  14  draws taut, at which point further expansion is restricted. The reader will therefore appreciate that extension limiting devices having changing restoring force characteristics are useful in many applications. 
     BRIEF SUMMARY OF THE PRESENT INVENTION 
     The present invention comprises a self-retracting extension limiting device. It is designed to be attached between a first object and a second object, where the distance between the first and second objects is variable. The device exerts a restoring force tending to draw the two objects together. A first, relatively weak restoring force is exerted over a range of distance. Then, when the objects have been separated to a defined limit of travel, the device exerts a much greater restoring force. The invention includes overlapping and self-contained elements so that unwanted slack is minimized and alignment of the elements is maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view, showing a prior art check cable. 
         FIG. 2  is a perspective view, showing a prior art check cable with a slack-eliminating feature. 
         FIG. 3  is a perspective view, showing a different type of slack-eliminating feature. 
         FIG. 4  is a perspective view, showing the use of a planar spring to create a staged restoring force. 
         FIG. 5  is a perspective view, showing the use of two parallel elements to create a staged restoring force. 
         FIG. 6  is a perspective view, showing a portion of the present invention. 
         FIG. 7  is a perspective view, showing the addition of tension springs to the present invention. 
         FIG. 8  is a perspective view, showing an alternate embodiment of the present invention using sling loops. 
         FIG. 9  is a perspective view, showing an alternate embodiment of the present invention where each loop is created by coupling one end of a cable to its opposite end. 
         FIG. 10  is a perspective view, showing a version of the present invention using apex blocks. 
         FIG. 11  is a perspective view with a cutaway, showing the internal details of an apex block. 
         FIG. 12  is a perspective view, showing the use of apex blocks incorporating stabilizing bores. 
         FIG. 13A  is a perspective view, showing one type of apex block in more detail. 
         FIG. 13B  is a perspective view with a cutaway, showing internal features of the apex block shown in  FIG. 13A . 
         FIG. 14  is a perspective view with a cutaway, showing how the cables are attached to the apex block of  FIG. 13A . 
         FIG. 15  is a perspective view with a cutaway, showing a different type of apex block. 
         FIG. 16  is a perspective view, showing the present invention in a retracted state. 
         FIG. 17  is a perspective view, showing the present invention in an extended state. 
         FIG. 18  is a perspective view, showing the use of offset apex blocks. 
         FIG. 19  is a perspective view, showing the use of dogleg apex blocks. 
         FIG. 20  is a perspective view, showing a modified version of the present invention having a greater extension length. 
         FIG. 21  is a detailed perspective view, showing some of the components of the embodiment of  FIG. 20  in greater detail. 
         FIG. 22  is a perspective view, showing an embodiment in which a loop of cable is looped around an apex block. 
         FIG. 23  is a perspective view with a cutaway, showing internal details of the apex block used in the embodiment of  FIG. 22 . 
         FIG. 24  is a perspective view, showing the attachment of an elastic tensile member to the apex block of  FIG. 23 . 
     
    
    
     REFERENCE NUMERALS IN THE DRAWINGS 
     
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                 10 
                 first block 
                 12 
                 second block 
               
               
                 14 
                 check cable 
                 16 
                 spring 
               
               
                 18 
                 attachment point 
                 20 
                 slack portion 
               
               
                 22 
                 retracting drum 
                 24 
                 planar spring 
               
               
                 26 
                 elastic member 
                 27 
                 compressive element 
               
               
                 28 
                 first cable loop 
               
               
                 30 
                 second cable loop 
                 32 
                 interlock 
               
               
                 34 
                 tension spring 
                 36 
                 loop apex 
               
               
                 38 
                 first sling loop 
                 40 
                 second sling loop 
               
               
                 42 
                 pulley 
                 44 
                 coupling device 
               
               
                 46 
                 termination 
                 48 
                 first loop cable 
               
               
                 50 
                 second loop cable 
                 52 
                 first apex block 
               
               
                 54 
                 second apex block 
                 56 
                 cable strands 
               
               
                 58 
                 potted region 
                 59 
                 expanding passage 
               
               
                 60 
                 spring anchor 
                 62 
                 stabilizing first apex block 
               
               
                 64 
                 stabilizing second apex block 
               
               
                 66 
                 cable guide bore 
                 68 
                 anchor access port 
               
               
                 70 
                 anchor bulkhead 
                 72 
                 cable exit 
               
               
                 74 
                 anchor 
                 80 
                 second offset block 
               
               
                 82 
                 first offset block 
                 84 
                 second dogleg block 
               
               
                 86 
                 first dogleg block 
                 90 
                 first loop end block 
               
               
                 92 
                 second loop end block 
                 94 
                 extension limiting device 
               
               
                 96 
                 first cable 
                 98 
                 second cable 
               
               
                 100 
                 loop apex block 
                 102 
                 loop retainer 
               
               
                 104 
                 loop access port 
                 106 
                 anchor block 
               
               
                 108 
                 elastic tensile member 
               
               
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 6  shows the primary components of the present invention (designated as extension limiting device  94 ). A connection is made between first block  10  and second block  12 . The blocks represent any two items that are to be attached (such as a ship and a mooring point). The connection is made by creating an interlock  32  between first cable loop  28  and second cable loop  30 . The two free ends of cable loop  30  are attached to second block  12 . Likewise, the two free ends of cable loop  28  are attached to first block  10 . 
     First block  10  and first cable loop  28  thereby form a first loop which moves in unison. Second block  12  and second cable loop  30  form a second loop which likewise moves in unison. The reader will observe in this disclosure how the two loops can be formed with various components and means of connection. However, the concept of the elements within a loop moving in unison remains throughout. The two loops shown in  FIG. 6  are angularly offset (in this case by 90 degrees) to form the interlock. The arrangement shown limits how far apart the two blocks can travel. When the two cable loops draw taut, interlock  32  will prevent further travel. 
     The device of  FIG. 6  lacks features to take up slack in the assembly.  FIG. 7  shows these features. Each cable loop has a loop apex  36 . A tension spring  34  is attached between each loop apex and the opposite block, as shown (Although coil springs are shown in  FIG. 7 , any type of elastically expanding tensile member could be used). 
     The tension springs maintain tension on the cable loops, even as the two blocks are moved closer together. The result is a staged restoring force between the two blocks. In the position shown in  FIG. 7 , the restoring force is created by the extension of the two tension springs  34 . The restoring force will increase linearly as the two blocks are pulled away from each other. However, at some point the two loop apexes will collide to form interlock  32 . Once this occurs, the cables will draw taut and the restoring force will substantially increase (assuming that the cables are made of a relatively inelastic material). 
       FIG. 8  shows the same type of assembly using slings instead of conventional round cables. Interlock  32  is created by the overlap between first sling loop  38  and second sling loop  40 . The tension springs are connected between each block and its opposing loop apex, as for the embodiment of  FIG. 7 . 
       FIG. 9  shows a different method of attaching the cable loops to the blocks. Instead of attaching each loop&#39;s free ends to the blocks, the free ends are joined by coupling devices  44 . Thus, each cable loop is a complete oval. A portion of each oval is passed around a pulley  42 . The two pulleys  42  are connected to the blocks by rotary joints. Tension springs  34  are again placed between the loop apexes and the appropriate block. 
     While the devices shown in  FIGS. 6-9  are functional, those skilled in the art will realize that attaching one end of a tension spring to a loop apex presents practical problems. The cables used in the loops will often be flexible. Simply hooking one end of a spring around the apex will greatly stress the cable at that one point. Another component is preferably employed to eliminate this problem. 
       FIG. 10  shows this additional component. First apex block  52  and second apex block  54  are placed in the position of the loop apexes shown in  FIG. 9 . Tension springs  34  connect to these apex blocks rather than to the cables themselves. First block  10  is connected to first apex block  52  by a pair of first loop cables  48  (The combination of the pair of first loop cables  48 , first apex block  52 , and first block  10  forming a loop). Likewise, second apex block  54  is connected to second block  12  by a pair of second loop cables  50 . The cable ends are attached to the apex blocks using any known method. 
       FIG. 11  illustrates one approach to attaching the cables and tension springs to an apex block. First apex block  52  is shown sectioned in half to reveal its internal features. It includes two expanding passages  59 . The free ends of the two first loop cables  48  are placed within the two expanding passages. The strands comprising the cable are then splayed and infused with liquid potting compound. At some point prior to the hardening of the liquid potting compound, the cable ends are placed within expanding passages  59 . The liquid potting compound then transitions to a solid, creating potted region  58 . Each potted region  58  contains a solidified composite of cable strands and hardened potting compound. This then creates a mechanical interference with the expanding passage in first apex block  52 , thereby locking the cable to the first apex block. The completed interlock is referred to as termination  46 . 
     Tension spring  34  may likewise be attached to the apex block by a variety of methods. One approach is to form spring anchor  60  on the end of the spring (using a forging or heading process) then cast the first apex block around spring anchor  60 . Another approach would simply be to provide an eye on the portion of first apex block  52  facing the tension spring. A hook on one end of the spring could then be inserted through this eye. 
     Returning to  FIG. 10 , the reader will observe how the use of the apex blocks allows a better connection between the cables within a loop and the corresponding tension spring. Within the invention&#39;s prescribed range of motion, the distance between a block and its corresponding apex block will remain constant. First block  10  and first apex block  52  will remain separated by a fixed distance, even as the distance between first block  10  and second block  12  varies. Likewise, second block  12  and second apex block  54  will remain separated by a fixed distance. 
     The two tension springs  34  maintain tension within the two “loops,” so that the cables remain taut and aligned. The tension within the springs also tends to contract the overall length of the device (Second block  12  is urged toward first block  10 ). The springs provide a restoring force. This is important even where rigid materials are used for the cables. As an example, the cables could be made of stiff fiberglass. Such cables would remain straight even without tension springs  34 . However, as explained initially, an important feature of the invention is the provision of a staged restoring force. Tension springs  34  provide the first stage of the restoring force. If, as an example, extension limiting device  94  is connected between a corner of a floating dock and a fixed point on shore, the tension springs will tend to urge the dock toward the fixed point on shore. The first stage of the restoring force may be quite weak in some applications. In fact, in some cases only enough restoring force to retain the general alignment of the components will be needed. 
     In the case of a floating dock, wave and possibly tidal action will tend to extend and retract extension limiting device  94 . Those skilled in the art will also realize that lateral flexing motions will occur. The geometry used in the embodiment of  FIG. 10  has some disadvantages, in that the two apex blocks can be pushed laterally into the cables. This action can cause wear. Thus, additional features are desirable. 
       FIG. 12  shows another embodiment of extension limiting device  94 . In this embodiment, stabilizing apex blocks are used. Stabilizing first apex block  62  is connected to a pair of first loop cables  48 . However, it also contains a pair of passages that allow the two second loop cables  50  to pass through it. These passages are a loose sliding fit, so that stabilizing first apex block  62  can slide back and forth along second loop cables  50 . 
     Stabilizing second apex block  64  is attached to the two second loop cables  50 . It includes passages allowing it to accommodate and slide back and forth along the two first loop cables  48 . The use of the stabilizing apex blocks keeps all the components appropriately separated as the extension limiting device expands and contracts, thereby reducing wear. 
       FIG. 13A  shows stabilizing second apex block  64 . It includes two cable guide bores  66 , which pass completely through the block. These are sized to provide a loose sliding fit with first loop cables  48  (thereby stabilizing the motion of the two loops with respect to each other). Two anchor access ports  68  are also provided.  FIG. 13B  shows the same apex block with a cutaway positioned to reveal internal details. The reader may readily observe how the nearer of the two cable guide bores  66  passes through the block from one side to the other. The reader may also observe the location of spring anchor  60 . As for the previous embodiments, the spring could be attached using many different methods, including the provision of an eye or hook on the block. 
     The two anchor access ports  68  do not pass all the way through the apex block. Anchor bulkhead  70  blocks the far end of each anchor access port. A cable exit  72  passes through each anchor bulkhead. This configuration allows the attachment of stabilizing second apex block  64  to cables having anchors affixed to their free ends. 
       FIG. 14  shows stabilizing second apex block  64  with cables attached. The upper of the two second loop cables  50  is shown passing through cable exit  72 . Anchor  74 —which is securely attached to the end of the cable, bears against anchor bulkhead  70 . Cable exit  72  is large enough to allow the passage of the cable, but not the anchor. The anchor can be attached to the end of the cable by any suitable means, including mechanical fasteners, swaging, potting, etc. While a discussion of these techniques is beyond the scope of this disclosure, the reader may learn more about such anchors by reviewing U.S. Pat. No. 7,076,853 to Campbell (2006), which is incorporated herein by reference. 
       FIG. 14  shows how each of the two second loops cables  50  is attached to the apex block, while each of the two first loop cables  48  pass through the apex block. The approach of using anchors on the end of the cable is suitable for transmitting tensile loads. The use of anchors (as opposed to directly potting the cable ends into the apex block) also allows the device to be more easily assembled in the field. The cables can be passed through the two cable exits  72  before the anchors are added to establish the desired cable length. 
     Referring back to  FIG. 12 , the reader will recall that the two cable loops are rotationally offset to avoid mechanical interference between the loops. An offset of only a few degrees may be suitable in some circumstances. However, in the embodiment of  FIG. 12 , the offset is 90 degrees. The same 90 degree offset is used for the illustrated embodiments of the stabilizing apex blocks.  FIG. 15  shows stabilizing first apex block  62 . The reader will observe that this includes the same features as stabilizing second apex block  64 . The stabilizing first apex block is simply the stabilizing second apex block turned around to face the other direction, then rotated 90 degrees. 
     Stabilizing first apex block  62  includes two cable guide bores  66 . These form a loose sliding fit over the two second loop cables  50 . The two first loop cables  48  are attached by passing the cables through cable exits  72  and securing the two anchors  74  against anchor bulkheads  70 . Thus attached, the invention is able to extend and retract while keeping its components properly aligned. 
       FIGS. 16 and 17  show the invention in a retracted and extended state respectively. In  FIG. 16 , the two tension springs  34  are in a relaxed state. Thus, there is no restoring force tending to urge second block  12  toward first block  10 . As second block  12  is initially moved further away from the position shown in  FIG. 16 , the tension springs will extend and the restoring force will increase linearly (according to the spring coefficients used). This linear increase is the first stage in the restoring force. 
     The linear increase in the restoring force will continue until the position shown in  FIG. 17  is reached. In this position, stabilizing second apex block  64  is bearing against stabilizing first apex block  62 . If additional force tending to pull second block  12  away from first block  10  is then applied, this force will be counteracted by tension in the cables themselves. This phenomenon creates the second stage in the restoring force. The cables—unless very elastic materials are used—will create a much larger restoring force than the tension springs. 
     The nature of the two stages can be adjusted as desired, by selecting appropriate materials and dimensions for the tension springs and the cables comprising the loops. In most applications there will be a significant rise in the restoring force during the transition from the first stage to the second stage. However, this need not always be the case. For example, a particular application might call for the use of very stiff tension springs and relatively elastic cables. With this arrangement, the transition from one stage to the next would be more gradual. 
     Those skilled in the art will realize that the arrangements shown in  FIGS. 6 through 17  could be realized using a variety of different geometries.  FIGS. 16 and 17  use cable loops which are angularly offset by 90 degrees.  FIG. 18  shows an embodiment using parallel loops which are linearly offset. First offset block  82  moves in unison with first block  10 , whereas second offset block  80  moves in unison with second block  12 . Each offset block attaches a pair of cables in one loop, and slidably fits over a pair of cables in the other loop. This type of arrangement has some disadvantages, in that tension on the offset blocks can produce a twisting force. This is a relatively small force, however, which is acceptable in some applications. 
       FIG. 19  shows still another geometry, using first dogleg block  86  and second dogleg block  84 . First dogleg block  86  moves in unison with first block  10 , whereas second dogleg block  84  moves in unison with second block  12 . First cable  96  passes through second dogleg block  84  and attaches to first dogleg block  86 . Second cable  98  passes through first dogleg block  86  and attaches to second dogleg block  84 . This arrangement allows extension and retraction without creating significant twisting forces. 
     All the embodiments disclosed in  FIGS. 6-19  are limited to a maximum 2:1 fully extended to fully retracted ratio (The fully extended length is twice the fully retracted length). Even this ratio is only achievable through carefully designing the tension springs and providing recessed attachment points. Many versions will only achieve a ratio of about 1.8:1. In the event that a higher ratio is needed, still more components must be added. 
       FIG. 20  shows an embodiment wherein two loops are linearly offset (as for the embodiment of  FIG. 18 ). However, in this version, the cables comprising the loops actually extend through the two end blocks. First block  10  includes a pair of cable guide bores  66  allowing each first loop cable  48  to slidably pass through first block  10 . Likewise, second block  12  includes a pair of cable guide bores allowing the two second loop cables  50  to pass through. 
       FIG. 21  shows some of the components in more detail. The two first loop cables  48  pass through cable guide bores  66  in first block  10 . The two free ends of first loop cables  48  are attached to first loop end block  90 . Compressive element  27  is sandwiched between first loop end block  90  and first block  10 . (Likewise, the two free ends of second loop cables  50  are attached to second loop end block  92 . A second compressive element  27  is sandwiched between second loop end block  92  and second block  12 ). 
     The compressive elements act as a compression spring. They may assume many forms. One example would be a compression spring resting within a hollow cylindrical sleeve. The sleeve prevents the compression spring from buckling. Another example would be a spring wound around a guide rod, with the guide rod preventing buckling. 
     In the embodiment of  FIGS. 20 and 21 , the first and second blocks can move apart until first offset block  82  bears against second offset block  80 . This would customarily be the fully extended position (other than additional extension through the stretching of the cables). However, in the embodiment of  FIGS. 20 and 21 , the presence of the two compressive elements  27  allows further extension. The first and second blocks can move further apart until the two compressive elements  27  are compressed to their maximum extent. 
     As mentioned previously, there are many possible approaches to securing the cables to the apex blocks.  FIGS. 22 through 24  show another example. In  FIG. 22 , first loop cable  48  is passed through the interior of loop apex block  100 , around a securing feature, and back toward first block  10 . In this embodiment, the two free ends of first loop cable  48  are attached to first block  10 .  FIG. 23  shows loop apex block  100  cut in half to show its internal details. The cable passes around loop retainer  102 , which is given a radius large enough to prevent unduly stressing the cable. The cable can be manipulated through loop access port  104 . During assembly, one end of the cable is first attached to block  10 . The free end is then passed through loop apex block  100 , around loop retainer  102 , and back toward block  10 . The free end is then attached to block  10 . 
     Loop apex block  100  preferably contains other features disclosed in the prior embodiments, such as a pair of cable guide bores  66 . The use of the loop attachment method does not change the operation of the completed assembly, which is the same as illustrated in  FIGS. 16-18 . However, it does introduce additional requirements as concerns the attachment of the elastic tensile member.  FIG. 24  illustrates how a separate anchor block  106  can be attached to loop apex block  100 . This anchor block attaches to loop apex block  100  in the vicinity of loop access port ( 104 ) (and typically covers the loop access port). Elastic tensile member  108  is attached to anchor block  106 . The far end of the elastic tensile member  108  is attached to second block  12 . 
     Elastic tensile member  108  performs the same function as tension spring  34 . It is typically an extruded section of elastic material which is used to maintain tension on the assembly. Elastic straps—having a rectangular cross section—can also be used. In fact, any element capable of elastically extending and retracting while maintaining appropriate tension can be used in any of the embodiments disclosed. 
     The reader will thereby appreciate that all the embodiments disclosed have certain common elements and features. The elements are as follows: 
     1. The first and second blocks. The term “block” should be broadly understood to represent a point where extension limiting device  94  interacts with the rest of the world. The block could actually be part of an external device, such as a part of a floating dock. On the other hand, the block could be part of the extension limiting device. As an example, the block could include a hook configured to attach to an eye on a floating dock. 
     2. The elastic tensile members. These are the tension springs  34  in most of the embodiments disclosed. While they are shown as conventional springs, any device which can elastically expand while creating tensile forces could be used. Rubber tubing is one example. 
     3. The cables. The cables are tension-carrying elements which—when the device approaches its limit of extension—provide a portion of the restoring force. These will generally be selected to provide a much higher proportion of the restoring force than the elastic tensile members (near the limit of extension) but this need not always be the case. 
     4. The connective elements. The cables and the elastic tensile members must be linked together by at least two connective elements. In the embodiment of  FIG. 7 , the “connective elements” are the mechanical joints between the two loop apexes  36  and the two tension springs  34 . The connective elements are more easily visualized in the embodiment of  FIG. 12 . The connective elements in  FIG. 12  are the two stabilizing apex blocks ( 62  and  64 ). 
     The invention is also defined in terms of how the elements must interact with each other. Using the elements explained above, the following must be true: 
     1. A first block must be connected to a first connective element by a first cable; 
     2. A second block must be connected to a second connective element by a second cable; 
     3. The first block must be connected to the second connective element by a first elastic tensile member; and 
     4. The second block must be connected to the first connective element by a second elastic tensile member. 
     Most of the embodiments use pairs of cables linking the same block to the same connective element. However, the embodiment of  FIG. 19  uses only a single cable for this purpose. Three, four, or more cables could be used for the purpose in other applications. 
     Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. Thus, the scope of the invention should be fixed by the following claims rather than the specific examples provided.