Parking brake safety mechanism for automatic transmissions

A parking brake for automatic transmissions increases the safety of such brakes by arranging the cam, control rod, pawl and lost motion spring so that whenever the shift lever is not moved to its full park position with the engine running, the lost motion spring exerts a force on the mechanism blocking return thereof to reverse position while urging the mechanism in the direction of its park position, thereby preventing the vehicle thought to have been parked from suddenly moving in reverse possibly injuring anyone standing behind the vehicle.

This invention relates to parking brake mechanism and more particularly to 
means for increasing the safety of a parking brake mechanism for vehicle 
automatic transmissions. 
Automatic transmissions for vehicles are controlled by a driver operated 
shift lever which is movable in one direction from a neutral position into 
one or more drive positions and in an opposite direction from neutral, 
first, to a reverse position and then to a park position. In every 
position of the shift lever except park, valve means in the automatic 
transmission are operated to control the transmission. In the park 
position the transmission is, of course, placed in a non-drive state and 
at the same time the shift lever operates a control rod carrying a cam 
which reacts against a pawl to urge it in the direction of a toothed wheel 
connected to the transmission out-put shaft. Because the spaces between 
the teeth of the wheel are not always aligned with the pawl, the pawl may 
land on the radial outer surface of a tooth and there must be lost motion 
mechanism including resilient means which enables the shift lever to be 
moved into its park position while the pawl is still not yet in its park 
position, the resilient means exerting a force on the pawl to move it into 
an inter-tooth space on the parking gear as soon as the vehicle has moved 
a slight distance sufficient to align a space with the pawl. 
The above arrangement is satisfactory so long as the driver makes certain 
that the shift lever is positioned completely in its park position. 
Occasionally, a driver is careless and the shift lever is accidently 
positioned intermediate its park and reverse positions. Under these 
circumstances, should the pawl hang up on a tooth surface, as the pawl 
must do about 50% of the time, and should the engine be still running, the 
resilient means of the lost motion connection can exert a force on the 
control rod in a direction away from "park" and drive the driver 
controlled lever back to its "reverse" position causing the car to 
suddenly and unexpectedly move in reverse. Should someone be behind the 
car he may and has been badly injured. 
The broad object of this invention is to substantially eliminate the above 
described problem. 
More particularly, it is an object of the invention to eliminate the 
problem by so arranging the pawl, cam and lost motion spring that in the 
event the shift lever is accidently moved only partially towards "park" 
position, the force exerted by the spring on the control rod always blocks 
its return to "reverse" while urging it in the direction of "park" rather 
than urging it in the direction of "reverse," as is now almost universally 
the practice.

Referring now to the drawings the numeral 10 designates the output shaft of 
an automatic transmission having connected thereto, as by splines (not 
shown), a toothed parking gear 12 having teeth 14 defining spaces 16 
therebetween adapted to receive the tooth 18 at one end of a parking pawl 
20 which is pivoted at 22 to a relatively fixed part of the transmission 
housing 24. The pawl tooth 18 is urged at all times in the direction of 
the parking gear 12 by resilient means which may be the coil tension 
spring 26 shown which reacts between the end of the pawl remote from the 
tooth and the transmission housing 24. 
In FIGS. 1 and 2, the pawl end to which the spring 26 is attached is urged 
by the spring into engagement with a first surface 28 of an axially 
movable cam member 30 which is slideably guided in a guide block 32 fixed 
to the transmission housing 24 adjacent the pawl 20. Whenever the pawl is 
engaged with the surface 28 the pawl tooth 18 is retained in a non-braking 
position clear of the pawl teeth 14. The cam 30 has a second surface 34 
spaced a greater distance from the pawl than the first surface 28 to 
enable the spring 26 to move the pawl tooth, when aligned with the space 
16 between teeth 14 of the parking gear 12, into its braking position. As 
is clear in FIG. 2, the surfaces 34 and 28 are joined by a sloping cam 
surface 36 and preferably the arrangement is as shown whereby the sloping 
surface 36 is defined by a cone connected at its smaller end to a control 
rod 38 and at its larger end merges into an elongated cylindrical part 40 
whose outer surface defines the above described first surface 28 of the 
cam. 
With reference now to FIG. 4 it will be observed that the end of the 
control rod 38 remote from the cam member 30 is pivotally connected to a 
conventional detent plate 40' rotatably mounted at 42 to a fixed part of 
the transmission housing. Connected to the detent plate intermediate the 
control rod 38 and pivot 42 is one end of a link 44 whose opposite end is 
connected to a driver controlled shift lever 46 which is movable in a 
slotted member (not shown) having lever positions marked thereon 
corresponding to "neutral" and one or more "drive" positions when the 
lever is moved in one direction from "neutral" and "reverse" and "park" 
positions when the lever 46 is moved in the opposite direction from 
neutral. The lower edge of the detent plate is provided with notches 48, 
marked "D", "N", "R" and "P" in the drawing and corresponding to the first 
drive, neutral, reverse and park positions, respectively, of the shift 
lever 46. A spring loaded detent 50 engages with the respective notches 
when the plate 40' is rocked by the lever 46 to the selected position 
appearing on the slotted plate. 
In operation, with the driver controlled shift lever in any non-park 
position, the pawl is urged by the spring 26 against the surface 28 of the 
cylindrical part 40 which, it will be observed, has sufficient axial 
length that for every possible position of the shift lever and detent 
plate, except "park", the pawl engages the surface 28 and is thereby 
retained in its non-park position of FIG. 1. 
When the shift lever and detent plate are moved to "park," the conical 
surface 36 of the cam member is slid beneath the pawl until the region 34 
is reached whereby the pawl tooth 18 may be moved by the spring 26 into an 
intertooth space 16 on the parking gear if the tooth is in alignment with 
a space. Should the pawl tooth not be in alignment with a space 16 it 
engages the outer radial surface of a tooth substantially as shown in FIG. 
1 and the pawl is retained by the tooth surface out of engagement with the 
cam surface 34 until the vehicle rolls a slight distance sufficient to 
bring an inter-tooth space 16 into registration with the pawl tooth and 
lock the parking gear. 
Assume now that the driver accidently fails to move the shift lever fully 
into the park position so that the pawl engages the conical surface 36 
somewhere between the surfaces 34 and 28. Under these circumstances, the 
reverse valve will have been operated so that the vehicle is not, at this 
time, driven in a reverse direction yet the pawl may not be able to move 
enough towards the parking gear to ensure that the output shaft is locked 
against movement. The force of the spring 26, however, acting on the pawl, 
causes it to re-act against the sloping cam surface 36 and positively 
prevent the cam and hence the control rod and detent plate from moving in 
the direction of "reverse", thus preventing dangerous unexpected reverse 
movement of the vehicle. By selecting the cone slope pitch so that 
frictional resistance does not exceed the force of the spring loaded pawl 
on the sloping surface, it is entirely possible to design the arrangement 
that the cam is actually moved by the pawl into its full "park" position 
as soon as an inter-tooth space 16 aligns with the pawl tooth. Even if 
this does not take place, any possibility of the cam being driven back 
into its reverse position through the action of a yielding lost-motion 
spring is eliminated due to the fact that there is substantially no force 
acting in that direction while there is a preponderant force acting in the 
opposite direction, if not strong enough to bodily move the mechanism to 
"park" at least to block positively any tendency to move towards 
"reverse". 
Many of the prior art arrangements, while possessing the safety problems 
explained above, require the use of both a pawl spring, usually urging the 
pawl away from the parking gear, and a separate lost motion spring, as 
illustrated, for example, in U.S. Pat. No. 3,601,230. In the present 
invention, the pawl spring itself also serves as the lost motion spring 
thereby decreasing cost and complexity. It will be apparent, of course, 
that the tension spring 26 shown is merely representative since any type 
of resilient means such as a torsion spring, or a compression spring on 
the appropriate sides of the pivot and pawl would serve equally well. 
It should be noted that it is desirable that the surface 34 be on the 
sloping part of the cone, or its equivalent, to ensure that when the shift 
lever is accidently not moved into its full park position, the spring 
loaded pawl will always bear on the sloping part of the cam to urge it in 
the direction of its park position. Should the pawl align only with the 
end of control rod 38 under such circumstances, though the pawl might 
block the return of the mechanism to its reverse position, the cam would 
not be urged by the spring loaded pawl towards its park position. This 
explains why the surface 34 is shown on the conical surface itself. 
Applicant has specifically described an arrangement using axially movable 
control rod and cam means. The invention is by no means restricted to such 
an arrangement but applies equally to systems utilizing rotary control 
rods and cams as shown, for example, in U.S. Pat. No. 3,703,941. Thus, the 
invention is susceptible of a wide variety of changes and modification 
without, however, departing from the scope and spirit of the appended 
claims.