Patent Application
20140116835
1. Field of the Invention
This invention relates to a parking brake mechanism for an automatic transmission and, more particularly, to a park mechanism actuated by a servo to alternately engage and disengage the park gear of a vehicle.
2. Description of the Prior Art
Parking mechanisms or assemblies are used in automotive vehicles to allow the vehicle to enter into and maintain a “parked” condition or state. Parking assemblies typically include a park rod or cam which selectively actuates a parking pawl which operatively engages the vehicle's park gear, thereby maintaining the vehicle in a “parked” position or state.
The park rod is typically mechanically linked to the vehicle's shifter and moves in response to shifter movement. When the shifter is moved to the “park” position, the park rod engages the parking pawl, which pivots or rotates into a position where it is disposed between a pair of teeth on the park gear, thereby substantially preventing the park gear and the output shaft from further rotating.
In a shift-by-wire park system, a spring apply-servo release system requires a force of about 100 pounds to pull the apply rod out of park when the vehicle is fully loaded on a grade. This force requires a large servo to conduct the event. A need exits to reduce the magnitude of force needed such that a smaller servo mechanism can be employed.
A park assembly includes a park gear, a pawl that engages and disengages the park gear, a park rod that causes the pawl to engage with the park gear and to disengage from the park gear, a servo, and a bellcrank transmitting a first force to the park rod causes the pawl to disengage from the park gear in response to a second force substantially greater than the first force and produced by the servo.
By utilizing a cam driven park rod and servo valve, the force amplification effects of the cam profile on the bellcrank reduces the size of the servo valve 28, thereby reducing its cost less, weight, and packaging difficulty.
The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art.
The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which:
Park gear 12 includes several peripherally disposed and substantially identical teeth or projections 16. Each tooth 16 is separated by a cavity or recess 18. A parking pawl 20, which includes a tooth 22, pivots about axis 24 to engage park gear 12, thereby substantially preventing the park gear 12 and output shaft from rotating. When pawl 20 is disengaged from park gear 12, output shaft is free to rotate about axis 25.
The shifter is operatively and selectively movable among several positions, such as a “park” position, a “reverse” position, a “neutral” position, and a “drive” position, which respectively correspond to various operating states or operating ranges modes of the transmission. The shifter may be a push-button shifter.
Park assembly 10 further includes a park rod 26, and a servo 28, which is preferably a hydraulic valve that responds to movement of the shifter to actuate park rod 26.
A bellcrank 30 includes a slot 32, which extends radially from the axis 34 of a pin 36, about which bellcrank 30 is supported in rotation. Valve 28 includes a pin 38, located in slot 32. As valve 28 moves along axis 40 in response to movement of the shifter, pin 38 causes bellcrank 30 to pivot about axis 34. Park rod 26, fitted in an arcuate slot 42 formed in bellcrank 30, displaces the park rod 26. A bracket 46, secured against displacement, is formed with mutually parallel slots 48, 50. Locating park rod 26 in slot 48 ensures that the end of park rod moves parallel to axis 40. Locating pin 38 in slot 50 ensures that valve pin 38 moves parallel to axis 40.
A bracket 54, secured against displacement, supports a return spring 56. The force of return spring 56 biases park pawl 20 to pivot toward engagement with a cam 58 and away from engagement with park gear 12. Cam 58 can move along park rod 26. A compression spring 60 on park rod 26 continually urges cam 58 toward an end of the park rod toward a stop 62, which limits that displacement. Spring 60 also allows cam 58 to move along the park rod 26 and away from pawl 20 by compressing the spring.
A plate 64, fixed against displacement, is formed with a longitudinal recess 66 directed toward pawl 20. Cam 58 is fitted in recess 66 and is guided to move along the recess alternately in the direction of the arrows 74, 75, thereby defining the location of cam 58 and direction of its movement.
As park rod 26 moves in the direction of arrow 74 cam 58 disengages from pawl 20, causing the pawl tooth 22 to disengage from and releasing park gear 12. Spring 56 holds pawl member 20 in the disengaged position until the next engagement sequence.
This force amplification allows a small valve to pull with a large force. After the pull-out-of-park event has occurred, cam profile 72 accelerates movement of the park rod to the no-park position, where the end of park rod 26 is at the end of slot 42 at the base of profile 72.
In the park assembly 10, ratcheting which may occur when a vehicle is unintentionally shifted to park while the vehicle is still moving, is prevented. If a shift into park is allowable, valve 28 and park rod 26 move in the direction of arrow 75, thereby causing cam 58 to engage parking pawl 20 and forcing the pawl into engagement with park gear 12.
By utilizing a cam driven park rod 26 and servo valve 28, the force amplification effects of the cam profile 70 reduce the size of the servo valve 28. The assembly 10 provides the travel needed to move the park rod 26 from the park position to the no-park position.
F2 cos θ=F1 (1)
F2 sin θ=F3 (2)
From a summation of moments about pivot axis 34
F2(b)=F4(a) (3)
Substituting from equations (1) and (2) into equation (3)
[F1/cos θ][c sin θ]=F4(a)
Tan θ=F4(a)/F1(c) (4)
In this example θ=9.3 degrees; the servo force F4 is 22 lb.; distance a is 25 mm. and distance c is 30 mm. Substituting these variables in equation (4) shows that F1 is about 112 lbs, indicating that bellcrank 30 produces a mechanical advantage of about five. The force F1 applied by the bellcrank to park rod 26 is about five times greater than the servo force F1 applied to the bellcrank.
In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.
