Fan-Drive Control for Open Circuit Variable Displacement Pump
INTRODUCTION
Hydraulically driven devices (e.g., fans and motors) are used in various
mechanical systems. For example, internal combustion engines may include one or more
hydraulically driven fans to draw air through an engine radiator. Such fans may be
driven by fixed or variable displacement pumps. Advantages of variable displacement
pumps in such applications include the ability to control the fan based on temperature,
even if the temperature is changing due to environmental or use conditions. Variable
displacement pumps having load sensing and pressure cutoff controls are particularly
useful to control the fan speed in response to sensed temperature of the engine. In these
cases, fan speed is increased and decreased based on increases and decreases in
temperature. A fluid pressure signal is directed to a displacement controller of the pump
via an electrohydraulic valve, thus controlling the fan speed.
SUMMARY
One aspect of the present disclosure relates to a valve used to control the
discharge pressure and/or the discharge rate of a variable displacement pump. The
displacement of the variable displacement pump is controlled by a control piston. The
valve includes a spool that moves between first and second positions. When the spool is
in the first position, the control piston is connected to tank pressure. When the spool is in
the second position, the control piston is connected to pump discharge pressure. A spring
biases the spool toward the first position. The valve includes a pin carried with the spool.
Pump discharge pressure acts an on an end face of the pin to move the spool from the
first position to the second position. The pump discharge pressure required to move the

spool from the first position to the second position can be manually adjusted by adjusting
the spring pressure applied to the spool. The pump discharge pressure required to move
the spool from the first position to the second position can also be adjusted by using an
electro-proportional reducing valve to regulate a pressure of a regulated pressure
chamber. The pressure within the regulated pressure chamber acts on a portion of the
spool to force the spool toward the second position.
Another aspect of the present disclosure relates to a valve including a valve body
defining a bore. The valve body also defines a discharge pressure port, a control pressure
port and a tank pressure port that are in fluid communication with the bore. The
discharge pressure port is adapted for fluid communication with a discharge of a pump,
the control pressure port is being adapted for fluid communication with a control piston
that controls a discharge rate of the pump, and the tank pressure port is adapted for fluid
communication with a tank. The valve also includes a seal member positioned within the
bore that divides the bore into a first portion and a second portion, and a main spool
positioned within the first portion of the bore. The main spool defines a first axial flow
path and a second axial flow path that extend along a spool axis of the main spool. The
main spool is movable along the spool axis within the first portion of the bore between a
first position and a second position. When the main spool is in the first position the first
axial flow path provides fluid communication between the tank pressure port and the
control pressure port and the main spool blocks fluid communication between the
discharge pressure port and the control pressure port. When the main spool is in the
second position the second axial flow path provides fluid communication between the
control pressure port and the discharge pressure port and the main spool blocks fluid
communication between the tank pressure port and the control pressure port. The main
spool has a first end and an opposite second end. A main spring of the valve is
positioned adjacent the first end of the main spool for biasing the main spool toward the
first position. A discharge pressure chamber of the valve positioned at the second portion

of the bore. The discharge pressure chamber is in fluid communication with the
discharge pressure port. A pin of the valve is carried with the main spool. The pin
extends from the second end of the main spool through the seal member to the discharge
pressure chamber. The pin has an exposed end face positioned within the discharge
pressure chamber and the pin and the main spool move together relative to the seal
member. A regulated pressure chamber of the valve is defined within the first portion of
the bore at the second end of the main spool such that the second end of the main spool
defines a portion of the regulated pressure chamber. An electro-proportional pressure
reducing valve of the valve is mounted to the valve body. The electro-proportional
pressure reducing valve is in fluid communication with the discharge pressure port and
the tank. A regulated pressure flow line is defined by the valve body and provides fluid
communication between the electro-proportional pressure reducing valve and the
regulated pressure chamber. The electro-proportional pressure reducing valve is operable
in a first configuration where the regulated pressure chamber is in fluid communication
with the tank pressure port and is fluidly disconnected form the discharge pressure port.
The electro-proportional pressure reducing valve also is operable in a second
configuration where the regulated pressure chamber is in fluid communication with the
discharge pressure port and is fluidly disconnected from tank.
A variety of additional aspects will be set forth in the description that follows.
These aspects can relate to individual features and to combinations of features. It is to be
understood that both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not restrictive of the broad
concepts upon which the examples disclosed herein are based.
BRIEF DESCRIPTION OF THE DRAWINGS
There are shown in the drawings, embodiments which are presently preferred, it
being understood, however, that the technology is not limited to the precise arrangements
and instrumentalities shown.

FIG. 1 is a top perspective view of a drive control valve that can be used in a fan
drive control system to control a discharge pressure of a pump used power the fan drive.
FIG. 2 is a bottom perspective view of the drive control valve of FIG. 1.
FIG. 3 is an exploded top perspective view of the drive control valve of FIG. 1.
FIG. 4 is a top view of the drive control valve of FIG. 1.
FIG. 5 A is a side sectional of the drive control valve of FIG. 1 incorporated into a
fan drive control system, the valve is shown with a main spool of the valve in a first
position where a pump pressure control port of the valve is adapted to be in fluid
communication with tank pressure.
FIG. 5B is a side sectional of the drive control valve and fan drive control system
of FIG. 1, the valve is shown with the main spool of the valve in a second position where
the pump pressure control port of the valve is adapted to be in fluid communication with
a pump discharge pressure.
FIG. 6 A is a partial schematic diagram of the fan drive control system of FIG. 5 A
with the main spool in the first position of FIG. 5 A and an electro-proportional pressure
reducing valve is operating in a first configuration in which a regulated pressure chamber
corresponding to the main spool is in fluid communication with tank pressure.
FIG. 6B is a partial schematic diagram of the fan drive control system of FIG. 5 A
with the main spool in the second position of FIG. 5B and the electro-proportional
pressure reducing valve is operating in the first configuration in which the regulated
pressure chamber corresponding to the main spool is in fluid communication with tank
pressure.
FIG. 6C is a partial schematic diagram of the fan drive control system of FIG. 5 A
with the main spool in the first position of FIG. 5 A and the electro-proportional pressure
reducing valve is operating in a second configuration in which the regulated pressure
chamber corresponding to the main spool is in fluid communication with pump discharge
pressure.
FIG. 7 is a plot of current versus regulated pressure in a drive control valve.

DETAILED DESCRIPTION
Reference will now be made in detail to the exemplary aspects of the present
disclosure that are illustrated in the accompanying drawings. Wherever possible, the
same reference numbers will be used throughout the drawings to refer to the same or like
structure.
The technology described below has application in systems that utilize
hydraulically driven devices such as fans or motors. Accordingly, this technology has
applications in vehicles, especially those that utilize internal combustion engines or other
systems that generate excess heat that should be cooled. For clarity, however, the
following embodiments will be described in the context of a fan system for cooling an
internal combustion engine.
FIGS. 1 and 2 depict a drive control valve 100 having a valve body 102. The
drive control valve 100 is adapted to interface with a control cylinder to limit a pressure
output by a variable displacement pump. On the underside of the valve body are a
number of ports, including a discharge pressure port 104, a control pressure port 106, and
a tank pressure port 108. An electro-proportional pressure reducing valve 110 is secured
to the valve body 102 with bolts 111, screws, or other fasteners. A pressure adjustment
screw 112 projects from a side of the valve body and is configured for attachment of a
control knob. As described below, turning this pressure adjustment screw 112 allows the
pressure level setting of the valve 100 to be manually adjusted such that the maximum
pressure level of fluid discharged from the pump is manually adjusted. For example, the
pressure adjustment screw 112 can be used to adjust a spring force of applied to a spool
of the valve. The adjustment screw 112 passes through an adjustment cap 114, which is
secured in place by a lock nut 116. A plug assembly 118 is inserted into a port on a rear
side of the valve body 102, and is described in more detail below. Additionally, plugs
seal other ports 120 that were formed in the valve body 102 during the manufacturing
process.

FIG. 3 depicts an exploded perspective view of the valve 100 of FIG. 1. A
regulating valve spool 605 projects from and is controlled by the pressure reducing valve
110. The regulating valve spool 605 projects into a plunger bore 124. The operation of
the pressure reducing valve 110 is described in more detail below. The valve body 102
also defines a main bore 126 in which a number of components are located. Proximate
the plug assembly 118, which seals an opposite end of the bore 126, are a pin 128 and a
seal member 130 through which the pin 128 passes. Once inserted, the pin 128 engages
with a hole defined by a main spool 132 such that the pin 128 is carried with the main
spool 132 as the main spool 132 moves along a spool axis 133 that extends through the
main bore 126. Thus, the main spool 132 and the pin 126 can move as a unit relative to
the seal member 130 within the main bore 126. The main spool 132 defines an interior
channel 138 that defines a first axial flow path. The main spool 132 includes a number of
lands 134 and a groove 136. The groove 136 defines a second axial flow path. A main
spring 140 is mounted within the main bore 126 adjacent the adjustment cap 114. Spring
seats 142 are positioned at opposite ends of the main spring 140. An O-ring 144 seals the
end of the main bore 126 proximate the adjustment cap 114.
FIG. 4 depicts a top view of the valve 100. The view associated with the section
line is depicted in FIGS. 5A and 5B, and shows the valve 100 in an assembled state and
in conjunction with a fan drive control system 500. Additional components of the fan
drive control system 500 include a variable displacement pump 502, a hydraulic fan
motor 504, and a control piston 506. Each of the pump 502, the fan motor 504, and the
tank pressure port 108 are connected to a tank 508, with the pump 502 connected via a
first flow line 502a. Discharge from the pump 502 drives the fan motor 504 via a second
flow line 504a and is directed into the discharge pressure port 104 such that the valve 100
can use the discharge pressure of the pump 502 as an input for controlling the pump
displacement and pump discharge pressure. The control pressure port 106 is connected to
the control piston 506 to control a displacement of the pump 502. For example, pressure
at the control piston 506 can control an angle of a swash plate in the pump 502 so as to

shorten or lengthen the stroke length of the pump 502. In one example, the valve 100 can
place the control pressure port 106 and the control piston 506 in fluid communication
with the discharge pressure of the pump 506 such that the discharge pressure of the pump
is applied to the control piston 506 and provides feedback for controlling pump
displacement. In is regard, displacement of the pump 502 can be indirectly related to a
control pressure applied to the control piston 506. That is, the displacement of the pump
502 can increase as the control pressure decreases, and the displacement of the pump 502
can decrease as the control pressure increases. A solenoid of the electro-proportional
reducing valve 110 is connected to a controller 510, which receives a signal from one or
more temperature sensors 512. The temperature sensor(s) 512 detect a temperature (e.g.,
an engine temperature or a temperature representative of the temperature of the engine)
and send signals related thereto to the controller 510. Various ports formed in the valve
body 102 during manufacturing are sealed with plugs 120'.
The seal member 130 divides the bore 126 into a first portion 514 (to the right of
the seal member 130 in FIG. 5 A) and a second portion 516 (to the left of the seal member
130 in FIG. 5A). The pin 118 extends from an end of the spool 132, through the seal
member 130, and into the second portion 516 of the bore 126. The other components are
located within the first portion 514 of the bore 126.
A number of flow passages and chambers within the valve body 102 direct the
flow from various system components through the valve 100. For example, a discharge
pressure chamber 518 is positioned at the second portion 516 of the bore 126 and is in
fluid communication with the discharge pressure port 104. Additionally, a regulated
pressure flow line 520, from the pressure reducing valve 110 is connected to a regulated
pressure chamber 522 located at the first portion 514 of the bore 126. The pressure
reducing valve 110 is also in fluid communication with the discharge pressure port 114
via a discharge pressure passage arrangement 524, and is in fluid communication with the
tank pressure port 108 via a tank pressure passage arrangement 526. Accordingly, the

hydraulic discharge pressure produced by the pump 520 may act upon an exposed end
128' of the pin 128 at the discharge pressure chamber 518, and a regulated hydraulic
pressure from the reducing valve 110 may act on an end 521 of the spool 132 that is
exposed to fluid pressure in the regulated pressure chamber 522.
In a first position along the spool axis 133 within the main bore 126 (see FIGS.
5A and 6A), the main spool 132 is oriented so as to provide fluid communication
between the tank pressure port 108 and the control pressure port 106, and is also
configured to block fluid communication between the discharge pressure port 104 and the
control pressure port 106. For example, the first axial flow path defined by interior
channel 138 provides fluid communication between the control pressure port 106 and the
tank pressure port 108. One of the lands 134 of the main spool 132 blocks fluid
communication between the discharge pressure port 104 and the control pressure port
106. The main spring 140 biases the main spool 132 toward the first position of FIG. 5 A.
In a second position along the spool axis 133 within the main bore 126 (see FIGS.
5B and 6B), the main spool 132 is oriented so as to provide fluid communication between
the discharge pressure port 104 and the control pressure port 106, and is also configured
to block fluid communication between the tank pressure port 108 and the control pressure
port 106. For example, the second axial flow path defined by groove/passage 136
between two of the lands 124 provides fluid communication between the control pressure
port 106 and the discharge pressure port 104. Other lands 134 of the main spool 132
block fluid communication between the tank pressure port 108 and the control pressure
port 106. The main spring 140 biases the main spool 132 from the second position (FIG.
5B) back toward the first position (FIG. 5A).
Normally, without significant discharge pressure being generated by the pump
502, the main spring 140 holds the main spool 132 in the first position of FIG. 5 A. In the
first position, the control piston 506 is in fluid communication with tank pressure and the
pump 502 has a maximum displacement. As the discharge pressure of the pump 502

increases, a force applied to the end face 128' of the pin 128 increases accordingly. The
force applied to the pin 128 by the pump discharge pressure opposes the spring force
applied to the main spool 132 by the main spring 140. Once the force applied to the pin
128 by the discharge pressure exceeds the spring force of the spring 140, the main spool
140 is forced from the first position (FIG. 5 A) to the second position (FIG. 5B). With the
spool 132 at the second position, the control piston 506 is placed in fluid communication
with the pump discharge pressure causing the pressure acting on the control piston 506 to
increase thereby causing the displacement of the pump (i.e., the stroke length of the
pump) to be reduced. During continued use, the main spool 132 can modulate between
the first and second positions to maintain the pump discharge pressure at a level equal to
or less than the level set by the force of the main spring 140. The pump discharge
pressure level set by the spring 140 can be manually adjusted by using the knob 112 to
adjust the spring force applied to the main spool 132. By increasing the spring force
applied to the main spool 132, the maximum pump discharge pressure permitted by the
valve 100 is increased. By decreasing the spring force applied to the main spool 132, the
maximum pump discharge pressure permitted by the valve 100 is decreased.
The pump discharge pressure level can also be adjusted by the adjusting the
regulated pressure in the regulated pressure chamber 522 via the electro-proportional
pressure reducing valve 110 so that a regulated pressure force applied to the end 521 of
the spool 132 is adjusted. The regulated pressure force opposes the spring force of the
main spring 140. By increasing the pressure in the regulated pressure chamber 522, the
amount of force that must be applied to the pin 128 by the pump discharge pressure to
overcome the spring force is decreased. By decreasing the pressure in the regulated
pressure chamber 522, the amount of force that must be applied to the pin 128 by the
pump discharge pressure to overcome the spring force is increased. Thus, by increasing
the pressure in the regulated pressure chamber 522, the maximum pump discharge

pressure level permitted by the valve 100 is decreased. Similarly, by decreasing the
pressure in the regulated pressure chamber 522, the maximum pump discharge pressure
level permitted by the valve 100 is increased.
The electro-proportional pressure reducing valve 110 is operable in a first
configuration (see FIG. 6A) and a second configuration (see FIG. 6C). When the electro-
proportional pressure reducing valve 110 is in the first configuration, the regulated
pressure chamber 522 is in fluid communication with the tank pressure port 108 and is
fluidly disconnected from the discharge pressure port 104. In this first configuration, the
regulated pressure chamber 522 is at tank pressure and no significant regulated pressure
force is applied to the end 521 of the main spool 132. Thus, the maximum pump
discharge pressure level setting of the valve 100 is set only by the spring force of the
main spring 140. This represents the highest maximum pump discharge pressure level
setting of the valve 100.
When the electro-proportional pressure reducing valve 110 is in the second
configuration, the regulated pressure chamber 522 is in fluid communication with the
discharge pressure port 104 and is fluidly disconnected from the tank port 104. In this
configuration, the regulated pressure chamber 522 is at pump discharge pressure. This
represents the lowest maximum pump discharge pressure level setting of the valve 100.
It will be appreciated that the electro-proportional pressure reducing valve 110 can
modulate between the first and second configurations to maintain a desired regulated
pressure at the regulated pressure chamber 522 that is between tank pressure and pump
discharge pressure. In this way, the maximum pump discharge level setting of the valve
can be set to a value between the lowest and highest maximum pump discharge pressure
level settings of the valve 100.
Referring to FIGS. 6A and 6C, the electro-proportional pressure reducing valve
110 includes a pressure regulating spool 605 that moves to switch the valve 110 between
the first and second configurations. The pressure regulating spool 605 has opposite ends

adjusted by adjusting the regulated pressure at the regulated pressure chamber 522 with
the electro-proportional pressure reducing valve 110. If the current supplied to the
solenoid 607 of the pressure reducing valve 110 is sufficient to overcome the spring force
applied to the end 603 of the spool 605 by the spring 611, the valve 110 moves to the
second configuration of FIG. 6C. In this way, pressure from the pump 502 is delivered to
the chamber 522 via the control pressure flow line 520, and is also delivered to the end
603 of the spool 605. The valve 110 modulates to set the regulated pressure at the
regulated pressure chamber 522 equal to pressure setting of the valve 110. The pressure
setting of the valve 110 can be determined by a current provided to the valve solenoid
(e.g., as depicted in FIG. 7).
As the main spool 132 moves to the second position, the control piston 506 is
placed in fluid communication with pump discharge pressure, thus reducing the
displacement of the pump 502 (e.g., by destroking the pump 502). The fluid pressure
applied to the end 603 of the spool 605 opposes the force applied to the spool 605 by the
solenoid of the reducing valve 110 such that the spool 605 can modulate between axial
positions to maintain a regulated pressure in the pressure line 520. As indicated above,
the regulated pressure is dependent upon the force applied to the spool 605 by the
solenoid of the reducing valve 110 and is therefore dependent upon the electric current
supplied to the solenoid. When no current is being provided to the solenoid 607 of the
reducing valve 110, the spring 602 biases the spool 605 of the reducing valve 110 to a
position where the control pressure flow line 520 is connected to tank 508 such that the
pressure in the regulated pressure chamber 522 is tank pressure. Thus, the regulated
pressure provided by the reducing valve 110 to the regulated pressure chamber 522 can
range from tank pressure as a minimum to the discharge pressure of the pump 502 as a
maximum. FIG. 7 shows an example relationship between the current supplied to the
solenoid and the corresponding regulated pressure provided to the pressure line 520 by
the pressure reducing valve 110.

The system 500 utilizes the spool 132, the pin 128, the electro-proportional
pressure reducing valve 110, and the spring 140, in combination, to adjust pump pressure
based on the temperature of the system 500, as detected by temperature sensor 512. FIG.
7 depicts a plot of current to the electro-proportional pressure reducing valve 110 versus
regulated pressure delivered to regulated pressure chamber 522 of the spool 132. Based
on the temperature detected by sensor 512, the controller 510 will send a signal to the
reducing valve 110. A greater current causes a higher regulated pressure to be delivered
to the regulated pressure chamber 522. This pressure will force the spool 132 to the right
in FIG. 5A, and is in addition to the pump discharge pressure acting on the exposed end
128' of the pin 128. Of course, due to the relationship between the pin 128 and the spool
132, force applied to the end 128' of the pin 128 will also force the spool 132 to the right
in FIG. 5A.
Returning to FIG. 7, as current increases, regulated pressure increases. This will
cause a decrease in the pump pressure setting in accordance with the following equation:
Pd * X * Ap + Pr * (Aspool - Ap) = Kx
where Pd is the pump discharge pressure, Ap is the pin end area, Pr is the regulated
pressure from the pressure reducing valve, Aspool is the spool area, Kx is spring
stiffness, and X is the preset spring deflection. Thus, when no current is delivered to the
solenoid, and therefore, the regulated pressure from the reducing valve is zero, the
maximum pump pressure setting is obtained. Conversely, increases in current decrease
the pump pressure.
Other advantages of the technology described herein would be apparent to a
person of skill in the art. For example, power required for the electro-proportional
pressure reducing valve is reduced compared to similar systems. That is, all the force
required to actuate the valve is not generated by increased current from the electro-

proportional pressure reducing valve. This also allows for use of a smaller pressure
reducing valve. The control provided by the mechanical spring force adjuster and the
pressure reducing valve in combination provide for improved performance at both higher
and lower temperatures. Additionally, power loss to the electro-proportional valve will
not adversely affect operation of the system, as mechanical operation is still available and
is even adjustable due to the presence of the mechanical adjustment screw.
The materials used for the spool, pin, and other components described herein may
be the same as those typically used for hydraulic valves or other similar applications.
These include metals such as steel, stainless steel, titanium, bronze, cast iron, and
platinum, as well as robust plastics or fiber-reinforced plastics.
While there have been described herein what are to be considered exemplary and
preferred embodiments of the present technology, other modifications of the technology
will become apparent to those skilled in the art from the teachings herein. The particular
methods of manufacture and geometries disclosed herein are exemplary in nature and are
not to be considered limiting. It is therefore desired to be secured in the appended claims
all such modifications as fall within the spirit and scope of the technology. Accordingly,
what is desired to be secured by Letters Patent is the technology as defined and
differentiated in the following claims, and all equivalents.

WE CLAIM:
1. A valve comprising:
a valve body defining a bore, the valve body also defining a discharge pressure
port, a control pressure port and a tank pressure port that are in fluid communication with
the bore, the discharge pressure port being adapted for fluid communication with a
discharge of a pump, the control pressure port being adapted for fluid communication
with a control piston that controls a discharge rate of the pump, and the tank pressure port
being adapted for fluid communication with a tank;
a seal member positioned within the bore that divides the bore into a first portion
and a second portion;
a main spool positioned within the first portion of the bore, the main spool
defining a first axial flow path and a second axial flow path that extend along a spool axis
of the main spool, the main spool being movable along the spool axis within the first
portion of the bore between a first position and a second position, wherein when the main
spool is in the first position the first axial flow path provides fluid communication
between the tank pressure port and the control pressure port and the main spool blocks
fluid communication between the discharge pressure port and the control pressure port,
and wherein when the main spool is in the second position the second axial flow path
provides fluid communication between the control pressure port and the discharge
pressure port and the main spool blocks fluid communication between the tank pressure
port and the control pressure port, the main spool having a first end and an opposite
second end;
a main spring positioned adjacent the first end of the main spool for biasing the
main spool toward the first position;
a discharge pressure chamber positioned at the second portion of the bore, the
discharge pressure chamber being in fluid communication with the discharge pressure
port;

a pin carried with the main spool, the pin extending from the second end of the
main spool through the seal member to the discharge pressure chamber, the pin having an
exposed end face positioned within the discharge pressure chamber, the pin and the main
spool being moveable relative to the seal member;
a regulated pressure chamber defined within the first portion of the bore at the
second end of the main spool such that the second end of the main spool defines a portion
of the regulated pressure chamber;
an electro-proportional pressure reducing valve mounted to the valve body, the
electro-proportional pressure reducing valve being in fluid communication with the
discharge pressure port and the tank; and
a regulated pressure flow line defined by the valve body that provides fluid
communication between the electro-proportional pressure reducing valve and the
regulated pressure chamber, the electro-proportional pressure reducing valve being
operable in a first configuration where the regulated pressure chamber is in fluid
communication with the tank pressure port and is fluidly disconnected form the discharge
pressure port, the electro-proportional pressure reducing valve also being operable in a
second configuration where the regulated pressure chamber is in fluid communication
with the discharge pressure port and is fluidly disconnected from the tank port.
2. The valve of claim 1, wherein the electro-proportional reducing valve modulates
between the first and second configurations to provide a regulated pressure in the
regulated pressure chamber.
3. The valve of claim 1, wherein the electro-proportional reducing valve includes a
proportional valve spool that moves between positions to switch the electro-proportional
reducing valve between the first and second configurations, wherein the electro-
proportional reducing valve includes a solenoid that acts on a first end of the proportional
valve spool and a proportional valve spring that acts on a second end of the proportional

valve spool, wherein the proportional valve spring opposes the solenoid and biases
proportional valve spool toward a position corresponding to the first configuration of the
electro-proportional valve, and wherein the regulated pressure of the regulated pressure
chamber acts on the second end of the proportional valve spool to generate a force that
opposes the solenoid.
4. The valve of claim 1, wherein each of the pump, control piston, and tank are
separate from the valve body.
5. The valve of claim 1, further comprising a mechanical spring force adjuster for
manually adjusting a spring force applied to the main spool by the main spring.
6. The valve of claim 5, wherein the mechanical spring force adjuster has a threaded
configuration.
7. The valve of claim 1, wherein the valve body defines a discharge pressure passage
arrangement that provides fluid communication between the second axial flow path of the
main spool and the discharge pressure chamber and also provides fluid communication
between the second axial flow path of the main spool and the electro-proportional
pressure reducing valve.
8. The valve of claim 7, wherein the valve body defines a tank pressure passage
arrangement that provides fluid communication between the electro-proportional pressure
reducing valve and the tank port of the valve body.
9. The valve of claim 1, wherein the second axial flow path of the main spool is
defined between first and second sealing lands of the main spool.

10. The valve of claim 9, wherein the first axial flow path of the main spool extends
through an interior of the main spool and includes an access port defined by the main
spool between the second sealing land of the main spool and a third sealing land of the
main spool.
11. A fan drive control system comprising:
a hydraulic motor for driving a fan;
a temperature sensor;
a controller that interfaces with the temperature sensor;
a variable displacement pump having a discharge port and an intake port;
a first flow line that connects the intake port to a tank;
a second flow line that connects the discharge port to the hydraulic motor;
a control piston for controlling a displacement of the variable displacement pump,
wherein the displacement of the pump is indirectly related to a control pressure applied to
the control piston such that the displacement of the variable displacement pump increases
when the control pressure decreases and the displacement of the variable displacement
pump decreases when the control pressure increases;
a valve body defining a bore, the valve body also defining a discharge pressure
port, a control pressure port and a tank pressure port that are in fluid communication with
the bore, the discharge pressure port being in fluid communication with the second flow
line, the control pressure port being in fluid communication with the control piston and
the tank pressure port being in fluid communication with the tank;
a seal member positioned within the bore that divides the bore into a first portion
and a second portion;
a spool positioned within the first portion of the bore, the spool defining a first
axial flow path and a second axial flow path that extend along a spool axis of the spool,
the spool being movable along the spool axis within the first portion of the bore between

a first position and a second position, wherein when the spool is in the first position the
first axial flow path provides fluid communication between the tank pressure port and the
control pressure port and the spool blocks fluid communication between the control
pressure port and the discharge pressure port, and wherein when the spool is in the
second position the second axial flow path provides fluid communication between the
control pressure port and the discharge pressure port and the spool blocks fluid
communication between the tank pressure port and the control pressure port, the spool
having a first end and an opposite second end;
a spring positioned adjacent the first end of the spool for biasing the spool toward
the second position;
a discharge pressure chamber positioned at the second portion of the bore, the
discharge pressure chamber being in fluid communication with the discharge pressure
port;
a pin carried with the spool, the pin extending from the second end of the spool
through the seal member to the discharge pressure chamber, the pin having an exposed
end face positioned within the discharge pressure chamber, the pin and the spool being
moveable relative to the seal member;
a regulated pressure chamber defined within the first portion of the bore at the
second end of the spool;
an electro-proportional pressure reducing valve mounted to the valve body, the
electro-proportional pressure reducing valve being in fluid communication with the
discharge pressure port and the tank;
a regulated pressure flow line defined by the valve body that provides fluid
communication between the electro-proportional pressure reducing valve and the
regulated pressure chamber; and
wherein the controller controls operation of the electro-proportional pressure
reducing valve to regulate a pressure in the regulated pressure chamber based on a
temperature sensed by the temperature sensor.

12. The fan drive control system of claim 11, wherein the controller varies an
electrical current provided to the electro-proportional pressure reducing valve based on
the temperature sensed by the temperature sensor.
13. The fan drive control system of claim 11, further comprising a mechanical spring
force adjuster for manually adjusting a spring force applied to the spool by the spring.
14. The fan drive control system of claim 13, wherein the mechanical spring force
adjuster has a threaded configuration.
15. The fan drive control system of claim 11, wherein the valve body defines a
discharge pressure passage arrangement that provides fluid communication between the
second axial flow path of the spool and the discharge pressure chamber and also provides
fluid communication between the second axial flow path of the spool and the electro-
proportional pressure reducing valve.
16. The fan drive control system of claim 15, wherein the valve body defines a tank
pressure passage arrangement that provides fluid communication between the electro-
proportional pressure reducing valve and the tank port of the valve body.
17. The fan drive control system of claim 11, wherein the second axial flow path of
the spool is defined between first and second sealing lands of the spool.

18. The fan drive control system of claim 17, wherein the first axial flow path of the
spool extends through an interior of the spool and includes an access port defined by the
spool between the second sealing land of the spool and a third sealing land of the spool.

ABSTRACT

The present disclosure relates to a valve used to control the discharge pressure
and/or the discharge rate of a variable displacement pump. The displacement of the
variable displacement pump is controlled by a control piston. The valve includes a spool
that moves between first and second positions. When the spool is in the first position, the
control piston is connected to tank pressure. When the spool is in the second position, the
control piston is connected to pump discharge pressure. A spring biases the spool toward
the first position. The valve includes a pin carried with the spool. Pump discharge
pressure acts an on an end face of the pin to move the spool from the first position to the
second position. The pump discharge pressure required to move the spool from the first
position to the second position can be manually adjusted by adjusting the spring pressure
applied to the spool. The pump discharge pressure required to move the spool from the
first position to the second position can also be adjusted by an electro-proportional
reducing valve used to regulate a pressure of a regulated pressure chamber. The pressure
within the regulated pressure chamber acts on a portion of the spool to force the spool
toward the second position.