Abstract:
A shock-isolation structure for supporting a relatively light load on a supporting surface and having a plurality of spring units operable in both tension and compression oriented in a truss configuration with first ends of said spring units connected to the supporting surface for universal movement and with second ends of said spring units connected to the load for universal movement, with each of the spring units including a coil spring, an end cap, and a rod which extends outwardly through the end cap with a clearance fit in a nonextended position, a clevis body on the end of each of the rods, a groove in each of the end caps, and a ridge on each of the clevis bodies for mating engagement when the rod is in the nonextended position to thereby center each of the rods in each of the end caps.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a shock-isolation structure for mounting a relatively light device on a support which can be subjected to high shock forces. 
     A shock-isolation system of the present type is known in the art, and is shown in U.S. Pat. No. 4,892,051. However, the shock-isolation system shown in this patent is for mounting a relatively heavy device, and to this end, its tension-compression units utilize liquid springs. In addition, the shaft within the liquid spring is in tight sealing engagement with its supporting structure which produces friction therebetween. However, this friction can be tolerated because it is still a relatively small proportion of the spring force. However, liquid springs operable in tension and compression cannot be used when the device which is being carried by the shock-isolation structure is of relatively light weight, possibly on the order of about ten pounds, because the liquid springs will act as rigid links and will not be actuated into their tension and compression modes when subjected to shocks. This is the case because the weight of the supported device is not sufficiently great to actuate the springs into their tension and compression modes when the device is subjected to large shocks. In other words, a relatively rigid shock-isolation system, such as shown in U.S. Pat. No. 4,892,051 will not operate to isolate a device if the device which is being supported is of relatively light weight. Therefore, for supporting devices of relatively light weight, the tension compression units of the shock-isolation structure cannot utilize liquid springs, but must use springs which will yield when the light device carried by the shock-isolation system is subjected to shock forces. However, when relatively light springs in the tension compression unit are used as part of the shock-isolation system, there must be a loose fit between the shafts of the tension compression units and their guiding structure. This permits the shafts to move somewhat radially in operation, and this can be tolerated when the tension compression units are actuated in tension from their at-rest midstroke positions. However, when they return to their at-rest midstroke positions, they must be in a predetermined alignment relative to their guiding structure to support the carried device in its proper position. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a shock-isolation structure utilizing a plurality of tension compression units containing coil springs for supporting relatively light loads against shocks which would cause the device to move relative to its supporting surface and which will cause the device to return to its original position after the shock is no longer present. Other objects and attendant advantages of the present invention will readily be perceived hereafter. 
     The present invention relates to a shock-isolation structure wherein a load is supported on a supporting surface with a plurality of spring units operable in both tension and compression therebetween and wherein said spring units are oriented in a truss configuration with first ends of said spring units connected to said supporting surface for universal movement and with second ends of said spring units connected to said load for universal movement, the improvement wherein each of said spring units includes a coil spring, an end cap, a rod which extends outwardly through said end cap with a clearance fit in a nonextended position, a body on the end of each of said rods, a depression in one of each of said end caps and said body, and a protrusion on the other of each of said end caps and said body for mating engagement when said rod is in said nonextended position to thereby center each of said rods in each of said end caps. 
     The various aspects of the present invention will be more fully understood when the following portions of the specification are read in conjunction with the accompanying drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a fragmentary side elevational view of the front end of a naval vessel having a shock-isolation structure mounting a device which is to be protected against severe shocks; 
     FIG. 2 is an enlarged fragmentary cross sectional view taken substantially along line  2 — 2  of FIG.  1  and showing the geometric orientation of the tension-compression type of spring units which mount the device of FIG. 1; 
     FIG. 3 is an enlarged fragmentary side elevational view of the tension-compression units mounting the device; 
     FIG. 4 is an enlarged fragmentary view, partially in cross section, showing the ball joint at the upper end of each of the tension-compression units; 
     FIG. 5 is an enlarged fragmentary side elevational view of the ball joint at the lower end of each of the tension-compression units; 
     FIG. 6 is a cross sectional view of the tension-compression unit in a normal midstroke unstressed condition; 
     FIG. 7 is a fragmentary cross sectional view of the right end of the tension-compression unit when it is subjected to tension; 
     FIG. 8 is a cross sectional view of the tension-compression unit when it is subjected to compression; 
     FIG. 9 is an end elevational view of the end of the cylinder of the tension-compression unit taken substantially in the direction of arrows  9 — 9  of FIG. 7; 
     FIG. 10 is an end elevational view of the portion of the clevis assembly taken substantially in the direction of arrows  10 — 10  of FIG. 7; 
     FIG. 11 is a fragmentary enlarged section of FIG. 6; and 
     FIG. 12 is a fragmentary enlarged section of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Summarizing briefly in advance, the present invention relates to a shock-isolation structure for supporting a relatively light device  10 , possibly on the order of about 10 pounds, on a hexapod system consisting of a plurality of tension-compression units  11  mounted on a deck  12  of a ship  13 . The tension-compression units  11  support the device  10  in such a manner that it follows the normal movements and ordinary vibrations of the ship. However, if the ship is subjected to a major shock, the tension-compression units  11  will be activated to permit the device  10  to move relative to the ship without being injured by the shock, and after the termination of the shock, the tension-compression units  11  will accurately recenter to their midstroke positions, to thereby hold their preshock midstroke alignment. The device  10  may be any type of electronic, mechanical or optical device which must be supported in the foregoing manner. Also, the device need not be restricted to being mounted on a ship but may be mounted on any type of member on which it must be supported so that it follows the normal forces to which the device is subjected but which will be permitted to be moved by its tension-compression units so that it will not be injured by excessive shock forces such as explosions, severe seismic shocks or the like. 
     As can be seen from FIG. 2, the tension-compression units  11  are oriented in a hexapod truss configuration containing three pairs of tension-compression units  11  with each pair oriented in an inverted V-configuration spaced 120° from an adjacent inverted V-configuration. Also, the lower ends of the tension-compression units  11  lie on a perimeter which is larger than the upper ends. However, if desired, the pairs of tension-compression units  11  can be oriented in a V-configuration. 
     Each tension-compression unit  11  includes a sleeve  14  having an end portion  15  which is slidably mounted relative to a cylinder  17 . The normal midstroke unstressed position of each tension-compression unit  11  is shown in FIG. 6, and in this position the device  10  is held so that it moves with the ship  13  irrespective of normal shocks and abrasions to which the ship is subjected. More specifically, a helical or coil spring  19  is located within sleeve  14 . The right end of coil spring  19  bears against end cap  20  which is suitably retained within sleeve  14 . The left end of spring  19  bears against the flared disc-like end  21  of member  22  which encircles the enlarged portion  23  of rod  24  having a fluidic head  25  suitably mounted thereon. Head  25  may be of the type described in U.S. Pat. No. 3,722,640, which is incorporated herein by reference. Rod  24  also has a reduced portion  27  and an annular shoulder  30  (FIG. 11) against which the annular end  31  (FIG. 11) of member  22  is engaged when the tension-compression unit  11  is in its normal position of FIG.  6  and its tension position of FIG.  7 . The outer end of rod  23  has a ball-type clevis assembly  32 ′ suitably mounted thereon which provides universal movement, and the smaller end  27  of rod  24  passes through a bore  32  (FIG. 11) in end cap  20  with a sufficient clearance fit so that there is no appreciable friction where the shaft moves through bore  32 . The clearance fit may be on the order of a few thousandths of an inch. Cylinder  17  functions as a damper and it has suitable hydraulic fluid therein. The end of cylinder  17  includes an accumulator  33  having a wall  34  mounted at the end of rod  36  with a suitable valve arrangement mounted therein which may be identical to that shown in U.S. Pat. No. 5,727,663 which is incorporated herein by reference. A ball-type clevis assembly  35  is mounted integrally with cylinder  17  and it provides universal movement. 
     Each of the cylinders  11  is mounted between the device  10  and the deck  12  in a hexapod configuration as shown in FIGS. 2 and 3. The clevis assemblies  32 ′ of tension-compression units  11  are mounted in clevis portions  37  mounted on deck  12  (FIGS.  2  and  5 ), and the upper portions of tension-compression units  11  are mounted in brackets  39  (FIGS.  2  and  4 ). Thus the opposite ends of each unit  11  is mounted for universal movement. 
     When the device  10  is subjected to a sufficiently large shock which causes it to move relative to the deck  12  of ship  13 , the tension-compression units  11  will be activated, and some may move in tension, and some may move in compression, and some may just retain their normal position. After the shock is no longer present, the internal mechanisms of each of the tension-compression units  11  will cause them to return to their normal midstroke position of FIG.  6 . 
     When a tension-compression unit  11  is placed in tension, the parts will move to the position of FIG. 7 wherein the small section  27  of shaft  24  is pulled out of bore  32 , and the engagement between annular shoulder  30  (FIG. 11) and annular end  31  of member  22  will pull member  22  to the right (FIG. 7) from its position shown in FIG. 6, thereby compressing the spring  19 . The piston head  25  will also move to the right to the position shown in FIG.  7 . During the movement of shaft  24  to the right, the portion  23  of shaft  24  will move through a conventional elastomerically energized U-cup seal  26  having a compressed O-ring  28  therein, as is well known. This seal prevents leakage of hydraulic fluid from cylinder  17 . The seal  26  is maintained in position because member  40  bears against one side thereof, and the other side bears against annular shoulder  36  (FIG. 12) of cylinder  17 . While the unit  10  is in tension, the force of coil spring  19  on end cap  20  will maintain sleeve  14  in a position wherein the annular shoulder  43  (FIG. 11) of sleeve  14  continues to bear against the annular end  44  of member  40 , which is threaded onto cylinder  17  at  45 . When the tensile force is removed from tension-compression unit  11 , the spring  19  will expand and the parts will return to the position shown in FIG.  6 . 
     As noted above, there is a loose fit with clearance between the reduced portion  27  of shaft  23  and bore  32  in end cap  20 , as depicted in FIG.  11 . This clearance is desirable so that there will be practically no friction between portion  27  of shaft  24  and bore  32  of end cap  20 , even if portion  27  of shaft  24  should hit the side of bore  32  when the shaft is in the position of FIG.  7 . However, it is imperative that the tension-compression unit always return to the same position wherein the small end  27  of rod  24  is centered relative to the remainder of the end cap  20  to insure that the device  10  is always supported in the same position. In accordance with the present invention, the body  49  of clevis assembly  32 ′ includes a protrusion in the form of an annular ridge  50  (FIGS. 7 and 9) which seats in complementary mating engagement in a depression in the form of an annular groove  51  in end cap  20 , thereby assuring perfect centering of the small portion  27  of shaft  23  within sleeve  14 . 
     When the tension-compression unit  11  is placed in compression, the parts will assume the positions of FIG. 8 wherein spring  19  is compressed between end cap  20  and portion  21  of member  20 . In this respect, the right end of spring  19  will bear against end cap  20 , and the left end of spring  19  will bear against portion  21  of member  22  which in turn will bear against member  40  which is threaded onto cylinder  17 , and the parts of shaft  23  and head  25  will assume the positions shown in FIG.  8 . When the compressive force is released, the parts will return to the position of FIG. 6 when spring  19  expands to the position of FIG.  6 . 
     In the above-described system the springs have to be preloaded to the extent that the supported device does not move relative to the supporting surface in response to forces which may normally be experienced on a routine basis. This preloading maintains the spring units in their normal midstroke unstressed positions of FIG.  6 . The total spring force produced by the tension-compression units can normally be between about three and four times the load. However, there are installations where the total spring force can be as low as 2.5 times the load or as high as ten times the load, but most of the time the spring force would be as low as about three times the load but it could be as high as about six times the load. The load would be the weight of the device including the member or platform above the tension-compression units on which the device rests. 
     Therefore, for example, if the load is ten pounds and the desired spring force is three times the load, the total spring force should be thirty pounds, and where the spring force is four times the load, the spring force would be 40 pounds. Therefore, on the basis of the foregoing, the ratio of total spring force to weight of the load can be between about 3:1, or it can be 4:1. By way of example and not of limitation, a system wherein the load is ten pounds and a 3-4G system preload is desired, the required preload can be 3×10 (30 pounds minimum), or it can be 4×10 (forty pounds maximum). 
     When six tension-compression units are at a 45° mounting angle between the supporting surface and the load, the required spring preload for each tension-compression unit or strut will be:          Ratio                 3        :        1                 for                 ten                 pound                 load     =       30     6      sin                 45      °       =     7.1                 pounds                 Ratio                 4        :        1                 for                 ten                 pound                 load     =       40     6      sin                 45      °       =     9.5                 pounds                              
     In the above equations, sin 45° is 0.7. Also, as noted above, the shaft clearance at bore  32  should be a few thousandths of an inch, preferably about ten thousandths of an inch. 
     As noted above, the total spring force can be as low as 2.5 times the load or as high as ten times the load, depending on the type of installation. Thus, there is a range where the ratio of spring force to load can be as low as 2.5:1 or as high as 10:1. 
     Also as noted above, the range of spring force to load in many instances can be as low as about three times the load or as high as six times the load, depending on the type of installation. Thus, there can be a range wherein the ratio of spring force to load can be as low as 3:1 or as high as 6:1. However, as noted above, the ratio of spring force to load would normally be between about 3:1 to 4:1. 
     While the above tension-compression unit utilized a protrusion in the form of a circular ridge  50  associated with a depression in the form of a circular groove  51  to center the shaft portion  27  in bore  32 , it will be appreciated that any other type of centering structure can be used including but not limited to one or more conical protrusions on body  49  or end cap  20  and a complementary mating depression on the other of body  49  and end cap  20 , or any other interfitting structure which causes the body  49  to return to a predetermined orientation relative to end cap  20 . 
     It is to be noted that the tension-compression unit  11  itself is extremely similar to a prior art device, and the only substantial difference resides in the self-centering construction resulting from the coaction of the ridge  50  and groove  51 , as explained above. Also, the prior art device did not have a clearance between the shaft and the end cap of the unit. 
     While a preferred embodiment of the present invention has been disclosed, it will be appreciated that it is not limited thereto, but may be otherwise embodied within the scope of the following claims.