Patent Publication Number: US-2016245404-A1

Title: Device for an emergency park release selector

Description:
TECHNICAL FIELD 
     The present invention relates to an automatic transmission system having an emergency park release selector and particularly, but not exclusively, to a device for introducing free play into the system. The invention finds advantageous application in automotive applications but may be employed in other applications such as but not limited to industrial and marine applications. Aspects of the invention relate to a mechanism, to a driving position selector, to an automatic transmission, to a vehicle and to a method. 
     BACKGROUND 
     It is known to provide a vehicle with an automatic transmission system typically such systems employ a driving position selector to shift between the four driving positions; ‘park’ (P), ‘reverse’ (R), ‘neutral’ (N) and ‘drive’ (D). A mechanical linkage may couple the driving position selector and a control device such as shift lever disposed in the vehicle cabin. Recently “shift-by-wire” transmission systems have been developed; “shift-by-wire” is an electromechanical or electrohydraulic system which replaces the mechanical linkage between the automatic transmission system and the shift lever in the vehicle cabin. In a shift-by-wire transmission system shifting between the four driving positions; ‘park’, ‘reverse’, ‘neutral’ and ‘drive’ is electronically controlled. 
     Typically, when a shift-by-wire automatic transmission system disengages park, the hydraulic pressure withdraws a park rod from a park pawl and so releases a park pawl tooth from a park wheel. In this condition the ‘neutral’, ‘drive’ or ‘reverse’ gear selections can now be selected. These system interactions often require a high pressure to overcome the park system loads and so any external loads must be kept to a minimum so as not to exceed the transmission limits. 
     It is known in shift-by-wire transmission systems for the transmission system to default to the ‘park’ position in the event of a loss of electrical power. Other systems may remain in the driving position selected at the time of the loss of electrical power, which may be the park position. It is therefore desirable to provide an Emergency Park Release (EPR) mechanism such that the vehicle can be moved under such conditions. The Emergency Park Release mechanism may be employed in other situations in which the driving position selector fails to operate and the parking position is engaged. Typically, the Emergency Park Release mechanism comprises a selector lever coupled to a selector shaft which forms part of the transmission system. The selector lever is coupled by a cable, such as a Bowden cable, to a release lever or handle. The release lever or handle is typically disposed in the vehicle cabin or engine bay. The release lever or handle is manually operated to disengage the parking position. 
     When the shift-by-wire transmission system is operated electronically each disengagement &amp; re-engagement of ‘park’, rotates the selector shaft, thus forcing the EPR selector lever to rotate and forcing the EPR cable system to cycle through its nominal routing. This imposes an unwanted additional load on the park system hydraulics and park return spring through, compression loads in springs and bellows and friction load in the Bowden cable. As such, it is desirable to provide a free-play device at this interface to assume negligible load from the EPR to the park system hydraulics and park return spring. 
     The present invention seeks to provide an improvement in the field of emergency park release systems for automatic transmissions that has particular application for vehicles. The invention may be utilised in applications other than for vehicles. 
     SUMMARY 
     Aspects of the invention provide a mechanism, a driving position selector, an automatic transmission, a vehicle and a method as claimed in the appended claims. 
     According to one aspect of the invention for which protection is sought, there is provided a mechanism for releasing an electronically controlled automatic transmission from a parking position, the automatic transmission comprising a selector shaft for selecting an operating position of the automatic transmission, the mechanism comprising a manual actuator coupled to a release lever by a cable, the release lever being coupled to the selector shaft such that the selector shaft is rotatable with respect to the release lever up to a predetermined angle. 
     Advantageously, the mechanism may provide that the park system hydraulics of the automatic transmission are not required to do additional work to overcome system loads (the automatic transmission does not need to move the release lever, compress the cable return spring or overcome a frictional load when moving the cable) of the emergency park release mechanism when operating normally, that is to say electronically. The emergency park release mechanism is therefore effectively disconnected from the automatic transmission during normal operating conditions. 
     One of the release lever and the selector shaft may comprise an aperture and the other of the release lever and the selector shaft may comprise a projection received within the aperture, wherein the aperture is shaped such that the selector shaft is rotatable with respect to the release lever up to the predetermined angle. 
     The selector shaft may be rotatable between a first position in which the automatic transmission is in a parking position and a second position in which the automatic transmission is disengaged from the parking position without rotating the release lever. 
     The release lever may be mounted upon the selector shaft and comprise an aperture for receiving the selector shaft, wherein the aperture is shaped such that the selector shaft is rotatable with respect to the release lever up to the predetermined angle. 
     The aperture may be shaped such that further rotation of either one of the release lever or selector shaft causes both the release lever and selector shaft to rotate synchronously. 
     The aperture shape may be a combination of the cross-sectional shape of the selector shaft at the first position and the cross-sectional shape of the selector shaft after rotation to the second position. The aperture may be substantially butterfly shaped. 
     The aperture may comprise a first pair of diagonally opposed surfaces for engaging opposing sides of the selector shaft for rotating the selector shaft in a first direction and wherein the diagonally opposed surfaces are arranged so as to engage the selector shaft on opposing sides of the rotational axis of the selector shaft. 
     The aperture may comprise a second pair of diagonally opposed surfaces for engaging on opposing sides of the selector shaft for rotating the selector shaft in a second direction and wherein the diagonally opposed surfaces are arranged so as to engage the selector shaft on opposing sides of the rotational axis of the selector shaft. 
     In some embodiments, only one pair of the first and second pairs of diagonally opposed surfaces may be engaged with the selector shaft when the selector shaft is rotated in either of the first or second directions. 
     The release lever may be coupled to the selector shaft proximate to a first end thereof and to the cable proximate to a second end thereof. 
     The mechanism may comprise a cable return spring for biasing the cable and the release lever to a non-operative position. 
     In use, the cable may be arranged to pull the second end of the release lever thereby rotating the release lever about the rotational axis of the selector shaft. 
     The manual actuator may comprise a latching device for retaining the manual actuator, the cable and the release lever in an operative position when actuated, so as to maintain the selector shaft in the second position. 
     When the release lever is disposed in a non-operative position, the first pair of diagonally opposed surfaces may be substantially disposed in engagement with the selector shaft when disposed in a parking position such that the selector shaft rotates substantially simultaneously with the release lever when rotated in the first direction. 
     According to another aspect of the invention for which protection is sought, there is provided a driving position selector for an electronically controlled automatic transmission comprising the mechanism described herein above. 
     According to a further aspect of the invention for which protection is sought, there is provided an automatic transmission for a vehicle comprising an electronically activated driving position selector having the mechanism described herein above. 
     The automatic transmission may comprise a housing wherein the selector shaft extends through the housing and the cable is slidably mounted within a bracket mounted upon the housing. 
     The cable may comprise an end connector for coupling the cable to the release lever and the cable return spring may be mounted between the end connector and the bracket. 
     The cable return spring may be disposed with a bellows for preventing or reducing ingress of dirt or moisture into the cable. 
     According to a still further aspect of the invention for which protection is sought, there is provided a vehicle comprising the automatic transmission described in the foregoing paragraphs or the mechanism described herein above. 
     According to yet a further aspect of the invention for which protection is sought, there is provided a method of releasing an automatic transmission from a park position comprising;
         providing an automatic transmission comprising an electronically activated driving position selector, the driving position selector having a selector shaft and a locking mechanism for locking the selector shaft in a park position; and   providing a mechanism for releasing the automatic transmission from a parking position, the mechanism having a release lever;   wherein, in a first mode the method comprises;
           activating the driving position selector electronically;   rotating the selector shaft between a first position in which the automatic transmission is in a parking condition, and second position in which the automatic transmission is disengaged from the parking condition;   where, in the first mode the selector shaft rotates with respect to the release lever between the first and second positions; and   
           in a second mode the method comprises;
           activating the driving position selector manually;   rotating the release lever in a first direction;   thereby rotating the selector shaft, synchronously with the release lever, between the first position in which the automatic transmission is in the parking condition, and the second position in which the automatic transmission is disengaged from the parking condition.   
               

     In the second mode the method may comprise locking the mechanism to prevent the release lever and selector shaft returning to the first position so as to prevent the automatic transmission engaging the parking condition. 
     In the second mode the method may comprise unlocking the mechanism to allow the release lever and selector shaft to return to the first position so as to allow the automatic transmission to engage the parking condition. 
     According to another aspect of the present invention for which protection is sought, there is provided a mechanism for releasing an electronically controlled automatic transmission from a parking position, the automatic transmission comprising a selector shaft for selecting an operating position of the automatic transmission, the mechanism comprising a manual actuator coupled to a release lever by a cable, the release lever being mounted upon the selector shaft and the release lever comprising an aperture for receiving the selector shaft, wherein the aperture is shaped such that the selector shaft is rotatable with respect to the release lever up to a predetermined angle 
     Optionally, the aperture is shaped such that further rotation of either one of the release lever or selector shaft causes both the release lever and selector shaft to rotate synchronously. 
     This has the advantage of providing an emergency park release mechanism for releasing the transmission from the park position. 
     In some embodiments the selector shaft may rotate between a first position in which the automatic transmission is in a parking position and a second position in which the automatic transmission is disengaged from the parking position without rotating the release lever. 
     Optionally, the aperture shape is a combination of the cross-sectional shape of the selector shaft at the first position and the cross-sectional shape of the selector shaft after rotation to the second position. The aperture may be substantially butterfly shaped. 
     The aperture may comprise a first pair of diagonally opposed surfaces for engaging on opposing sides of the selector shaft for rotating the selector shaft in a first direction and wherein the diagonally opposed surfaces are optionally arranged so as to engage the selector shaft on opposing sides of the rotational axis of the selector shaft. 
     Optionally, the aperture may also comprise a second pair of diagonally opposed surfaces for engaging on opposing sides of the selector shaft for rotating the selector shaft in a second direction and wherein the diagonally opposed surfaces are optionally arranged so as to engage the selector shaft on opposing sides of the rotational axis of the selector shaft. 
     In some embodiments only one pair of the first or second pairs of engaging surfaces is engaged with the selector shaft when rotating the selector shaft in the first or second direction. 
     The release lever may be coupled to the selector shaft proximate to a first end thereof and may be coupled to the cable proximate to a second end of the selector shaft. 
     Optionally, the mechanism comprises a cable return spring for biasing the cable to the first position. 
     In some embodiments the cable is arranged to pull the second end of the release lever whereby rotating the release lever about the rotational axis of the selector shaft. 
     The manual actuator may comprise a latching device for retaining the manual actuator, cable and the release lever in the second position. 
     In some embodiments whereupon the release lever is disposed in the first position, the first pair of diagonally opposed surfaces is substantially disposed in engagement with the selector shaft when disposed in a parking position such that the selector shaft rotates substantially simultaneously with the release lever when rotated in the first direction. 
     According to another aspect of the invention for which protection is sought, there is provided a driving position selector for an electronically controlled automatic transmission comprising the mechanism described herein above. 
     According to a further aspect of the invention for which protection is sought, there is provided an automatic transmission for a vehicle comprising an electronically activated driving position selector having the mechanism described herein above. 
     Optionally, the automatic transmission comprises a housing wherein the selector shaft extends through the housing and the cable is slidably mounted within a bracket mounted upon the housing. 
     Optionally, the cable comprises an end connector for coupling the cable to the release lever and the cable return spring is mounted between the end connector and the bracket. 
     In some embodiments the cable return spring is disposed with a bellows for preventing or reducing ingress of dirt or moisture into the cable. 
     According to a still further aspect of the invention for which protection is sought, there is provided a vehicle comprising the automatic transmission described in the foregoing paragraphs or the mechanism described herein above. 
     According to yet a further aspect of the invention for which protection is sought, there is provided a method of releasing an automatic transmission from a park position comprising;
         providing an automatic transmission comprising an electronically activated driving position selector, the driving position selector having a selector shaft and a locking mechanism for locking the selector shaft in a park position;   providing a mechanism for releasing the automatic transmission from a parking position the mechanism having a release lever,   in a first mode the method comprises;   activating the driving position selector electronically,   rotating the selector shaft between a first position in which the automatic transmission is in a parking condition, and second position in which the automatic transmission is disengaged from the parking condition,   where, in the first mode the selector shaft rotates with respect to the release lever between the first and second positions,   in a second mode the method comprises;   activating driving position selector manually,   rotating the release lever in a first direction,   whereby, rotating selector shaft, synchronously with the release lever, between the first position in which the automatic transmission is in the parking condition, and the second position in which the automatic transmission is disengaged from the parking condition.       

     Optionally, in the second mode the method comprises locking the mechanism to prevent the release lever and selector shaft returning to the first position so as to prevent the automatic transmission engaging the parking condition. 
     Optionally, in the second mode the method comprises unlocking the mechanism to allow the release lever and selector shaft to return to the first position so as to allow the automatic transmission to engage the parking condition. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives, and in particular the features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination thereof. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of an automatic transmission having an emergency park release mechanism according to an embodiment of the invention; 
         FIG. 2  is an enlarged view of the emergency park release mechanism of  FIG. 1 ; 
         FIG. 3  is a perspective view from above of part of the emergency park release mechanism according to an embodiment of the invention; 
         FIG. 4  is an exploded view of the part of the emergency park release mechanism shown in  FIG. 3 ; 
         FIG. 5A  is a side view of a release lever according to an embodiment of the invention; 
         FIG. 5B  is a top view of a selector shaft according to an embodiment of the invention; 
         FIG. 5C  is an end view of a selector shaft according to an embodiment of the invention; 
         FIG. 6A  is a side view of the part of the emergency park release mechanism shown in  FIG. 3  in a first condition; 
         FIG. 6B  is a side view of the part of the emergency park release mechanism shown in  FIG. 3  in a second condition; 
         FIG. 6C  is a side view of the part of the emergency park release mechanism shown in  FIG. 3  in a third condition; 
         FIG. 7A  is a schematic representation of the profile of a selector shaft according to an embodiment of the invention; 
         FIG. 7B  is a schematic representation of the profile of a selector shaft according to an embodiment of the invention at a first orientation and at a second rotated orientation; 
         FIG. 7C  is a schematic representation of the shape of the aperture defined in the release lever for providing free play according to an embodiment of the invention; 
         FIG. 7D  is a schematic representation of the profile of a selector shaft according to another embodiment of the invention; 
         FIG. 7E  is a schematic representation of the shape of an aperture in a release lever of an emergency park release mechanism for use with the selector shaft of  FIG. 7D ; 
         FIG. 7F  is a schematic representation of the profile of a selector shaft according to a further embodiment of the invention; and 
         FIG. 7G  is a schematic representation of the shape of an aperture in a release lever of an emergency park release mechanism for use with the selector shaft of  FIG. 7F . 
     
    
    
     DETAILED DESCRIPTION 
     Detailed descriptions of specific embodiments of the emergency park release mechanism of the present invention are disclosed herein. It will be understood that the disclosed embodiments are merely examples of the way in which certain aspects of the invention can be implemented and do not represent an exhaustive list of all of the ways the invention may be embodied. Indeed, it will be understood that the emergency park release mechanism described herein may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimised to show details of particular components. Well-known components, materials or methods are not necessarily described in great detail in order to avoid obscuring the present disclosure. Any specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the invention. 
       FIG. 1  illustrates schematically a shift-by-wire automatic transmission system  1  including an emergency park release mechanism  2  according to an embodiment of the invention. The automatic transmission  1  is coupled to an electronic driving position selector  9  disposed within the cabin of a vehicle (not shown). The transmission system  1  comprises an actuator (not shown) which is in communication with, electrically coupled, to the electronic driving position selector  9  for selecting between predefined driving positions, including ‘park’, ‘reverse’, ‘neutral’ and ‘drive’. The automatic transmission system  1  comprises a locking mechanism (not shown) for locking the transmission system  1  in a park position. The transmission system  1  comprises resilient biasing means in the form of a park return spring (not shown) for biasing the transmission system  1 , in particular a selector shaft  28 , to return to the park position, for example in the case of electrical failure, so as to engage with the locking mechanism. In other embodiments the park return spring may be omitted such that the selector shaft  28  remains in the selected driving position at the time of an electrical failure. 
     The emergency park release mechanism  2  comprises a lever or handle  8  which is manually operable by a user. The handle  8  may be disposed in the vehicle cabin, preferably in a concealed location so as to prevent inadvertent activation of the emergency park release mechanism  2 . In alternative embodiments the handle  8  may be located elsewhere on the vehicle for example in the engine bay or in storage compartment such as the boot or trunk. The handle  8  is coupled to a release lever  16  by a cable  10  such as a Bowden cable. The release lever  16  is mounted upon a selector shaft  28 , (best illustrated in  FIG. 3 ) and the selector shaft  28  is rotatable about an axis  18 . The cable  10  is mounted to a housing  25  of the automatic transmission  1 , optionally by a bracket  14 . An end of the cable  10  is coupled to a first end  24  (see  FIG. 3 ) of the release lever  16 . The cable  10  is pivotally or rotatably mounted to the first end of the release lever  16 . Optionally, the end of the cable  10  comprises a connector  12  (see  FIG. 2 ), for example a socket for receiving a ball mounted on the release lever  16  proximate the first end  24  of the release lever  16 . In alternative embodiments a pin or rod may pass through an aperture in the connector  12  mounted on the end of the cable  10 . Additionally or alternatively in other embodiments the pin or rod may extend through the release lever  16  such that the release lever  16  may rotate with respect to the connector  12  and cable  10 . 
     The emergency park release mechanism  2  comprises a resilient biasing means in the form of a spring to form a cable return mechanism  20  for overcoming friction in the cable when returning the cable  10  and release lever  16  to a default position, for example the default position is a ‘non-operative’ position, i.e. the release lever position in which the release lever  16  does not disengage the transmission system  1  from the park position. The cable return mechanism  20  comprises a helical spring  21  mounted between the connector  12  and the bracket  14 . An inner core  23  of the cable  10  passes through the helical spring so as to extend substantially along a helical axis of the helical spring  21 . The inner core  23  of the cable  10  and helical spring  21  are disposed within a sheath in the form of bellows  27  so as to prevent or reduce the ingress of moisture and/or dirt into the cable  10 . The bellows  27  is mounted between the connector  12  and the bracket  14 . 
     The release lever  16  is mounted on the selector shaft  28  and secured thereto by a mechanical fixing. Optionally, the mechanical fixing comprises an external screw thread disposed on the end of the selector shaft  28 . A nut  22  comprising an internal screw thread complimentary to the external screw thread disposed on the end of the selector shaft  28  is mounted upon the selector shaft  28  so as to retain the release lever  16 . Optionally, a washer may be provided between the nut  22  and the release lever  16 . The nut  22  may be a locking nut. 
       FIGS. 3 to 5C  illustrate the release lever  16  and the selector shaft  28 . The selector shaft  28  comprises an end section  31  configure to be inserted into the aperture  36  in the release lever  16 . The end section  31  comprises a shoulder  33  against which a first side of the release lever  16  abuts, see  FIG. 4 . The end section  31  comprises a pair of opposing flat sections  34  which extend parallel to the longitudinal axis  18  of the selector shaft  28 . 
     The release lever  16  comprises an aperture  36 . In the illustrated embodiment the aperture  36  has a “butterfly” shape, best shown in  FIG. 5A . The shape of the aperture  36  is defined by the cross-sectional shape of the end section  31  of the selector shaft  28  and the desired amount of free play to be incorporated. The aperture  36  is a combination of the cross-sectional shape or profile of the end  32  of the selector shaft  28  at a first position P 1  and cross-sectional shape or profile of the end  32  of the selector shaft  28  at a second position P 2 . To get to the second position P 2  the selector shaft  28  has been rotated about the longitudinal axis  18  by an angle α, as illustrated in  FIG. 7B . The angle α is determined by the angle of rotation of the selector shaft  28  necessary to disengage the automatic transmission from the park position. Optionally, this angle of rotation is up to about 30°, in some embodiments the angle of rotation is between about 20° and about 30° and may be 24°. In other embodiments other angles of rotation are envisaged. 
       FIGS. 6A, 6B and 6C  illustrate the release lever  16  in operation. In  FIG. 6A  the release lever  16  is in the default inactive condition (i.e. a non-operative position) and the selector shaft  28  is in the park position. In  FIG. 6B  the release lever  16  is in the default inactive condition and the selector shaft  28  has been rotated, within the aperture  36 , in the direction indicated by direction arrow D 1 . The selector shaft  28  has been rotated through an angle α that is sufficient to disengage the automatic transmission from the park position in which the transmission is locked. It is envisaged that the rotation of the selector shaft  28  will have been initiated by an electronic control unit (not shown) following an input from the driver. This is normal operation of the transmission system  1 ; the emergency release system  2  is inactive since the transmission system  1  disengages from the park position by employing an electrical actuator (not shown). The electronic control unit will instruct the transmission to disengage park, for example using a mechanical or hydraulic system. 
     The selector shaft  28  may rotate freely between the first position shown in  FIG. 6A  and the second position shown in  FIG. 6B  without causing rotational movement of the release lever  16 . 
     Additionally, the selector shaft  28  may be rotated, from the first position shown in  FIG. 6A  to the second position shown in  FIG. 6C , through an angle α, so as to disengage the automatic transmission from the park position, by rotating the release lever  16  through an angle α as indicated by direction arrow D 2  (i.e. by moving the release lever to an operative position). This provides that the transmission system  1  can be manually operated by the emergency park release system  2  to disengage the transmission system  1  from the park position. A user can, by pulling the cable  10 , effect rotation of the release lever  16 , this in turn acts upon the selector shaft  28  to disengage the transmission system  1  from the park position. In doing so the user overcomes the bias of the park return spring which is acting upon the selector shaft to return the selector shaft  28  to the park position. The release lever  16  and cable  10  must be held or locked in a release position (i.e. the operative position) in order to prevent the park return spring returning the selector shaft  28  to the park position. The release lever  16  and cable  10  may be locked by providing the handle  8  with a locking mechanism  3   a / 3   b  at the end of the cable  10  disposed in the vehicle cabin or other location on the vehicle. 
     The aperture  36  comprises four surfaces S 1 , S 2 , S 3 , S 4 , as illustrated in  FIG. 7C . First surface S 1  and second surface S 2  engage with the selector shaft  28  to rotate the selector shaft in a first direction, clockwise as illustrated in  FIGS. 6A to 6C . The selector shaft  28  when in the park position as illustrated in  FIG. 6A  is in contact with the first and second surfaces S 1 , S 2 . First surface S 1  engages with or abuts an upper portion of a first one of the flat sections  34  on a first side of the selector shaft  28 , the upper portion being above the longitudinal axis  18  about which the selector shaft  28  rotates. Second surface S 2  engages with a lower portion of a second one of the flat sections  34  on a second, opposing, side of the selector shaft  28 , the lower portion being below the longitudinal axis  18  about which the selector shaft  28  rotates. 
     The selector shaft  28  is brought substantially into contact with the third and fourth surfaces S 3 , S 4  when the electronic control unit activates the automatic transmission  1  to rotate the selector shaft  28  to disengage the transmission from the park position, as shown in  FIG. 6B . 
     The selector shaft  28  and the release lever  16  may be returned to the park position shown in  FIG. 6 a    from the disengaged position shown in  FIG. 6C  by releasing the cable  10 , for example by disengaging the locking mechanism provided upon the handle such that the park return spring acts upon the selector shaft  28  to return the selector shaft  28  to the park position. In doing so the selector shaft  28  returns the release lever  16  to the park position as shown in  FIG. 6A . The selector shaft  28  acts upon first and second surfaces S 1  and S 2  of the aperture  36  provided in the release lever  16  to return the release lever  16  to the park position (i.e. such that the release lever is in the non-operative position). 
     In other embodiments it will be appreciated that the selector shaft  28  may be rotated in the counter-clockwise direction by counter-clockwise rotation of the release lever  16  such that the third and fourth surfaces S 3 , S 4  of the release lever aperture  36  are in contact with the selector shaft  28 . Third surface S 3  engages or abuts with an upper portion of the second one of the flat sections  34  on the second side of the selector shaft  28 , the upper portion being above the longitudinal axis  18  about which the selector shaft  28  rotates. Fourth surface S 4  engages with a lower portion of the first one of the flat sections  34  on the first side of the selector shaft  28 , the lower portion being below the longitudinal axis  18  about which the selector shaft  28  rotates. It will also be appreciated that to rotate the selector shaft  28  in the counter-clockwise direction from the position illustrated in  FIG. 6C  it will be necessary to rotate the release lever  16  counter clockwise about an angle α to bring the third and fourth surfaces S 3 , S 4  into contact with the respective portions of the flats  34  of the selector shaft  28  whereby taking up the free play. In order to effect rotation of the selector shaft  28  back to the first, engaged, position, as shown in  FIG. 6A  the release lever  16  must be rotated counter clockwise about a further angle α. 
     Other exemplary embodiments of aspects of the invention are illustrated in  FIGS. 7D, 7E, 7F, 7G , wherein like reference numerals have, where possible, been used to denote like parts, albeit with the addition of the prefix “100” and “200” to indicate that these features belong to the alternative embodiments respectively. 
       FIG. 7D  illustrates a cross-section of an end portion of a selector shaft  128  according to an alternative embodiment, in this embodiment the selector shaft  128  has a hexagonal shape.  FIG. 7E  illustrates the shape of an aperture  136  for a release lever (not shown) for incorporating the selector shaft  128  having the hexagonal shape illustrated in  FIG. 7D . The aperture  136  includes a predetermined degree of free play, in this example about 24°. 
       FIG. 7F  illustrates a cross-section of an end portion of a selector shaft  228  according to yet another alternative embodiment. In this embodiment the selector shaft  228  has a triangular shape.  FIG. 7G  illustrates the shape of an aperture  236  for a release lever (not shown) for incorporating the selector shaft  228  having the triangular shape illustrated in  FIG. 7F . The aperture  236  includes a predetermined degree of free play, in this example about 24°. 
     It can be appreciated that various changes may be made within the scope of the present invention, for example, it will be appreciated that in yet other embodiments the selector shaft  28  may have an alternative cross-sectional shape and that the release lever  16  may have an alternatively shaped aperture in dependence upon the shape of the selector shaft  28  and the degree of free play desired. The shape of the aperture  36  can be derived by rotating the cross-sectional shape of the selector shaft  28  through an angle α which provides the desired degree of free play. 
     Whereas the side  34  of the end portion  31  of the selector shaft  28  are shown as substantially flat in the illustrated embodiments, in alternative embodiments the sides  34  may have another suitable contour and in such embodiments, the aperture of the release lever  16  may have appropriately shaped surfaces S 1 , S 2 , S 3 , S 4 . 
     Further it will be appreciated that the maximum degree of free play which can be achieved whilst still providing the aperture  36  with an engaging or driving surface for effecting rotation of the release lever  16  will be limited by the cross-sectional shape of the selector shaft  28 . The selector shaft  28  must not be able to rotate continuously in one direction without hindrance from the aperture  36 . 
     In an alternative embodiment (not shown), the aperture  36  may be provided in the end of the selector shaft  28  and a suitably shaped projection (i.e. having a shape corresponding to the end section  31  of the selector shaft  28  in the previously described embodiments of  FIGS. 1 to 7G ) may be provided proximate to the second end  26  of the release lever  16  to be received within the aperture in the selector shaft. In this alternative embodiment, the relationship between the geometry of the aperture in the selector shaft and the projection on the release lever is the same as for the previously described embodiments in that the aperture is shaped such that the selector shaft is rotatable with respect to the release lever up to a predetermined angle. Rotation on the release lever beyond the predetermined angle causes the selector shaft and release lever to rotate synchronously. 
     In the case that the aperture  36  is provided in the release lever  16 , it has been explained previously that a threaded nut  22  may be mounted on the end of the selector shaft  28  so as to retain the release lever  16  thereon. In the alternative embodiment where the aperture is provided in the end of the selector shaft and the release lever is provided with a projection to be received in the aperture, an alternative mounting arrangement for the release lever is required. For example, the release lever may be rotatably mounted on a bracket or other mounting member attached to the side of the transmission casing. Other arrangements for rotatably mounting the release lever adjacent to the end of the selector shaft are possible. 
     It will be recognised that as used herein, directional references such as “top”, “bottom”, “front”, “back”, “end”, “side”, “inner”, “outer”, “upper” and “lower” do not limit the respective panels to such orientation, but merely serve to distinguish these panels from one another.