Patent Publication Number: US-8113733-B2

Title: System having a high axial stiffness swivel joint

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of and claims priority of U.S. patent application Ser. No. 11/835,264, filed Aug. 7, 2007 (now U.S. Pat. No. 7,802,939), the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     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 clevises 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, a 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. U.S. Pat. No. 6,758,623 to Bushey discloses a high axial stiffness swivel joint that can transmit compressive and tension loads. 
     SUMMARY 
     A system includes a support, an actuator and a swivel joint coupling the actuator to the support. The swivel joint includes a first base member, a second base member, and a spider positioned between the first and second base members. The spider includes a center support and first and second bearing support elements. Each bearing support element has an arcuate surfaces adapted to form joints with the first and second base members. In one embodiment, at least one shim element disposed is between at least one of the first and second bearing support elements and the center support. 
     These and various other features and advantages will be apparent from a reading of the following Detailed Description using the exemplary embodiment therein described. This Summary and Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a simulation system for use with the present invention. 
         FIG. 2  is a perspective view of a swivel joint in accordance with one embodiment. 
         FIG. 3  is a cross-sectional view of the swivel joint of  FIG. 2 . 
         FIG. 4  is a partially exploded view of a swivel joint in accordance with one embodiment. 
         FIG. 5  is an exploded view of the swivel joint of  FIG. 4 . 
         FIG. 6  is a side view of a swivel joint in accordance with one embodiment. 
         FIG. 7  is a top plan view of the swivel joint of  FIG. 6 . 
         FIGS. 8-11  are schematic illustrations of alternative embodiments of a swivel joint. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing the swivel joint in detail, an explanation of an exemplary operating environment for the swivel 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. 2-5  illustrate swivel joint  20  in more detail, and further illustrate multiple ways to secure swivel joint  20  to table  12  and strut  18 . As illustrated, swivel joint  20  can be mounted to table  12  by inserting fasteners through apertures  13  formed in table  12 . The fasteners engage corresponding apertures  17  ( FIG. 4 ) formed in swivel joint  20 . Swivel joint  20  can be mounted to strut  18  using fasteners  23  ( FIGS. 2 and 5 ). Fasteners  23  are inserted through apertures  22  formed in swivel joint ( FIGS. 2 and 5 ) and engage corresponding apertures  24  formed in strut  18 . In one embodiment, notches  59  ( FIGS. 2 and 5 ) are provided in collars  58  for enabling access to fasteners  23  when swivel  20  is assembled. In this manner, fasteners  23  can be inserted into and removed from apertures  22  when swivel joint  20  is assembled. Further, it is noted that any suitable means can be utilized to secure swivel joint  20  to table  12  and strut  18 . For instance, apertures similar to apertures  13  can be formed in strut  18  for inserting fasteners through strut  18  and into swivel  20 . Further, apertures similar to apertures  22  can be formed in swivel  20  for securing swivel  20  to table  12 . 
     Referring to  FIG. 2 , swivel joint  20  is rotatable about two axes of rotation,  27  and  28 . In one embodiment, swivel  20  allows rotation simultaneously about axes  27  and  28  through angles greater than +/−20°; however other ranges can be provided depending on the desired application. 
       FIG. 3  is a cross section of the swivel joint  20  illustrated in  FIG. 2  taken at line  3 - 3 . In the embodiment illustrated in  FIG. 3 , swivel joint  20  comprises first and second base members  30  and  32  and a spider  33  positioned between the first and second base members  30  and  32 . The spider  33  includes a center support  34  and first and second bearing support elements  36  and  38 . Each bearing support element  36 ,  38  includes an arcuate surface  37 ,  39 , respectively, adapted to form a movable joint with first and second base members  30 ,  32 , respectively. 
     Referring also to  FIG. 5 , connector assemblies  49 A,  49 B,  49 C, and  49 D join base members  30 ,  32  to center support  34 . In particular, first and second connector assemblies  49 A and  49 B connect the center support  34  to the first base member  30  such that the first base member  30  is rotatable with respect to the center support  34  about axis  27 , while third and fourth connector assemblies  49 C and  49 D connect the center support  34  to the second base member  32  such that the second base member  32  is rotatable with respect to the center support  34  about axis  28 . As described below, each of the connector assemblies  49 A- 49 D is supported on at least one of the shafts  52 . In the embodiment illustrated in  FIG. 5 , shafts comprise a plurality of cantilevered cylindrical shafts supported by center support  34 . In some embodiments, the center support  34  and shafts  52  are integral being formed of a single unitary body. 
     In the embodiment illustrated in  FIG. 5 , connector assemblies  49 A and  49 B connect the first base member  30  with the center support  34  using fasteners  50 . Each connector assembly  49 A and  49 B comprises a collar  58  rotatably supported on a shaft  52  by a bearing assembly  56 . Fasteners  54  secure the shafts  52  to support  34  while fasteners  50  are inserted into apertures  60  formed in the base member and engage apertures  62  formed in collars  58 . Bearings  56  provide suitable assemblies for rotation of collars  58 . Bearings  56  can be any suitable configuration such as, but not limited to, parallel needle rollers. As those skilled in the art will recognize, alternative bearing assemblies such as hydrostatic bearings, balls, or the like, can also be used in place of bearing assemblies  56  herein illustrated. 
     Fasteners  54  are received in apertures  55  of center support  34 . In one embodiment, fasteners  54  are bolts that are received by threaded apertures  55 . Further, in one embodiment, each shaft  52  includes a pilot  57  that extends toward the center support  34 . Center support  34  includes corresponding apertures  35  configured to received pilots  57 . Pilots  57  and apertures  35  provide a connection between shafts  52  and center support  34  that inhibits vertical movement (i.e., shear) of shafts  52  with respect to support  34 . 
     Fasteners  50 , herein illustrated as a pair of bolts, draw their respective collars  58 , and thus connector assemblies  49 A and  49 B, towards base member  30 , providing a compressive force between base member and center support  34 . The force created is preferably at a level greater than the force that results at that location from a maximum tension force expected to be placed on swivel joint  20 . Base member  32  and connector assemblies  49 C and  49 D (shown in  FIGS. 4 and 5 ) are similar to base member  30  and connector assemblies  49 A and  49 B such that fasteners  50 , along with connector assemblies  49 C and  49 D, providing a compressive force between base member  32  and center support  34 . 
     Bearing support elements  36  and  38  are configured to provide bearing support surfaces for the base members  30  and  32 , respectively. In one embodiment, arcuate surfaces  37  and  39  of bearing support elements  36  and  38  directly support partial rotation of the first and second base members  30  and with respect to the center support  34 . In one embodiment, each arcuate surfaces  37 ,  39  comprises a hydrostatic bearing surface for supporting base members  30 ,  32 , respectively. In another embodiment, bearing support elements  36  and  38  are configured to support bearing assemblies positioned between bearing support elements  36  and  38  and the first and second base members  30  and  32 . In the embodiment illustrated, a first bearing assembly  40  is positioned between the first base member  30  and arcuate surface  37 , while a second bearing assembly  42  is positioned between the second base member  32  and arcuate surface  39 . Bearing assemblies  40  and  42  can be any suitable configuration such as, but not limited to, parallel needle rollers. As those skilled in the art will recognize, alternative bearing assemblies such as hydrostatic bearings, other rolling elements such as balls, or the like, can also be used in place of bearing assemblies  40  and  42  herein illustrated. 
     Further, in the embodiment illustrated bearing cages  41  and  43  are utilized and are shaped similar to arcuate surfaces  37  and  39 , respectively. Bearing cages  41  and  43  are disposed between arcuate surfaces  37  and  39  and base members  30  and  32 , respectively. Bearing assemblies  40  and  42  are retained in bearing cages  41  and  43 . At least one set of bearings are positioned on each arcuate surface  37  and  39 . In the embodiment illustrated, four sets of bearings are positioned on each arcuate surface  37  and  39 . Bearing assemblies  40  and  42 , herein illustrated as parallel needle rollers, provide suitable assemblies for rotation of base members  30  and  32  along arcuate surfaces  37  and  39 . 
     In any of the embodiments described herein, at least one shim  46  can be positioned between the arcuate surfaces  37  and  39  and the center support  34 . As discussed below, use of shims  46  is advantageous to establish clearance between components in swivel joint  20  such that a desired preload force is achieved when fasteners  50  are tightened. 
     In the illustrated embodiment, bearing support elements  36  and  38  include arcuate surfaces  37  and  39  that are shaped substantially cylindrical, which herein includes cylindrical. Nevertheless, material deformation in some cases may cause uneven coupling between arcuate surfaces  37  and  39  and base members  30  and  32 . As such, bearing support elements  36  and can include a slight taper at each end to even coupling contact between surfaces  37  and  39  and corresponding base members  30  and  32  when assembled. 
     In one embodiment, center support  34  has opposed major surfaces that have substantially planar portions. Further, each bearing support element  36  and  38  includes a surface that has substantially planar portions that are adapted to engage one of the major surfaces of the center support  34 . For instance, in one embodiment center support  34  includes a substantially flat first surface  67  facing upper bearing support element  36 . Upper bearing support element  36  can also be configured to include a substantially flat surface (not shown) that is adapted to engage the surface  67 . In another embodiment, center support  34  includes a substantially flat second surface (not shown) facing lower bearing support element  38 . Similarly, lower bearing support element  38  can also be configured to include a substantially flat surface  69  that is adapted to engage the substantially flat surface of center support  34 . While center support  34  is illustrated as substantially square, it is noted that center support  34  can be any suitable shape such as, but not limited to, rectangular, circular, elliptical, etc. Further, center support  34  can be symmetrical or asymmetrical. 
     In one embodiment, spider  33  includes at least one aligning device configured to align a bearing support element ( 36 , 38 ) and/or a shim  46  (if present) with the center support  34 . Further, the at least one aligning device is configured to react lateral forces applied to the respective bearing support element ( 36 , 38 ). For instance, when swivel joint  20  is deflected to some extent (i.e., rotated about axes  27  and/or  28 ), a portion of the axial load can create a shearing force between at least one of the bearing support elements ( 36 , 38 ) and the center support  34 . In this manner, the at least one aligning device operates as a shear key to prevent lateral movement of the respective bearing support element ( 36 , 38 ) with respect to the center support  34 . Stated another way, tension on a top mounting surface of base member  30  when joint  20  is deflected to an angle about axis (and/or  28 ) results in an upward force on shafts  54  at a corresponding deflection angle. A component of that force proportional to the sine of the deflection angle will try to shear the center support  34  relative to the bearing support element along the plane of the shim. The aligning device augments the friction resistance of this joint. It is also noted that in some embodiments of joint  20  movement of the bearing support element along the orthogonal axis can be constrained by collars  58 . 
     For example, a suitable aligning device includes protrusions and corresponding apertures formed on the center support  34 , bearing support elements  36 , 38  and/or shims  46 . In the exemplary embodiment illustrated in  FIG. 3 , the at least one aligning device comprises locating pins. One locating pin  44  is inserted into an aperture  66 A in center support  34  and an aperture  66 B in a bottom surface of bearing support element  36 . Similarly, a second pin  44  can be inserted into a second aperture  68 A in a bottom surface of center support  34  and an aperture  68 B in bearing support element  38 . 
     As described above, connector assemblies  49 A,  49 B,  49 C and  49 D along with fasteners  50 , couple center support  34  to their respective base members  30  and  32 . Coupling of center support  34  with base members  30  and  32  creates a direct (axial) primary load path from base member  30  through bearing support element  36 , center support  34 , bearing support element  38 , and into base member  32 , and vise-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  through connector assemblies  49 A and  49 B, center support  34 , connector assemblies  38 C and  38 D, and into base member  32 . This secondary load path is less rigid than the primary load path. However, 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. 
     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 engaging surfaces of the collars  58  and their respective base members  30  and  32  to create a desired preload force from fasteners  50  upon assembly. In one embodiment, shims elements are used to adjust the gap. In the embodiment illustrated, a first shim element  46  is positioned between the first bearing support element  36  and center support  34 , and a second shim element  46  is positioned between the second bearing support element  38  and the center support  34 . As appreciated by those skilled in the art, one or more shims can be used in one or both locations. 
     The dimensions of shims  46  (i.e., thickness) and their number can be adjusted based on the desired preload forces. In order to transmit tension forces effectively without backlash, the compressive force between the base members  30  and  32  and spider assembly  33  is preferably at least 500 pounds. In another embodiment, the compressive force is at least 1,000 pounds. In a further embodiment, the compressive force is at least 5,000 pounds. In yet a further embodiment, the compressive force is at least 25,000 pounds. 
     It is noted that other means can be utilized to create a gap between collars  58  and their respective base member. For example, shims can be positioned in alternate locations such as, but not limited to, engaging collars  58 . Additionally, in some embodiments bearing assemblies  40  and  42  can be configured to include larger bearing elements to increase the gap between base members  30  and  32  and their respective connector assemblies ( 49 A and  49 B) and ( 49 C and  49 D). Further, in some embodiments, shafts  52  can be adjusted to shift connector assemblies  49 A- 49 D with respect to center support  34  and create a gap between collars  58  and their respective base members. 
       FIGS. 6-7  illustrate an embodiment of a swivel joint in which hydrostatic bearings are utilized. Fluid source  72  is in fluid communication with a port  74  in base member  30 ′. 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. 7 . Fluid is delivered to bearing  80  through auxiliary port  84 . A channel  90  is recessed in base member  30 ′ to provide fluid communication to bearing  80 . Channel  90  forms pads  92 . Although the number of pads herein illustrated is four, 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  30 ′ 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  33  and the base members  30 ′ and  32 ′ 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  37  and  39  to respective base members  30 ′ and 32′. 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 for 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  37  and  39  to respective base members  30 ′ and  32 ′ in the presence of external tension loads placed on swivel  20 . 
     The spring elements can take many forms.  FIGS. 8-11  schematically illustrate alternative embodiments of a spring element that provides a preload force between the spider and a base member of a clevis. In each of the following figures, only one half of the joint is illustrated. 
       FIG. 8  illustrates swivel  100  including spring elements  102 . As illustrated, spring elements  102  are pins extending from or to a center support  104 . The spring elements  102  extend into collars  107  of connector assemblies  106 A and  106 B. Fasteners  108  draw connector assemblies  106 A and  106 B towards base member  110 . This causes a bending moment in spring elements  102 . If desired, connector assemblies  106 A and  106 B can include enlarged apertures to provide clearance for the bending spring elements  102 . 
     In  FIG. 9 , a swivel joint  111  includes spring elements  112  that are compliant and draw base member  114  towards connector assemblies  116 A and  116 B. In this case, pin portions  118  are more rigid than spring elements  112 . As desired, spring elements  112  could be fasteners, retainers or any other element to provide a preload force. Depending on the material used for fasteners  54 , shafts  52 , or fasteners  50  and  51 , the swivel joint  20  can operate according to the principles of  FIGS. 8 and 9  individually or in combination. 
     In yet another embodiment,  FIG. 10  illustrates a side view of swivel joint  120  having a connector assembly  122  that is a spring element. A similar construction would be provided on the other side. In this case, connector assembly  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 connector assembly  122  towards base element  129 . 
     Compressive spring forces may also be used. In  FIG. 11 , 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 fastener  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. 11  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 subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above as has been determined by the courts. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims and can be varied in a number of ways within the scope of the claims.