1. Field of Invention
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.
2. Description of Related Art
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.
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'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)).
This parking lock device includes a parking gear, a parking rod, a parking lock pole, and a drive lever.
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.
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.
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.
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)).
The range selector is generally configured as follows. That is, when the shift lever installed near the driver'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.
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.
The above-mentioned related art still leaves room for improvements in the following respects.
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.
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.
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.
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.
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.
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.
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.