Abstract:
A controllable or selectable coupling assembly includes a plurality of forward locking elements, at least one reverse locking element and first and second coupling members supported for relative rotation about a common rotational axis. The coupling members include a first, second and third coupling faces. The first coupling face has a set of forward pockets angularly spaced about the axis. The second coupling face has a set of reverse locking formations adapted for abutting engagement with the at least one reverse locking element. The third coupling face opposes the first coupling face and has a set of forward locking formations. Either forward backlash is a non-zero integer multiple of reverse backlash or the reverse backlash is a non-zero integer multiple of the forward backlash to prevent the coupling assembly from inadvertently binding in both directions (i.e. a “lock-lock” condition) about the axis.

Description:
TECHNICAL FIELD 
     This invention generally relates to controllable coupling or clutch assemblies and, in particular, to such assemblies which have forward and reverse backlash. 
     OVERVIEW 
     Coupling assemblies such as clutches are used in a wide variety of applications to selectively couple power from a first rotatable driving member, such as a driving disk or plate, to a second, independently rotatable driven member, such as a driven disk or plate. In one known variety of clutches, commonly referred to as “one-way” or “overrunning” clutches, the clutch engages to mechanically couple the driving member to the driven member only when the driving member rotates in a first direction relative to the driven member. Further, the clutch otherwise permits the driving member to freely rotate in the second direction relative to the driven member. Such “freewheeling” of the driving member in the second direction relative to the driven member is also known as the “overrunning” condition. 
     One type of one-way clutch includes coaxial driving and driven plates having generally planar clutch faces in closely spaced, juxtaposed relationship. A plurality of recesses or pockets is formed in the face of the driving plate at angularly spaced locations about the axis, and a strut or pawl is disposed in each of the pockets. Multiple recesses or notches are formed in the face of the driven plate and are engageable with one or more of the struts when the driving plate is rotating in a first direction. When the driving plate rotates in a second direction opposite the first direction, the struts disengage the notches, thereby allowing freewheeling motion of the driving plate with respect to the driven plate. 
     When the driving plate reverses direction from the second direction to the first direction, the driving plate typically rotates relative to the driven plate until the clutch engages. As the amount of relative rotation increases, the potential for an engagement noise also increases. 
     Controllable or selectable one-way clutches (i.e., OWCs) are a departure from traditional one-way clutch designs. Selectable OWCs add a second set of locking members in combination with a slide plate. The additional set of locking members plus the slide plate adds multiple functions to the OWC. Depending on the needs of the design, controllable OWCs are capable of producing a mechanical connection between rotating or stationary shafts in one or both directions. Also, depending on the design, OWCs are capable of overrunning in one or both directions. A controllable OWC contains an extremely controlled selection or control mechanism. Movement of this selection mechanism can be between two or more positions which correspond to different operating modes. 
     U.S. Pat. No. 5,927,455 discloses a bi-directional overrunning pawl-type clutch, U.S. Pat. No. 6,244,965 discloses a planar overrunning coupling, and U.S. Pat. No. 6,290,044 discloses a selectable one-way clutch assembly for use in an automatic transmission. U.S. Pat. Nos. 7,258,214 and 7,344,010 disclose overrunning coupling assemblies, and U.S. Pat. No. 7,484,605 discloses an overrunning radial coupling assembly or clutch. 
     A properly designed controllable OWC can have near-zero parasitic losses in the “off” state. It can also be activated by electro-mechanics and does not have either the complexity or parasitic losses of a hydraulic pump and valves. 
     In a powershift transmission, tip-in clunk is one of most difficult challenges due to absence of a torque converter. When the driver tips-in, i.e., depresses the accelerator pedal following a coast condition, gear shift harshness and noise, called clunk, are heard and felt in the passenger compartment due to the mechanical linkage, without a fluid coupling, between the engine and powershift transmission input. Tip-in clunk is especially acute in a parking-lot maneuver, in which a vehicle coasting at low speed is then accelerated in order to maneuver into a parking space. 
     In order to achieve good shift quality and to eliminate tip-in clunk, a powershift transmission should employ a control strategy that is different from that of a conventional automatic transmission. The control system should address the unique operating characteristics of a powershift transmission and include remedial steps to avoid the objectionable harshness yet not interfere with driver expectations and performance requirements of the powershift transmission. There is a need to eliminate shift harshness and noise associated with tip-in clunk in a powershift transmission. 
     For purposes of this disclosure, the term “coupling” should be interpreted to include clutches or brakes wherein one of the plates is drivably connected to a torque delivery element of a transmission and the other plate is drivably connected to another torque delivery element or is anchored and held stationary with respect to a transmission housing. The terms “coupling”, “clutch” and “brake” may be used interchangeably. 
     A pocket plate may be provided with angularly disposed recesses or pockets about the axis of the one-way clutch. The pockets are formed in the planar surface of the pocket plate. Each pocket receives a torque transmitting strut, one end of which engages an anchor point in a pocket of the pocket plate. An opposite edge of the strut, which may hereafter be referred to as an active edge, is movable from a position within the pocket to a position in which the active edge extends outwardly from the planar surface of the pocket plate. The struts may be biased away from the pocket plate by individual springs. 
     A notch plate may be formed with a plurality of recesses or notches located approximately on the radius of the pockets of the pocket plate. The notches are formed in the planar surface of the notch plate. 
     Another example of an overrunning planar clutch is disclosed in U.S. Pat. No. 5,597,057. 
     Some U.S. patents related to the present invention include: U.S. Pat. Nos. 4,056,747; 5,052,534; 5,070,978; 5,449,057; 5,486,758; 5,678,668; 5,806,643; 5,871,071; 5,918,715; 5,964,331; 5,979,627; 6,065,576; 6,116,394; 6,125,980; 6,129,190; 6,186,299; 6,193,038; 6,386,349; 6,481,551; 6,505,721; 6,571,926; 6,814,201; 7,153,228; 7,275,628; 8,051,959; 8,196,724; and 8,286,772. 
     Yet still other related U.S. patents include: U.S. Pat. Nos. 4,200,002; 5,954,174; and 7,025,188. 
     U.S. Pat. No. 6,854,577 discloses a sound-dampened, one-way clutch including a plastic/steel pair of struts to dampen engagement clunk. The plastic strut is slightly longer than the steel strut. This pattern can be doubled to dual engaging. This approach has had some success. However, the dampening function stopped when the plastic parts became exposed to hot oil over a period of time. 
     Metal injection molding (MIM) is a metalworking process where finely-powdered metal is mixed with a measured amount of binder material to comprise a ‘feedstock’ capable of being handled by plastic processing equipment through a process known as injection mold forming. The molding process allows complex parts to be shaped in a single operation and in high volume. End products are commonly component items used in various industries and applications. The nature of MIM feedstock flow is defined by a science called rheology. Current equipment capability requires processing to stay limited to products that can be molded using typical volumes of 100 grams or less per “shot” into the mold. Rheology does allow this “shot” to be distributed into multiple cavities, thus becoming cost-effective for small, intricate, high-volume products which would otherwise be quite expensive to produce by alternate or classic methods. The variety of metals capable of implementation within MIM feedstock are referred to as powder metallurgy, and these contain the same alloying constituents found in industry standards for common and exotic metal applications. Subsequent conditioning operations are performed on the molded shape, where the binder material is removed and the metal particles are coalesced into the desired state for the metal alloy. 
     Other U.S. patent documents related to at least one aspect of the present invention includes U.S. Pat. Nos. 8,813,929; 8,491,440; 8,491,439; 8,286,772; 8,272,488; 8,187,141; 8,079,453; 8,007,396; 7,942,781; 7,690,492; 7,661,518; 7,455,157; 7,455,156; 7,451,862; 7,448,481, 7,383,930; 7,223,198; 7,100,756; and 6,290,044; and U.S. published application Nos. 2015/0000442; 2014/0305761; 2013/0277164; 2013/0062151; 2012/0152683; 2012/0149518; 2012/0152687; 2012/0145505; 2011/0233026; 2010/0105515; 2010/0230226; 2009/0233755; 2009/0062058; 2009/0211863; 2008/0110715; 2008/0188338; 2008/0185253; 2006/0124425; 2006/0249345; 2006/0185957; 2006/0021838; 2004/0216975; and 2005/0279602. 
     Some other U.S. patent documents related to at least one aspect of the present invention includes U.S. Pat. Nos. 8,720,659; 8,418,825; 5,996,758; 4,050,560; 8,061,496; 8,196,724; and U.S. published application Nos. 2014/0190785; 2014/0102844; 2014/0284167; 2012/0021862; 2012/0228076; 2004/0159517; and 2010/0127693. 
     A problem has arisen with some controllable one-way clutches (i.e. mechanical diodes (MD&#39;s)) which are meant to lock in one direction and lock or free wheel in the opposite direction, depending upon the position of a selector. In certain positions or locations, the clutch may not come out of a “lock-lock” condition (i.e. may inadvertently bind in both directions about the axis). 
     SUMMARY OF EXAMPLE EMBODIMENTS 
     An object of at least one embodiment of the present invention is to provide a controllable coupling assembly having forward and reverse backlash and which is prevented from inadvertently binding. 
     In carrying out the above object and other objects of at least one embodiment of the present invention, a controllable coupling assembly having forward and reverse backlash is provided. The assembly includes a plurality of forward locking elements. Each of the forward locking elements has a load-bearing surface. The assembly also includes at least one reverse locking element and first and second coupling members supported for relative rotation about a common rotational axis. The coupling members include a first coupling face having a set of forward pockets angularly spaced about the axis. Each of the forward pockets receives one of the forward locking elements and defines a forward load-bearing surface adapted for abutting engagement with the load-bearing surface of its respective forward locking element. The members also include a second coupling face which has a set of reverse locking formations adapted for abutting engagement with the at least one reverse locking element to prevent the relative rotation in a reverse direction about the axis and a third coupling face that opposes the first coupling face. The third coupling face has a set of forward locking formations. Each of the set of forward locking formations is adapted for abutting engagement with one of the forward locking elements to prevent the relative rotation in a forward direction about the axis. The number of forward locking elements is different than the number of reverse locking elements. The number of forward locking formations is different than the number of reverse locking formations. Either the forward backlash is a non-zero integer multiple of the reverse backlash or the reverse backlash is a non-zero integer multiple of the forward backlash to prevent the coupling assembly from inadvertently binding in both directions about the axis. 
     The assembly may further include a fourth coupling face that opposes the second coupling face. The fourth coupling face may have at least one reverse pocket. Each reverse pocket may receive a reverse locking element and may define a reverse load-bearing surface adapted for abutting engagement with a load-bearing surface of its respective reverse locking element. 
     The forward pockets may be grouped into at least one set wherein the forward pockets in each set are uniformly angularly spaced. 
     The forward pockets may be grouped into two or more sets. 
     The first coupling member may have the first and second coupling faces and the second coupling member may have the third coupling face. 
     The first coupling face may be oriented to face axially in a first direction along the axis, wherein the second coupling face may be oriented to face axially in a second direction opposite the first direction along the axis. 
     The forward and reverse locking elements may be locking struts. 
     The forward and reverse locking formation may be notches. 
     The first coupling member may be a splined ring. 
     The first and third coupling faces may be annular coupling faces that oppose each other. 
     The assembly may further include a plurality of reverse locking elements and a third coupling member having a fourth coupling face that opposes the second coupling face. The fourth coupling face may have a set of reverse pockets angularly spaced about the axis. Each of the reverse pockets may receive one of the reverse locking elements and defines a reverse load-bearing surface adapted for abutting engagement with a load-bearing surface of its respective reverse locking element. 
     The reverse pockets may be grouped into at least one set wherein the reverse pockets in each set are uniformly angularly spaced. 
     The reverse pockets may be grouped into two or more sets. 
     The assembly may further include a control member mounted for controlled, shifting movement between the second and fourth coupling faces relative to the set of reverse pockets and operable for controlling position of the reverse locking elements. The control member allows at least one of the reverse locking elements to engage at least one of the reverse locking formations in a first position of the control member wherein the control member maintains the reverse locking elements in their pockets in a second position of the control member. 
     The control member may comprise a slide plate controllably rotatable about the rotational axis between the first and second positions. 
     The assembly may further include a control element coupled to the control member to controllably shift the control member. 
     The assembly may further include a generally round snap ring that is received by an annular groove in the third coupling member to retain the members together and prevent axial movement of the members relative to one another. 
     Further in carrying out the above object and other objects of at least one embodiment of the present invention, a controllable clutch assembly having forward and reverse backlash is provided. The assembly includes a plurality of forward locking elements. Each of the forward locking elements has a load-bearing surface. The assembly also includes at least one reverse locking element and first and second clutch members supported for relative rotation about a common rotational axis. The clutch members include a first coupling face having a set of forward pockets angularly spaced about the axis. Each of the forward pockets receives one of the forward locking elements and defines a forward load-bearing surface adapted for abutting engagement with the load-bearing surface of its respective forward locking element. The members also include a second coupling face which has a set of reverse locking formations adapted for abutting engagement with the at least one reverse locking element to prevent the relative rotation in a reverse direction about the axis and a third coupling face that opposes the first coupling face. The third coupling face has a set of forward locking formations. Each of the set of forward locking formations is adapted for abutting engagement with one of the forward locking elements to prevent the relative rotation in a forward direction about the axis. The number of forward locking formations is different than the number of reverse locking formations. Either the forward backlash is a non-zero integer multiple of the reverse backlash or the reverse backlash is a non-zero integer multiple of the forward backlash to prevent the coupling assembly from inadvertently binding in both directions about the axis. The forward pockets are grouped into at least one set wherein the forward pockets in each set are uniformly angularly spaced. 
     The forward pockets may be grouped into two or more sets. 
     Yet still further in carrying out the above object and other objects of at least one embodiment of the present invention, a controllable coupling assembly having forward and reverse backlash is provided. The assembly includes a plurality of forward locking elements. Each of the forward locking elements has a load-bearing surface. The assembly also includes a plurality of reverse locking elements. Each of the reverse locking elements has a load-bearing surface. The assembly further includes first, second and third coupling members supported for relative rotation about a common rotational axis. The coupling members have a first coupling face with a set of forward pockets angularly spaced about the axis. Each of the forward pockets receives one of the forward locking elements and defines a forward load-bearing surface adapted for abutting engagement with the load-bearing surface of its respective forward locking element. The members also have a second coupling face with a set of reverse locking formations. Each of the set of reverse locking formations are adapted for abutting engagement with one of the reverse locking elements to prevent the relative rotation in a reverse direction about the axis. A third coupling face opposes the first coupling surface. The third coupling face has a set of forward locking formations. Each of the set of forward locking formations is adapted for abutting engagement with one of the forward locking elements to prevent the relative rotation in a forward direction about the axis. A fourth coupling face opposes the second coupling face. The fourth coupling face has a set of reverse pockets angularly spaced about the axis. Each of the reverse pockets receives one of the reverse locking elements and defines a reverse load-bearing surface adapted for abutting engagement with a load-bearing surface of its respective reverse locking element. The number of forward locking elements is different than the number of reverse locking elements. The number of forward locking formations is different than the number of reverse locking formations. Either the forward backlash is a non-zero integer multiple of the reverse backlash or the reverse backlash is a non-zero integer multiple of the forward backlash to prevent the coupling assembly from inadvertently binding in both directions about the axis. 
     The reverse pockets may be grouped into at least one set wherein the reverse pockets in each set are uniformly angularly spaced. 
     The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings and in view of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a controllable clutch or coupling assembly constructed in accordance with one embodiment of the present invention; 
         FIG. 2  is a view similar to the view of  FIG. 1  but taken from a different direction to illustrate the bottom surfaces of the assembly; 
         FIG. 3  is a top plan view of a coupling member of another embodiment; and 
         FIG. 4  is a top plan view of a coupling member of yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring now to the drawing figures,  FIGS. 1 and 2  are exploded perspective views (taken from different directions to illustrate different surfaces of the components of the assembly) of a controllable one-way clutch or coupling assembly, generally indicated at  10 , and constructed in accordance with one embodiment of the present invention. The assembly  10  includes an annular reverse pocket plate or first outer coupling member, generally indicated at  12 . An outer axially-extending surface  14  of the plate  12  has external splines  16  for coupling the plate  12  to the inner surface of a transmission case (not shown). An inner radially extending surface or coupling face  18  of the plate  12  is formed with spaced pockets  20  in which reverse struts  22  are pivotally biased outwardly by coil springs  27 . Preferably, sixteen reverse struts  22  are provided. However, it is to be understood that a greater or lesser number of reverse struts may be provide as will be described in greater detail herein below. 
     The assembly  10  also includes a control member or selector slide plate, generally indicated at  26 , having a plurality of spaced apertures  28  extending completely therethrough to allow the reverse struts  22  to pivot in their pockets  20  and extend through the apertures  28  to engage spaced locking formations or ramped reverse notches  30  formed in a radially extending surface or coupling face  32  ( FIG. 2 ) of an inner pocket plate or coupling member, generally indicated at  34 , when the plate  26  is properly angularly positioned about a common central rotational axis  36  by a shift fork or control element  38  which extends through a notch or slot (not shown) formed through an outer circumferential end wall  42  of the plate  12 . Preferably, 28 reverse notches are provided. However, it is to be understood that a greater or lesser number of reverse notches may be provided as will be described in greater detail herein below. 
     The fork  38  is secured or coupled to the control plate  26  so that movement of the fork  38  in the slot between different angular positions causes the plate  26  to slide or shift between its control positions to alternately cover or uncover the struts  22  (i.e., to engage or disengage the reverse struts  22 , respectively). 
     The plate  34  preferably comprises a splined ring having internal splines  46  formed at its inner axially extending surfaces  48  ( FIG. 2 ). A radially extending surface or coupling face spaced from the surface  32  of the plate  34  has a plurality of spaced pockets  52  ( FIG. 1 ) formed therein to receive a plurality of forward struts  54  therein which are pivotally biased by corresponding coil springs  55 . Preferably, fourteen forward struts  54  are provided. However, it is to be understood that a greater or lesser number of forward struts may be provided as will be described in greater detail herein below. 
     Referring collectively to  FIGS. 1 and 2 , assembly  10  also includes a second outer coupling member or notch plate, generally indicated at  58 , which has a plurality of locking formations, cams or notches  60  formed in a radially extending surface or coupling face thereof by which the forward struts  54  lock the plate  34  to the notch plate  58  in one direction about the axis  36  but allow free-wheeling in the opposite direction about the axis  36 . Preferably, thirty two forward notches are provided. However, it is to be understood that a greater or lesser number of forward notches may be provided as will be described in greater detail herein below. 
     The notch plate  58  includes external splines  64  which are formed on an outer axial surface  66  of the plate  58  and which are received and retained within axially extending recesses  68  formed within an inner axially extending surface  70  of the end wall  42  of the plate  12  ( FIG. 1 ). 
     As shown in  FIG. 1 , the assembly  10  further includes a snap ring, generally indicated at  72 , having end portions  74  and which fits within an annular groove  76  formed within the inner surface  70  of the end wall  42  of the plate  12  to hold the plates  12 ,  26 ,  34  and  58  together and limit axial movement of the plates relative to one another. 
     The shift fork  38 , in one control position of its control positions, disengages the reverse struts  22 . The shift fork  38  is rotated about 7° in a forward overrun direction about the axis  36  to rotate the selector plate  26  to, in turn, allow the reverse struts  22  to move from their disengaged positions in their pockets  20  to their engaged positions with the notches  30 . 
     As previously mentioned, many clutch assemblies (such as the assembly  10  described in  FIGS. 1 and 2  as well as the assemblies  300  and  400  described herein below with respect to  FIGS. 3 and 4 , respectively) are meant to lock in one direction and lock or free wheel in the opposite direction depending on the position of its selector or selector plate. In certain clutch locations, the clutch assembly would not come out of a “lock-lock” condition (i.e. would inadvertently bind in both directions about the rotational axis). This is due to the transitional backlash (i.e. distance the clutch can move between forward and reverse directions) was extremely low. This extremely low transitional backlash did not allow the locking elements or struts to drop out of their locking or binding position upon command thereby resulting in the “lock-lock” condition. 
     It was discovered for clutch assemblies having: 1) the number of forward locking elements different than the number of reverse locking elements; 2) the number of forward locking formations different than the number of reverse locking formations; and 3) either the forward backlash is a non-zero integer multiple of the reverse backlash or the reverse backlash is a non-zero integer multiple of the forward backlash that the coupling or clutching assembly was prevented from inadvertently binding in both directions about the rotational axis of its assembly. 
     When the above noted conditions are satisfied, the number of forward and reverse locking elements and the number of forward and reverse locking formations can be selected, so that minimum transitional backlash is substantially equal in all positions. The following table illustrates example possible combinations of forward and reverse locking elements (struts) and forward and reverse locking formations (notches): 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Single/Dual 
                 Forward 
                   
                   
                 Single/Dual 
                 Reverse 
                   
                 Function 
               
               
                   
                 Forward 
                 Forward 
                 Strut 
                 Resolution 
                 Reverse 
                 Reverse 
                 Strut 
                 Resolution 
                   
                 without 
               
               
                 Entry # 
                 Notches 
                 Struts 
                 Engagement 
                 (deg) 
                 Notches 
                 Struts 
                 Engagement 
                 (deg) 
                 n 
                 tie-up? 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 36 
                 14 
                 Dual 
                 1.4276 
                 84 
                 1 
                 Single 
                 4.2857 
                 3 
                 Yes 
               
               
                 2 
                 32 
                 14 
                 Dual 
                 1.6071 
                 28 
                 16 
                 Dual 
                 1.6071 
                 1 
                 Yes 
               
               
                 3 
                 24 
                 14 
                 Dual 
                 2.1429 
                 28 
                 12 
                 Dual 
                 2.1429 
                 1 
                 Yes 
               
               
                 4 
                 26 
                 18 
                 Dual 
                 1.5385 
                 78 
                 1 
                 Single 
                 4.6154 
                 3 
                 Yes 
               
               
                 5 
                 32 
                 14 
                 Dual 
                 1.6071 
                 112 
                 1 
                 Single 
                 3.2143 
                 2 
                 Yes 
               
               
                 6 
                 38 
                 12 
                 Dual 
                 1.5789 
                 36 
                 10 
                 Dual 
                 2.0000 
                 1.267 
                 No 
               
               
                 7 
                 34 
                 12 
                 Dual 
                 1.7647 
                 26 
                 14 
                 Dual 
                 1.9780 
                 1.121 
                 No 
               
               
                   
               
             
          
         
       
     
     Entry #2 is represented in  FIGS. 1 and 2 . Entries 6 and 7 are examples of coupling assemblies which would experience the “lock-lock” or binding condition. The type of “engagement” (either “single” or “dual”) indicates either a single or two struts provide the locking function in one of the directions of rotation. 
     The above-noted discovery is applicable to any controllable ratcheting clutch assembly both radial and planar configurations such as the configurations shown in  FIGS. 3 and 4 .  FIG. 3  shows a planar concentric configuration of an assembly, generally indicated at  300 , including a reverse member or plate, generally indicated at  302 , which is grounded at  301 . The plate  302  has an outer surface or coupling face  304  with reverse struts (not shown) in reverse pockets  308  and an inner surface or coupling face  306  with forward notches  310 . The opposing clutch member which has the forward pockets and struts, coupling faces, and the corresponding reverse notches is not shown for simplicity. The resulting assembly  300  is a dual engagement assembly having 6 reverse pockets, 44 reverse notches (not shown), a reverse backlash of 2.727, 12 forward pockets (not shown), 22 forward notches, a forward backlash of 2.727° and a transitional backlash of 1.364°. 
       FIG. 4  shows a second planar concentric coupling assembly, generally indicated at  400 , including a reverse coupling member or plate, generally indicated at  402 , which is grounded at  401 . The plate  402  has an outer surface or coupling face  404  with reverse struts (not shown) in reverse pockets  408  and an inner surface or coupling face  406  with forward struts (not shown) in forward pockets  410 . The opposing coupling member or plate (not shown for simplicity) has corresponding coupling faces and forward and reverse locking formations. The resulting assembly  400  is a single engagement assembly having 8 reverse pockets (and struts), 25 reverse notches (not shown), a reverse backlash of 1.8°, 4 forward pockets (and struts), 50 forward notches (not shown), a forward backlash of 1.8° and a transitional backlash of 0.9°. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.