Hydrostatic transmission having an overspeed control

An underspeed actuator (18) is connected to the displacement control (17) for controlling the displacement of a variable displacement pump (11) of a hydrostatic transmission (10) in response to the pressure differential between low and high pressure control signals (A,B) wherein the pressure differential is directly proportional to the engine speed. An overspeed control valve means (41) is connected to the low and high pressure signals and decreases the pressure level of the high pressure signal received by the underspeed actuator (18) in response to the pressure differential exceeding a predetermined magnitude. This reduces the displacement of the variable displacement pump (11) thereby controlling the amount of engine overspeed caused by the hydrostatic transmission (10) driving the engine when the vehicle is descending a steep hill.

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.

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 drive wheels or tracks of the vehicle, not shown, in the usual 
manner. 
A control means 16 is connected to the variable displacement pump 11 for 
controlling the displacement thereof and includes a displacement control 
17 connected to the variable displacement pump 11 and an underspeed 
actuator 18 mechanically connected to the displacement control 17. The 
underspeed actuator 18 includes a piston 19 slidably positioned in a body 
21 defining first and second fluid actuating chambers 22,23. First and 
second springs 24,26 are positioned within the first and second chambers 
22,23 respectively at opposite ends of the piston 19. 
For an understanding of the operation of the underspeed actuator 18 and its 
connection to the displacement control 17 it will suffice to state that 
the piston 19 is movable between a first position at which the 
displacement of the pump 11 is adjusted to its minimum displacement and a 
second position at which the displacement of the pump 11 is adjusted to 
its maximum displacement. As shown in the drawing the piston 19 is at its 
first position. The piston 19 is controllably moved between the first and 
second positions in response to pressure differential existing between the 
first and second chambers 22,23 as well as the force of the spring 24. The 
first and second positions of the piston 19 are commonly referred to as 
"full underspeed" and "zero underspeed", respectively. 
A signal means 27 controllably delivers low and high pressure control 
signals A,B to the first and second actuating chambers 22,23 respectively 
at a pressure differential that is substantially proportional to the 
operating speed of the engine. The pressure differential between the low 
and high pressure signals A,B is increased in response to an increase in 
the engine speed and decreased in response to a decrease in the engine 
speed. 
The signal means 27 can be, for example, a fixed displacement control pump 
28, a venturi 29 and a relief valve 31. The control pump 28 is driven by 
the engine to produce fluid flow proportional to the operating speed of 
the engine. The venturi 29 is connected to the control pump 28 through a 
pump output line 32. A high pressure signal conduit 33 is connected to the 
pump output line 32 and to the second actuating chamber 23 for delivering 
the high pressure signal B thereto. A low pressure signal conduit 34 
connects a low pressure port 36 to the first fluid actuating chamber 22 
for delivering the low pressure signal A thereto. 
A venturi bypass valve 37 is positioned in parallel to the venturi 29 for 
fine tuning the signal means 27 for providing a predetermined pressure 
differential between the low and high pressure signals A,B at a 
preselected engine speed. 
A restriction means for example, an orifice 38 is positioned within high 
pressure signal conduit 33 for restricting fluid flow thereto for a later 
defined purpose. 
An overspeed control valve means 41 is connected to the signal means 27 for 
decreasing the pressure level of the high pressure signal B received by 
the control means 16 in response to the pressure differential between the 
low and the high pressure signals A,B, exceeding a predetermined 
magnitude. The valve means 41 includes a pressure reducing type valve 42 
having a valve spool 43 slidably positioned in a bore 44 of a valve body 
46 defining a pair of fluid chambers 47,48 at opposite ends of the valve 
spool 43. A first port 49 communicates with the chamber 48 while second 
and third axially spaced ports 51,52 communicate with the bore 44. The 
third port 52 is connected to the pump output line 32 through a conduit 
53. The second port 51 is connected through a conduit 54 to a portion 33a 
of high pressure signal conduit 33 between the orifice 38 and the 
underspeed actuator 18. The first port 49 is connected to the low pressure 
signal conduit 34 through a conduit 56. A passage 57 connects the second 
port 51 to the fluid chamber 47. 
The valve spool 43 has a first annular groove 58 in continuous 
communication with the second port 51 and a second annular groove 59 
axially spaced from the first annular groove and connected thereby by a 
passage 61. A piston 62 is slidably positioned in a bore 63 at one end of 
the valve spool 43 and in communication with the fluid chamber 47 defining 
a chamber 64. An annular groove 66 in the valve spool is in continuous 
communication with the third port 52 and with the fluid chamber 64 through 
a plurality of radial passages 67. 
The valve spool 43 is movable between a first position at which the portion 
33a of high pressure signal conduit 33 is blocked from communication with 
the fluid chamber 48 and hence the low pressure signal conduit 34 and a 
second position at which the portion 33a of the high pressure signal 
conduit 33 is in variable communication with the fluid chamber 48 and 
hence the low pressure signal conduit 34. A spring 68 is positioned in the 
fluid chamber 48 for resiliently urging the valve spool 43 to the first 
position. 
Alternately, the first port 49 can be connected to the reservoir so that 
the portion 33a of the high pressure signal conduit 33 is in variable 
communication with the reservoir at the second position of the valve spool 
43. 
OPERATION OF THE FIRST EMBODIMENT 
Under normal operating conditions the underspeed actuator 18 functions to 
control the displacement of the variable displacement pump 11 in response 
to variations in engine speed caused by varying lug conditions on the 
engine. Thus, when the engine is lightly loaded and operating at a maximum 
predetermined operating speed, the signal means 27 produces a pressure 
differential between the low and high pressure signals A,B sufficient for 
the high pressure signal B in the fluid chamber 23 to urge the piston 19 
upwardly to its second position at which the variable displacement pump 11 
is at its maximum displacement. However, in a lug condition sufficient to 
cause the engine speed to drop, the output flow of control pump 28 also 
decreases so that the pressure differential between the low and high 
pressure signals A,B also decreases. When the pressure differential drops 
below a first predetermined magnitude the piston 19 moves downwardly 
toward its first position thereby reducing the displacement of the 
variable displacement pump 11. 
During the above noted normal operating condition the pressure differential 
between the low and high pressure signals A,B is felt at three areas of 
the valve 42. The high pressure signal B in the high pressure conduit 33 
is delivered to the fluid chamber 64 where it acts directly on the piston 
62. The high pressure signal B in portion 33a downstream of the orifice 38 
is delivered to the fluid chamber 47 where it acts on the end of the valve 
spool 43 minus the area of the piston 63. The low pressure signal A in low 
pressure signal conduit 34 is delivered to the fluid chamber 48. However, 
the bias of the spring 68 resiliently prevents movement of the spool 43 to 
its second position so long as the engine speed remains within its normal 
operating range. Should the engine speed exceed the normal operating range 
such as might occur when the vehicle is descending a steep hill with the 
pump 11 at its maximum displacement, the pressure differential between the 
low and high pressure signals A,B, increases. When the pressure 
differential exceeds a second predetermined magnitude, the valve spool 43 
will move to the second position causing variable communication between 
conduits 54 and 56 and hence between portion 33a of high pressure signal 
conduit 33 and low pressure signal conduit 34. The orifice 38 establishes 
a pressure drop in the portion 33a of high pressure signal conduit 33 and 
hence second chamber 23 thereby causing the piston 19 to move toward its 
second position and reduces the displacement of the variable displacement 
pump 11. Reducing the displacement of the variable displacement pump 11 
(which is acting as a motor in a downhill situation) reduces its output 
torque capability and thereby controls the amount of engine overspeed. 
The piston 62 is sized so that the decrease in pressure of the high 
pressure signal B in portion 33a is proportional to the increase in the 
pressure differential between the low and high pressure signals A,B 
generated by the signal means 27. 
After the vehicle reaches the bottom of the hill or other procedures taken 
to slow the engine speed to its normal operating range the signal means 27 
automatically decreases the pressure differential between the low and high 
pressure signals A,B. This causes the valve spool 43 to move back to its 
first position blocking communication between conduits 54 and 56 so that 
the underspeed actuator 18 is again under the direct control of the signal 
means 27. 
SECOND EMBODIMENT 
A second embodiment of a hydrostatic transmission having an overspeed 
control is disclosed in FIG. 2. It is noted that the same reference 
numerals of the first embodiment are used to designate similarly 
constructed counterpart elements of this embodiment. In this embodiment, 
however, the valve body 46 of valve 41 has a pair of axially spaced 
annuluses 71,72. The high pressure signal conduit 33 is connected directly 
to the third port 52 which in turn is in communication with the annulus 
71. The second port 51 is in communication with the annulus 72 and is 
connected to the conduit 54 which is in turn in direct communication with 
the second chamber 23 of the underspeed actuator 18. The valve spool 43 is 
movable between a first position at which the high pressure signal conduit 
33 is in communication with the conduit 54 and is blocked from 
communication with the low pressure signal conduit 34 and a second 
position at which communication between the high pressure signal conduit 
33 and the conduit 54 and between the conduit 54 and the low pressure 
conduit 34 is controllably modulated. 
OPERATION OF THE SECOND EMBODIMENT 
The operation of the second embodiment is essentially as described above 
with the difference being that instead of the valve 41 reducing the 
pressure in a portion of the high pressure signal conduit 33 and hence the 
second fluid chamber 23, the high pressure signal B is normally delivered 
from the signal means 27 to the second fluid chamber 23 through the valve 
41 at the first position of the valve spool 43. Thus at the second 
position of the valve spool 43 delivery of the high pressure signal B to 
the second fluid chamber 23 and communication between the second fluid 
chamber 23 and the low pressure signal conduit 34 is controllably 
modulated to reduce the fluid pressure in the second fluid chamber 23. 
THIRD EMBODIMENT 
A third embodiment of a hydrostatic drive having overspeed control is 
disclosed in FIG. 3. It is noted that the same reference numerals of the 
first embodiment are used to designate similarly constructed counterpart 
elements of this embodiment. In this embodiment, however, a limiting means 
75 is included as part of the valve means 41 for limiting the extent of 
decrease in the pressure level of the high pressure signal B received by 
the control means 16 in response to the pressure differential between the 
low and high pressure signals exceeding a third predetermined magnitude 
greater than the second predetermined magnitude. The limiting means 75 can 
include, for example, another valve spool 76 slidably positioned in a bore 
77 in the end of the valve spool 43 extending into the fluid chamber 48. A 
plug 78 is sealingly secured within the bore defining a fluid chamber 79 
at one end of the valve spool 76 with the fluid chamber 79 being a fluid 
communication with the fluid chamber 48 through a passage 81. Another 
fluid chamber 82 at the other end of the valve spool 76 is in 
communication with the annular groove 58 of the spool 43 through a pair of 
radial passages 83, a pair of radial passages 84 connect the bore 77 and 
the annular groove 59. The valve spool 76 is movable between a first 
position at which the passages 83 are blocked from communication with 
radial passages 84, a second position at which the passages 83 are in 
fluid communication with radial passages 84 and has an intermediate 
position at which communication between the passages 83 and the radial 
passages 84 is controllably modulated. A spring 86 is positioned in the 
chamber 79 and resiliently urges the valve spool 76 to the first position. 
OPERATION OF THE THIRD EMBODIMENT 
The basic operation of the valve means 41 of this embodiment is essentially 
the same as the operation of the first embodiment and therefore, only the 
operation of the limiting means 75 will be described in detail. During 
normal operation of the hydrostatic transmission 10 the high pressure 
signal B is delivered through conduit 54, the second port 51, annular 
groove 58, and passages 83 into the fluid chamber 82. Also, the low 
pressure signal A is delivered through conduit 56, first port 49, fluid 
chamber 48, passage 81, and into the fluid chamber 79. When the engine 
speed reaches the maximum operating speed the fluid pressure in fluid 
chamber 82 will be sufficient to move the valve spool 76 rightwardly to 
the second position against the resiliency of the spring 86. This 
establishes communication between the annular grooves 58 and 59 through 
the passages 83, bore 77 and passages 84. 
As the valve spool 43 moves rightwardly to its second position in response 
to an increase in the pressure differential in the low and high pressure 
signals A,B, in response to an increase in engine speed above the maximum 
operating speed, the high pressure signal B is communicated with the low 
pressure signal A to controllably decrease the fluid pressure in the 
annular groove 58, the conduit 54, and hence the second fluid chamber 23 
of the underspeed actuator 18. As the fluid pressure in the annular groove 
58 continues to decrease as the engine speed increases, the fluid pressure 
in the fluid chamber 82 also decreases resulting in movement of valve 
spool 76 to the left toward its intermediate position. When the fluid 
pressure in the annular groove 58 has decreased to a predetermined level, 
the valve spool 76 will have reached its intermediate position and will 
thereafter maintain the pressure in the annular groove 58, conduit 54 and 
the second fluid chamber 23 at the predetermined level. This predetermined 
level is preferably selected so that the displacement of the variable 
displacement pump 11 is reduced to an intermediate displacement between 
the maximum and minimum displacement of the pump. 
INDUSTRIAL APPLICABILITY 
The present invention has particular utility on all vehicles having a 
hydrostatic transmission as a control for preventing the transmission from 
accelerating the engine and the components of the transmission to an 
overspeed condition when the vehicle is descending a hill. The overspeed 
control operates automatically without operator input and functions by 
reducing the displacement of the variable displacement pump thereby 
reducing its drive torque capability when the pump is acting as a motor in 
a downhill condition. By reducing the drive torque capability of the pump, 
its ability to drive the engine is reduced thereby limiting the degree of 
engine overspeed. 
Other aspects, objects and advantages of this invention can be obtained 
from a study of the drawings, the disclosure and the appended claims.