Abstract:
Drive engagement apparatus for engaging a driven gear ( 18 ) with a shaft ( 14 ) connected to a load. Engagement is achieved by interengaging an axially movable sleeve ( 19 ) with a dog. One of the sleeve ( 19 ) or dog is drivably connected to the gear ( 18 ) and the other to the shaft ( 14 ). Compressed air acts on a piston ( 21 ) which is engaged via a fork ( 22 ) with the sleeve ( 19 ) to effect axial movement of the sleeve ( 19 ). However, axial movement is delayed by a friction drive, which prevents engagement between the sleeve ( 19 ) and dog until the rotational speeds of those elements is equalised. The friction drive is actuated by pressing together a plurality of friction plates ( 24 ) using the compressed air. The dog comprises an engagement unit ( 231 ) that is interengageable with the sleeve ( 19 ) and a base unit ( 232 ) that is axially movable under the air supply to activate and deactivate the friction drive. The base unit ( 232 ) and engagement unit ( 231 ) are rotatable relative to one another within a predetermined angle to adjust for misalignment between the sleeve ( 19 ) and dog during interengagement.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention described herein relates to engagement devices for enabling a rotating driving member to be brought into smooth positive engagement with a load to be driven by e.g. synchronising the driving member with a driven member. 
       BACKGROUND TO THE INVENTION 
       [0002]    A known clutch arrangement includes friction plates enclosed in a clutch housing. The friction plates are compressed mechanically or pneumatically by a circular piston pressing on a pressure plate, which comes into contact with the friction plates and pushes them together. An output shaft having the load to be driven attached to it, e.g. by a coupling, has alternate friction plates in mechanical cooperation with it. The other alternate friction plates (i.e. those not in cooperation with the output shaft) are in mechanical cooperation with a driving sleeve which rotates as part of the driving shaft. The clutch works by friction acting between the friction plates as they are pushed together. 
         [0003]    GB 2216203 discloses an example of the above type of engagement device. It describes an internally splined driving sleeve movable under the action of a pneumatic ram to engage an output dog drivably connected to an output shaft—this engagement effectively makes the driving sleeve and output shaft a single mechanical member, thereby avoiding the dependence on operating air pressure. The ram has an actuating rod with a fork element attached to it, the fingers of the fork element engaging an annular groove in the outer surface of the driving sleeve. Thus, when compressed air acts on an end of the pneumatic ram, the actuating rod slides axially, moving the driving sleeve with it. The driving sleeve has a pressure plate located inside it and releasably engaged to it by means of steel balls resiliently urged into depressions formed in the inner surface of the sleeve. There are a set of friction plates, alternate ones of which are engaged with the internal splines of the driving sleeve, the remainder being engaged with the output shaft. When the sleeve is initially moved towards engagement with the output dog, the pressure plate moves axially with it and loads the friction plates against one another to begin turning the output shaft. A large torque is required to start the rotation because of the inertia of the load attached to the output shaft. This torque manifests itself as friction between the friction plates and the internal splines of the driving sleeve. This friction is enough to prevent further sliding motion of the driving sleeve until the rotational speeds of the driving sleeve and output shaft are more or less equal. The torque required to turn the output shaft is then less, so the friction acting on the splines of the driving sleeve is reduced and sliding recommences. 
         [0004]    In WO 2004/109137, the present inventor proposed a improved arrangement of the type described in GB 2216203 wherein the functions of (i) engaging a driving member with a driven member, and (ii) activating a friction drive e.g. by pushing friction plates together were separated by incorporating a valve arrangement in the pneumatic ram, which allowed a force to act through the ram without necessarily moving the driving member. Thus, the clutch was used only to synchronise the driving and driven members, which meant that it was less likely to burn out through overloading. The valve arrangement in the piston also allowed an air controlled friction drive to be deactivated just before the moment of positive engagement of the clutch, in order for engagement to proceed smoothly. 
         [0005]    The driving member and driven member typically have toothed projections which interlock to provide the positive engagement. On rare occasions, the driving member and driven member are synchronised so that the toothed projections become aligned and abut one another at the point of engagement, i.e. instead of intermeshing, the projection are pressed together point-to-point. One disadvantage of this is that positive engagement is not properly achieved because there is no physical interlock between the driving member and the driven member. There is therefore a risk of the driving member and the driven member slipping relative to one another, e.g. if the size of the driven load reduces for any reason. Such slipping can jar the apparatus, and may cause damage e.g. to the interlocking features. Furthermore, when engagement is missed the pneumatic ram is prevented from moving along its full axial extent but continues to be urged towards that position by the air pressure. This puts a load on the connection between the pneumatic ram and the element (e.g. fork element) that connects the ram to the member (driven or driving) which it moves. This connection may be weakened or otherwise damaged (e.g. bent out of alignment) by this force. 
       SUMMARY OF THE INVENTION 
       [0006]    At its most general, the present invention provides a two-part engagement device at the interlock point between the driving member and driven member. The engagement device serves a dual purpose of activating the friction drive and adjusting for misalignment between the driving member and the driven member at interengagement. The two parts of the engagement device are rotatable relative to one another within a predetermined angle to enable interlockable features associated with the driving member and driven member to shift into a meshing configuration. 
         [0007]    Thus, according to the present invention there may be provided apparatus for engaging a rotary driving member with an element to be driven, the apparatus including: first and second rotatable members each having an engagement portion, the first rotatable member being axially movable to interengage the respective engagement portions, and one of the first or the second rotatable members being drivably connectable to the rotary driving member and the other of the first or the second rotatable members being drivably connectable to the element to be driven such that interengagement of the respective engagement portions effects positive engagement of the rotary driving member with the element to be driven; a slidable piston connected to the first rotatable member such that pressure acting on one end of the piston causes axial movement of the piston and the first rotatable member; a clutch device having a plurality of axially movable friction plates, a first set of which are rotatably engaged with the first rotatable member and a second set of which are rotatably engaged with the second rotatable member, the plurality of friction plates being arranged to provide a friction drive when the first and second set are pushed together; wherein one of the engagement portions includes a base unit drivably connected to the rotary driving member or the element to be driven and an engagement unit mounted on the base unit and arranged to interengage with the other of the engagement portions, the base unit and the engagement unit being rotatable relative to each other within a predetermined angle to permit the respective engagement portions to occupy an orientation suitable for interengagement, and wherein the slidable piston includes a passageway arranged to permit pressure acting on the end of the piston also to act on the base unit, wherein the base unit is axially movable to activate the friction drive to substantially synchronise the rotation speeds of the first and second rotatable members before interengagement of the respective engagement portions. Thus, if the engagement portions are misaligned at engagement, the engagement unit can shift with respect to the base unit to an orientation in which the engagement portions are better aligned. 
         [0008]    The shift of the engagement unit may be caused by contact between the two engagement portions. Preferably, the axial force of the engagement portion associated with the first rotatable member when it contacts the other engagement portion is deflected to cause the engagement unit to shift relative to the base unit. 
         [0009]    Preferably, the apparatus includes a bias unit arranged to urge the base unit and engagement unit towards an equilibrium position. The base unit and the engagement unit may be relatively rotatable both clockwise and anticlockwise from the equilibrium position. The direction of relative rotation may be that in which the engagement unit travels a shorter distance into an orientation suitable for interengagement. 
         [0010]    Preferably, the base unit comprises an inner annular element and the engagement unit comprises an outer annular element mounted coaxially on the inner annular element, the inner and outer annular elements having one or more interlocking stopper elements arranged to limit the rotation of the outer annular element relative to the inner annular element. The interlocking stopper elements are preferably spaced, i.e. have predetermined play, in order to achieve the limited rotation. The rotation may be limited to only a few degrees, e.g. less than 10°, more preferably less than 8°. This may correspond to a limitation of less than 5° (preferably less than 4°) in each rotation direction. Preferably, however, the rotation is limited according to the spacing of interengaging elements on the engagement portions. Thus, the rotation is preferably limited to less that the angular separation of two adjacent interengaging elements, i.e. the angular spacing between the centre of two adjacent interengaging elements on one of the engagement portions. This preferably corresponds to rotation limited to half the angular separation of two adjacent interengaging elements in each rotation direction. 
         [0011]    Preferably, the inner annular element has a plurality of radially projecting stopper tabs which are receivable in corresponding recesses formed in the outer annular element. The circumferentially spaced walls of each recess may define a space within with a stopper tab can move. The amount of movement preferably corresponds to the difference between the circumferential spacing of the recess walls and the circumferential extent of the stopper tab. The inner annular element may comprise a star-shaped element. 
         [0012]    Preferably, the bias unit includes biasing means arranged to urge the stopper tabs into an equilibrium position within their corresponding recesses. Each stopper tab may be urged to a substantially central position within its recess. The biasing means may include a cushion plug at each circumferential interface between a stopper tab and its corresponding recess, each cushion pad being arranged to resist rotational movement of the stopper tab away from the equilibrium position. Thus, the base unit and engagement unit may only rotate relative to one another when a torque greater than a predetermined value is applied between them. As described below, the torque may be caused by deflecting the axial force experienced between the engagement portions when the first rotatable member moves axially to interengage the engagement portions. 
         [0013]    Preferably, each stopper tab includes a circumferential through-hole, blocked at each end by a protruding plug, each protruding plug being urged out of the through-hole by a spring to form a pair of cushion pads at each circumferential interface between a stopper tab and its corresponding recess. To rotate the engagement unit relative to the base unit in this arrangement, the stopper tab must overcome the spring force. 
         [0014]    Preferably, the engagement unit includes a first set of projecting teeth arranged to mesh with a second set of projecting teeth on the other engagement portion. The teeth of one of the first or second set of projecting teeth may have angled outer surfaces arranged to rotatably deflect the projecting teeth of the other set when the first and second set of projecting teeth are urged against one another. For example, the projecting teeth on the engagement portion of the first rotatable member may be provided with pointed tips, which, when urged against an angled (e.g. sloping or curved) surface, move sideways. 
         [0015]    The apparatus of the present invention preferably resembles to the drive engagement apparatus of WO 2004/109137. Thus, the piston may be slidably mounted in a housing, the piston being operably connected with the first rotatable member such that pressure acting on one end of the piston causes axial movement of the piston with respect to the housing, thereby effecting axial movement of the first rotatable member. 
         [0016]    The friction drive actuator is preferably urged away from activating the friction drive so that the friction plates are disengaged when at rest. 
         [0017]    Preferably, the first rotatable member moves in a first direction into positive engagement with the second rotatable member; the friction drive actuator moves in a second direction to activate the friction drive; and the first direction is substantially opposite the second direction. 
         [0018]    Preferably, the apparatus includes restraining means to restrict initial axial movement of the piston, such that pressure acting on the piston produces movement of the friction drive actuator prior to movement of the piston. 
         [0019]    Preferably, the piston includes a valve arrangement adapted to cause the pressure to cease acting on the base unit during interengagement between the first and second rotatable members, thereby to deactivate the friction drive. At this point, the load is taken by the second rotatable member. 
         [0020]    Thus, at activation pressure acts on the piston in an axial direction to push the first rotatable member towards engagement with the second rotatable member. However, the pressure also activates the friction drive, and the pressure from the friction plates on the first rotatable member is enough to delay axial movement. During the delay, the friction drive causes the second rotatable member (coupled to the load) to rotate. When the rotation speeds of the first and second rotatable members are substantially synchronised, the pressure on the first rotatable member from the friction plates is reduced enough to be overcome by the pressure acting on the piston. The first rotatable member may then move into engagement with the second rotatable member. When this has occurred, since the friction drive has served its purpose, the pressure on the friction drive actuator is cut off to set free the friction plates. 
         [0021]    Preferably, the valve arrangement maybe arranged to provide an additional air pressure supply to the piston just before engagement. This additional air supply may act as a boost to ensure the axial force exerted between the engagement portions is enough to cause the necessary relative rotation to an orientation suitable for interengagement. 
         [0022]    The axially movable first rotatable member is preferably drivably connected to the rotary driving member and the second rotatable member is drivably connected to the element to be driven. The opposite configuration is also possible. 
         [0023]    The second rotatable member preferably includes a gear mounted on a shaft, and the first rotatable member preferably includes a sleeve coaxial with the shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    An example of the present invention is described below with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  shows a cross-sectional view of a known drive engagement apparatus of the type disclosed in WO 2004/109137; 
           [0026]      FIG. 2  shows a cross-sectional view of drive engagement apparatus which is an embodiment of the present invention; 
           [0027]      FIG. 3  shows a cross-sectional view of the two-part engagement device of the present invention taken along the line Y-Y in  FIG. 2 ; 
           [0028]      FIG. 4  shows the inner member of the engagement device shown in  FIG. 3 ; 
           [0029]      FIG. 5  shows a cross-section of the inner member taken along the line A-A in  FIG. 4 ; 
           [0030]      FIG. 6  shows a cross-section of a tab portion of the inner member taken along the line B-B in  FIG. 5 ; 
           [0031]      FIG. 7  shows the outer member of the engagement device shown in  FIG. 3 ; 
           [0032]      FIG. 8  shows a cross-section of the outer member taken along the line C-C in  FIG. 7 ; 
           [0033]      FIG. 9  shows a front view of the axially movable sleeve of the present invention; and 
           [0034]      FIG. 10  shows a cross-section of the sleeve taken along the line D-D in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
     Further Options and Preferences 
       [0035]      FIG. 1  shows a known drive arrangement, and is described to explain how the engagement apparatus works in general and to highlight the improvement proposed by the present invention.  FIG. 1  shows a housing consisting of two parts  11 ,  12 . A driving gear  18  is externally splined to be permanently engaged to a rotating member (not shown) of an engine. Thus, when the engine is running, driving gear  18  rotates. The driving gear  18  has splines around its external surface which are keyed into internal splines on a driving sleeve  19 , which is axially slidable relative to the driving gear  18 . 
         [0036]    The housing also holds an output shaft  14 . The output shaft  14  extends through the centres of the driving sleeve  19  and driving gear  18 , where it is located in a roller bearing  53 , which allows independent rotation of the shaft  14  and driving gear  18 . A protruding end of the shaft  14  has coupling  15  attached to it by means of which the drive arrangement can be attached to an external device (e.g. centrifugal fire engine pump), which needs to be driven. 
         [0037]    Driving sleeve  19  is axially movable by piston  21 , which has a fork member  22  that engages an annular groove  20  in the surface of the driving sleeve  19 . Thus, when compressed air is supplied through input port  34  to space  35 , piston  21  is pushed to the left as shown in  FIG. 1 ; this would serve also to push the driving sleeve  19  to the left. 
         [0038]    The output shaft  14  is externally splined, and an output dog  23  is slidably keyed via internal splines on to it. Output dog  23  and driving sleeve  19  are arranged so that they can be drivably connected to one another via a dog tooth connection  28 ,  29 . In other words, driving sleeve  19  can be pushed into engagement with output dog  23  to effect mechanical connection between the driving gear  18  and output shaft  14 . 
         [0039]    If the driving sleeve  19  were pushed into immediate engagement with the output dog  23 , the inertia of the load connected to the output shaft  14  would give the system a large shock, which could easily damage components. It is better for the output shaft  14  (and therefore the output dog  23 ) to be already rotating at a similar (if not the same) speed as the driving sleeve  19  when engagement occurs, to minimise any shock loading. To delay the moment of engagement, friction plates  24  are provided between the driving gear  18  and the output dog  23 . Alternate ones of the friction plates have internal splines which engage on the external splines of output shaft  14 , therefore rotate with that shaft. The other alternate friction plates have external splines that engage with the internal splines of the driving sleeve  19 ; the friction plates  24  are able to slide axially relative to one another. 
         [0040]    When the piston  21  is in the rightmost position in  FIG. 1  (i.e. disengaged or ‘parked’), there is a gap between the output dog  23  and the driving gear  18  so that there is free play between the friction plates  24 . Thus, when the friction plates are not in use, the two sets of plates can rotate relative to one another relatively easily. 
         [0041]    On the opposite side of the output dog  23  from the friction plates  24 , there is a annular ring  25  mounted in a cylinder formed in the housing. The ring  25  has a pressure plate  27  attached to it which engages the output dog  23  via roller bearing  44 . The arrangement is such that when e.g. compressed air is provided to the cylinder, the ring  25  is pushed to the right as seen in  FIG. 1 . Thus, the pressure plate  27  pushes the output dog  23  via thrust bearing  44  to the left; the output dog  23  pushes the friction plates  24  together, squeezing them between the output dog  23  and the driving gear  18 , thereby activating a friction drive on the output shaft. 
         [0042]    One set of friction plates  24  are axially slidably engaged with the internal splines of the sleeve  19 . These plates are interposed by another set of friction plates which are axially slidably engaged with external splines on the output shaft  14 . The friction drive is actuated when both sets of friction plates are pressed together. A passageway  60  and radial hole  62  are provided in the output shaft to enable lubricant (e.g. oil) to be delivered to the friction plates. 
         [0043]    Fluid (e.g. compressed air) is provided to the cylinder by a bore  32  drilled in the housing. The compressed air for moving the ring  25  comes from the same port  34  as the compressed air for moving piston  21 . The piston  21  has a passageway  31  drilled in it which has a port  36  at one end that opens into space  35 . At the other end, a radial hole  33  links the passageway  31  to bore  32 , i.e. it allows compressed air communication between the port  34  and cylinder  26 . 
         [0044]    As shown in  FIG. 1 , the intermediate portion of the piston  21  has a coiled spring  30  fitted around it that pushes against the wall of the housing and an upstanding ridge  26  on the piston, i.e. it acts to push the piston  21  to the right in  FIG. 1 , i.e. it acts to stop driving sleeve  19  from being pushed immediately into engagement with output dog  23 . In fact, the spring is of a particular biasing strength so that, when e.g. compressed air is provided from port  34  to space  35 , travel of the piston  21  is restricted enough by the spring  30  so that the compressed air communicates first with the ring  25  and therefore acts on the output dog  23  first. In other words, the spring  30  ensures that the friction drive on the output shaft is initiated by movement of the output dog  23  before the driving sleeve  19  moves significantly. 
         [0045]    The leftmost end of piston  21  also includes annular grooves located on either side of the radial hole  33 . The grooves contain sealing rings  55 ,  56  which define a zone around the end portion when it is located in the housing in which the pressure from radial hole  33  can act. Thus, the piston  21  itself can act as a valve for the pressure acting through the passageway  31 . When the zone is positioned over the bore  32 , the pressure through the passageway  31  can act on the ring  25 , whereas if the piston  21  is moves axially so that one of the sealing rings  55 ,  56  moves over the entrance to the bore  32 , the ring  25  will be isolated from the pressure. 
         [0046]      FIG. 1  shows the arrangement in a disengaged position. Piston  21  is at its rightmost position. Output shaft  14  is thus not driven. To move to an engaged state, compressed air is provided into space  35  via port  34 . The spring  30  restricts the movement of the piston  21  under this pressure, such that the pressure acts first on ring  25  in cylinder  26  via passageway  31  and radial hole  33  and bore  32 . The ring  25  pushes pressure plate  27  against output dog  23 , which slides so as to push the friction plates  24  together. This movement is relatively small: the output dog  23  is unable to slide into engagement with the drive sleeve  19 ; the sleeve itself must move to effect engagement. Friction between the alternate plates that rotate with the sleeve and the plates engaged with the output shaft  14  makes the shaft  14  start to turn. However, the torque required for this means high contact pressures act against the side surfaces of the internal splines of the driving sleeve  19  which prevent it from moving to the right (i.e. to engage with the output dog  23 ). However, as the output shaft  14  increases in speed, the torque required lessens so that the contact pressures reduce to allow the pneumatic force on the driving sleeve  19  to overcome the restraining force of the spring  30  so that it begins to slide into full engagement with output dog  23 . The pneumatic force through radial hole  33  acts from the zone defined by sealing rings  55 ,  56 . That zone is positioned so that as the driving sleeve  19  begins to slide into full engagement with the output dog  23 , the compressed air supply to output dog  23  is cut off. 
         [0047]    Pressure plate  27  has a pull-back mechanism where it (and ring  25 ) are urged fully back towards the housing when pressure is removed. The pull-back mechanism has a bolt  50  fixed in a recess in the housing. The bolt has a cylinder  51  slidably mounted on it and biased away from it (to the left in  FIG. 1 ) by a spring  52 . Cylinder  51  is attached to pressure plate  27  such that it acts to pull the plate towards the housing. 
         [0048]    In detail, as the piston  21  moves axially, sealing ring (e.g. o-ring)  56  moves over the radial hole(s)  58  to stop the compressed air from reaching the ring  25 . The ring  25  is then pulled away from the output dog  23  by pressure plate  27 , which acts under the influence of spring  52  contained between bolt  50  and cylinder  51  as in  FIG. 1 . The friction drive is therefore deactivated at the point of engagement between the sleeve  19  and output dog  23 . 
         [0049]    The relatively large area on which air pressure may act on ring  25  allows a greater force than previously known devices to be applied here, which may allow the clutch to start up with a certain amount of load already connected. This is in contrast to the previous device, where a zero-load condition was recommended for start up to avoid clutch plate slippage. 
         [0050]      FIG. 2  shows a driving engagement apparatus which is a modified version of the apparatus shown in  FIG. 1  and is an embodiment of the present invention. The basic operation of the engagement apparatus is the same as in  FIG. 1 , and parts which perform the same function are given the same reference numerals. The modification concerns the engagement between the sleeve  19  and the output dog  23 . In  FIG. 2 , the output dog comprises a plurality of parts. Firstly, there is a thrust plate  230  which provides a surface for the bearing  44  to abut. Thus, when compressed air acts on the ring  25  to push the pressure plate  27  outwards, the thrust plate  230  (which is in splined engagement with the shaft  14 ) moves axially along the shaft  14  toward the friction plates  24 . Adjacent the thrust plate  230 , and also in splined engagement with the shaft  14 , is a star plate  232  which has an outer engagement plate  231  mounted on it. This structure is described in detail below. Both the thrust plate  230  and the star plate  232  are drivably connected to (and therefore rotate with) the shaft  14 . The outer engagement plate  231  is mounted on the star plate  232  in a way that permits it to rotate relative to the star plate  232  and the thrust plate  230  within a predetermined angle. The engagement plate  231  and sleeve  19  are arranged so that they can be drivably connected to one another via a dog tooth connection  193 ,  252  (see  FIGS. 7 and 10 ). Thus, sleeve  19  is axially moved into engagement with outer engagement plate  231  in order to drivably connect the driving gear  18  to the shaft  14 . 
         [0051]    Since engagement plate  231  may only rotate relative to star plate  232  within a predetermined angle, it rotates with the shaft  14  when the friction drive is activated. Similarly to  FIG. 1 , therefore, engagement plate  231  is rotating at a similar (if not identical) speed to sleeve  19  at the point of engagement. 
         [0052]    The clutch will have a non-zero torque capacity which enables it to start a certain amount of load. In this case, the sleeve  19  would axially slide into the engagement plate  231  immediately. This is undesirable as it would cause the dog tooth connections on the sleeve  19  and engagement plate  231  to rotate against on another and take a large proportion of the loading force. This can lead to damage. To prevent this, the air pressure is selected to ensure that the force exerted by the thrust plate  230  on the clutch exceeds the torque capacity of the clutch. This ensures that the rotation speeds of the sleeve  19  and engagement plate  231  are substantially equalised before they interengage, i.e. before the dog tooth connectors (splines) contact one another. 
         [0053]    Finally, just before the full engagement, i.e. when the piston  21  has almost completed its travel (to the leftmost position in  FIG. 2 ), the piston valve represented by o-rings  55 ,  56  cuts off the air pressure to the thrust plate  230 , thereby allowing the clutch plates freedom to move (i.e. effectively disengaging the friction drive). O-rings  55 ,  56  define the zone  37  in which air pressure through radial hole  33  acts. Just before full engagement, o-ring  56  moves over the hole  58  to isolate the zone  37  from the passageway  32  to the ring  25 . 
         [0054]    Thus, activating the clutch when air is supplied to the piston  21  causes the axial movement of the piston  21  to be delayed sue to the pressure on the friction plates  24  in the clutch housing (sleeve  19 ). Only when the rotation of the self-centring movable dog (star plate  232  and engagement plate  231 ) is substantially synchronised with the sleeve  19  does the pressure on the sleeve  19  reduce enough to allow the piston  21  to continue its axial travel. Interengagement of the sleeve  19  and self-centring movable dog  231 , 232  then occurs, at which point the clutch has served its purpose, as the sleeve  19  is halfway into full engagement. Thus, the clutch can be deactivated, which is achieved by cutting off the air supply to the operating piston  25 . 
         [0055]    The self-centring movable dog  231 , 232  has a dual function. Firstly it comprises two parts that are rotatable relative to one another to provide smooth interengagement. Secondly, it is axially movable to activate the synchronisation of the sleeve  19  with the rotatable dog. 
         [0056]    The difference between the apparatus in  FIG. 1  and  FIG. 2  is that the engagement plates  231  may shift slightly relative to the star plate  232  at the point of engagement to ensure that the dog tooth connection properly engages. The mechanism for this is illustrated in  FIG. 3 .  FIG. 3  shows a cross-section view through the star plate  232  and engagement plates  231 . The star plate has a central bore  235  for receiving the shaft  14 . The bore  235  has internal splines (illustrated figuratively at  242 ) which are keyed into external splines on the outer surface of the shaft  14 . The star plate  232  has an inner annular element from which three regularly spaced stopper tabs  241  protrude in radial directions. Each stopper tab extends for about 70 degrees around the circumference of the inner annular element. This arrangement is shown in detail in  FIG. 4 . The outer engagement plate  231  is shown in  FIG. 7 . It comprises a central bore  250  which receives the inner annular element of the star plate  232 . Surrounding the bore  250  are three equally spaced projections  254  which define circumferential recesses  255  between them. The projections  254  and recesses  255  are shaped so that the stopper tabs  241  of the star plate  232  each fit into a recess  255  of the outer engagement plate  231 . The angular extent of the recess  255  is greater than the angular extent of the stopper tabs  241  so that relative angular movement (rotation about an axis perpendicular to the bores  235 ,  250 ) between the outer engagement plate  221  and the star plate  232  is permitted. The amount of relative angular rotation permitted is governed by the difference in angular extent between the stopper tab  234  and its recess  255 . 
         [0057]      FIG. 7  shows that the outer engagement plate has a plurality of teeth  252  around its outer edge. These are adapted to inter engage with inwardly projecting teeth  193  on the sleeve  19  (see  FIGS. 9 and 10 ). The surface of the teeth  252  on the outer engagement plate  231  facing the sleeve  19  are chamfered or otherwise angled e.g. as two angled surfaces  258  sloping away from a ridge. Similarly, the parts of the teeth  193  on the sleeve  19  that face towards the teeth  252  on the outer engagement plate  231  are shaped into points  194 . When the points  194  contact the angled (sloped) surfaces  258 , the axial force on the sleeve is deflected to cause the outer engagement plate  231  to rotate relative to the star plate  232 . The direction of rotation depends on the orientation of the angled surface  258 . 
         [0058]      FIG. 3  also illustrates a cushioning mechanism that urges the outer engagement plates  231  into an equilibrium position relative to the star plate  232 . The cushioning mechanism stabilises the system and prevents rattling due to loose movement between the outer engagement plate  231  and the star plate  232 . The cushioning mechanism comprises three pairs of cushion plugs  260 , each pair associated with one of the stopper tabs  241  of the star plate  232 . As shown in  FIGS. 4 ,  5  and  6 , each stopper tab  241  comprises a circumferential passageway  244  which can communicate with the side walls  257  of each recess via slots  246 . Each cushion plug  260  has an abutment portion  261  shaped to protrude through a slot  246  to contact the recess side wall  257 . Each cushion plug  260  is urged out of its slot  246  by a spring  262  located in the circumferential passageway  244 . To prevent the cushion plug from being completely pushed through its slot  246 , each plug  260  has a central ridge  263  whose diameter is greater than the diameter of the slot  246 . By urging each pair of cushion plugs  260  outwards into contact with the walls of their respective recess  255 , the outer engagement plate  231  is maintained in a equilibrium position where each stopper tab  241  of the star plate  232  is held substantially centrally in each recess  255  of the outer engagement plate  231 . 
         [0059]    To cause relative movement between the outer engagement plate  231  and the star plate  232 , a torque must be applied to the outer engagement plate  231  that overcomes the force due to the springs  262 . When this occurs, one of each pair of cushion plugs  260  will be pushed back into the stopper tab  241  by the rotating outer engagement plate  231 . 
         [0060]    Thus, the force of the sleeve  19  on the engagement plate  231  due to the air pressure on the piston  21  is enough to overcome the force of the springs  262 , therefore permitting full mechanical contact by the dog tooth connectors (splines) on the sleeve  19  and engagement plate  231 . 
         [0061]      FIG. 8  shows that the back side of the outer engagement plate  231  includes a bore  256  shaped to enclose the thrust plate  230 . There is no mechanical engagement between these two plates. 
         [0062]      FIGS. 9 and 10  show in detail the sleeve  19 . The teeth  193  are discussed above. These project inwardly from the outer body  191  of the sleeve  19 . The outer body  191  includes upstanding projections  192  which form the groove  20  for receiving the selector fork  22 .