Automatic return mechanism for hydrostatic transmission pressure release valve

A hydrostatic transmission pressure release valve control mechanism is provided to enable an operator to quickly release the transmission pressure and manually move the vehicle when the engine is not operating. The pressure release mechanism, which permits vehicle movement for repairs, maintenance and similar activities, is automatically deactivated to restore transmission pressure as the vehicle brake is depressed in preparation for restarting the engine.

FIELD OF THE INVENTION 
The present invention relates to small vehicles having hydrostatic 
transmissions such as lawn and garden tractors. Specifically, the 
invention provides for a hydrostatic transmission fluid pressure release 
valve control mechanism for allowing the vehicle, when the engine is not 
operating, to be manually pushed for maintenance or similar activities. 
BACKGROUND OF THE INVENTION 
Small vehicles such as lawn and garden tractors are commonly equipped with 
hydrostatic transmissions. The hydrostatic pump-motor circuits in these 
transmissions generally remain pressurized when the engine is not 
operating. Consequently, movement of the unpowered vehicle when routine 
maintenance and/or other repairs are to be performed is difficult since 
fluid flow through the pump is not possible without a line being provided 
to bypass the pump. 
While it is common for lawn and garden-type vehicles with hydrostatic 
transmissions to provide hydrostatic fluid pressure release circuits and 
valve controls that allow the operator to manually move the vehicle for 
maintenance and other activities when the engine is not operating, the 
controls for opening and closing those circuits present problems. 
One such vehicle, identified as the John Deere Model 300/400 Series Lawn 
and Garden Tractor, provides a hydrostatic fluid pressure release lever 
which is set and then locked in place by the operator. Should the operator 
fail to return the lever to its original position before restarting the 
engine, the hydrostatic fluid release valve will not permit the 
transmission to build up pressure, and accordingly, the transmission will 
have no forward or reverse speed. Once the engine is started and the 
pressure release control lever is returned to its original position, the 
tractor may lurch as hydrostatic pressure is restored. 
A second type of hydrostatic fluid pressure release control is found on the 
200 Series John Deere Model Lawn and Garden Tractors. Once the pressure 
release lever is activated on this vehicle, it remains in its released 
position. When the engine is subsequently started, the pump pressure in 
the hydrostatic transmission closes the pressure release valve, and the 
transmission functions normally. This type of release control system 
however requires a high charge pump pressure to close the hydrostatic 
fluid pressure release valve automatically and, of course, adds additional 
manufacturing and maintenance costs to the vehicle. 
A third type of hydrostatic fluid pressure release control mechanism is 
found in the 100 Series John Deere Model Tractors. With this control, the 
operator depresses a fluid pressure release valve lever beneath his seat 
to release the fluid pressure. The lever has a spring return and therefore 
must be held in the released position during the entire time the tractor 
is being pushed. While this system overcomes the need for the operator to 
release the lever after manually moving the vehicle, it can be 
inconvenient because the operator must simultaneously hold the release 
lever in place, push the tractor and steer it at the same time. 
SUMMARY OF THE INVENTION 
Accordingly, there is provided herein a hydrostatic transmission pressure 
release valve control system which permits the operator to quickly release 
the pressure in the hydrostatic transmission to permit manual movement of 
the vehicle for repair, maintenance and/or similar activities and then 
automatically restores the pressure as the brake pedal is depressed to 
restart the engine. 
This control system does not require a high pressure charge pump nor does 
it require that the operator hold the pressure release valve lever in 
place or manually return it to its original position before restarting the 
engine. 
The pressure release valve control of the present invention allows the 
operator to release the hydrostatic transmission pressure by simply 
shifting a control lever beneath his seat. Subsequently, when the brake 
pedal is depressed to deactivate the engine electrical interlock before 
restarting the engine, a mechanical linkage between the brake pedal and 
pressure release valve closes the valve to restore hydrostatic fluid 
pressure to the transmission. 
In the preferred embodiment, the pressure release valve means includes a 
pair of ball check valves provided in a fluid line which interconnects the 
high and low pressure fluid lines that form a loop between the hydrostatic 
pump and motor. The check balls are mechanically unseated by a pair of 
control levers which are then held in place by a detent means. Once the 
check balls are unseated, the hydrostatic fluid can be circulated in a 
loop through the interconnecting line and the motor as the vehicle is 
manually pushed. With the hydrostatic pressure relieved, the transmission 
will not retard rotation of the vehicle drive wheels and it can be easily 
moved for maintenance, repairs, loading and/or similar activities. 
The detent means is automatically deactivated when the operator depresses 
the brake pedal in preparation for restarting the vehicle. As the brake 
pedal is depressed to deactivate the engine electrical interlock, a 
linkage between the pedal and the control levers causes the detent means 
to be released and the control levers to be shifted out of contact with 
the check balls. As the check balls reseat and the engine is started, 
hydrostatic fluid flow between the transmission pump and motor is 
re-established and the operator can proceed to drive the vehicle in 
forward or reverse as desired.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Looking first to Fig. 1, there is illustrated a typical lawn and garden 
tractor 10 having a mower 12 mounted therebeneath. The tractor 10 is 
equipped with a brake pedal 14, front and rear wheels 16 and 18, the rear 
wheels 18 being equipped with brakes (not shown). The tractor 10 is 
further equipped with an engine beneath the cowling 20, an operator's 
station 22, and a steering wheel 24. 
Illustrated schematically in FIG. 2 is the transmission hydrostatic fluid 
pressure release valve control mechanism 26. In this embodiment, the 
transmission includes a pump 28 and motor 30 as illustrated. A supply of 
fluid is provided in the reservoir 32 which supplies, through an inlet 
filter 34, first and second pressure lines 36 and 38 which interconnect 
the pump 28 and motor 30. The pump 28 is bi-directional as is the motor 
30, allowing it to move hydrostatic fluid through either line 36 or 38 and 
receive it from either line 36 or 38. 
A bypass line 40 is provided between the first and second pressure lines 36 
and 38. Provided in the bypass line 40 are bypass valve means 42, composed 
in the preferred embodiment of check valves. Other bypass means such as a 
spool valve could also be utilized and the invention is not intended to be 
limited to the use of check valves. The check valve balls 44 and 46, are 
shiftable between closed and open or first and second positions for 
respectively blocking and permitting flow of hydrostatic fluid through the 
bypass valve means 42. To shift the check balls 44 and 46 between their 
seated and unseated positions is a pair of valve control means or lever 
arms 48. 
The control means or lever arms 48 cooperate with a detent means designated 
by the numeral 50. The detent means 50 serves to hold the lever arms 48 in 
place once activated to maintain the ball valve means 42 in their open 
positions, as shown in FIG. 3. Any detent mechanism which would serve to 
hold the lever arms 48 in place could be utilized, such as a spring loaded 
ball receivable in a recess carried in the lever arms 48. 
The lever arms 48 are pivotally supported at 52 for swinging movement and 
coupled at their ends 54 to a hand activated lever 56 which can be shifted 
by the operator. The hand lever 56 and control lever arms 48 are 
operatively coupled with a brake means. The brake means includes the 
control rod 58 which is connected to the brake pedal 14 carried on the 
vehicle 10. As the brake pedal 14 is actuated, the control rod 58 is 
pulled to the right as viewed in FIG. 2, swinging the lever arms 48 
counterclockwise and disengaging the detent means 50 to move the lever 
arms 48 out of contact with the check balls 46. 
The control rod 58 is in turn connected through the brake link 60 to an 
electrical interlock means 62. The electrical interlock means 62 takes the 
form in the present invention of an electrical circuit interrupter, that 
circuit being completed (see FIG. 2) as the brake link 60 makes contact 
with a switch 64 to permit electrical current to flow from the battery to 
the engine. 
The operation of the hydrostatic fluid pressure release valve control 
system will now be reviewed. 
When an operator wants to perform maintenance on his vehicle or move the 
vehicle around the shop without the engine being powered, he will first 
turn the engine off. The pressure release control mechanism will be in the 
configuration illustrated in FIG. 2. Once the engine has died, he will 
pull the hand lever 56 to rotate the valve control lever arms 48 to the 
position illustrated in FIG. 3. The control lever arms 48 will contact the 
check balls 44 and 46, depress them and permit fluid in the transmission 
to circulate through the lines 36 and 38 to the motor 30. Since the fluid 
will then be able to move through the motor 30 and lines under low 
pressure, the pump 28 will not provide resistance to rotation of the drive 
wheels 18 as the operator moves the vehicle 10. 
When the operator has finished with the activity related to servicing or 
maintaining the vehicle and wants to start the engine again, he will 
depress the brake pedal 14 to apply the wheel brakes. The brake pedal 14, 
which is connected to the control rod 58 and link 60, will shift them to 
the right as viewed in FIG. 2. As the link 60 is swung to the right, it 
applies the wheel brakes 66 and through contact with the electrical 
interrupter circuit 62, completes the circuit between the battery and 
engine. Accordingly, the engine can then be started with this safety 
feature. Simultaneously, the control rod 58 will swing into contact with 
the control levers 48 (see FIGS. 2, 4 and 5) and urge them 
counterclockwise, as viewed in FIG. 2. The force of the control rod 58 on 
the lever arms 48 disengages the detent means 50 and moves the lever arms 
48 out of contact with the check balls 44 and 46, thereby allowing them to 
reseat and the interconnecting line 40 to be blocked. As the check balls 
44 and 46 reseat, fluid is forced to follow the loop through the pump 28 
and motor 30, and the pressure will be restored to the lines 36 and 38 as 
the engine is started. 
Accordingly, the operator does not have to remember that the hydrostatic 
fluid pressure release valve control lever 56 must be shifted when he 
starts the engine to continue his work, since it will automatically be 
released as the operator depresses the brake pedal 14 to start the engine. 
With the present invention, there is provided a simple and positive 
mechanism for relieving pressure inside the hydrostatic transmission and 
permitting easy and manual movement of the vehicle as may occasionally be 
desired. There is further provided a system whereby the transmission's 
hydrostatic fluid pressure is restored to its normal pressurized state as 
the operator starts the vehicle in preparation for further activities.