Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement
Patent
1980-08-20
1982-12-28
Michalsky, Gerald A.
Power plants
Pressure fluid source and motor
Condition responsive control of pump or motor displacement
60449, F15B 700
Patent
active
043654738
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
This invention relates generally to a hydrostatic transmission for a vehicle and more particularly to an overspeed control for preventing the transmission from accelerating the engine and thus the components of the hydrostatic transmission to an overspeed condition when the vehicle is descending a steep hill.
2. Background Art
Some industrial vehicles have a hydrostatic transmission for delivering power from the engine to the wheels or tracks. Such a hydrostatic transmission commonly has a variable displacement pump and a fixed displacement drive motor, or in some cases a variable displacement motor, interconnected through a closed loop fluid circuit. When the vehicle having such a transmission is descending a hill, the drive motor tends to act as a hydraulic pump and directs pressurized fluid to the pump which acts as a hydraulic motor. The engine, being directly connected to the pump normally provides a braking or retarding action by resisting the driving action of the pump. However, when the vehicle is descending a steep hill the driving power generated by the transmission may be sufficient to cause the engine to overspeed under some conditions due to the reverse operation of the pump and drive motor. For example, with the pump in a maximum displacement condition it is in a condition for providing maximum torque when it acts as a motor and being driven by fluid directed thereto from the drive motor which is acting as a pump and being driven by the wheels or tracks. Thus, the torque generated by the pump under this condition over powers the engine and drives it at a speed commensurate with the fluid flow in the closed loop of the hydrostatic transmission. Such an overspeed condition of the engine could damage either the engine or the pump and drive motor since they are also being rotated faster than their designed operating speed.
The present invention is directed to overcoming one or more of the problems as set forth above.
DISCLOSURE OF THE INVENTION
In one aspect of the present invention a hydrostatic transmission has a variable displacement pump driven by an engine, signal means for controllably delivering low and high pressure signals with the pressure differential between the low and high pressure signals being increased in response to an increase in the engine speed and decreased in response to a decrease in the engine speed, and control means for receiving the low and high pressure signals and reducing the pump displacement in response to a decrease in the pressure differential below a predetermined magnitude. A valve means is provided for decreasing the pressure level of the high pressure signal received by the control means in response to the pressure differential between the low and high pressure signals exceeding a second predetermined magnitude.
Controlling the overspeed of an engine caused by the reverse operation of the pump and drive motor of a hydrostatic transmission of a vehicle descending a steep hill is accomplished by decreasing the pressure level of the high pressure signal received by the control means connected to the pump. Decreasing the pressure level of the high pressure signal causes the displacement of the pump to be reduced which in turn reduces the drive torque capability of the pump which is acting as a motor when the vehicle is descending a hill.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an embodiment of the present invention with certain components shown in section.
FIG. 2 is a schematic of a second embodiment of the present invention and;
FIG. 3 is a cross sectional view of an alternate valve construction.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to FIG. 1 hydrostatic transmission is generally indicated by reference numeral 10 and includes a variable displacement pump 11 and a fixed displacement drive motor 12 interconnected through a closed loop having first and second conduits 13,14. The variable displacement pump 11 is driven by an engine, not shown, while the drive motor 12 is connected to the
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patent: 3952514 (1976-04-01), Habiger
patent: 4080850 (1978-03-01), Bubula et al.
patent: 4094145 (1978-06-01), Habiger
patent: 4106292 (1978-08-01), Habiger
Schexnayder Lawrence F.
Spivey, Jr. William J.
Caterpillar Tractor Co.
Grant John W.
Michalsky Gerald A.
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