Patent Publication Number: US-2010108460-A1

Title: Parking mechanism of automatic transmission

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a parking mechanism of an automatic transmission. 
     2. Description of the Related Art 
     As described in Japanese Utility Model Application Publication No. 6-49849 (JP-U-6-49849), an automatic transmission is provided with a parking mechanism for locking a rotating shaft of the automatic transmission to inhibit rotation of the rotating shaft when the vehicle driver selects a parking range with a shift lever, for example. 
     The parking mechanism includes a parking gear that is secured to the rotating shaft, a parking lock pole for switching the parking gear between a locked state, in which the parking gear cannot rotate, and an unlocked state, in which the parking gear can rotate, and a cam for swinging the parking lock pole that is made of a generally elongated rectangular plate about a fulcrum at an end thereof. 
     The parking lock pole is swingably supported at one end and has a pawl that engages with notches between the teeth of the parking gear at the other end. 
     The parking lock pole is urged in the unlocking direction by a suitable reverse spring. The cam is disposed in contact with the other end of the parking lock pole, and the parking lock pole is moved toward or away from the parking gear by moving the cam. 
     The parking mechanism operates as follows. When the vehicle driver selects the parking range with the shift lever, the apex of the cam is moved to a position where it presses the parking lock pole. Then, the parking lock pole is moved toward the parking gear against the elasticity of the reverse spring until the parking lock pole is engaged with one of the notches of the parking gear to establish a locked state in which the parking gear and the rotating shaft cannot rotate. 
     When a range other than the parking range is selected, the apex of the cam is moved to a position where it cannot contact the parking lock pole. Then, the parking lock pole is moved away from the parking gear by the elastic restoring force of the reverse spring until the parking lock pole is disengaged from the parking gear to establish an unlocked state in which the parking gear and the rotating shaft can rotate. 
     A parking mechanism as described above is shown in  FIG. 8 . In such a parking mechanism, when the parking lock pole  22  is moved from a position indicated by broken lines to a position indicated by solid lines in  FIG. 8 , the widthwise center of the distal end of the pawl  22   a  is displaced from point X 1  to point X 2 . That is, the movement locus R that connects the displacement points X 1  and X 2  is an arc having the radius of curvature r, which is the distance from the swinging fulcrum O at the one end of the parking lock pole  22  to the widthwise center of the distal end of the pawl  22   a.    
     It is necessary to reduce the space occupied by the parking lock pole  22  for a compact design. However, when the length of the parking lock pole  22  is decreased, the radius of curvature r of the arc as the movement locus R of the pawl  22   a  also decreases. 
     When the radius of curvature r decreases, the divergence of the widthwise center of the distal end of the pawl  22   a  of the parking lock pole  22  from a radial line L extending from the axis of rotation P of the parking gear  21  through the circumferential center of the corresponding notch  21   b  and the inclination of the pawl  22   a  with respect to the corresponding notch  21   b  increase as the displacement of the pawl  22   a  in its width direction increases. 
     Then, the pawl  22   a  is likely to interfere with the teeth  21   a  of the parking gear  21  when engaging the pawl  22   a  of the parking lock pole  22  with one of the notches  21   b  of the parking gear  21  and the process of disengaging the pawl  22   a  from the corresponding notch  21   b,  and therefore the pawl  22   a  and the teeth  21   a  may be subjected to fatigue damage relatively early. 
     Although the pawl is tapered toward its distal end and the notches are increased in width toward their open ends in JP-U-6-49849, the possibility of the interference as described above still remains. 
     If the length of the parking lock pole is maximized to increase the radius of curvature of the arc as the movement locus of the pawl as much as possible, the divergence of the widthwise center of the distal end of the pawl  22   a  may be decreased and the interference as described above is reduced or avoided. However, this is not practical because the space occupied by the parking lock pole increases against the intention of compact design. 
     SUMMARY OF THE INVENTION 
     The present invention provides a parking mechanism of a vehicle automatic transmission in which the interference between a pawl (protrusion) on a parking lock pole and notches of a parking gear when engaging or disengaging the pawl with the notches is reduced to improve the service life of both the pawl and the parking gear. 
     A first aspect of the present invention relates to a parking mechanism of an automatic transmission which includes: a parking gear, that is provided on a rotating shaft in the automatic transmission; a parking lock pole having a protrusion that removably engages with the parking gear, and switch means that moves the parking lock pole toward the parking gear until the protrusion engages the parking gear to switch the parking lock pole to a locked state and moves the parking lock pole away from the parking gear until the protrusion is disengaged from the parking gear to switch the parking lock pole to an unlocked state. The parking mechanism of an automatic transmission includes a fulcrum cam that moves the fulcrum of the parking lock pole. 
     In the parking mechanism of an automatic transmission, when the switch means switches the parking lock pole between the locked and unlocked states, the fulcrum cam moves the fulcrum of the parking lock pole such that the widthwise center of the protrusion of the parking lock pole moves along a radial line that extends from the axis of rotation of the parking gear to the circumferential center of a notch of the parking gear. 
     According to this configuration, when the parking lock pole is moved by the switch means, the widthwise center of the protrusion of the parking lock pole is displaced substantially linearly along the radial line. 
     Therefore, the divergence of the widthwise center of the protrusion from the radial line may be decreased, and the angle of the protrusion relative to the corresponding notch does not change significantly. As a result, the protrusion is less likely to interfere with the teeth of the parking gear in the process of engaging the protrusion of the parking lock pole with the notches of the parking gear and in the process of disengaging the protrusion from the corresponding notch, and the protrusion can be therefore engaged and disengaged from the notches of the parking gear relatively easily and smoothly. Thus, it is possible to reduce fatigue damage to the protrusion and the notches. 
     In the above aspect, the fulcrum cam may be received in a circular hole formed through one end of the parking lock pole in which the fulcrum cam smoothly rotates in the circular hole, have a rotating shaft at an eccentric position, and be of a circular shape as seen in a plan view. 
     This configuration has a defining feature that the fulcrum cam is located at the one end of the parking lock pole where it is supported and the fulcrum of the parking lock pole may be moved by rotating the rotating shaft of the fulcrum cam. With this defining feature, the structure is simplified and the occupied space may be reduced. 
     In the above aspect, the parking mechanism of an automatic transmission may further include detection means for detecting a shift range selected in the automatic transmission, and control means for controlling the switch means in response to the detected shift range by the detection means. When the detection means detects that a parking range is selected, the control means controls the switch means to switch the parking lock pole to the locked state. 
     According to this configuration, because shift operation means such as shift lever and the switch means may be connected electrically instead of mechanically, the structure of the parking mechanism can be simplified. 
     In the above aspect, the fulcrum cam may be provided at the one end of the parking lock pole, and the parking lock pole may be made of a generally elongated rectangular plate and have an other end which is moved toward and away from the parking gear. 
     This configuration has a defining feature that the fulcrum cam is provided at the one end of the parking lock pole where it is supported, and the other end of the parking lock pole is moved toward and away from the parking gear. With this defining feature, the structure may be simplified and the occupied space can be reduced. 
     In the above aspect, the switch means may include a switching cam that applies a biasing force to the parking lock pole in a direction toward the parking gear, and an elastic member that pulls the parking lock pole away from the parking gear may be provided. The switching cam has a convex portion, which is formed on a specific part of the outer periphery of the switching cam that applies a pressure to the parking lock pole so that the parking lock pole is moved toward the parking gear against the biasing force of the elastic member. The switching cam also has a base circular portion that is formed on a specific part of the outer periphery of the switching cam that does not apply a pressure to the parking lock pole so that the parking lock pole may be moved away from the parking gear by the biasing force of the elastic member. 
     In the above aspect, the switch means may allow the parking lock pole to be switched to an unlocked state by the elastic member when the detected shift range is a shift range other than the parking range. The switch means switches the parking lock pole to a locked state by the switching cam when the detected shift range is the parking range. 
     In the above aspect, the fulcrum cam may be moved by the switch means or may be moved by a drive means other than the switch means. 
     In the above aspect, the fulcrum may be located at the center of the fulcrum cam as seen in a plan view, and the fulcrum may be displaced toward the other end when the parking lock pole is switched to the locked state, and displaced toward the one end when the parking lock pole is switched to the unlocked state. 
     In the above aspect, the fulcrum cam may be rotated in one direction or both directions to switch the parking lock pole between the locked state and the unlocked state. 
     A second aspect of the present invention relates to a parking mechanism of an automatic transmission which includes: a parking gear, that is provided on a rotating shaft in the automatic transmission; a parking lock pole having a protrusion that removably engages with the parking gear, and switch means that moves the parking lock pole toward the parking gear until the protrusion engages the parking gear to switch the parking lock pole to a locked state and moves the parking lock pole away from the parking gear until the protrusion is disengaged from the parking gear to switch the parking lock pole to an unlocked state. The parking mechanism of an automatic transmission includes guide means that supports the parking lock pole such that the parking lock pole may be moved linearly toward and away from the axis of rotation of the parking gear without changing the angle with respect to the parking gear. 
     According to this configuration, when the parking lock pole is moved by the switch means, the widthwise center of the protrusion of the protrusion is displaced substantially linearly along the radial line. 
     Therefore, the divergence of the widthwise center of the protrusion of the parking lock pole from the radial line can be decreased, and the angle of the protrusion relative to the corresponding notch does not change significantly. As a result, the protrusion is less likely to interfere with the teeth of the parking gear in the process of engaging the protrusion of the parking lock pole with the notches of the parking gear and in the process of disengaging the protrusion from the corresponding notch, and the protrusion can be therefore engaged with and disengaged from the notches relatively easily and smoothly. Thus, it is possible to minimize fatigue damage to the protrusion and the notches. 
     In the above aspect, the parking mechanism of an automatic transmission may further include detection means for detecting a shift range selected in the automatic transmission, and control means for controlling the switch means in response to the detected shift range by the detection means. When the detection means detects that a parking range is selected, the control means controls the switch means to switch the parking lock pole to the locked state. 
     According to this configuration, because shift operation means such as a shift lever and the switch means may be connected electrically instead of mechanically, the structure of the parking mechanism may be simplified. 
     In the above aspect, the switch means may include a switching cam that applies a biasing force to the parking lock pole in a direction toward the parking gear, and an elastic member that pulls the parking lock pole away from the parking gear may be provided. Each switching cam has a convex portion formed on a specific part of the outer periphery of the switching cam that applies a pressure to the parking lock pole so that the parking lock pole can be moved toward the parking gear against the biasing force of the elastic member, and a base circular portion formed on a specific part of the outer periphery of the switching cam that does not apply a pressure to the parking lock pole so that the parking lock pole can be moved away from the parking gear  21  by the biasing force of the elastic member. 
     In the above aspect, the switch means may allow the parking lock pole to be switched to an unlocked state by the elastic member when the detected shift range is a shift range other than the parking range. The switch means may allow the parking lock pole to be switched to a locked state by the switching cam when t detected he shift range is the parking range. 
     In the above aspect, the guide means may include a guide pin and a guide hole for guiding the parking lock pole to slide in parallel to the axis of rotation of the parking gear. 
     In the above aspect, the guide pin may have a larger dimension in the sliding direction of the parking gear. 
     In the above aspect, the parking lock pole may be supported generally perpendicular to a line extending from the axis of rotation of the parking gear to the longitudinal center of the parking lock pole, the switching cam may be disposed in contact with the longitudinal center of the parking lock pole, and the elastic member is provided at each end of the parking lock pole. 
     In the above aspect, the parking lock pole may be supported generally perpendicular to a line extending from the axis of rotation of the parking gear to the longitudinal center of the parking lock pole, the switching cam may be disposed in contact with each end of the parking lock pole, and the elastic member may be provided at the longitudinal center of the parking lock pole. 
     In the above aspect, the parking lock pole may be supported generally along a line extending from the axis of rotation of the parking gear to the longitudinal center of the parking lock pole, the switching cam may be disposed in contact with an end of the parking lock pole opposite the parking gear, and the elastic member may be provided on transverse opposite sides of the parking lock pole at a longitudinal intermediate portion of the parking lock pole. 
     According to the above aspects, interference between the protrusion of the parking lock pole and notches of the parking gear when engaging or disengaging the protrusion with the notches of the parking gear may be minimized without increasing the occupied space. Therefore, because fatigue damage to the protrusion and the notches is minimized, the service life of the parking gear, the parking lock pole and so on is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic view of an automatic transmission to which a parking mechanism according to the present invention is applied. 
         FIG. 2  is a perspective view illustrating the parking mechanism shown in  FIG. 1 . 
         FIG. 3  is a front view of the parking mechanism shown in  FIG. 2 , which is in an unlocked state. 
         FIG. 4  is a front view of the parking mechanism shown in  FIG. 2 , which is in a locked state. 
         FIG. 5  is a side view of the parking mechanism shown in  FIG. 3 , which is in an unlocked state. 
         FIG. 6  is a side view of the parking mechanism shown in  FIG. 4 , which is in a locked state. 
         FIG. 7  is a view illustrating the operation of the parking mechanism according to the present invention. 
         FIG. 8  is a view illustrating the operation of a related art parking mechanism. 
         FIG. 9 , which corresponds to  FIG. 3 , shows a second embodiment of the parking mechanism according to the present invention. 
         FIG. 10 , which corresponds to  FIG. 4 , shows the parking mechanism shown in  FIG. 9 . 
         FIG. 11  is a view illustrating the operation of the parking mechanism shown in  FIG. 9 . 
         FIG. 12 , which corresponds to  FIG. 11 , shows another embodiment of the parking mechanism according to a first modification of the second embodiment of the present invention. 
         FIG. 13 , which corresponds to  FIG. 11 , shows another embodiment of the parking mechanism according to a second modification of the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention are described below with reference to 
       FIG. 1  through  FIG. 13   
     First Embodiment  
     Referring to  FIG. 1 , an outline of one example of an automatic transmission to which a parking mechanism of the present invention is applicable is described. 
       FIG. 1  shows an engine  1 , and an automatic transmission  2 . In this embodiment, the automatic transmission  2  is mounted in a front engine-rear drive (FR) vehicle. 
     In the FR vehicle, the automatic transmission  2  changes the speed of the rotational driving force generated by the engine  1  and transmits the driving force to the right and left rear wheels via a propeller shaft and a differential (which are not shown). 
     The automatic transmission  2  essentially includes an input shaft  5 , a torque converter  6 , a speed change mechanism  7 , an output shaft  8 , and other components. 
     The automatic transmission  2  operates as follows. When the rotation of the crankshaft (not shown) of the engine  1  is transmitted to the input shaft  5  via the torque converter  6 , the rotational driving force is converted at a desired gear ratio by the speed change mechanism  7  and output from the output shaft  8 . 
     Although not specifically shown, the speed change mechanism  7  may be constituted of a multi-speed planetary mechanism, a gear mechanism having a plurality of gears, or a continuously variable mechanism. 
     The speed change operation of the automatic transmission  2  is controlled by a transmission control device  3 . The transmission control device  3  may be a generally known Electronic Control Unit (ECU). 
     When a shift lever  11  which is located, for example in the vicinity of the driver&#39;s seat of the vehicle, is operated by the driver to select a desired range (parking range, reverse range, neutral range, drive range etc.), the selected range is detected by a shift position sensor  12 . Based on the detected range position, the transmission control device  3  controls a hydraulic control device via a given drive system (not shown) to establish the desired gear drive. 
     The shift position sensor  12  is an embodiment of the detection means of the present invention, and the transmission control device  3  is an embodiment of the control means of the present invention. 
     The automatic transmission  2  is provided with a parking mechanism  20 . 
     The parking mechanism  20  inhibits rotation of the output shaft  8  of the automatic transmission  2  when the parking range is selected, for example by the driver, and essentially includes a parking gear  21 , a parking lock pole  22 , a reverse spring  23 , a switching cam (not shown), a fulcrum cam  24 , and an electric actuator  25 , as shown in  FIG. 2  to  FIG. 6 . The arrangement and shapes of these components are suitably selected and not limited to those described in this embodiment. 
     The parking gear  21  is fixed to the exterior of the output shaft  8 , and has teeth  21   a  on its outer periphery. 
     The parking lock pole  22  is made of a generally elongated rectangular plate and is supported at one end in a cantilever fashion so that the other end may be displaced toward and away from the parking gear  21 . At the other end of the parking lock pole  22   a  pawl  22   a  is formed that removably engages with notches  21   b  between the teeth  21   a  of the parking gear  21 . 
     The pawl  22   a  is tapered toward its distal end, and the notches  21   b  gradually increase in width toward their open ends. The pawl  22   a  may serve as the protrusion of the present invention. 
     The reverse spring  23  biases the parking lock pole  22  in a direction away from the parking gear  21 , and may be any type of spring such as coil spring or plate spring. 
     The switching cam switches the parking lock pole  22  between an unlocked state, in which the parking gear  21  and the output shaft  8  are allowed to rotate, and a locked state, in which rotation of the parking gear  21  and the output shaft  8  are prohibited. The mechanism for switching the parking lock pole  22  between the unlocked and locked state is not limited to a cam. For example, a piston that is driven by hydraulic pressure or a structure in which the parking lock pole  22  is directly driven by the shift lever  11  may be employed. 
     The switching cam may serve as the switch means of the present invention. 
     The fulcrum cam  24 , which moves the fulcrum of the parking lock pole  22 , is of a circular shape and has a rotating shaft  24   a  at a position offset from its center. 
     The fulcrum cam  24  is slidably received in a circular hole  22   b  formed through the one end of the parking lock pole  22  in the thickness direction. To enable the fulcrum cam  24  to be slidably received in the hole  22   b,  at least one of the outer peripheral surface of the fulcrum cam  24  and the inner peripheral surface of and the hole  22   b  may be coated with a suitable lubricating film or that at least one of the fulcrum cam  24  and the parking lock pole  22  be made of a material having lubricity. The fulcrum cam  24  and the rotating shaft  24   a  may be formed integrally with each other or formed separately and joined together. 
     The electric actuator  25  is a motor that rotates the fulcrum cam  24 . The electric actuator  25  has an output shaft, which may be connected directly to the rotating shaft  24   a  of the fulcrum cam  24  or connected to the fulcrum cam  24  via a suitable reducer mechanism (worm gear or the like). 
     An actuator serving as a power source for driving the switching cam may be provided in addition to the electric actuator  25  for rotating the fulcrum cam  24 , or the electric actuator  25  may be used to drive both the switching cam and the fulcrum cam  24 . For example, the switching cam and the rotating shaft of the fulcrum cam  24  may be operatively connected by a gear or chain. 
     The fulcrum cam  24  is an embodiment of the fulcrum cam of the present invention. 
     Referring now to  FIG. 3  to  FIG. 7 , the operation of the parking mechanism  20  is described below. 
     When the parking range is selected, the shift position sensor  12  detects the range position and outputs a signal that indicates the detected range position to the transmission control device  3 . 
     The transmission control device  3  controls the switching cam such that the parking lock pole  22  in the unlocked state as shown in  FIG. 3  and  FIG. 5  is moved toward the parking gear  21  until the pawl  22   a  engages one of the notches  21   b  of the parking gear  21  as shown in  FIG. 4  and  FIG. 6  to bring the parking lock pole  22  into the locked state to inhibit rotation of the parking gear  21  and the output shaft  8 . 
     At this time, the transmission control device  3  simultaneously controls the electric actuator  25  to rotate the fulcrum cam  24  in the clockwise direction, so that the fulcrum of the parking lock pole  22  is moved from the position shown in  FIG. 3  to the position shown in  FIG. 4 . That is, the fulcrum is moved, as shown in  FIG. 3  and  FIG. 4 , generally to the lower left (from its rightmost position to its lowermost position) as shown in the drawings. 
     Referring to  FIG. 7 , the movement of the parking lock pole  22  is described in detail. 
     Because the reverse spring  23  pulls the other end of the parking lock pole  22  away from the parking gear  21 , when the fulcrum cam  24  rotates about the central axis O of the rotating shaft  24   a,  the fulcrum cam  24  slides in the hole  22   b  of the parking lock pole  22 . 
     Thus, the apex of the fulcrum cam  24  (the point on the outer peripheral surface of the cam farthest away from the rotating shaft  24   a  is defined as the apex) is displaced from point Q 1  through point Q 2  to point Q 3  as shown in  FIG. 7 . Because of the displacement of the apex of the fulcrum cam  24 , the parking lock pole  22  does not take a rotary motion as in the related art shown in  FIG. 8  but is gradually moved toward the parking gear  21  from the position shown by broken lines in  FIG. 7  through the position shown by double-dot dash lines in  FIG. 7  to the position shown by solid lines in  FIG. 7 . 
     Because of the movement of the parking lock pole  22 , the widthwise center of the distal end of the pawl  22   a  is displaced substantially linearly along a radial line L extending from the axis of rotation P of the parking gear  21  through the circumferential center of a corresponding one of the notches  21   b  from point X 1  through point X 2  to point X 3  as shown in  FIG. 7 . In other words, the widthwise center of the distal end of the pawl  22   a  is displaced generally linearly toward the axis of rotation P of the parking gear  21 . 
     Therefore, the divergence of the widthwise center of the distal end of the pawl  22   a  from the radial line L is decreased, and the angle of the pawl  22   a  relative to the corresponding notch  21   b  does not change significantly. As a result, the pawl  22   a  of the parking lock pole  22  is less likely to interfere with the teeth  21   a  of the parking gear  21 , and the pawl  22   a  may be engaged with the corresponding notch  21   b  relatively easily and smoothly. 
     When a range other than the parking range is selected, the shift position sensor  12  detects the range position and outputs a signal that indicates the detected range position to the transmission control device  3 . Then, in contrast to the above, the electric actuator  25  rotates the rotating shaft  24   a  of the fulcrum cam  24  in the counterclockwise direction as shown in  FIG. 4 . 
     When the fulcrum cam  24  is rotated in the counterclockwise direction, the fulcrum of the parking lock pole  22  is moved from the position shown in  FIG. 4  to the position shown in  FIG. 3 . That is, the fulcrum of the parking lock pole is moved generally to the upper right (from its lowermost position to its rightmost position) as shown in the drawings. 
     At this time, the transmission control device  3  simultaneously controls the switching cam. Therefore, because the pressure of the switching cam on the parking lock pole  22  is gradually decreased, the parking lock pole  22  is pulled away from the parking gear  21  by the restoring force of the reverse spring  23  until the pawl  22   a  is disengaged from the corresponding notch  21   b  of the parking gear  21 . As a result, the parking lock pole  22  is brought into the unlocked state. 
     Referring now to  FIG. 7 , the movement of the parking lock pole  22  is described in detail. 
     The apex of the fulcrum cam  24  (the point on the outer peripheral surface of the cam farthest away from the rotating shaft  24   a  is defined as the apex) is displaced from point Q 3  through point Q 2  to point Q 1  as shown in  FIG. 7 . Because of the displacement of the apex of the fulcrum cam  24 , the parking lock pole  22  does not take a rotary motion as in the related art shown in  FIG. 8  but is gradually moved toward the parking gear  21  from the position shown by solid lines in  FIG. 7  through the position shown by double-dot dash lines in  FIG. 7  to the position shown by broken lines in  FIG. 7 . 
     Because the pawl  22   a  is also displaced generally linearly along the radial line L in the unlocking process, the pawl  22   a  is less likely to interfere with the teeth  21   a  of the parking gear  21 , and the pawl  22   a  can be disengaged from the notch  21   b  relatively easily and smoothly. 
     As described above, the parking mechanism  20  of this embodiment is configured such that, when the parking mechanism  20  is brought into the locked state or the unlocked state, the widthwise center of the distal end of the pawl  22   a  of the parking lock pole  22  is displaced generally linearly along the radial line L extending from the axis of rotation P of the parking gear  21  through the circumferential center of a corresponding one of the notches  21   b.    
     Therefore, the divergence of the widthwise center of the distal end of the pawl  22   a  of the parking lock pole  22  from the radial line L can be decreased, and the angle of the pawl  22   a  relative to the corresponding notch  21   b  does not change significantly. As a result, the pawl  22   a  is less likely to interfere with the teeth  21  a of the parking gear  21  when engaging the pawl  22   a  of the parking lock pole  22  with the corresponding notch  21   b  of the parking gear  21  and in the process of disengaging the pawl  22   a  from the corresponding notch  21   b,  and the pawl  22   a  can be therefore engaged with and disengaged from the notches  21   b  relatively easily and smoothly. 
     Thus, with the above parking mechanism, it is possible to minimize fatigue damage to the pawl  22   a  and the notches  21   b  and improve the service life of the parking gear  21 , the parking lock pole  22  and so on. 
     In addition, because the angle of the pawl  22   a  is not changed significantly in the process of displacing the pawl  22   a  toward or away from the notches  21   b  as described above, even if the shift lever  11  is accidentally shifted into the parking range while the vehicle is running, ratcheting, a situation where the pawl  22   a  of the parking lock pole  22  is repelled by the teeth of the parking gear  21 , occurs and the parking lock pole  22  is displaced away from the parking gear  21 , the pawl  22   a  is less likely to be caught in any of the notches  21   b  of the rotating parking gear  21 . As a result, damage to the pawl  22   a  and the notches  21   b  may be minimized. 
     First Modification of the First Embodiment  
     The basic configuration of the automatic transmission  2  to which this embodiment is applicable is not limited to that of an automatic transmission of the type mounted in an FR vehicle as described above, and is applicable to other configurations such as that of an automatic transmission mounted in an FF vehicle. 
     Although the parking gear  21  of the parking mechanism  20  is mounted on the output shaft  8  thereof in the automatic transmission  2  for an FR vehicle as described in the above embodiment, the parking gear  21  may be provided on the counter driven gear, for example, in the case of an automatic transmission for an FF vehicle. In this case, the output shaft  8  and the counter driven gear may correspond to the rotating shaft of the present invention. 
     Second Modification of the First Embodiment  
     Although a configuration using the fulcrum cam  24  and the electric actuator  25  is used as the means for moving the fulcrum of the parking lock pole  22  in the first embodiment, a mechanical power transmission mechanism having a shift cable (not shown) extending from the shift lever  11  may be used to rotate the fulcrum cam  24  of the parking lock pole  22  instead of the electric actuator  25 . Such a mechanical power transmission mechanism is well known and hence its description is omitted here. 
     When the shift lever  11  and the parking lock pole  22  are operatively connected by a shift cable (not shown), the pawl  22   a  is less likely to interfere with the teeth  21   a  of the parking gear  21  in the locking and unlocking processes as described above, and the pawl  22   a  may be engaged and disengaged from the notches  21   b  relatively easily and smoothly. In addition to the effect, the force necessary to shift the shift lever  11  may be reduced. 
     Third Modification of the First Embodiment  
     Although the electric actuator  25  is controlled such that the electric actuator  25  rotates the fulcrum cam  24  in the clockwise direction to move the apex of the fulcrum cam  24  from its rightmost position to its lowermost position, as shown in  FIG. 7 , when the parking lock pole  22  is switched from the unlocked state to the locked state in the first embodiment, what is necessary is that the widthwise center of the distal end of the pawl  22   a  of the parking lock pole  22  is displaced lineally along the radial line L extending from the axis of rotation P of the parking gear  21  through the circumferential center of one of the notches  21   b  by moving the fulcrum of the parking lock pole  22 . 
     The fulcrum cam  24  may be rotated in the clockwise direction by the electric actuator  25  to displace the apex of the fulcrum cam  24  shown in  FIG. 7  from its rightmost position to its leftmost position, and from its lowermost position to its left most position, for example. Also, the fulcrum cam  24  may be rotated in the counterclockwise direction to displace the apex of the fulcrum cam  24  shown in  FIG. 7  from its rightmost position to its uppermost position, from its rightmost position to the leftmost position, and from its uppermost position to its leftmost position, for example. 
     Fourth Modification of the First Embodiment  
     Although the fulcrum cam  24  is rotated in the clockwise direction when the parking lock pole  22  is switched from the unlocked state to the locked state and the fulcrum cam  24  is rotated in the counterclockwise direction when the parking lock pole  22  is switched from the locked state to the unlocked state in the first embodiment, control may be provided such that the fulcrum cam  24  is rotated only in one direction. That is, what is necessary is that the fulcrum of the parking lock pole  22  is moved to the left as shown in  FIG. 3 ,  FIG. 4  and so on when the parking lock pole  22  is switched from the unlocked state to the locked state, and the fulcrum of the parking lock pole  22  is moved to the right as shown in  FIG. 3 ,  FIG. 4  and so on when the parking lock pole  22  is switched from the locked state to the unlocked state. 
     Second Embodiment  
     A second embodiment of the present invention is described with reference to  FIG. 9  to  FIG. 11 . The same components as those of the first embodiment are designated by the same reference numerals and their description is not repeated. The differences from the first embodiment are primarily described below. 
     As shown in  FIG. 9 , the parking mechanism has a parking lock pole  22  with slots  22   c,  extending in the transverse direction of the parking lock pole  22 , formed at each end, guide pins  31  formed on a case or the like of the automatic transmission  2  and received in the slots  22   c,  a switching cam  32  disposed in contact with the longitudinal center of the lower side of the parking lock pole  22 , and two reverse springs  23 , which serve as elastic members, provided at each end of the parking lock pole  22  that pull the parking lock pole  22  away from the parking gear  21 . The guide pins  31  and the slots  22   c  may correspond to the guide means of the present invention. The arrangement and shapes of these components are suitably selected and not limited to this embodiment. As examples, modifications 1 and 2 are described later. 
     The switching cam  32  has a rotating shaft  32   a,  a convex portion  32   b  which is formed on a specific part of the outer periphery thereof and applies a pressure to the parking lock pole  22  to move the parking lock pole  22  toward the parking gear  21  against the biasing force of the reverse springs  23 , and a base circular portion  32   c  which is formed on a specific part of the outer periphery thereof and does not apply a pressure to the parking lock pole  22  so that the parking lock pole  22  can be moved away from the parking gear  21  by the biasing force of the reverse springs  23 . 
     When the transmission control device  3  controls the rotation of the rotating shaft  32   a  of the switching cam  32  in the state shown in  FIG. 9 , the convex portion  32   b  of the switching cam  32  displaces the parking lock pole  22  toward the parking gear  21  against the biasing force of the reverse springs  23  until the pawl  22   a  engages one of the notches  21   b  of the parking gear  21 , as shown in  FIG. 10 , to bring the parking lock pole  22  into the locked state. 
     Because each end of the parking pawl  22  is supported by the slots  22   c  and the guide pins  31 , the parking pawl  22  is displaced linearly along a radial line L that extends from the rotational axis P of the parking gear  21  through the circumferential center of the corresponding notch  21   b  without changing its angle, as shown in  FIG. 11 . Thus, the pawl  22   a  of the parking lock pole  22  does not interfere with the teeth  21   a  of the parking gear  21 , and the pawl  22   a  may engage the corresponding notch  21   b  relatively easily and smoothly. 
     The parking lock pole  22  is also displaced linearly without changing its angle when unlocking the transmission. Thus, the pawl  22   a  of the parking lock pole  22  does not interfere with the teeth  21   a  of the parking gear  21 , and the pawl  22   a  is disengaged from the notch  21   b  relatively easily and smoothly. 
     Thus, according to this embodiment, it is possible to reduce fatigue damage to the pawl  22   a  and the notches  21   b  and improve the service life of the parking gear  21 , the parking lock pole  22  and so on. 
     First Modification of the Second Embodiment  
     Although one switching cam  32  contacts the longitudinal center of the lower side of the parking lock pole  22 , and two reverse springs  23  are provided at longitudinal opposite ends of the parking lock pole  22  in the above second embodiment, one reverse spring  23  may be provided at the longitudinal center of the parking lock pole  22  and two switching cams  32  may be disposed in contact with the longitudinal opposite ends of the lower side of the parking lock pole  22  as shown in  FIG. 12 . 
     Second Modification of the Second Embodiment  
     Although the parking lock pole  22  is moved in its width direction in the second embodiment, the parking lock pole  22  and associated members may be so arranged that the parking lock pole  22  can be moved in its longitudinal direction. 
     As shown in  FIG. 13 , the parking mechanism has a parking lock pole  22  arranged at an angle of 90° from the position shown in  FIG. 9  and has a slot  22   d  extending in its longitudinal direction at its longitudinal center, a guide pin  31  formed on the case of the automatic transmission  2  or the like and received in the slot  22   d,  a switching cam  32  disposed in contact with the longitudinal lower end of the parking lock pole  22 , and two reverse springs  23  provided on transverse opposite side of the parking lock pole  22  at a longitudinal intermediate portion of the parking lock pole  22  that pull the parking lock pole  22  away from the parking gear  21 . The slot  22   d  and the guide pin  31  may be provided at any position. Furthermore, a slot  22   d  and the associated guide pin  31  may be provided at each end of the parking pawl  22 . 
     Third Modification of the Second Embodiment  
     In the above second embodiment and the modifications thereof, the diameter of the guide pin or pins  31  may have a larger dimension in the sliding direction of the parking lock pole  22  than in the other directions. With this configuration, the parking lock pole  22  may be linearly moved toward and away from the parking gear more reliably. 
     While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.