Abstract:
A swivel joint according to the present invention includes first and second base members. A spider includes first and second arcuate surfaces which form joints with the first and second base members, respectively. Furthermore, the spider includes first and second pins both opposed from each other and third and fourth pins opposed from each other. A first mechanism is coupled to the spider and the first base member and is adapted to effect a force between the first arcuate surface and the first base member. Also, a second mechanism is coupled to the spider and the second base member and is adapted to effect a force between the second arcuate surface and the second base member.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to swivel joints. More particularly, the present invention relates to a swivel joint for transferring tension and compression forces. 
     Universal joints are well-known devices that couple members together, yet allow motion in two degrees of freedom. Commonly, the universal joint includes two yokes or devises with a cross or spider member disposed therebetween. Bearing surfaces on ends of the spider allow relative angular motion about two orthogonal axes. 
     Universal joints can be used in a variety of different applications. In many instances, the universal joint is used to transfer torque loads between coupled members. However, an universal joint having high axial stiffness for transmitting tension and compression forces would be particularly beneficial for yet other applications. However, current universal joints experience low strength and stiffness in the axial direction due to bending stresses and deflection of the spider. 
     SUMMARY OF THE INVENTION 
     A swivel joint according to one aspect of the present invention includes first and second base members. A spider further includes first and second arcuate surfaces that form joints with the first and second base members, respectively. Furthermore, the spider includes first and second pins opposed from each other and third and fourth pins opposed from each other. A first mechanism, coupling the spider and the first base member, is adapted to effect a compressive force between the first base member and the first arcuate surface. Also, a second mechanism, coupling the spider and the second base member, is adapted to effect a compressive force between the second base member and the second arcuate surface. The forces between the base members and the corresponding arcuate surfaces maintain coupling of the base members and the corresponding arcuate surfaces when subjected to external working forces, thereby preserving axial stiffness throughout the working force range in tension and compression and through the range of motion of the joint. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a simulation system for use with the present invention. 
     FIGS. 2 and 3 are perspective views of a swivel joint according to the present invention. 
     FIG. 4 is a perspective view of a swivel joint with certain elements illustrated in dashed lines. 
     FIGS. 5 and 6 illustrate exploded views of a swivel joint according to the present invention. 
     FIG. 7 is an exploded view of an alternative embodiment of a swivel joint according to the present invention. 
     FIG. 8 is a side view of a swivel of an alternative embodiment according to the present invention. 
     FIG. 9 is a top plan view of the swivel illustrated in FIG.  8 . 
     FIGS. 10-13 are schematic illustrations of alternative embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing the universal joint in detail, an explanation of an exemplary operating environment for the universal joint and forming another aspect of the invention, may be helpful. FIG. 1 schematically illustrates an exemplary simulation system  10 . The system  10  includes a table  12  for supporting a specimen  11  under test and a plurality of actuators  15  for driving the table  12  in response from a system controller  16 . Struts  18  are commonly provided to couple the table  12  to each of the actuators  15 . In the embodiment illustrated, swivels  20  are provided between the actuators  15  and the struts  18  and the table  12 . In the embodiment illustrated, system  10  includes twelve swivels  20 , however, it will be appreciated that a swivel  20  can be provided in the system  10  where needed depending upon operating parameters thereof. In one embodiment discussed below, swivels  20  can include hydrostatic bearings where fluid for the bearings can be provided by actuators  15 . In addition, struts  18  can be adapted to provide fluid communication between swivels  20  through port  21 . Swivels  20  are particularly useful in simulation system  10 , where the swivels  20  are used for transmitting forces to a specimen or table proportionate to command inputs from controller  16 . In particular, swivels  20  are configured to transmit compression and tension forces with high load capacity, high axial stiffness, and minimized backlash. Other systems can utilize swivel joint  20  including, but not limited to, other actuator assemblies (e.g. hydraulic, pneumatic, electric), robotic mechanisms and machine tools, to name a few. 
     FIGS. 2-3 illustrate swivel joint  20  in more detail. Swivel  20  is secured to a portion of table  12  and strut  18  (or alternatively actuator  15 ) with a plurality of fasteners  23 . The plurality of fasteners  23 , herein illustrated as bolts, can be of any type to secure swivel joint  20  to table  12 , struts  18 , actuators  15  or any other member in which swivel joint  20  is useful. Swivel joint  20  is rotatable about two axes of rotation,  24  and  26 . FIG. 3 illustrates swivel joint  20  rotated about axis  24 . Rotation of swivel joint  20  about axes  24  and  26  can be achieved through a range of angles in order to transmit forces to table  12 , as desired. In one embodiment, swivel  20  allows rotation simultaneously about axes  24  and  26  through angles greater than +/−20°; however other ranges can be provided depending on the desired application. 
     With reference to FIG. 4, swivel joint  20  includes two yokes or clevises  30  and  32  and a spider or cross  34  disposed between clevises  30  and  32 . By way of example, clevis  32  includes base member  32 A, retainers  32 B and  32 C, and fasteners  32 D. Fasteners  32 D, herein illustrated as a pair of bolts, draw retainers  32 B and  32 C, and thus spider  34 , toward base member  32 A. Fasteners  32 D, along with retainers  32 B and  32 C, effect a compressive force between base member  32 A and spider  34 . The force created is preferably at a level greater than a maximum tension force expected to be placed on swivel joint  20 . Clevis  30  is constructed similar to clevis  32  such that base member  30 A and retainers  30 B and  30 C effect a compressive force between base member  30 A and spider  34 . Clevises  30  and  32  are herein illustrated orthogonal to each other. Also, the orientation of base member  30 A is opposite that of the base member  32 A. 
     The retainers  30 B and  30 C,  32 B and  32 C, along with fasteners  30 D and  32 D, couple spider  34  to their respective base members  30 A and  32 A. Coupling of spider  34  with base members  30 A and  32 A creates a direct (axial) primary load path from base member  30 A through spider  34  and into base member  32 A and vice-versa. Presence of the primary load path allows high compressive forces to be placed on swivel joint  20 , while swivel joint  20  remains rigid in an axial direction. In addition, a secondary load path exists from the base member  30 A through retainers  30 B and  30 C, spider  34 , retainers  32 B and  32 C and into base member  32 A. This secondary load path is less rigid than the primary load path. In addition, the secondary load path preloads the primary load path so the primary load path can handle external tension forces placed on swivel joint  20  in a rigid manner. 
     With reference to FIGS. 5 and 6, exploded views of an exemplary form of swivel joint  20  are illustrated. In this embodiment, spider  34  includes spider body  35  having a lower body portion  35 A and an upper body portion  35 B. Lower body portion  35 A includes a first arcuate surface  36  and upper body portion  35 B includes a second arcuate surface  38 . Arcuate surfaces  36  and  38  are shaped substantially cylindrical, which includes cylindrical. Nevertheless, material deformation in some cases may cause uneven coupling between arcuate surfaces  36  and  38  and base members  30 A and  32 A. In yet another alternative embodiment, arcuate surfaces  36  and  38  can include a slight taper at its ends to even coupling between surfaces  36  and  38  and corresponding base members  30 A and  32 A. Pin portions  40  and  42  include pairs of opposed pins (( 40 A,  40 B) and ( 42 A,  42 B)) which extend away from spider body  35  and are disposed in apertures  41  of each of the retainers  30 B,  30 C,  32 B and  32 C. Fasteners  44  secure spider body portions  35 A and  35 B together with pin portions  40  and  42  disposed therebetween. Although spider body  35  is herein illustrated and described with separate component elements, an integral assembly wherein two or more assembly elements are formed as a single body can also be used. 
     Plate members  46  and  48  are shaped similar to arcuate surfaces  36  and  38 , respectively, and are disposed between arcuate surfaces  36  and  38  and base members  30 A and  32 A, respectively. Bearing surface assemblies  50  and  52  are retained in plate members  46  and  48 . Bearing surface assemblies  50  and  52 , herein illustrated as parallel needle rollers, provide suitable assemblies for rotation of clevises  30  and  32  along arcuate surfaces  36  and  38 . A plurality of bearing surface assemblies  54  are also provided for each of the retainers  30 B,  30 C,  32 B and  32 C. Bearing surface assemblies  54  (herein needle rollers) provide suitable assemblies for rotation of pin portions  40  and  42  in apertures  41  of each retainer  30 B,  30 C,  32 B and  32 C. As those skilled in the art will recognize, alternative bearing surface assemblies such as hydrostatic bearings, balls or the like may also be used in place of bearing surface assemblies  50 ,  52  and  54  herein illustrated. 
     Construction of the individual components of swivel joint  20  can be provided in order to establish clearance between components such that a desired preload force is achieved when the components are assembled. For example, a gap can be produced between the retainers  30 B,  30 C,  32 B and  32 C and the base members  30 A,  32 A to create a desired preload force from fasteners  30 D and  32 D upon assembly. This gap is adjustable (i.e. via shims or close manufacturing tolerances) to provide different preload forces. In order to transmit tension forces effectively without backlash, the compressive force between the base members  30 A,  32 A and spider  34  is preferably at least 500 pounds. In another embodiment, the compressive force is at least 1000 pounds. In a further embodiment, the compressive force is at least 5000 pounds. In yet a further embodiment, the compressive force is at least 10,000 pounds. 
     FIG. 7 illustrates an alternative embodiment of the present invention. In this embodiment, spider  60  has arcuate surfaces that are substantially spherical, which includes spherical. The spherical surfaces of spider  60  couple to base members  30 A and  32 A, which hereby includes concave spherical surfaces. In one embodiment, spider  60  can be constructed of two body portions similar to spider body  35  as illustrated or alternatively one integral body wherein the pins  40 A,  403 ,  42 A,  42 B can be provided or inserted. Ball bearings  62  provide suitable bearing assemblies for spider  60 . 
     FIGS. 8-9 illustrate an embodiment of the present invention in which hydrostatic bearings are utilized. In addition, a spider  70  having an integral body is illustrated. Fluid source  72  is in fluid communication with a port  74  in base member  32 A. Port  74  provides fluid to a commutator  76 . Commutator  76  includes a seal  77  in order that fluid passes to spider  70  without leaking to hydrostatic bearings  80  and  81  and, in the embodiment illustrated, a friction reducing member  78 . Commutator  76  is adapted to be in fluid communication with a central port  82  in spider  70 . Auxiliary ports  84  and  86  carry fluid from central port  82  to bearings  80  and  81 . Commutator  76  minimizes the number of external connectors in order to provide fluid to the hydrostatic bearing elements. Although, if desired, separate fluid couplings can be provided for each of the base members. 
     Fluid can also pass through central port  82  to commutator  87 , which is constructed similar to commutator  76 , but orthogonally oriented relative thereto. Commutator  87  allows fluid communication to a port  88 . In this manner, port  88  can be in fluid communication with another swivel  20  through passageway  21  in strut  18  such as illustrated in FIG.  1 . This eliminates separate hoses for each of the swivels  20 . 
     By way of example, bearing  80  is illustrated in FIG.  9 . Fluid is delivered to bearing  80  through auxiliary port  84 . A channel  90  is recessed in base member  32 A to provide fluid communication to bearing  80 . Channel  90  forms pads  92 . Although the number of pads herein illustrated is three, any number of pads can be used. As fluid collects in channel  90 , fluid pressure develops in channel  90  and eventually leaks to pads  92 . Fluid pads  92  creates a suitable bearing surface for rotation between base member  32 A and spider  70  and also creates a squeeze film to minimize backlash in swivel  20  when compression and tensile forces are applied. 
     The preload forces between the spider  34  and the base members  30 A and  32 A can be achieved through use of spring elements. The spring elements, as discussed earlier, create two load paths. A working load path (primary) is established by coupling arcuate surfaces  36  and  38  to respective base members  30 A and  32 A. This is a rigid load path that handles compressive forces. The preload or secondary load path is transferred through spring elements to provide a compressive preload to the working load path. The secondary load path is more compliant than the rigid working path. The preload path allows the rigid working load path to maintain coupling of arcuate surfaces  36  and  38  to respective base members  30 A and  32 A in the presence of external tension loads placed on swivel  20 . 
     The spring elements can take many forms. FIGS. 10-13 schematically illustrate alternative embodiments of a spring element that provides a preload force between the spider  34  and a base member of a clevis. In each of the following figures, only one half of the joint is illustrated. FIG. 10 illustrates swivel  100  including spring element  102 . As illustrated, spring element  102  is a pin or pins extending between retainers  104 B and  104 C. Fasteners  104 D draw retaining elements  104 B and  104 C towards base member  104 A. This causes a bending moment in spring element  102 . If desired, retaining elements  104 B and  104 C can include enlarged apertures to provide clearance for the bending spring element  102 . 
     In FIG. 11, a swivel joint  110  includes spring elements  112  that are compliant and draw base member  114 A towards retainer elements  114 B and  114 C. In this case, pin portion  116  is more rigid than spring elements  112 . As desired, spring element  112  could be fasteners, retainers or any other element to provide a preload force. Depending on the material used for pins  40 ,  42 , or fasteners  30 D,  32 D, the swivel joint  20  can operate according to the principles of FIGS. 10 and 11 individually or in combination. 
     In yet another embodiment, FIG. 12 illustrates a side view of swivel joint  120  having spring element  122  comprising a retainer. A similar construction would be provided on the other side. In this case, retainer  122  includes a slot or gap  124 . Element  126 , herein a bolt, can be provided to increase the width of slot  124 , which provides a force in the direction of arrow  128 , which forces retainer  122  towards base element  129 . 
     Compressive spring forces may also be used. In FIG. 13, swivel joint  130  includes a spring element  132  forcing an upper retainer element  134  towards a lower retainer element  136 , which is in this case integral with a base member. For example, each fasteners  137  can include a bolt  139  inserted through an external spring element  132  (herein a spring washer stack), the bolt  139  extending through an upper retainer element  134  and threaded into a lower retainer element  136 . External spring element  132  can also be a coil spring or other external spring element. FIG. 13 illustrates one side of one half of the swivel joint  130 . A similar construction would be provided for each side portion of the joint. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.