Abstract:
A power-assisted gearshift mechanism for a manual transmission, comprising a shift rail mounted for axial movement to select one or more gears, a control arm engageable with the shift rail to cause axial movement of the shift rail in response to operation of a operator control, and a fluid pressure actuator, the fluid pressure actuator comprising a piston moveable in a cylinder, wherein fluid pressure in the cylinder causes force to be applied to the shift rail, the cylinder having a fluid supply connection and a fluid exit connection, and wherein the fluid exit connection is closeable to cause the fluid pressure within the cylinder to change, and the fluid exit connection comprising a bore within the shift rail, wherein movement of the control linkage is operable to open or close the bore.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC 119 (a)-(d) to United Kingdom Patent Application No. GB 0617365.2 filed on Sep. 2, 2006, which is incorporated by reference in its entirety herein. 
     BACKGROUND TO THE INVENTION 
     This invention relates to a power assisted gear shift mechanism for a manual transmission and a transmission having such a gear shift mechanism. 
     In a conventional manual transmission, the operator is able to select a particular gear or torque path through the transmission by operating a gear lever. This operates a mechanism to act physically on the appropriate synchroniser within the gearbox to provide the desired gear. The operator has to apply sufficient force to the gear lever to enable the clutches in the synchroniser to operate effectively. In some cases, this can require a long gear lever to provide the driver with the appropriate mechanical advantage to apply sufficient force to synchromesh clutches, resulting in the operator having to move the lever over the greater distance. A long gear lever thus provides a noticeable delay in the gear being selected, due to the greater distance over which the lever is moved by the operator. 
     An aim of the invention is to reduce or overcome one or more of the above problems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, we provide a power-assisted gearshift mechanism for a manual transmission, comprising a shift rail mounted for axial movement to select one or more gears, a control arm engageable with the shift rail to cause axial movement of the shift rail in response to operation of a operator control, and a fluid pressure actuator, the fluid pressure actuator comprises a piston moveable in a cylinder, wherein fluid pressure in the cylinder causes force to be applied to the shift rail, the cylinder having a fluid supply connection and a fluid exit connection, and wherein the fluid exit connection is closeable to cause the fluid pressure within the cylinder to change, and the fluid exit connection comprising a bore within the shift rail, wherein movement of the control linkage is operable to open or close the bore. 
     The fluid exit connection may be closeable to cause the fluid pressure within the cylinder to increase. 
     The control arm may comprise an end part engageable with a control part of the shift rail wherein movement of the end part closes an outlet of the bore. 
     The control part may comprise a block having a recess in which the end part of the control arm may be received and wherein the outlet of the bore may be located with the recess. 
     An end part of the shift rail may provide part of the fluid pressure actuator. 
     The control arm may comprise an end part engageable with a control part of the shift rail, the control part comprising a valve mechanism operable to close an outlet of the bore, wherein movement of the end part may act on the valve mechanism to open the outlet of the bore. 
     An end part of the actuator may comprise the piston moveably mounted within the cylinder. 
     The gearshift mechanism may comprise two fluid pressure actuators, each fluid pressure actuator may be operable to apply an axial force to the shift rail in an opposite direction. 
     The power-assisted gearshift may comprise one or more shift rails, each shift rail being moveable by the control linkage. 
     According to a second aspect of the invention, we provide a transmission comprising a casing, an input and an output, a gearbox having a plurality of clutches and a gearshift mechanism according to the first aspect of the invention, the gearshift mechanism being operable to cause operation of the clutches to provide a drive path between the input and the output. 
     The clutches may comprise synchromesh clutches. 
     The transmission may comprise a supply of hydraulic fluid under pressure, wherein the hydraulic fluid may be supplied to the fluid pressure actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, wherein: 
         FIG. 1  is an end view of a transmission embodying the present invention. 
         FIG. 2  is a section through the transmission of  FIG. 1 . 
         FIG. 3  is a plan view of part of a gear shift mechanism embodying the present invention. 
         FIG. 4  is a side view of a part of the gear shift mechanism of  FIG. 2 . 
         FIG. 5  is a plan view similar to  FIG. 3  showing a further gear shift mechanism. 
         FIG. 6  is a plan view similar to  FIG. 3  showing a still further gear shift mechanism. 
         FIG. 7  is a view in more detail of the part of the mechanism in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a transmission embodying the invention is generally shown at  10  in end view, comprising a casing  11 . A gear lever column  12  projects upwardly of the casing  11  and has a connection  13  at the upper end thereof for connection to a operator control comprising a gear lever of conventional type. As shown in  FIG. 2 , the transmission  10  has an output connection  13 , an input connection  14  for connection to for example a torque converter or directly or indirectly to an engine and a plurality of gears mounted on a first shaft  15  and second shaft  16 . The transmission  10  further has a pair of synchro-mesh clutches  17 ,  18 , each of which is operable through the gear shift mechanism described in more detail below to select one of forward or reverse drive paths through the transmission  10 . 
     Referring now to  FIGS. 3 and 4 , a gear shift mechanism is shown for operating the clutches  17 ,  18 . The gear shift mechanism  20  comprises a first shift rail  21  and a second shift rail  22 , each of the shift rails being mounted for axial movement. Each of the shift rails  21 ,  22  has a shift fork  23 ,  24  respectively attached thereto to cause operation of the clutches  17 ,  18  respectively. The shift rails are located within the casing in the position generally shown at  19  in  FIG. 1 . 
     To enable the operator to slide the shift rails  21 ,  22 , each of the shift rails  21 ,  22  has a control part comprising a shift block  25 ,  26  respectively. The shift blocks  25 ,  26  have recesses  27 ,  28  in an upper surface thereof, in which an end part  29  of a control lever generally shown at  30  in  FIG. 4  is received. By appropriate movement of the gear lever, the control linkage  30  engages one base of the recess  27 ,  28  and acts on it to move the respective shift rail  21 ,  22  in an axial direction to operate the clutches as desired. 
     As shown in  FIG. 3 , a fluid pressure actuator is provided on each shift rail to apply a force to the shift rail in response to operation of the operator control. The first shift rail  21  has a first fluid pressure actuator generally shown at  31  comprising a cylinder  32  in which a piston  33  connected to an end part of the first shift rail is attached. The first shift rail further has a second fluid pressure activator  34  comprising a cylinder  35  and a piston  36  moveable in the cylinder  35  and mounted on an end part of the first shift rail  21 . Similarly, the second shift rail  22  at one end has a third fluid pressure actuator generally shown at  37  comprising a cylinder  38  within which a piston  39  is moveably received, the piston  39  being attached to an end of the second shift rail  22 . The second shift rail  22  further has a fourth fluid pressure actuator generally shown at  40  comprising a cylinder  41  within which a piston  42  is slidably moveable and is connected to the other end of the second shift rail  22 . Each of the first, second, third and fourth fluid pressure actuators  31 ,  34 ,  37 ,  40  has a corresponding inlet  43 ,  44 ,  45 ,  46  for connection to a source of fluid under pressure, in this example, hydraulic fluid. The first shift rail  21  has a first internal bore  47  and a second internal bore  48 . The first internal bore  47  leads from the left hand end of the shift rail  21  as seen in  FIG. 3  to an outlet  47   a  on the right hand face of recess  27 , while the second internal bore leads from the right hand end of the first shift rail  21  as shown in  FIG. 3 , to an outlet  48   a  on the left hand face of recess  27 . Similarly, second shift rail  22  has a third internal bore  49  and a fourth internal bore  50 . The third internal bore  49  leads from the left hand end of the second shift rail  22  as shown in  FIG. 3  to an outlet  49   a  on the right hand face of recess  28 . The fourth internal bore  50  extends from the right hand end of the second shift rail  22  to an outlet  50   a  on the left hand face of the recess  28 . Consequently, when the outlets  47   a ,  48   a ,  49   a ,  50   a , are unobstructed, each of the cylinders  32 ,  35 ,  38 ,  41  is supplied with hydraulic fluid under pressure on the respective connection  43 ,  44 ,  45 ,  46 , and the fluid under pressure passes along the corresponding internal bore  47 ,  48 ,  49 ,  50  where it vents through the corresponding outlet. Accordingly, when the gear lever is in its neutral position, i.e. is not engaging either face of either recesses  27 ,  28  the supply of fluid pressure to the respective fluid pressure actuator  31 ,  34 ,  37 ,  40  results in no net force being applied to the respective shift rail  21 ,  22 . 
     In operation, when the operator moves the gear shift lever to select a desired gear, the control linkage  30  will be moved within one of the recesses  27 ,  28  and engage a end face, to cause axial movement of the shift rails  21 ,  22  and corresponding movement of the appropriate shift from  23 ,  24  to operate the clutch. The end part  29  of the control lever  30 , in contacting the end face of the recess  27 ,  28 , will close the outlet  47   a ,  48   a ,  49   a ,  50   a  in that face of the recess  27 ,  28 . By closing the outlet, the flow of fluid under pressure from the corresponding cylinder  32 ,  35 ,  38 ,  41  will be blocked or restricted, causing a pressure increase in that cylinder and a resulting force on the corresponding piston  33 ,  36 ,  39 ,  42  applying a further axial force to the shift rail  21 ,  22  in the direction of movement of the control lever  30 . So for example, when it is desired to move the first shift rail  21  to the left as shown in  FIG. 3 , the control linkage  30  is moved to bring the end part  29  into engagement with the left hand face of the recess  27 . In doing so, the outlet  48   a  connected to the second internal bore  48  will be closed, thus restricting the escape of fluid under pressure from cylinder  35 , increasing the pressure within the cylinder  35  and causing a corresponding force to be applied to the piston  36 , urging the shift rail  21  to the left. Because the outlet  47   a  will be unobstructed, the flow of fluid under pressure from cylinder  32  through internal bore  47  will not be hindered, and so there will be no substantial resistance to the movement of the shift rail  21  to the left due to fluid within the cylinder  32 . 
     The end part  29  of the control linkage  30  does not need to create a complete seal with the outlet  47   a ,  48   a ,  49   a ,  50   a , but needs simply obstruct the outlet sufficiently to provide a back pressure within the corresponding cylinder  32 ,  35 ,  38 ,  41 . 
     In the present example, the force applied by the driver through the shift rail and shift fork is about 800 Newtons, and the fluid pressure actuators supply a further 400 Newtons of force. The increase in force will reduce the time taken for the mating surfaces of the clutches  17 ,  18  to synchronise and engage, reducing the time taken for a gear change and the apparent force required to be applied by the operator, thus improving operator control and feel of the vehicle. The fluid pressure actuators can advantageously be provided as part of the case of the transmission  10 , and as the mechanism is compact and requires no external control, it may be provided relatively simply and inexpensively without requiring any increase in the overall envelope of the transmission. Advantageously, where the transmission has a hydraulic pump integrally provided, as shown at  60  in  FIG. 2 , the fluid pressure supplied to the fluid pressure actuators may come from that fluid pressure source. If it is desired not to divert fluid pressure from other functions when no gear is being selected, an appropriate valve may be provided so that no fluid under pressure is supplied to actuators  31 ,  34 ,  37 ,  40  during normal operation, but when a gear change is required, for example when the operator disconnects a master clutch, then fluid pressure may be applied to the actuators and the gear shift mechanism operated as discussed herein before. 
     Although the configuration described herein shows the end part of each shift rail supporting a piston moveable within a cylinder, it will be apparent that fluid pressure actuators may be provided otherwise as desired, for example as separate cylinders and pistons which are drivingly connected between the shift rail and a fixed part of the casing, or by the end part of the shift rail comprising a cylinder as part of an internal bore and a piston being mounted in that cylinder fixed relative to the casing, or indeed otherwise as desired. Although the system shown above has two shift rails, it may be provided with one, two, three or more shift rails as desired, or required by the number of gears provided in the transmission. 
     An alternative system embodying the present Invention is illustrated in  FIG. 5 . The system is similar to that disclosed in  FIG. 4 , with the exception of the internal bores which are configured to not require a cross-over as shown in  FIG. 3 . Referring to  FIG. 5 , the shift rails are shown at  21 ′,  22 ′ supporting shift forks  23 ,  24  in like manner to the shift forks of  FIG. 3 . The first shift rail  21 ′ has a first fluid pressure actuator generally shown at  61  comprising a cylinder  62  in which a piston  63  is slidably movable. The first shift rail  21 ′ further has a second fluid pressure activator  64  comprising a cylinder  65  and a piston  66  moveable in the cylinder  35  mounted on an end part of the first shift rail  21 ′. Similarly, the second shift rail  22 ′ has a third fluid pressure actuator generally shown at  67  comprising a cylinder  68  within which a piston  69  is slidably movable and attached to an end part of the second shift rail  22 ′. The second shift rail  22 ′ further has a fourth fluid pressure actuator generally shown at  70  comprising a cylinder  71  within which a piston  72  is slidably movable and is mounted to the other end of the second shift rail  22 ′. Each of the fluid pressure actuators  61 ,  64 ,  67 ,  70  has a corresponding inlet  73 ,  74 ,  75 ,  76  for a connection to the source of fluid under pressure. However, in contrast to the embodiment of  FIG. 3 , the inlets  73 ,  74 ,  75 ,  76  are connected to the annular end of the respective cylinder  62 ,  65 ,  68  and  71 , and consequently act on the face of the piston  63 ,  66 ,  69 ,  72  to which the respective shift rail  21 ′,  22 ′ is connected. Thus, the pressure within the cylinders  62 ,  68  will be acting to urge the respective shift rail  21 ′,  22 ′ to the left, whilst the pressure within cylinders  65 ,  71  will be acting to urge the respective shift rail  21 ′,  22 ′ to the right. 
     In like manner to the embodiment of  FIG. 3 , the shift rails  21 ′,  22 ′ have a respective shift block  25 ′,  26 ′ connected thereto and comprising a corresponding recess  27 ′,  28 ′ to receive an end part of the control lever  30 . In the embodiment of  FIG. 5 , the first shift rail  21 ′ has a first internal bore  77  leading from an inlet  77   a  in flow communication with the cylinders  62  to an outlet  77   b  provided in the left hand phase of the recess  27 ′. The first shift rail  21 ′ has a second internal bore  78  leading from an inlet  78   a  in flow communication with the cylinder  65  to an outlet  78   b  in the right hand side wall of the recess  27 ′. The second shift rail  22 ′ has a similar pair of internal bores. Consequently, movement of the control linkage will close one of the outlets and cause a corresponding pressure increase in the corresponding cylinder  62 ,  65 ,  68 ,  71 , urging the respective shift rail  21 ′,  22 ′ to move the direction required by the operator. For example, when the end  29  of the control linkage  30  is moved to the left In the first recess  27 ′, it will close the outlet  77   b , thus preventing or hindering fluid from leaving the cylinder  62  and causing the pressure within the cylinder  62  to act on the piston  63  urging the shift rail  21 ′ to the left and thus assisting the movement of the shift rail  21 ′ in a similar manner to the embodiment of  FIG. 3 . Consequently, the same assistance is provided with the operator when changing gear, but without needing to engineer the shift blocks  25 ′,  26 ′ to provide a cross-over of the Internal bores as shown in  FIG. 3 . 
     A yet further embodiment is shown in  FIGS. 6 and 7 . Referring to  FIG. 6 , the shift rails  21 ″,  22 ″ support shift forks  23 ,  24  in like manner to the embodiment of  FIG. 3 , and pressure is supplied to cylinders  32 ,  35 ,  38  and  41  to act on a piston provided by the end face of the respective shift rail  21 ″,  22 ″. 
     The shift rails  21 ″,  22 ″ have a respective shift block  25 ″,  26 ″ attached thereto. The first shift rail  21 ″ has a first internal bore  47 ″ and a second internal bore  48 ″ in like manner to the embodiment of  FIG. 3 , and the second shift rail  22 ″ has a third internal bore  39 ″ and a fourth internal bore  50 ″, again in like manner to the embodiment of  FIG. 3 . Referring now to  FIG. 7 , one of the shift blocks  25 ″ is shown in more detail in sectional view. The shift block  25 ″ has a first inlet  80  which is in flow communication with the first internal bore  47 ″, and a second inlet  81  which is in flow communication with the second internal bore  48 ″. A first fluid outlet is provided at  82  which is closed by a first valve mechanism  83 . A second fluid outlet  84  is provided closed by a second valve  85 . Each of the valves  83 ,  85  comprise a valve element  83   a ,  85   a  which acts to close the respective fluid outlet  82 ,  84  and is biased to a closed position by a respective biasing element  83   b ,  85   b . The valve elements  83 ,  85  are provided with a respective end face  83   c ,  85   c , for engagement with the end  29  of the control linkage  30 . Accordingly, it will be apparent that when the end of the control linkage is moved to the left or right as shown in  FIG. 7 , it will act on the end face  83   c ,  85   c  of the respective valve element  83 ,  85  and connect the respective inlet  80 ,  81  to the respective fluid outlet  82 ,  84 , thus connecting the respective internal bore  47 ″,  48 ″ to atmosphere and so causing the pressure to fall in the corresponding cylinder  32 ,  35 . Because the other cylinder  32 ,  35  will remain at a relatively higher pressure, there will be a corresponding force applied to the shift rail  21 ″ acting to urge it to the left or the right as seen in  FIG. 6  by virtue of movement of the control linkage  30 . The second shift block  26 ″ has the same structure and means of operation as the first shift block  25 ″. 
     Although the system described herein is intended for use with the hydraulic fluid, it will be apparent that the fluid under pressure may comprise pneumatic pressure or any other fluid if desired and appropriate. 
     In the present specification “comprise” means “includes or consists of” and “comprising” means “including or consisting of”. 
     The features disclosed In the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.