Abstract:
A coupler for connecting a drive shaft to a driven member and accommodating a substantial misalignment between the drive shaft and the driven member, while minimizing audible noise and vibration. The coupling includes a hollow core nest element having a plurality of slots formed in one end, two substantially identical elongate body elements, and a plurality of spring elements. Each elongate body element has two ends, a first end being adapted to couple with either the drive shaft or the driven member, and a second end that includes a plurality of posts that engage the slots of the hollow core nest. An aperture in the hollow core nest is larger than the diameter of the elongate bodies, allowing for the components to move radially relative to each other to accommodate for any offset and misalignment between the longitudinal axes of the drive shaft and the driven member, while the spring elements ensure that the components positively contact one another to reduce audible noise that is otherwise produced by loose fitting components. In one embodiment, a spring element is integrally formed in the slots of the hollow core nest, such that the posts of the body elements are held in engagement with sides of the slots by the spring elements. This embodiment includes an end cap that prevents the posts from becoming disengaged from the slots. Another embodiment employs helical coil springs mounted externally around the hollow core nest.

Description:
FIELD OF THE INVENTION 
     This invention generally pertains to a connector for use in coupling a motor drive shaft to a driven member, and more specifically, to a universal style coupling for connecting a motor drive shaft to a driven member such that the coupling accommodates misalignment between the drive shaft and the driven member while minimizing a noise associated with rotation of the driven member by the motor drive shaft. 
     BACKGROUND OF THE INVENTION 
     In many portable motor-driven devices, small direct current (DC) motors are connected to rotatably driven shafts using solid metal couplings. Such couplings typically comprise a short section of thick-walled tubing having two radially-extending threaded orifices formed in the wall of the tubing, adjacent to each end. Set screws are threaded into the orifices and are tightened to engage the drive shaft of the motor that is inserted into one end of the coupling, and to secure a driven shaft that is inserted into the other end of the coupling. Even if a fastener locking substance is applied, the set screws often loosen with use, enabling the drive shaft and/or driven shaft to slip within the coupling, causing scoring of the shafts and possible failure of the devices in which the couplings are installed, as the driven shaft will no longer be rotatably driven by the drive shaft. 
     Couplings are generally available from suppliers in only a limited range of sizes. If the coupling used to join two shafts is too large, it cannot properly connect the shafts and can cause vibration during rotation, because its mass is not symmetrically distributed around the center lines of the two shafts. In addition, conventional couplings generally require that the center lines of the two shafts that are joined be relatively closely aligned. Any misalignment between a motor drive shaft and a driven shaft, even if slight, is likely to cause side loading of one or both the drive shaft and driven shaft, producing increased wear of bearings or journals in which the shafts are rotatably supported. Solid couplings also transmit noise and vibration from the motor to other parts of the device in which they are used. 
     Ideally, it would be preferable to provide a coupling that is more tolerant of misalignment between a motor drive shaft and a driven shaft. Furthermore, such a coupling should not cause vibration of the assembly or produce noise while in operation. It will therefore be apparent that a simple coupling, which addresses the problems noted above and is relatively low in cost, would be desirable for use in small electric, motor-powered devices. 
     SUMMARY OF THE INVENTION 
     In accord with the present invention, a coupling is defined for connecting a drive shaft to a driven member. The coupling includes a hollow core nest and two elongate bodies that pass partially through a central aperture within the hollow core nest. One end of the hollow core nest has a plurality of slots formed therein that the posts engage. The posts are sized to correspond to the width of the slots. The end of each elongate body opposite that with the posts has an opening with a cross-sectional size generally corresponding to that of the drive shaft and the driven member. The openings in the elongate bodies for the drive shaft and the driven member are disposed on opposite ends of the coupler. 
     The central aperture of the hollow core nest is larger in cross-sectional size than that of the elongate body that passes through the central aperture, thereby enabling the coupling to accommodate radial offset and longitudinal axial misalignment between the drive shaft and the driven member. The plurality of slots have a depth which is greater than the corresponding dimension of the plurality of posts, enabling the coupling to accommodate axial movement. The coupler also includes a plurality of spring elements that provide a biasing force for ensuring that the posts of the first and second elongate bodies are positively in contact with one side of the slots of the hollow core nest, thereby substantially reducing a level of noise that would otherwise exist during rotation of the coupling. The coupler is thereby adapted to drivingly couple the drive shaft to the driven member, such that the coupler accommodates misalignment between the drive shaft and the driven member, and as a result, the coupler operates relatively noiselessly. 
     In one embodiment the plurality of spring elements are integral to the hollow core nest. In this embodiment, the plurality of spring elements are preferably a narrow strip of material disposed within each slot, such that each slot is separated into two sub-slots, one sub-slot being slightly smaller in size than the plurality of posts on the elongate bodies, such that when one of the plurality of posts is engaged into the slightly smaller sub-slot, said narrow strip firmly engages said post into the sub-slot, thereby reducing a noise level associated with loose fitting coupling components. This embodiment includes an end cap disposed at the end of the hollow core nest that covers the slots, preventing the posts of the elongate bodies from disengaging from the slots. Preferably, elastomeric shims are disposed in the sub-slots not engaged by the plurality of posts. 
     When properly assembled, the plurality of posts of the elongate body connected to the drive shaft drivingly engage the solid side of the sub-slots, and the plurality of posts of the elongate body connected to the driven member are driven by the solid sides of the sub-slots, when the drive shaft is rotated in a preferred forward direction. To aid in the proper assembly of the coupling, markings are provided on the hollow core nest and on the elongate bodies. Also, the nest preferably includes four slots, each slot having a strip that serves as the spring element, and the elongate bodies each include two posts. 
     In another preferred embodiment, the plurality of spring elements comprise helical coil springs, each having opposed ends that apply the biasing force between a first post on one elongate body and a second post on the other elongate body. In this embodiment, each post includes a hook that engages an end of one of the helical coil springs. The hollow core nest preferably includes a plurality of channels on its outer surface in which the plurality of spring elements are disposed. 
     In a preferred embodiment, the slots are disposed of at about 90° intervals around the hollow core nest. Furthermore, the posts of each elongate body are preferably disposed about 180° apart. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction/with the accompanying drawings, wherein: 
     FIG. 1 is an isometric view of a first embodiment of a motor and sensor unit, and a coupling that incorporates an external spring element, in accord with the present invention; 
     FIGS. 2 and 3 are exploded isometric views of the coupling of FIG. 1, shown from opposite ends; 
     FIG. 4 is a side elevational view of the of the coupling of FIG. 1; 
     FIG. 5 is cross-sectional view of the coupling of FIG. 4 taken along section line  5 — 5  in FIG. 4; 
     FIG. 6 replicates the cross section view of FIG. 5, but illustrates the body of the coupling misaligned relative to a longitudinal axis of the rest of the coupling; 
     FIG. 7 is a side elevational view of the of the first embodiment of the coupling, rotated approximately 45° from the orientation of the side elevational view of the of the coupling shown in FIG. 4; 
     FIG. 8 is a cross-sectional view of the coupling taken along section line  8 — 8  of FIG. 7, illustrating both bodies of the coupling misaligned relative to a longitudinal axis of a nest of the coupling; 
     FIG. 9 is an isometric view of a second embodiment of a coupling in accord with the present invention, incorporating an integral spring element; 
     FIGS. 10 and 11 are exploded isometric views of the coupling of FIG. 9, from opposite ends of the coupling; 
     FIG. 12 is a cross-sectional, side elevational view of the second embodiment of the coupling; 
     FIG. 13 is a cross-sectional view of the coupling taken along section line  13 — 13  in FIG. 12, illustrating the relative positions of the slots within the nest of the coupling, the body posts, and the shims; 
     FIG. 14 is an enlargement of a portion of FIG. 13, illustrating how the insertion of a body post into a slot of the hollow core nest deflects an integral spring element; and 
     FIGS. 15A and 15B are side elevational views of the second embodiment of the coupling, illustrating the coupling attached to the drive shaft and the driven shaft. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In an exemplary initial application of the present invention, it is used for coupling a drive shaft of a small electric stepping motor to a driven shaft that is threaded and reciprocates a plunger. The plunger acts on an elastomeric membrane in a disposable cassette incorporated in a medical infusion pump, to displace fluid from a chamber formed in the cassette, forcing the fluid to flow through a line and into a patient&#39;s body. Details of this cassette infusion pump are not pertinent to the present invention and therefore are not shown or disclosed herein. It should be noted that the coupling of the present invention can be used in conjunction with almost any application in which a drive shaft needs to be coupled to a driven shaft, and thus, is not limited to the particular application disclosed herein. 
     A first embodiment of an externally sprung coupler  10  in accord with the present invention is shown in FIG.  1 . An electric stepping motor  17  has a drive shaft  17   a  that is rotated when the electric stepping motor is energized with an electrical current. Externally sprung coupler  10  includes a drive shaft adapter  12 , a drive shaft body  16 , a hollow core nest  18 , a driven body  20 , and a driven member  14 . The term “hollow core nest” as used herein and in the claims that follow is somewhat descriptive of the configuration and function performed by this member, which has a hollow or open central aperture and serves as a “nest” support for other elements of the coupler. It is anticipated that externally sprung coupler  10  can be used for many applications that do not require drive shaft adapter  12 , and thus, the drive shaft adapter is considered optional. It is certainly possible for drive shaft  17   a  to be directly coupled to drive shaft body  16 . However, drive shaft adapter  12  is useful in the exemplary application of externally sprung coupler  10  described herein. Radially extending flanges  26  of drive shaft adapter  12  are formed as flat vanes that pass through an optical sensor  19 , enabling the angular position of the drive shaft to be determined, which is useful in monitoring the rotational position of the drive shaft, to enable it to be properly controlled. Those of ordinary skill in the art will readily understand that depending on the application, drive shaft adapter  12  can be provided with only a single flange, or with more than two flanges. 
     In the specific application illustrated, driven member  14  includes a helical screw  15  that is used for reciprocating a plunger (not shown) relative to a disposable cassette (also not shown). It should be noted that drive shaft body  16 , hollow core nest  18 , and driven body  20  can be used in conjunction with a different driven member. 
     As clearly shown in FIGS. 2 and 3, drive shaft body  16  includes two drive shaft body posts  22 , and driven body  20  includes two driven body posts  24 , all of which are of an appropriate size to fit within four slots  34  of hollow core nest  18  (clearly shown in FIG.  3 ). When externally sprung coupler  10  is assembled, two coils springs  38  are mounted on the exterior of nest  18 , each coil spring  38  connecting one drive shaft body post  22  to one driven body post  24 . One such connection is shown in FIG. 1 (the other connection is hidden by externally sprung coupler  10 ). It should be noted that all drive shaft body posts  22  and driven body posts  24  include a hook  33  which the ends of coil springs  38  engage. The exterior of hollow core nest  18  is preferably curved along the path of coil springs  38 , and tabs  35   a  and  35   b , also on the exterior of hollow core nest  18 , ensure that coil springs  38  remain in the correct position (one set of tabs  35   a  and  35   b  being provided for each coil spring  38 ). 
     FIGS. 2 and 3 show how the components of externally sprung coupler  10  are assembled. The hollow core of nest  18  is sufficiently large to enable driven member  14  and driven body  20  to pass through its central aperture. Driven member  14  includes a spline  32  that is press fit into a generally hexagonal-shaped opening  30   a  formed in driven body  20 . Spline  32  forms an interference fit when forced into opening  30   a.  Opening  30   a  is hexagonal shaped near the exterior of the opening, and cylindrical shaped, with a smaller diameter, further within the opening, as evident in FIGS. 5 and 6. Spline  32  is of a size such that spline  32  passes freely through the hexagonal shaped portion of opening  30   a , and forms an interference fit in the smaller cylindrical portion. It should be noted that spline  32  and opening  30   a  can alternatively be configured in other shapes, as will be understood by those of ordinary skill in the art. 
     Drive shaft body  16  includes a notch  21 , which is clearly seen in FIG. 2, and driven body  20  includes a corresponding notch  23 , which is clearly seen in FIG.  3 . When properly assembled, notches  21  and  23  engage to ensure that the two drive shaft body posts  22  and two driven body posts  24  each fit into a unique slot  34  of hollow core nest  18 . 
     In this preferred embodiment, drive shaft adapter  12  incorporates a hexagonal shaft  36  that terminates in a smaller diameter cylindrical shaft  37 , both of which are sized to fit within an opening  30   b  in drive shaft body  16 . Opening  30   b  is clearly seen in FIG. 3.. Cylindrical shaft  37  is sized to fit freely into the corresponding cylindrical section of opening  30   b . Hexagonal shaft  36  includes ridges  36   a , one ridge per face of hexagonal shaft  36 . Ridges  36   a  are preferably located at each vertex, though ridges  36   a  can also be located on the center of each face. In the preferred embodiment ridges  36   a  are on the order of {fraction (5/1000)} of an inch in height. The purpose of ridges  36   a  is to form an interference fit between hexagonal shaft  37  and the hexagonal section of opening  30   b , and thus to more securely attach drive shaft adapter  12  to drive shaft body  16 . Those of ordinary skill in the art will readily understand that the location and dimensions of ridges  36   a  can be modified while still enabling a secure attachment to be achieved. 
     It should be noted that drive shaft body  16  and driven member body  20  are preferably identical, which both reduces manufacturing costs and facilitates assembly of externally sprung coupler  10 . Thus, drive shaft body posts  22  and driven body posts  24  are identical in configuration, as are openings  30   a  and  30   b . Only after the externally sprung coupler is assembled can driven body  20  and drive shaft body  16  be distinguished, based on their positions relative to drive shaft  17   a  and driven member  14 . 
     Drive shaft adapter  12  also includes an opening  28 , clearly visible in FIG.  3 . Opening  28  is of a size and shape that generally corresponds to the size and shape of drive shaft  17   a  and can vary depending on the corresponding size and shape of the drive shaft of the particular motor selected. 
     When externally sprung coupler  10  is fully assembled, driven member  14  passes completely through hollow nest  18 . Driven member body  20  passes part way through hollow core nest  18 , further advancement being precluded when driven body posts  24  seat within slots  34  of hollow core nest  18 . Once driven member  14  and driven body  20  are thus assembled, drive shaft body  16  is positioned such that notch  21  of drive shaft body  6  meshes with corresponding notch  23  of driven member body  20 . Drive shaft body posts  22  are inserted into the final two slots  34  of hollow core nest  18 . When assembled, each of slots  34  of hollow core nest  18  has either a drive shaft body post  22  or a driven body post  24  seated within it. Coil springs  38  are engaged onto hooks  33  such that each coil spring  38  connects one drive shaft body post  22  to one driven body post  24 . It should be noted that the fit between notch  21  and notch  23  is such that drive shaft body  16  and driven member body  20  can move relative to each other, enabling the assembled coupler to flex in a manner not possible with a solid coupler. FIG. 4 illustrates externally sprung coupler  10  fully assembled, with the driven shaft and drive shaft aligned along a common longitudinal axis. Driven member  14  and driven body  20  are joined together with an interference fit and do not move relative to each other. Similarly, drive shaft body  16  and drive shaft adapter  12  are joined together with an interference fit and do not move relative to each other. In contrast, driven body  20  and drive shaft body  16  can move out of longitudinal axial alignment relative to each other, as well as relative to hollow core nest  18 . Furthermore, the depth of the slots in the hollow core nest permits axial movement of the drive shaft and/or of the driven member relative to the hollow core nest. This freedom of movement enables externally sprung coupler  10  to drivingly couple a drive shaft to a driven member when the drive shaft and driven member are not in longitudinal axial alignment. Coil springs  38  insure that a force is applied that biases drive shaft body  16  and driven member body  20  in contact with hollow core  18 , so that noise is not created by loose fitting components when externally sprung coupler  10  is used to transfer force from the drive shaft to the driven member. It should be noted that in an exemplary application the present invention will be used with a stepping motor. The constant starting and stopping of a stepping motor can cause noise when components of a coupler move relative to one another. Coil springs  38  minimize such noise. 
     FIG. 5 shows a cross-sectional view of externally sprung coupler  10  in which the interference fit of spline  32  of driven member  14  into opening  30   a  of driven body  20  can be clearly seen. It can also be clearly seen that hollow core nest  18  is larger in diameter than either driven body  20  or drive shaft body  16 . The interference fit of hexagonal shaft  36  and cylindrical shaft  37  of drive shaft adapter  12  with opening  30   b  of drive shaft body  16  are clearly evident in this Figure. It is also apparent that driven body posts  24  can move radially relative to their corresponding slots  34  in hollow core nest  18 , by at least a small amount. Driven body  20  (and thus, driven body posts  24 ) is free to move downward relative to hollow core nest  18  until an upper gap  39   a  is eliminated (correspondingly increasing the size of a lower gap  39   b ). Drive shaft body  16  is also able to move radially in a similar manner relative to hollow core nest  18 . This radial accommodation enables externally sprung coupler  10  to adjust to a misalignment offset between the longitudinal axes of the drive shaft and the driven member. 
     FIG. 6 illustrates another type of misalignment of drive shaft body  16  relative to hollow core nest  18 . As illustrated, the end of drive shaft body  16  is deflected upwardly by α° relative to the longitudinal axis of hollow core nest  18 . It should be noted that drive shaft body  16  could be deflected in a downwardly direction as well. Similarly (although not shown), driven member body  20  can be deflected relative to the longitudinal axis of hollow core nest  18  by a similar degree, both toward the viewer of FIG. 6, as well as away from the viewer. The combination of these accommodations for radial and longitudinal axial movement of drive shaft body  16  and driven body  20  enable externally sprung coupler  10  to drivingly couple a drive shaft to a driven member when there is considerable misalignment and/or offset between the longitudinal axes of the drive shaft and the driven member. 
     FIG. 7 is another view of externally sprung coupler  10 , in which externally sprung coupler  10  has been rotated approximately 45° from the position illustrated in FIGS. 4,  5 , and  6 . In this position, only one driven body post  24  and one drive shaft body post  22  can be seen. When externally sprung coupler  10  is properly assembled, hooks  33  face away from the path of coil spring  38 . Preferably, as in FIG. 5, hook  33  of drive shaft body post  22  faces away from driven body post  24 , and the corresponding hook of driven body post  24  faces away from drive shaft body post  22 . This configuration provides a secure attachment for coil spring  38 . FIG. 8 is a cross-sectional view that illustrates both drive shaft body  16  and driven body  20  being deflected from longitudinal axial alignment relative to hollow core nest  18  through angles of α° and β°, respectively. 
     FIGS. 9-15B illustrate an internally sprung coupler  50 , which is a second preferred embodiment of the present invention. Internally sprung coupler  50  includes a hollow core nest  18 ′, a drive shaft body  16   a , and a driven member body  20   a . Also shown in FIG. 9 is drive shaft adapter  12  and driven member  14 . It should be understood that internally sprung coupler  50  could be used in association with a different type of driven member  14  as well as a different type of drive shaft adapter  12  (or with no drive shaft adapter). Drive shaft body post  22   a  and driven body post  24   a  are different than the corresponding posts of the externally sprung coupler in that the posts of the internally sprung coupler do not incorporate hook  33 . Preferably, both drive shaft body  16   a  and driven body  20   a  include a body orientation directional arrow  60  that is molded therein or otherwise applied, e.g., by the use of ink or other marking substance. Directional arrow  60  extends parallel to the longitudinal axis of the bodies and points away from the end of the body that incorporates the posts. The purpose of directional arrow  60  is to aid in the assembly of internally sprung coupler  50 , as will be explained below. Absent directional arrows  60  on the bodies (and corresponding directional arrows  56  and  58  on hollow core nest  18 ′), one might incorrectly assemble internally sprung coupler  50 . 
     Hollow core nest  18 ′ includes four fastener holes  54 . Fastener holes  54  are used to secure an end cap  40  to the hollow core nest. The purpose of end cap  40  is to insure that drive shaft body posts  22   a  and driven body posts  24   a  do not become disengaged from the corresponding slots  48  in hollow core nest  18 ′. This concern does not arise with respect to externally sprung coupler  10 , because external coil springs  38  of externally sprung coupler  10  prevent that from occurring. Hollow core nest  18 ′ of internally sprung coupler  50  includes four slots; however as described below, these slots have been significantly modified in internally sprung coupler  50  compared to the slot in externally sprung coupler  10 . The slots of internally sprung coupler  50  incorporate a leaf spring  46 , which separates each of the slots into two distinct sub-slots, including a sub-slot  48  into which drive shaft body posts  22   a  and driven body posts  24   a  are inserted, and a shim sub-slot  52 . An elastomeric shim  42  is disposed in each shim sub-slot  52 . 
     Hollow core nest  18 ′ also includes assembly direction arrows, including an “away from end cap” directional arrow  56  and a “toward end cap” directional arrow  58 . While FIG. 9 illustrates only one directional arrow  56  and one arrow  58 , it should be understood that a second directional arrow  56  is located on hollow core nest  18 ′, approximately 180° from the position of the first directional arrow  56 . Similarly, a second directional arrow  58  is located approximately 180° from the position of directional arrow  58  shown in FIG.  9 . The purpose of these directional arrows will be described more fully below; however as noted above, the purpose of the directional arrows on both hollow core nest  18 ′ and bodies  16   a  and  20   a  is to ensure that internally sprung coupler  50  is correctly assembled. Unlike externally sprung coupler  10 , internally sprung coupler  50  can be assembled in a less desirable configuration that significantly reduces the functionality of the coupler. When assembling externally sprung coupler  10 , hooks  33  provide assembly directional clues as to the preferred orientation and configuration. 
     As internally sprung coupler  50  is assembled, the orientation of the directional arrows  60  are matched to that of directional arrows  56  and  58  on hollow core nest  18 ′. For example, in FIG. 10, driven body  20   a  is positioned such that driven body posts  24   a  are inserted into slots  48  in hollow core nest  18 ′, adjacent to directional arrows  56 . If driven body  20   a  were rotated by  900  about its longitudinal axis (similar to drive shaft body  16   a ), then driven body posts  24   a  would engage slots  48  that are adjacent directional arrows  58 . As directional arrows  58  are opposed to directional arrow  60  on driven body  20   a , it will be immediately apparent that the wrong orientation has occurred during assembly. 
     FIGS. 10 and 11 provide an exploded view of internally sprung coupler  50 . As illustrated, each shim  42  is connected to a shim support structure  44 , which is preferable, as it aids in the assembly of the internally sprung coupler  50 , because each individual shim does not need to be separately placed. Support structure  44  fits into a groove in end cap  40 . Fasteners pass through fastener holes  54  to connect hollow core nest  18 ′ with end cap  40 . Preferably, the type of fasteners used will be removable so that internally sprung coupler  50  may be disassembled when desired. End cap  40  prevents shim support structure  44  (and thus shims  42 ), drive shaft body  16   a , and driven body  20   a , from being disconnected from hollow core nest  18 ′. Drive shaft body  16  includes a notch  21 , and driven body  20  includes a corresponding notch  23 . 
     FIG. 12 is a cross-sectional view of internally sprung coupler  50  as it appears when fully assembled. The accommodation for movement and both longitudinal axial misalignment and radial offset provided for drive shaft body  16   a  and driven body  20   a  relative to hollow core nest  18 ′ is quite similar to that provided by externally sprung coupler  10 . To accommodate an offset, driven body  20   a  can move radially upwardly and downwardly with respect to hollow core nest  18 ′, and drive shaft body  16   a  can move in a similar manner inwardly and outwardly, in regards to the view in FIG.  12 . Both drive shaft body  16   a  and driven body  20   a  can move through an angular misalignment relative to the longitudinal axis of hollow core nest  18 ′, generally as described above, with respect to externally sprung coupler  10 . The only significant difference is that if externally sprung coupler  10  were subject to severe angular distortion, coil springs  38  might become detached from hooks  33 , in which case, the coupler would likely come apart. In contrast, if internally sprung coupler  50  were subjected to a similarly severe angular misalignment, end cap  40  prevents the coupler from coming apart. As the loading caused by longitudinal angular misalignment increases excessively, it is likely that one of the drive shaft body posts  22   a  or driven body posts  24   a  would fail. 
     FIG. 13 is a cross-sectional view that shows how drive shaft body post  22   a , driven body post  24   a , and shims  42  fit into the slots of hollow core nest  18 ′. When used in a cassette infusion pump, the motor, by convention, will turn in a clockwise direction as viewed from the outward extending end of the drive shaft. Before assembling internally sprung coupler  50 , it is important to know the rotational direction of the motor with which the coupler is to be used. The directional arrows ( 56 ,  58 , and  60 ) shown in the Figures previously discussed are based on a drive shaft rotation in the clockwise direction, when the drive shaft is rotating in its normal forward direction (as opposed to being run in reverse). In FIG. 13, drive shaft body posts  22   a  engage solid surfaces  62  when rotated in a clockwise direction by a driveshaft. If the rotational force were applied in the counterclockwise direction, drive shaft body posts  22   a  would be applying force to leaf springs  46 , not to solid surfaces  62 , which is undesirable, because a small amount of compliance (determined by the elastic properties of leaf springs  46  and shim material  42 ) would occur. It is preferred that as drive shaft body  16   a  is rotated by the drive shaft in the clockwise direction, drive shaft body posts  22 a apply force against solid surface  62  of hollow core nest  18 ′, thereby providing a positive contact and eliminating any compliance. 
     When hollow core nest  18 ′ is caused to rotate in the clockwise direction due to the force exerted on solid surface  62  by drive shaft body posts  22   a , hollow ore nest  18 ′ in turn, exerts a force on driven body posts  24   a  via solid surface  64 . The purposes of directional arrows  56 ,  58 , and  60  are to insure that the coupler is assembled correctly relative to this rotational direction. If internally sprung coupler  50  were assembled incorrectly, drive shaft body posts  22   a  apply force to leaf springs  46  rather than solid surfaces  62 , which will also occur if the clockwise rotating motor is reversed. For the application of internally sprung coupler  50  to a disposable cassette pump, it is preferable that the coupler provide no compliance in the forward direction. Thus, care must be exercised to ensure that internally sprung coupler  50  is assembled in consideration of the motor normal rotational direction and the requirements of the particular application in which the coupler is used. 
     FIG. 14 illustrates further details showing how leaf spring  46  operates as an “internal spring” and positively engages drive shaft body posts  22   a  when these posts are inserted within slots  48 . As is clear from FIG. 14, slots  48  are just slightly smaller than drive shaft body posts  22   a  and driven body posts  24   a.  When one of posts  22   a  (or posts  24   a ) are inserted into any of slots  48 , leaf spring  46  is deflected slightly away from that post. Leaf springs  46  thus each positively engage the post within the slot. Shims  42  add additional resiliency to leaf springs  46 . In FIG. 14, drive shaft body post  22   a  is inserted into slot  48 , and leaf spring  46  is deflected to a new position  46 ′. This deflection causes shim  42  to compress to a new position  42 ′. The elasticity of this “internal spring” can be adjusted by changing the material used for shim  42  to provide a different elasticity, or by increasing or decreasing the thickness of leaf spring  46 . Preferably, leaf spring  46  is deflected by about 0.011″ when the post is inserted into slot  48 . 
     FIGS. 15A and 15B illustrate that if directional arrows  56 ,  58 , and  60  are used to ensure the correct assembly of the two bodies and to ensure that hollow core nest  18 ′ is in the proper orientation, it does not matter in which orientation assembled internally sprung coupler  50  is positioned before drive shaft adapter  12  (or only the drive shaft, if no adapter and flange are required) and driven member  14  are attached to the coupler. In FIG. 15A, end cap  40  is disposed adjacent to drive shaft adapter  12 . Internally sprung coupler  50  has been assembled and attached to driven member  14  and drive shaft adapter  12  as shown in the exploded views of FIGS. 10 and 11. When a drive shaft rotates in a clockwise direction, drive shaft body posts  22   a  will apply a force to solid surfaces  62  (FIG.  13 ), causing hollow core nest  18 ′ to rotate in a clockwise direction, further causing solid surfaces  64  to apply a force to driven body posts  24   a.    
     In FIG. 15B, an internally sprung coupler  50   a  that has been properly assembled, by correctly following the indication of directional arrows  56 ,  58 , and  60  is attached to driven member  14  with the driven member disposed adjacent to end cap  40 , which is opposite to the attachment of driven member  14  to internally sprung coupler  50  in FIG.  15 A. Thus in internally sprung coupler  50   a , driven body  20   a  is not attached to driven member  14 , but instead, to drive shaft adapter  12 . A clockwise rotation of the drive shaft attached to internally sprung coupler  50   a  causes driven body posts  24   a  to apply a force to solid surface  64 , thus causing hollow core nest to rotate. In each situation as shown in FIGS. 15A and 15B, the rotational force is transferred to a solid surface between the driven body post  24  and hollow core nest  18 . If the rotational of the motor is reversed, i.e., to a counterclockwise direction, then the rotational force transferred between driven body post  24   a  and hollow core nest  18  is via leaf spring  46 , rather than solid surface  64 , which results in a small amount of compliance. Such compliance, even to this limited extent, is undesirable in the application of the coupler to a cassette infusion pump when the motor is used to displace fluid, but is not so significant when retracting a plunger. Thus, the orientation of assembly directional arrows  56 ,  58 , and  60  are a function of the normal “forward” rotational direction of the motor selected. Furthermore, as mentioned above with respect to the bodies of the first embodiment, in internally sprung coupler  50  (or  50   a ), the bodies of the internally sprung coupler are identical and interchangeable, which facilitates production and assembly of the coupler. 
     Although the present invention has been described in connection with the preferred form of practicing it, those of ordinary skill in the art will understand that many modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.