Abstract:
An Oldham-style coupling assembly wherein a driving shaft ( 12 ) is rotatably coupled to a driven shaft ( 32 ) using a reduced-lash flexible coupling assembly. The coupling assembly includes an intermediate coupling plate ( 38 ) stationed between the driven ( 32 ) and driving ( 12 ) shafts. A driven lug ( 34 ) associated with the driven shaft ( 32 ) has tapered contact faces ( 36 ) and engages a driving slot ( 40 ) in the center of the coupling plate ( 38 ). Likewise, a pair of diametrically opposed half-lugs ( 18 ) associated with the driving shaft ( 12 ) engage in respective half slots ( 42 ) through a similar tapered interface. Springs ( 28 ) backing each of the half lugs ( 18 ) establish a continuous axial compression force within the coupling plate ( 38 ) to urge seating of the tapered interfaces but without frustrating transverse sliding in the respective slots ( 40, 42 ), thereby taking up lash from the system. The lugs ( 18, 34 ) are oriented perpendicular relative to each other, so as to simulate a traditional Oldham-style coupling. The coupling assembly is useful to rotatably unite a fuel pump ( 14 ) and a vacuum pump ( 10 ) in a vehicular engine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    None. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a coupling assembly for transmitting rotational motion between a driving shaft and a driven shaft, and more particularly toward an Oldham-style coupling with reduced lash. 
         [0004]    2. Related Art 
         [0005]    In various power transmission configurations, rotating shafts are coupled to each other with a coupling joint to accommodate small amounts of shaft misalignment from collinearity. Such coupling devices have been proposed in a variety of configurations. One design of fairly ancient origin is the Oldham coupling, as shown in  FIG. 7 , wherein a center torsion block is provided with diametric slots on opposite faces that are oriented perpendicular to each other. Sliding of the center block permits a substantial amount of lateral shaft offset, while built-in clearance permits some angular misalignment as well. 
         [0006]    A particular concern with such couplings can arise in certain applications where lash, i.e., clearance or play between contact faces, is undesirable. For example, vacuum pumps are used in some vehicular applications to assist operation of the brake system. For safety reasons, these vacuum pumps are not allowed to be driven from the front end accessory drive (FEAD) belt, and therefore are commonly driven from a timing chain, belt or gear from the cam shaft or other convenient power shaft. When driven from a cam shaft gear or timing gear, as an example, torsional vibrations inherent in the subsystem can introduce unwanted noise, vibration and harshness (NVH), as well as component wear. 
         [0007]    The source of backlash in prior art style Oldham couplings ( FIG. 7 ) is found at the interfaces between the drive lugs and the coupling plate slots. Because a slip fit condition is required to achieve proper sliding function and accommodate axial misalignment, lash necessarily exists in the prior art systems. Accordingly, there is a need to couple two rotary members having respective driving and driven shafts in close proximity while accommodating modest shaft misalignment, without introducing unwanted NVH or wear into the mechanical system. The terms “driving” and “driven” refer to the direction of power flow. A driving feature transmits torque to a driven feature in the same hand of rotation as the direction of rotation, whereas a driven member transmits torque to the driving member in a hand of rotation opposite to the direction of rotation. 
       SUMMARY OF THE INVENTION 
       [0008]    An Oldham-style coupling assembly is provided for transmitting rotational motion between two rotary shafts while accommodating modest axis misalignment therebetween. The coupling assembly comprises a driving shaft supported for rotation about a first axis. The driving shaft includes a driving lug having opposed contact faces disposed generally transverse to the first axis. These contact faces are tapered in a converging axial direction away from the driving shaft. A driven shaft is supported for rotation about a second axis generally collinear with the first axis. The driven shaft includes a driven lug located in the same plane as the driving lug, but orthogonally oriented to the driving lug. The driven lug has opposed contact faces disposed generally transverse to the second axis and tapered in a converging axial direction away from the driven shaft. A coupling plate is operatively disposed between the driving and driven shafts. The coupling plate includes a driving slot having tapered side walls corresponding to the contact faces of the driven lug for engaging the driven lug while enabling transverse relative sliding motion therebetween. Similarly, the coupling plate includes a driven slot having tapered side walls corresponding to the contact faces of the driving lug for engaging the driving lug while enabling transverse relative sliding motion therebetween. A biasing element is configured to establish a continuous axial compressive force between each of the driven and driving lugs and the coupling plate. 
         [0009]    By forming the driven and driving lugs and their mating driving and driven slots with tapers that converge toward the intermediate coupling plate, a compressive axial force introduced by the biasing element urges both sets of lugs into tighter wedging engagement with the coupling plate, thereby eliminating lash between the components. Thus, two rotary shafts can be coupled for transmitting rotational movement therebetween while accommodating modest shaft misalignment but eliminating or substantially reducing any lash in the coupling assembly. 
         [0010]    According to another aspect of this invention, a method is provided for coupling two rotary members having respective driven and driving shafts in close proximity while accommodating modest shaft misalignment therebetween. The method comprises the steps of rotatably supporting a driving shaft about a first axis and providing a driving lug on the driving shaft having tapered contact faces. Furthermore, a driven shaft is rotatably supported about a second axis. A driven lug is provided on the driven shaft having tapered contact faces. An intermediate coupling plate is provided between the driving and driven shafts having a driving slot with tapered side walls corresponding to the contact faces of the driven lug and a driven slot oriented transversely to the driving slot. The driven slot has tapered side walls corresponding to the contact faces of the driving lug. The driven and driving lugs are slidably engaged in their respective driving and driven slots. An axial compression is maintained between the driven and driving lugs and the intermediate coupling plate. As described above, the axial compression drives each tapered lug into tighter engagement with its respective slot, while still permitting relative sliding movement so that the Oldham-style coupling can properly accommodate modest lateral offset between the shafts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
           [0012]      FIG. 1  is an exploded view of a vacuum pump and fuel pump assembly joined for co-rotation via a coupling device according to the subject invention; 
           [0013]      FIG. 2  is an assembled view of a vacuum pump and a driving shaft coupled to each other using the subject Oldham-style coupling; 
           [0014]      FIG. 3  is a perspective view showing the vacuum pump and its driven shaft together with a coupling plate disposed thereon; 
           [0015]      FIG. 4  is an exploded view of the components shown in  FIG. 2 ; 
           [0016]      FIG. 5  is a cross-sectional view taken diametrically through the half-lugs, and as taken generally along line  5 - 5  in  FIG. 6 ; 
           [0017]      FIG. 6  is a cross-sectional view through one of the half-lugs as taken generally along line  6 - 6  in  FIG. 5 ; 
           [0018]      FIG. 7  is an exploded view of a traditional Oldham-style coupling; 
           [0019]      FIGS. 8 and 9  illustrate how lash can be introduced into the coupling assembly if the driving lugs and the driven lugs are not located in a common plane; 
           [0020]      FIG. 10  schematically illustrates an alternative configuration of the invention where the coupling plate is constructed with appropriate elasticity to generate an axial biasing force, negating the need for separate spring elements; and 
           [0021]      FIG. 11  is a view as in  FIG. 10  but showing a different perspective. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a rotary input style vacuum pump is generally indicated at  10  in  FIGS. 1-6 . The vacuum pump  10  is used throughout this specification as an exemplary accessory device having a rotary input. As common among all accessory devices having a rotary input, the vacuum pump  10  receives its power through an operative connection to a driving shaft  12  which, in the example of  FIG. 1 , is rotationally supported within the structure of a fuel pump assembly, generally indicated at  14 . The fuel pump  14  is presented in  FIG. 1  as one possible application for the subject invention, whereas the underlying concepts of this invention are applicable to a wide variety of fields and endeavors. Continuing in this example, however, a gear  16  is affixed to the driving shaft  12  and may be driven by any type of available and convenient torque input including a meshing gear, belt, chain or other transmission device. In this example, it is assumed that the gear  16  may be in meshing contact with a cam shaft gear or timing gear (not shown). The driving shaft  12  is supported for rotation about a first axis A. 
         [0023]    Gear  16 , which is fixed onto the end of the driving shaft  12  and held there to rotate with the shaft with a feature, such as a key as suggested in  FIG. 5 , includes a driving lug. In the described embodiment, the driving lug is bifurcated and presented as two half lugs  18  spaced apart from each other on opposite sides of the first axis A, although those of skill in the art will envision non-bifurcated versions of a driving lug. Each half lug  18  has opposed contact faces  20  disposed generally transverse to the first axis A. The contact faces  20  are tapered in such a manner as to converge in an axial direction away from the gear  16  and the driving shaft  12 . Moreover, the contact faces  20  of one half lug  18  are co-planar with the respective contact faces of the other half lug  18 . Thus, the driving half lugs  18  are thickest adjacent the gear  16  and thinnest at their outer ends. In the disclosed embodiment of this invention, each half lug  18  is supported in a half guide slot  22  formed as an embossment on the gear  16 , although alternative guide arrangements may be used. The half guide slots  22  are diametrically opposed from each other and equally spaced in the radial direction relative to the first axis A. 
         [0024]    Each half lug  18  includes flats  24  that are sized to fit between the guide slot faces  22  so as to hold the half lugs  18  in an axially slidable orientation and transmit rotary motion from the gear  16  to the half lugs  18 . A pocket-like counter bore  26  may be formed on the backside of each half lug  18 , as shown in  FIG. 4 , to seat a compression spring  28 . An axially extending through-hole is formed in each half lug  18 , centrally through the counter bore  26 , to receive a pin  30  which is anchored in a hole in the gear  16 . The pin  30  acts as a guide rod to permit axial sliding movement of each half lug  18  within the guide slot  22 . The spring  28  thus provides a biasing force continually urging each half lug  18  toward an axially extended condition relative to the gear  16 . The pin  30  is provided with a head that acts as a limit or stop so that the half lugs  18  are captured on the pin  30  and so that the flats  24  do not escape from the respective guide slots  22 . 
         [0025]    Referring again to the vacuum pump  10 , this exemplary rotary device is provided with a driven shaft  32  that is supported for rotation, within the vacuum pump  10 , about a second axis B. The first and second axes A, B are generally parallel to each other and preferably designed to be in collinear alignment with each other. However, during operation and particularly in a vehicular engine environment, the axes A, B may shift occasionally due to operating stresses, moments of inertia, unequal loading, and the like, and thus become slightly misaligned while in operation. Manufacturing tolerances as well as wear of the components may also cause misalignment of the axes. The driven shaft  32  includes a driven lug  34  supported orthogonally opposite the half lugs  18 . The driven lug  34  straddles the second axis B and is generally centered there about, but is not necessarily a unitary or non-bifurcated member. The driven lug  34  has opposed contact faces  36  disposed generally transverse to the second axis B and which converge in an axial direction away from the driven shaft  32 . In other words, the driven lug  34  is thickest adjacent the vacuum pump  10 . A plane located through the driven lug  34 , equidistant between its contact faces  36 , is oriented perpendicular to a plane located through the driving lug  18 , equidistant between its contact faces  20 . 
         [0026]    A coupling plate, generally indicated at  38 , is perhaps best shown in  FIGS. 3 and 4  comprising a unitary structure adapted to interconnect the driving shaft  12  to the driven shaft  32  while accommodating moderate shaft misalignment due to the reasons mentioned above. The coupling plate  38  includes a centrally located driving slot  40  having tapering side walls corresponding to, i.e. complementing, the contact faces  36  of the driven lug  34 . The driving slot  40  is longer than the length of the driven lug  34 , so that the driving slot  40  can slide relative to the driven lug  34  in a transverse direction. The coupling plate  38  also includes a pair of diametrically opposed driven half slots  42  spaced apart one from each other to receive the respective half lugs  18 . The half slots  42  have tapered side walls corresponding to the contact faces  20  of the half lugs  18  for slidably receiving the half lugs  18  while enabling relative sliding motion in the manner described above. In the disclosed embodiment, each half slot  42  is unbounded on its radially outer end. In other words, the half slots  42  are open at the ends, thereby giving the contact plate  38  somewhat of an H-shaped appearance when viewed from the front. 
         [0027]    When the driving shaft  12  and the driven shaft  32  are brought together, as depicted in  FIGS. 2 ,  5  and  6 , the coupling plate  38  serves to transmit rotational motion therebetween while accommodating modest misalignment of the axes A and B. More specifically, the lug and coupling plate structures mimic a traditional Oldham-style coupling, such as shown in  FIG. 7 , to enable a generally constant velocity transmission as the motion of coupling plate  38  describes a circle whose size is dictated by the degree of instantaneous shaft misalignment. Thus, with each revolution of misaligned shafts, the driving slot  40  will slide with respect to the driven lug  34 , and the half slots  42  will slide relative to their respective half lugs  18 . At all times, the respective lug contact faces  20 ,  36  remain in full surface-to-surface engagement with the respective slots  40 ,  42  in the coupling plate  38 , through which motion is transmitted. 
         [0028]    In order to eliminate or substantially reduce lash in this flexible coupling assembly, the springs  28  operating against each of the half lugs  18  provide a biasing effect that establishes a continuous axial compression force between the driven  34  and driving  18  lugs and the coupling plate  38 . More specifically, the springs  28  continually urge the respective half lugs  18  into tighter wedging engagement with their respective half slots  42  so as to maintain full surface-to-surface contact therebetween. The compressive force provided by the springs  28  biases the coupling plate  38  toward the vacuum pump  10 , thereby more tightly seating the tapered driven lug  34  into the driving slot  40  of the coupling plate  38 . Thus, through the biasing action of the springs  28 , lash is removed between the mating surfaces. As the shafts  12 ,  32  rotate, misalignment will cause the respective slots  40 ,  42  to slide relative to their respective lugs  18 ,  34 , but all the while the biasing action of the springs  28  operates to maintain full surface-to-surface contact and eliminates lash. Even as components wear due to attrition, the continual biasing force of the springs  28  dispels lash from the system. 
         [0029]    Of course, many mechanical equivalents to the disclosed coupling assembly may be envisioned by those of skill in the art. For example, the biasing element can be configured as something other than a coil spring or to act directly upon the driven lug  34  instead of the driving half lugs  18 . Alternatively, the biasing element can be configured entirely within the coupling plate  38  to expand the coupling plate against the respective lugs  18 ,  34 . This is illustrated in  FIG. 8 , where the coupling plate  38  is constructed from spring steel sheet metal. The coupling plate  38  is generally thin and flexible in the axial direction, yet is stiff and non-compliant in the torsional direction. Flexibility of the plate in the axial direction can be adjusted by adding flanges in certain areas. For instance, flanges may be added near the ends of the slots to reduce the flexing between pairs of slot contact faces. And further still, the particular configuration of the lugs  18 ,  34  can be redesigned according to any of the known forms and embodiments of Oldham-style couplings. Nonetheless, the particular configuration illustrated includes numerous advantages, including orientation of the transmitted forces in a common plane, i.e., through the body of the coupling plate  38 , which reduces or eliminates couples that could force the coupling plate  38  to tilt into a position that might otherwise introduce torsional compliance or lash into the system. However, by taking up as much slack as possible in the system, impact loading is reduced that may otherwise occur with reversals of torque transmission through the mechanical system. By this method, NVH issues are improved and wear is reduced on the coupling contact faces. 
         [0030]    The axial force generated by the springs  28  urge driving lug  18  into driven slot  42 , and is resisted by a parallel force of equal magnitude between driven lug  34  and its mating slot  40 . If the coupling plate  38  receives a tilting couple generated by the friction force at the driving lug  18 , the spring resisting force will be biased toward one end of the driven lug  34 , thereby generating a couple that opposes tilting of the coupling plate  38 . Tilting of the coupling plate  38  can be avoided if certain parameters are properly controlled. The tilting couple can be minimized by keeping all the lugs in a common plane to minimize or eliminate the tilting moment arm, and also by minimizing the sliding friction force with proper lubrication and surface finish of the contact faces. The maximum magnitude of the couple that opposes tilting of the coupling plate  38  can be enhanced by maximizing length of the driven lug  34  to produce a large moment arm, and also by installing springs  28  with an adequate force preload. 
         [0031]    Tilting of the coupling plate  38  causes backlash in the system because as the tapered driven lug  34  is withdrawn from its tapered drive slot  42 , clearance between the two parts is created in the same manner that clearance between the parts disappears when they are pressed into tighter contact with each other. A conventional Oldham coupling that has its drive faces parallel to each other does not see a change in clearance between the drive and driven faces with a change in axial position, and thus does not experience the same degradation of performance with a small tilting of the coupling plate. 
         [0032]    Accordingly, this invention provides a zero-lash, or substantially reduced lash, coupling mechanism to transmit rotational motion between two rotary shafts without introducing additional noise, vibration or harshness to the system due to the backlash. The source of backlash in prior art style Oldham couplings ( FIG. 7 ) is found at the interfaces between the drive and driven shaft lugs and their mating coupling plate slots. However, lash is taken up in the subject invention through the action of the springs  28  as a biasing element combined with tapered contact faces  20 ,  36  on the respective lugs  18 ,  34 . In one particular application of this invention, wherein a vacuum pump  10  is coupled to a fuel pump  14 , further and favorable advantages can be realized by a viscous damping effect provided by the vacuum pump  10  that will help soften the torsional signature generated in a high pressure fuel injection pump  14  such as used in diesel applications. 
         [0033]      FIGS. 8 and 9  schematically illustrate an alternative undesirable embodiment of the invention in which the driving and driven lugs are axially offset from each other instead of being located in a common plane. For convenience, reference numbers introduced previously are offset by 100 to represent like or corresponding parts. In this alternative embodiment, a couple applied to the coupling plate  138  introduces torsional backlash into the system. If the driven axis B is misaligned with the driving axis A, the sliding friction of driven lug  134  within the driven slot  140  produces a force that is opposed by an equal and opposite resisting force at the interface between the driving lug  118  and its mating driven slot  142 . If the contact faces  136  of the driven lug  134  are not in the same plane as the contact faces  120  of the driving lug  118 , the axial spacing between the contact faces  136 ,  120  create a moment arm that when multiplied by the magnitude of the friction force between driven lug  134  and its mating driving slot  140  produces a couple that tends to tilt the axis of the coupling plate  138 . If the coupling plate  138  tilts relative to axis A, the contact face  136  of the driven lug  134  will no longer have full surface-to-surface contact and torsional backlash will be introduced. 
         [0034]      FIGS. 10 and 11  depict an alternate embodiment, wherein the coupling plate  138  is formed as unitary sheet-like member such as can be formed in an inexpensive stamping operation. Of course, the unitary sheet-like nature of this alternative coupling plate  138  can be implemented within the context of the first disclosed embodiment of this invention in  FIGS. 1-8 , wherein springs  28  provide a biasing force as described. 
         [0035]    The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims.