Abstract:
A parking lock device for an automatic transmission includes: a parking gear externally fixed to the output shaft; a parking lock pole displaced between a lock position, where the parking lock pole latches onto the parking gear to disable rotation of the parking gear, and an unlock position, where the parking lock disengages from the parking gear to enable rotation of the parking gear; a parking rod for displacing the parking lock pole toward and away from the parking gear; a drive lever that moves the parking rod; and a booster mechanism interposed between the parking rod and the drive lever that increases the driving force of the drive lever. Thus, the parking lock device makes the driving force applied by the drive lever when unlocking as small as possible and prevents inadvertent unlocking when in a locked state, thereby achieving smooth operation and enhanced reliability.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The disclosure of Japanese Patent Application No. 2007-006099 filed on Jan. 15, 2007 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a parking lock device for an automatic transmission for a vehicle. The parking lock device causes a pawl of a parking lock pole to latch onto a parking gear, which is externally fixed to an output shaft of an automatic transmission, to lock and prevent rotation of the output shaft, and disengages the pawl from the parking gear to unlock and allow rotation of the output shaft. 
         [0004]    2. Description of Related Art 
         [0005]    In an automatic transmission for a vehicle, generally, a parking range (P), a reverse range (R), a neutral range (N), a drive range (D), and the like are set as the selectable shift ranges. 
         [0006]    For instance, such an automatic transmission has a parking lock device for bringing the output shaft of the automatic transmission into a non-rotatable locked state when the parking range P is selected with a shift lever installed near the driver&#39;s seat of the vehicle (see, for example, Japanese Patent Application Publication No. 62-218261 (JP-A-62-218261), Japanese Patent Application Publication No. 9-267726 (JP-A-9-267726), and Japanese Patent Application Publication No. 9-240442 (JP-A-9-240442)). 
         [0007]    This parking lock device includes a parking gear, a parking rod, a parking lock pole, and a drive lever. 
         [0008]    Operation of the parking lock device will be described. First, when the parking rod is pushed to one side in its axial direction to tilt the parking lock pole, and the pawl of this parking lock pole is latched onto the parking gear externally fixed to the output shaft, the output shaft is brought into a non-rotatable locked state. 
         [0009]    Further, when the parking rod is pulled to the other side in its axial direction to tilt the parking lock pole in a direction opposite to the above-mentioned direction, and the pawl of the parking lock pole is disengaged from the parking gear, the output shaft is brought into a rotatable unlocked state. 
         [0010]    It should be noted that the parking rod is pushed or pulled in the axial direction by the drive lever that is tilted in response to an operation of placing the shift lever into the parking range or an operation of releasing the shift lever from the parking range. This parking rod is directly connected to a predetermined portion of the drive lever. 
         [0011]    As the drive lever of this parking lock device, some related art devices use, for example, the manual valve lever of a range selector (see, for example, Japanese Patent Application Publication No. 9-267726 (JP-A-9-267726)). 
         [0012]    The range selector is generally configured as follows. That is, when the shift lever installed near the driver&#39;s seat of the vehicle is operated by the driver, the manual valve lever is tilted. The state of a manual valve, which is a constituent element of a hydraulic controller for switching shift ranges, is changed in accordance with this tilting movement of the manual valve lever, thereby establishing the shift range selected by the above-mentioned shift change operation. 
         [0013]    It should be noted that as the mode of power transmission between the manual valve lever and the shift lever, there is a type in which power is directly transmitted by mechanically coupling and connecting the manual valve lever and the shift lever by using a power transmission member such as a shift rod or shift cable, or the like, and a type in which the shift lever and the manual valve lever are not connected together by the power transmission member but are detached from each other, and when the shift lever is operated by the driver of the vehicle, the position of the selected shift range is detected by a sensor or the like, and the manual valve lever is driven by an actuator such as a motor so as to establish the detected shift range. 
         [0014]    The above-mentioned related art still leaves room for improvements in the following respects. 
         [0015]    As a general tendency, when a vehicle is stopped on a downhill slope, the same forward rotation torque as that at the time of forward movement is input to the output shaft from the driving wheels, whereas when the vehicle is stopped on an uphill slope, reverse rotation torque is input to the output shaft from the driving wheels. 
         [0016]    When, in a state with the parking lock device locked, the above-mentioned forward rotation torque is input to the output shaft, this makes the pawl of the parking lock pole prone to disengage from the parking gear. In this case, the driving force of the manual valve lever (drive lever) required for bringing the parking lock device into an unlocked state can be made relatively small. 
         [0017]    On the other hand, when, in a state with the parking lock device locked, the above-mentioned reverse rotation torque is input to the output shaft, the pawl of the parking lock pole bites into the parking gear. In this case, the driving force of the manual valve lever (drive lever) required for bringing the parking lock device into an unlocked state needs to be made relatively large. 
         [0018]    However, since this phenomenon is established on the basis of the positional arrangement of the tilting axis of the parking lock pole with respect to the parking gear, if this positional arrangement differs, a phenomenon reverse to that mentioned above is established. 
         [0019]    In view of the phenomenon as described above, generally, the engaging force or the like of a latch lever with each position groove of the manual valve lever is adjusted to allow suitable setting of the driving force of the manual valve lever required for achieving the above-mentioned unlocked state. 
         [0020]    From the viewpoint of safety, this setting is desired to ensure that unlocking does not easily occur. However, in that case, the driving force to be applied by the drive lever at the time of unlocking needs to be set large, resulting in an increase in the requisite operation force of the shift lever. 
         [0021]    In contrast, although it may be possible to use assist means such as a motor for facilitating the unlocking operation, this leads to an increase in equipment cost. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention provides a parking lock device that locks the output shaft of an automatic transmission in a manner that prevents or allows rotation of the output shaft, as appropriate, and, by means of a simple configuration, minimizes the driving force that is applied by the drive lever when unlocking, and also prevents inadvertent unlocking when the output shaft is locked, thereby achieving smooth operation and enhanced reliabilty. 
         [0023]    According to one aspect of the present invention, there is provided a parking lock device which brings an output shaft of an automatic transmission into a non-rotatable locked state or a rotatable unlocked state as required, the parking lock device includes: a parking gear externally fixed to the output shaft; a parking lock pole displaced between a lock position, where the parking lock pole approaches and latches onto the parking gear to thereby disable rotation of the parking gear, and an unlock position, where the parking lock pole separates and disengages from the parking gear to thereby enable rotation of the parking gear; a parking rod pushed and pulled to displace the parking lock pole toward or away from the parking gear; a drive lever supported in a tiltable manner that pushes or pulls the parking rod in a longitudinal direction of the parking rod; and a booster mechanism interposed between the parking rod and the drive lever, that increases a driving force of the drive lever applied when locking or unlocking the parking lock device and converts the driving force into a pressing force or a pulling force on the parking rod. 
         [0024]    According to this configuration, the use of the booster mechanism increases the pressing force or pulling force on the parking rod even when the driving force applied by the drive lever when locking or unlocking the parking lock device is small. Accordingly, the parking lock device can be easily and reliably locked or unlocked with a relatively small force. 
         [0025]    Therefore, the driving force of the drive lever required when stopping a vehicle on a uphill slope or a downhill slope to lock or unlock the parking lock device may be minimized. 
         [0026]    Furthermore, the overall configuration is simple, which only involves interposing a mechanical booster mechanism between the driver lever and the parking rod. This is advantageous in reducing equipment cost as compared with a case where power assist means such as an expensive motor is equipped as described in the related art. 
         [0027]    The booster mechanism may include a link which pivots about a position in the driver lever separated from a tilting axis and on one end of the parking rod. When the drive lever is tilted to a locking side, the link is positioned so that the link is aligned in a straight line with the parking rod and pushes the parking rod in the manner of a strut, and when the drive lever is tilted to an unlocking side, the link is positioned so that the link is inclined at a predetermined angle with respect to the parking rod and pulls the parking rod. 
         [0028]    According to this configuration, the link and the drive lever constitute a toggle joint type booster mechanism. 
         [0029]    In the booster mechanism as described above, due to the tilting movement of the drive lever, a tangential force (equivalent to a driving force) acts on the joint portion between the drive lever and the link, and the tangential force is increased by the boosting action and converted into a pressing force or pulling force that acts on the parking rod at the joint portion between the link and the parking rod. 
         [0030]    This increases the pressure force or pulling force on the parking rod even when the tangential force, that is, the driving force of the drive lever, is small. 
         [0031]    The booster mechanism may further include: a fixed link attached at both ends to a tilting axis and a position separated from the tilting axis in the drive lever; a movable link which is pivoted on the position in the driver lever separated from the tilting axis and on one end of the parking rod, and which, when the fixed link is tilted to the locking side integrally with the drive lever, is positioned so that the movable link is aligned in a straight line with the parking rod and pushes the parking rod in the manner of a strut, and when the fixed link is tilted to the unlocking side integrally with the drive lever, is positioned so that the movable link is inclined at a predetermined angle with respect to the parking rod and pulls the parking rod; a connecting bar whose both ends are engaged with elongated guide holes respectively provided in the fixed link and the movable link so as to extend between their both ends; and a stopper that prevents the fixed link and the movable link from bending opposite to an unlocking side in a state when the fixed link, the movable link, and the parking rod are aligned in a straight line. 
         [0032]    According to this configuration, the two links constitute the booster mechanism of a toggle joint type. Thus, in the same manner as mentioned above, it is possible to increase the pressure force or pulling force on the parking rod even when the tangential force, that is, the driving force of the drive lever is small. 
         [0033]    In addition, the movements of the two links are coordinated with each other by the connecting bar, thereby making the movements of the links smooth. Further, the stopper provides enhanced reliability of operation, such as by preventing the two links arranged in a straight line from bending inadvertently to the side opposite to the unlocking side. 
         [0034]    Further, the ends of the elongated guide hole in the fixed link and of the elongated guide hole in the movable link, which are located near each other, may be extended so as to be offset to a side opposite to the unlocking side with respect to a line connecting the tilting axis of the drive lever and a pivot point at which the movable link is pivoted on the one end of the parking rod, in the state when the fixed link, the movable link, and the parking rod are aligned in a straight line. 
         [0035]    According to this configuration, if, conversely to the normal operation, an axial load is applied toward the movable link from the parking rod, moments act to move the joint portions of the links to the stopper side. 
         [0036]    Due to this operating principle, it is possible to keep the links aligned in a straight line and prevent the links from becoming unlocked as a result of bending. 
         [0037]    With the parking lock device according to the present invention, it is possible, by means of a simple configuration, to make the driving force to be applied by the drive lever when shifting to an unlocked state as small as possible and prevent inadvertent unlocking when in a locked state. This makes it possible to achieve smooth operation and enhanced reliability of the parking lock device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein: 
           [0039]      FIG. 1  is a schematic diagram showing an example of an automatic transmission to which a parking lock device according to the present invention is applied; 
           [0040]      FIG. 2  is a perspective view showing a range selector device and the parking lock device shown in  FIG. 1 ; 
           [0041]      FIG. 3  is a side view showing the parking lock device shown in  FIG. 2 , illustrating a locked state; 
           [0042]      FIG. 4  is a side view showing the parking lock device shown in  FIG. 2 , illustrating an unlocked state; 
           [0043]      FIG. 5  is a front view showing the relative position of a parking gear with a parking lock pole of the parking lock device shown in  FIG. 4 ; 
           [0044]      FIG. 6  is a plan view showing a booster mechanism shown in  FIG. 3 ; 
           [0045]      FIG. 7  is an enlarged side view showing the booster mechanism shown in  FIG. 3 ; 
           [0046]      FIG. 8  is an enlarged perspective view showing the booster mechanism shown in  FIG. 3 ; 
           [0047]      FIG. 9  is an exploded perspective view of the booster mechanism shown in  FIG. 3 ; 
           [0048]      FIGS. 10A to 10D  are views illustrating the operation of the booster mechanism shown in  FIG. 3 , of which  FIG. 10A  shows operation for a parking range P,  FIG. 10B  shows operation for a reverse range R,  FIG. 10C  shows operation for a neutral range N, and  FIG. 10D  shows operation for a drive range D; 
           [0049]      FIG. 11  is a view illustrating the force for holding a locked state applied by the booster mechanism shown in  FIG. 10A ; 
           [0050]      FIG. 12  is a skeleton diagram illustrating the relationship between the driving force and the pressing force at the time of locking in the booster mechanism shown in  FIGS. 3 to 9 ; 
           [0051]      FIG. 13  is a skeleton diagram illustrating the relationship between the driving force and the pulling force at the time of unlocking in the booster mechanism shown in  FIGS. 3 to 9 ; 
           [0052]      FIG. 14  is a graph showing the relationship between displacements ΔX and AY shown in  FIGS. 10A to 10D ; and 
           [0053]      FIG. 15  is a graph showing the relationship between the angle θ formed between a fixed link and a movable link when bent as shown in  FIGS. 10A to 10D  and the magnification factor of a pressing force or pulling force on a parking rod. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0054]    An embodiment of the present invention will be described below with reference to the drawings.  FIGS. 1 to 15  show an embodiment of the present invention. 
         [0055]    Before describing a portion to which features of the present invention are applied, an overview of an example of an automatic transmission to which a parking lock device according to the present invention is to be applied will be described with reference to  FIGS. 1 and 2 . 
         [0056]      FIG. 1  is a schematic diagram of an automatic transmission to which the present invention is to be applied, and  FIG. 2  is a perspective view showing a range selector and a parking lock device shown in  FIG. 1 . 
         [0057]    In these figures, reference numeral  1  denotes an automatic transmission for a front-engine, rear-drive (FR) vehicle. 
         [0058]    As shown in  FIG. 1 , the automatic transmission  1  mainly includes an input shaft  2 , a torque converter  3 , an oil pump  4 , a transmission mechanism unit  5 , a hydraulic controller  6 , an output shaft  7 , and the like. A transmission controller  8  controls the gear changes in the automatic transmission  1 . 
         [0059]    The operation of the automatic transmission  1  can be summed up as follows. When the rotation of a crankshaft (not shown) of the engine  1  is input to the input shaft  2  via the torque converter  3 , the speed of the rotation input to the input shaft  2  is changed at a suitable gear ratio by the transmission mechanism unit  5  before being output from the output shaft  7 . 
         [0060]    Although not illustrated in detail, the transmission mechanism unit  5  is configured by, for example, a plurality of planetary mechanisms. Alternatively, the transmission mechanism unit  5  can be configured as, for example, a gear mechanism having a plurality of gears, a continuously variable transmission mechanism called CVT, or the like. 
         [0061]    The hydraulic controller  6  controls the gear change operation of the transmission mechanism unit  5  described above. Although not illustrated in detail, the hydraulic controller  6  includes at least a plurality of linear solenoid valves for controlling engaging operations of various brakes and clutches used in the transmission mechanism unit  5 , and a manual valve  9  for supplying hydraulic fluid to the respective linear solenoid valves as required. 
         [0062]    The manual valve  9  supplies hydraulic fluid to each linear solenoid valve as appropriate from a suitable port in order to establish a neutral range N, a drive range D, or a reverse range R in accordance with an operation of a shift lever  10  by the driver. 
         [0063]    The shift lever  10  is installed near the driver&#39;s seat of a vehicle, and is manually operated and placed into an arbitrary shift range (parking range P, reverse range R, neutral range N, or drive range D). 
         [0064]    The manual valve  9  is configured as a so-called spool valve, and includes a valve body  9   a  having various feed ports and exhaust ports (not shown), and a spool  9   b  accommodated in the valve body  9   a  so as to be axially displaceable. The valve body  9   a  is fixed to the case of the automatic transmission  1 . 
         [0065]    The manual valve  9  displaces the spool  9   b  to one side or the other in the axial direction of the spool  9   b  to position the spool  9   b  at a predetermined position, thereby establishing the parking range P, the reverse range R, the neutral range N, or the drive range D as appropriate. 
         [0066]    Further, the above-described automatic transmission  1  is equipped with a range selector  20  and a parking lock device  30 . 
         [0067]    The range selector  20  changes the state of the manual valve  9  of the above-described hydraulic controller  6  in order to establish a gear position (such as the parking range P, the reverse range R, the neutral range N, or the drive range D) of the automatic transmission  1  corresponding to a shift range selected by the shift lever  10 , for example. As shown in  FIGS. 2 and 3 , the range selector  20  mainly includes a manual valve lever  21 , a shift control shaft  22 , an actuator  23 , and a latch lever  24 . 
         [0068]    The manual valve lever  21  may be titled in, for example, four steps in coordination with shift ranges (the parking range P, the reverse range R, the neutral range N, and the drive range D) selected by the shift lever  10 . The manual lever  21  axially displaces the spool  9   b  of the manual valve  9  in accordance with its tilt position. 
         [0069]    Corrugated grooves are provided on one end side  21   a  of the manual valve lever  21 . The corrugated grooves include the number of grooves (four grooves) corresponding to the four shift positions (the parking range P, the reverse range R, the neutral range N, and the drive range D) of the shift lever  10 . As shown in  FIGS. 3 and 4 , for example, marks “P, R, N, D” are written near the four grooves. 
         [0070]    The front end of the spool  9   b  of the manual valve  9  is joined to the other end side  21   b  of the manual valve lever  21 . The front end of a parking rod  33  of the parking lock device  30  abuts against a projection member  21   c.    
         [0071]    When the manual valve lever  21  is tilted as appropriate, the spool  9   b  is axially displaced to advance and retract, thereby establishing the parking range P, the reverse range R, the neutral range N, or the drive range D. 
         [0072]    The manual valve lever  21  is integrally joined to one end of the shift control shaft  22 . 
         [0073]    The shift control shaft  22  is rotatably supported on the case or the like of the automatic transmission  1 , and is rotationally driven in a suitable direction by the actuator  23 . 
         [0074]    Although not illustrated in detail, the actuator  23  includes, for example, a suitable motor and reduction gear mechanism (warm gear or the like). The transmission controller  8  controls the operation of the actuator  23 . 
         [0075]    The latch lever  24  maintains the four tilt positions of the manual valve lever  21  individually. The latch lever  24  has, at the other end of its main body made of a leaf spring or the like whose one end is fixed to the case of the automatic transmission  1 , a pin  24   a  that is engaged with one of the corrugated grooves in a sector arm  21   a  of the manual valve lever  21 . 
         [0076]    The transmission controller  8  is configured as a commonly known ECU (Electronic Control Unit). For example, the transmission controller  8  detects the position of the shift lever  10  by a range position sensor  11 , and drives the actuator  23  to control the hydraulic controller  6 , thereby establishing a suitable gear position, that is, a power transmission path in the transmission mechanism unit  5 . 
         [0077]    The range position sensor  11  detects the range position, parking range (P), the reverse range (R), the neutral range (N), the drive range (D), and the like in which the shift lever  10  is placed. 
         [0078]    Next, a gear change operation will be briefly described. For example, when the shift lever  10  is operated by the driver, the selected range is detected by the shift position sensor  11 , and on the basis of the detected range position, the transmission controller  8  rotationally drives the actuator  23  in a suitable direction, thus causing the shift control shaft  22  and the manual valve lever  21  integrated with the shift control shaft  22  to rotate, that is, to tilt by a predetermined angle. 
         [0079]    This tilting of the manual valve lever  21  causes the spool  9   b  of the manual valve  9  to slide, and the manual valve  9  of the hydraulic controller  6  is switched to a range selected from “P”, “R”, “N”, and “D”, so a suitable gear position is established within the transmission mechanism unit  5 . 
         [0080]    The manual valve lever  21  is set and held in position as the pin  24   a  of the latch lever  24  engages with an arbitrary groove in its sector arm  21   a.    
         [0081]    Next, the basic configuration of the parking lock device  30  according to an embodiment of the present invention will be described in detail with reference to  FIGS. 3 to 5 . 
         [0082]    The parking lock device  30  disables rotation of the output shaft  7  of the automatic transmission  1  when the driver selects the parking range P with the shift lever  10 . The parking lock device  30  basically includes a parking gear  31 , a parking lock pole  32 , and the parking rod  33 . 
         [0083]    The parking gear  31  is externally fixed to the output shaft  7 , and the parking lock pole  32  is arranged near the parking gear  31  so that the lock pole  32  is able to tilt on its one end side as an axis. Provided at a halfway position in the longitudinal direction of the parking lock pole  32  is a pawl  32   a  that is fit into or detached from between teeth of the parking gear  31 . 
         [0084]    The parking rod  33  may be displaced toward its front end or its rear end substantially in parallel with the output shaft  7  due to tilting movement of the manual valve lever  21  that serves as a tilting lever. 
         [0085]    The front end of the parking rod  33  is connected to the manual valve lever  21  via a booster mechanism  40  described later. A taper cone  34  for allowing the parking lock pole  32  to tilt is provided at the rear end of the parking rod  33 . 
         [0086]    It should be noted that reference numeral  35  denotes a coil spring for pressing the taper cone  34  to the parking gear  31  side, and reference numeral  36  denotes a snap ring that is latched and fixed onto the parking rod  33  and receives one end of the coil spring  35 . Reference numeral  37  denotes a guide for guiding the axial displacement of the parking rod  33 . 
         [0087]    The basic operation of the parking lock device  30  will be described. 
         [0088]    First, when the manual valve lever  21  is tilted into a position as shown in  FIG. 3 , the parking rod  33  is pushed toward the rear, and the large-diameter-side portion of the taper cone  34  pushes the parking lock pole  32  upwards so that its pawl  32   a  is inserted in between the teeth of the parking gear  31 . As a result, the output shaft  7  is non-rotatably locked in this state. 
         [0089]    On the other hand, when the manual valve lever  21  is tilted into a position as shown in  FIG. 4 , the parking rod  33  and the taper cone  34  are pulled toward the front, and the force with which the parking lock pole  32  is pushed up by the taper cone  34  is released. Accordingly, the parking lock pole  32  moves downwards, and its pawl  32   a  comes out from between the teeth of the parking gear  31 . As a result, the output shaft  7  is unlocked and able to rotate. 
         [0090]    Now, the portion to which the features of the present invention is applied to the parking lock device  30  described above will be described in detail with reference to FIGS.  3  to  15 . 
         [0091]    First, to unlock the parking lock device  30 , the manual valve lever  21  is tilted. In this regard, a contrivance is made to make the requisite driving force at this time as small as possible while preventing inadvertent unlocking when in a locked state. 
         [0092]    Specifically, the booster mechanism  40  is interposed between the front end of the parking rod  33  and the manual valve lever  21 . 
         [0093]    The booster mechanism  40  is a toggle joint type booster mechanism, and mainly includes a fixed link  41 , a movable link  42 , a connecting bar  43 , and a stopper  44 . 
         [0094]    The fixed link  41  and the movable link  42  are each made of a two-ply strip plate, and are arranged adjacent to each other at a predetermined spacing with respect to the thickness direction. Further, the fixed link  41  and the movable link  42  are connected together at pivots  45  and  46  with the other end of the fixed link  41  and one end of the movable link  42  overlapped with each other. The other end of the fixed link  41  is thus pivoted on one end of the movable link  42 . 
         [0095]    In the body portions of the links  41  and  42 , elongated guide holes  41   a  and  42   a  are respectively provided so as to penetrate the body portions in the thickness direction and extend in the longitudinal direction. The shape of the elongated guide holes  41   a  and  42   a  will be described later in detail. 
         [0096]    The connecting bar  43  has guide pins  43   a  and  43   b  integrally provided at each end of the connecting bar  43  of a single stripe, respecitively. The guide pins  43   a  and  43   b  of the connecting bar  43  are respectively inserted in the elongated guide holes  41   a  and  42   a  of the links  41  and  42  with some play. The connecting bar  43  coordinates the movements of the links  41  and  42  to each other, thereby making the movements of the links  41  and  42  smooth. 
         [0097]    The stopper  44  plays the following role. That is, in a state with the links  41  and  42  aligned so as to be continuous in a straight line, that is, when the parking lock device  30  is brought into a locked state, the stopper  44  prevents the links  41  and  42  from being inadvertently bent to the opposite to the unlocking side under an excessive momentum while keeping the links  41  and  42  arranged in a straight line. As such, the stopper  44  functions as a support plate or the like fixed to the case or the like of the automatic transmission  1 . 
         [0098]    The fixed link  41  is integrally fixed to the manual valve lever  21 . Specifically, one end of the fixed link  41  is attached to the shift control shaft  22  serving as the tilting axis of the manual valve lever  21 , and the pivot  45  on the other end side of the fixed link  41 , that is, on the side connected to the movable link  42 , is attached to the projection member  21   c  of the manual valve lever  21 . 
         [0099]    The front end of the parking rod  33  is attached to the other end of the movable link  42  via a pivot  47 . The other end of the movable link  42  is thus pivoted on the front end of the parking rod. 
         [0100]    Prior to assembly, the above-described pivots  45 ,  46 , and  47  are each formed as, for example, a pin having a flange provided at one end. After the pin is passed through through-holes of the members to be connected together while leaving a predetermined gap as shown in  FIG. 9 , the distal end of the pin is caulked to form a flange, thereby preventing detachment. Further, the distal end of the shift control shaft  22 , and both ends of the guide pins  43   a  and  43   b  of the connecting bar  43  are also each assembled and caulked to form a flange in the same manner as mentioned above. It should be noted, however, the connecting structures may be fastened using a bolt and nut arrangement. 
         [0101]    The position of the booster mechanism  40  may be changed as shown in  FIG. 3 , where the fixed link  41  and the movable link  42  are aligned in a straight line, or as shown in  FIG. 4 , where they are bent to form a V-shape, as the fixed link  41  integrally attached to the manual valve lever  21  changes its position following the tilting movement of the manual valve lever  21 . 
         [0102]    The shape and placement of the elongated guide holes  41   a  and  42   a  of the links  41  and  42  are designed to prevent the links  41  and  42  from easily bending in a V-shape when an axial load is input from the parking rod  33  while the links  41  and  42  are aligned in a straight line. These arrangements will be described below with reference to  FIG. 7 . 
         [0103]    First, the elongated guide holes  41   a  and  42   a  of the links  41  and  42  extend in a direction orthogonal to the longitudinal direction at the ends positioned close to each other. The elongated guide holes  41   a  and  42   a  are thus formed in a substantially L-shape. 
         [0104]    Further, the centerline N of each of the elongated linear portions of the elongated guide holes  41   a  and  42   a  is arranged so as to be offset toward the unlocking side (side opposite to the stopper  44 ) by a predetermined amount a, with respect to each of the centerlines L of the fixed link  41  and movable link  42  that extend continuously in a single line in a state with the fixed link  41  and the movable link  42  aligned in a straight line with the parking rod  33 . 
         [0105]    Further, the extended ends of the elongated guide holes  41   a  and  42   a  are extended to a position where, when the guide pins  43   a  and  43   b  are arranged at the extended ends, the centers P of the guide pins  43   a  and  43   b  are offset to the side opposite to the unlocking side (stopper  44  side) with respect to the centerline L. 
         [0106]    With the elongated holes  41   a  and  42   a  formed as described above, if, as shown in  FIG. 11 , an axial load Fa is applied from the parking rod  33  toward the movable link  42 , moments M 1  and M 2  act on the links  41  and  42 , which cause pivoted portions ( 45  and  46 ) of the links  41  and  42  to move toward the stopper  44 , respectively. It should be noted that the axial load Fa is generated as a reaction force from the parking rod  33  when a ratcheting operation is performed to move the shift lever  10  from the drive range D into the parking range P, for example. 
         [0107]    Due to this operating principle, it is possible to keep the links  41  and  42  aligned in a straight line while preventing the links  41  and  42  from easily bending to the unlocking side. 
         [0108]    Next, the operation of the parking lock device  30  when the above-described booster mechanism  40  is provided will be described, with reference to  FIGS. 3 ,  4 , and  10 . 
         [0109]    First, for example, when the shift lever  10  is moved from the drive range D to the parking range P, as shown in  FIG. 3 , the shift control shaft  22  and the manual valve lever  21  are tilted in the arrow A 1  direction (counterclockwise direction) in the drawing by the actuator  23  as described above. The pin  24   a  of the latch lever  24  is thus engaged with the parking position groove of the manual valve lever  21 . 
         [0110]    Simultaneously, the fixed link  41  is tilted in the same direction integrally with the tilting movement of the manual valve lever  21 , so the pivots  45  and  46  that connects the fixed link  41  with the movable link  42  are pushed in the arrow B 1  direction (upwards) in the drawing. This causes the fixed link  41  and the movable link  42  to be positioned so that they lie continuous to the parking rod  33  in a straight line as shown in  FIGS. 3 and 10A . 
         [0111]    When the fixed link  41  and the movable link  42  lie continuous to the parking rod  33  in a straight line, the distance between the one end of the fixed link  41  to the other end of the movable link  42  is at a maximum, which means that the parking rod  33  is pushed to the maximum in the arrow C 1  direction (rearwards) in the drawing. The parking lock pole  32  is thus pushed up by the taper cone  34 , bringing the parking gear  31  into a non-rotatable locked state. 
         [0112]    On the other hand, when the shift lever  10  is moved from the parking range P to the reverse range R, as shown in  FIG. 4 , the shift control shaft  22  and the manual valve lever  21  are tilted in the arrow A 2  direction (clockwise direction) in the drawing by the actuator  23 . The pin  24   a  of the latch lever  24  is thus engaged with the reverse position groove of the manual valve lever  21 . 
         [0113]    Simultaneously, the fixed link  41  is tilted in the same direction together with the tilting movement of the manual valve lever  21 , so the pivots  45  and  46  that connect the fixed link  41  with the movable link  42  are pulled in the arrow B 2  direction (downwards) in the drawing. This causes the fixed link and the movable link  42  to be positioned so that they are bent in a V-shape as shown in  FIGS. 4 and 10B . 
         [0114]    In this state, the distance between the one end of the fixed link  41  to the other end of the movable link  42  decreases, which means that the parking rod  33  is pulled by a predetermined distance in the arrow C 2  direction (forwards) in the drawing. The parking lock pole  32  is thus lowered by the taper cone  34 , bringing the parking gear  31  into a rotatable unlocked state. 
         [0115]    In addition, when the shift lever  10  shifts into the neutral range N and when the shift lever  10  shifts into the drive range D, as shown in  FIGS. 10C and 10D , respectively, the angle formed between the fixed link  41  and the movable link  42 , which are bent in a V-shape, progressively decreases, and the stroke that pulls the parking rod  33  forwards increases. 
         [0116]    Next, in connection with the operation of the parking lock device  30  as described above, the advantage of providing the booster mechanism  40  will be described with reference to  FIGS. 12 and 13 . 
         [0117]    First, the relationship between the driving force of the manual valve lever  21  and the pressing force on the parking rod  33  in the above-described locked state will be described with reference to  FIG. 12 . 
         [0118]    When the manual valve lever  21  is tilted counterclockwise in the drawing, a tangential force F 0  (equivalent to a driving force) acts on the pivots  45  and  46  that connect the fixed link  41  with the movable link  42 . The tangential force F 0  is increased by the boosting action of the booster mechanism  40 , thereby generating a pressing force F P  at the pivot  47  that connects the movable link  42  to the parking rod  33 . Here, F 1  denotes a component force of the tangential force F 0 . 
         [0119]    In this way, when locking the parking lock device  30 , a very large pressing force F 0  is obtained with respect to the parking rod  33  by means of a small tangential force F 0 , that is, by means of a small driving force of the manual valve lever  21 . 
         [0120]    Further, the relationship between the driving force of the manual valve lever  21  and the pressing force on the parking rod  33  in the above-described unlocked state will be described with reference to  FIG. 13 . 
         [0121]    When the manual valve lever  21  is tilted clockwise in the drawing, a tangential force −F 0  (equivalent to a driving force) directed opposite to that mentioned above acts on the pivots  45  and  46  that connect the fixed link  41  with the movable link  42 . The tangential force −F 0  is increased by the boosting action of the booster mechanism  40 , which generates a pulling force −F P  at the pivot  47  that connects the movable link  42  to the parking rod  33 . It should be noted that −F 1  denotes a component force of the tangential force −F 0 . 
         [0122]    In this way, when unlocking the parking lock device  30 , a very large pulling force −F P  may be obtained with respect to the parking rod  33  by means of a small tangential force −F 0 , that is, by means of a small driving force of the manual valve lever  21 . 
         [0123]    In the boosting mechanism  40 , when unlocking from a locked state, in which the fixed link  41  and the movable link  42  are aligned in a straight line as shown in  FIG. 10A , by bending the links  41  and  42  in a V-shape as shown in  FIGS. 10B to 10D , and when locking from an unlocked state conversely, the displacement ΔX of the pivots  45  and  46  and the displacement ΔY of the pivot  47  can be determined by the following equation. 
         [0000]      Δ X =( Y/ 2)sin θ and Δ Y=Y− 2×[( Y/ 2)cos θ]= Y (1−cos θ) 
         [0124]    In the above equation, Y is the distance from the center of the shift control shaft  22  to the center of the pivot  47  in the booster mechanism  40 , and θ is the angle formed by the links  41  and  42  when bent in a V-shape. 
         [0125]    For reference, the relationship between ΔX and ΔY when Y is set as, for example, “10” is indicated by the graph of  FIG. 14 . Also, on the basis of the data shown in  FIG. 14 , the relationship between the angle θ of bend, and the amplification factor of the pressing force or pulling force on the parking rod  33  is shown in the graph of  FIG. 15 . 
         [0126]    As is apparent from the graphs of  FIGS. 14 and 15 , by using the booster mechanism  40  described above, the force with which the pivots  45  and  46  are pulled in the ΔX direction with the manual valve lever  21  in order to unlock the parking lock device  30  is amplified by a factor of several tens to several hundreds as the force pulling the parking rod  33  in the ΔY direction in early stages. 
         [0127]    In addition, for reference, the case where the vehicle is stopped on a slope will be described. 
         [0128]    As described above with reference to the related art, as a general tendency, when a vehicle is stopped on a downhill slope, the same forward rotation torque as that at the time of forward movement is input to the output shaft  7  from the driving wheels, whereas when the vehicle is stopped on an uphill slope, a reverse rotation torque is input to the output shaft  7  from the driving wheels. 
         [0129]    In the parking lock device  30  illustrated in this embodiment, if, for example, the above-mentioned forward rotation torque is input to the output shaft  7  while the parking lock device  30  locked, the pawl  32   a  of the parking lock pole  32  is prone to disengage from the parking gear  31 . Accordingly, as shown in  FIG. 11 , the axial load Fa directed to the unlocking side acts on the parking rod  33 . 
         [0130]    At this time, the moments M 1  and M 2  respectively act on the links  41  and  42  of the booster mechanism  40  to move the pivoted portions ( 45  and  46 ) of the links  41  and  42  to the stopper  44  side, thereby keeping the links  41  and  42  aligned in a straight line as it is and preventing the links  41  and  42  from bending to the unlocking side. The parking lock device  30  remains locked. 
         [0131]    On the other hand, if the above-mentioned reverse rotation torque is input to the output shaft  7  while the parking lock device  30  is locked, the pawl  32   a  of the parking lock pole  32  engages the parking gear  31 . 
         [0132]    Accordingly, a relatively large driving force of the manual valve lever  21  is required to unlock the parking lock device  30 . At this time, according to this embodiment, when shifting to an unlocked state, the driving force of the manual valve lever  21  is increased by the boosting action of the booster mechanism  40  as appropriate to act as a pulling force on the parking rod  33 , so that the parking rod  33  is pulled strongly even when the driving force of the manual valve lever  21  is small. The pawl  32   a  of the parking lock pole  32  may be thus disengaged from the parking gear  31 . 
         [0133]    As described above, in the parking lock device  30  according to the embodiment to which the features of the present invention are applied, the manual valve lever  21  is connected to the parking rod  33  via the booster mechanism  40 . It is thus possible to increase the pressing force or pulling force on the parking rod  33  even when the driving force applied by the manual valve lever  21  when shifting to a locked state or unlocked state is small. 
         [0134]    Accordingly, the parking lock device  30  can be easily and reliably locked or unlocked with a relatively small force. 
         [0135]    Therefore, the driving force to be applied by the manual valve lever  21  when locking the parking lock device or unlocking the parking lock device from an unlocked state can be made small. 
         [0136]    Furthermore, in this embodiment, the booster mechanism  40  adopts a relatively simple configuration of a toggle joint type using the plurality of links  41  and  42  and the like, which proves advantageous in reducing equipment cost as compared with a case where power assist means such as an expensive motor is equipped as described above with reference to the related art. 
         [0137]    It is to be understood that the present invention is not limited to the above-described embodiment, but covers all such modifications and applications as fall within the scope of the claims and their equivalents. Hereinafter, other embodiments of the present invention will be exemplified. 
         [0138]    (1) While the above-mentioned embodiment is directed to the parking lock device  30  equipped in a front-engine, rear-drive (FR) automatic transmission, this can also be a parking lock device equipped in a front-engine, front-drive (FF) automatic transmission. 
         [0139]    (2) While in the above-mentioned embodiment the manual valve lever  21  of the range selector  20  is used as the drive lever for driving the parking rod  33  of the parking rod device  30 , this may be another independent drive lever. 
         [0140]    (3) While the above-mentioned embodiment is directed to the case where the manual valve lever  21  of the range selector  20  is driven by the manual actuator  23 , this may be of a type in which, as in the related art, the manual valve lever  21  and the shift lever  10  are mechanically connected together by a power transmission member such as a shift rod or a shift cable, and the manual valve lever  21  is driven by the operating force on the shift lever  10  applied by the driver. 
         [0141]    (4) The booster mechanism  40  described in the above-mentioned embodiment can also adopt a configuration in which the fixed link  41  is omitted, and the fixed link  41  is substituted for by the manual valve lever  21  itself 
         [0142]    In that case, it is necessary to provide the elongated guide hole  41   a  of the fixed link  41  in the manual valve lever  21 . 
         [0143]    (5) The booster mechanism  40  described in the above-mentioned embodiment can also adopt a configuration in which the connecting bar  43  is omitted.