SERV1862
June 2008
GLOBAL SERVICE LEARNING
TECHNICAL PRESENTATION
3PC CONTROL VALVE
PROPORTIONAL PRIORITY, PRESSURE
COMPENSATED (PPPC) HYDRAULIC SYSTEM
OPERATION
Service Training Meeting Guide
(STMG)
3PC CONTROL VALVE
PROPORTIONAL PRIORITY, PRESSURE
COMPENSATED (PPPC) HYDRAULIC SYSTEM
OPERATION
AUDIENCE
Level II - Service personnel who understand the principles of machine systems operation,
diagnostic equipment, and procedures for testing and adjusting.
CONTENT
This presentation provides information on the new Priority Proportional Pressure (3PC)
Compensated Hydraulic main control valve group to be used in small and medium Caterpillar
prime products. This presentation may be used for self-paced and self-directed training.
OBJECTIVES
After learning the information in this presentation, the technician will be able to:
1. identify the major components in the hydraulic system; and
2. explain the function of the major components in the hydraulic system
REFERENCES
STMG 595 "Introduction to Load Sensing/Pressure Compensated Hydraulics
STMG 767 "924G and 924Gz Wheel Loaders Steering, Brake, Fan, and
Implement Hydraulic Systems"
STMG 760 "M313C, M315C, M316C, M318C, and M322C Hydraulic
Excavators - Systems Operation"
TIM "Telehandler 'B' Series Implement and Steering Systems"
Estimated Time: 1 Hour
Illustrations: 20
Handouts: 3
Form: SERV1862
Date: June 2008
© 2008 Caterpillar Inc.
SESV1595
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Text Reference
TABLE OF CONTENTS
INTRODUCTION ........................................................................................................................5
THE PPPC HYDRAULIC SYSTEM...........................................................................................6
Implement Pump.....................................................................................................................7
MAIN CONTROL VALVE GROUP............................................................................................9
Control Valve Operation .......................................................................................................12
CONTROL OF THE CONTROL SPOOL ................................................................................23
Pilot Control Valves ..............................................................................................................24
Solenoid Control Valves .......................................................................................................27
REPRESENTATIVE APPLICATIONS ......................................................................................29
930G Wheel Loader Hydraulic System................................................................................29
"H" Series Medium Wheel Loader Hydraulic System .........................................................31
963D Track-Type Loader Hydraulic System........................................................................33
CONCLUSION...........................................................................................................................35
VISUAL LIST ............................................................................................................................36
HYDRAULIC SCHEMATIC COLOR CODE...........................................................................37
HANDOUTS...............................................................................................................................38
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NOTES
Text Reference
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Text Reference
3PC CONTROL VALVE
PROPORTIONAL PRIORITY, PRESSURE
COMPENSATED (PPPC) HYDRAULIC SYSTEM
OPERATION
© 2008 Caterpillar Inc.
1
INTRODUCTION
A 3PC implement control valve has been introduced on Caterpillar products. The control valve
features a Proportional Priority, Pressure Compensated (PPPC) design that is similar to other
Caterpillar PPPC systems.
A PPPC system, provides precise control of the individual implements. A PPPC hydraulic
system divides the oil flow among all operating circuits in the system. The amount of flow sent
to a particular circuit is proportional to the position of the directional control spool, which is
controlled by a mechanical lever, joystick, or a solenoid.
Since the valves are pressure compensated, cylinder speeds will not change as the load varies
as long as the pump can meet system flow needs.
When the flow demands of the system exceed the total flow available from the pump, the flow
is divided proportionally among all activated circuits; however, the implements will all move
slower due to less flow available.
The new control valve will appear first on small and medium wheel loaders. In the future this
control valve will appear on additional Caterpillar products.
NOTE: Due to Service Training's move to focus on systems, engineering nomenclature
is being used to differentiate this control valve from other presentations with PPPC
control valves.
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Text Reference
3PC HYDRAULIC SYSTEM BLOCK DIAGRAM
HOLD
Manifold
Pump
Control
Valve
Third
Function
Pilot
Accumulator
Inlet
Manifold
Tilt
Cylinder
Tilt
Control
Valve
Fourth
Function
Lift
Cylinders
Lift
Control
Valve
Auxiliary
Control
Valve
Auxiliary
Control
Valve
Hydraulic
Piston
Pump
Tank
Wired to
Controller
Tilt
Pilot
Valve
Lift
Pilot
Valve
Auxiliary
Solenoid
Valves
Auxiliary
Solenoid
Valves
2
THE PPPC HYDRAULIC SYSTEM
The illustration above shows an example of the new Proportional Priority, Pressure,
Compensated (PPPC) hydraulic system. The components in the hydraulic system will include a
tank, variable displacement piston pump, and a main control valve group.
The main control valve group will feature: an inlet manifold, solenoid valves, and individual
control valve sections to operate the various circuits. The pilot control valves are not part of
the main control valve group. The solenoid valve could be part of the main control valve group
or they may be separate. Typically, they will be part of the main control valve group.
The hydraulic flow for the hydraulic system is provided by a variable displacement piston
pump. Depending on the application, all of the circuits could be controlled by pilot control
valves, by proportional solenoid valves, or any combination of the two. The system shown
above shows two valves controlled by pilot control valves and the other two valves by solenoid
valves.
NOTE: The individual control valve could also be designed to be controlled by
mechanical linkage; however, that type of design is not being planned.
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Text Reference
STEERING AND IMPLEMENT PUMP
CONSTANT FLOW
Margin
Spring
Actuator
Piston
Signal
Pressure
Bias
Spring
Stability
Orifice
Signal
Relief Valve
Swashplate
Piston and
Barrel Assembly
Control Valve
Group
Pressure
Cutoff Spool
Margin
Spool
3
Implement Pump
The flow requirements of the system and the available engine horsepower for the hydraulic
system will determine the type of variable displacement pump used. For lower flow systems,
pumps with a single actuator piston will be used. For higher flow systems, pumps with a small
and large actuator piston will be used.
Either type of pump will be controlled by a pump control valve. The pump control valve
contains two spools. The margin spool regulates output flow of the pump to keep the pump
supply pressure at a fixed value above the signal pressure. The difference between the supply
pressure and signal pressure is called "margin pressure." The pressure cutoff spool limits the
maximum system pressure and serves as a back-up relief valve in PPPC hydraulic systems.
The signal limiter and margin spring work together to control the maximum system pressure in
most instances.
The pump is designed to maintain flow. Whenever the forces above and below the margin
spool are not balanced due to changes in the flow demand, the pump will upstroke or destroke
to meet the flow demand.
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Text Reference
When the pump supply pressure equals the sum of the signal pressure plus the margin spring
value, the margin spool moves to a metering position to control oil to and from the actuator.
This action stabilizes the system. The swashplate is held at a relatively constant angle to
maintain the required flow. This condition is called "CONSTANT FLOW."
This pump control valve has a stability orifice in the passage to the actuator piston. The orifice
is used to regulate the response rate of the actuator piston by creating a constant leakage path to
drain.
NOTE: Control of variable displacement pumps is evolving. Solenoids could also be
incorporated in the control of the pump in order to improve the controllability of the
pump for an application.
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Text Reference
3PC MAIN CONTROL VALVE GROUP
HOLD
Pilot
Accumulator
Lift
Cylinders
Tilt
Cylinder
Pilot Shutoff
Solenoid
Lock
Valve
Pilot
Supply
Third
Function
Fourth
Function
Line Relief
Valve
Makeup
Valve
Tank
Pressure
Reducing
Valve
Pressure
Compensator
Valve
Bridge
Passage
Pump
Supply
Control
Spool
Signal
Duplication
Valve
Resolver
Orifice
Load
Signal
to Pump
Signal
Relief
Valve
Tank
Pilot Control
Valve or
Solenoid Valves
Pilot Control
Valve or
Solenoid Valves
Pilot Control
Valve or
Solenoid Valves
Pilot Control
Valve or
Solenoid Valves
4
MAIN CONTROL VALVE GROUP
The following illustrations will cover the operation for the main control valve group. This
illustration shows four control valves with their corresponding components.
The main control valve group features a parallel feeder path, closed-center hydraulic system.
The parallel feeder path in the main control valve group allows two or more functions to be
operated at the same time. The control valves use pressure compensating valves and a resolver
network. The resolver network is used to direct the highest workport pressure to the signal
duplication valve.
The individual control valves have a pressure compensator valve. The pressure compensator
maintains a controlled pressure differential across the spool to control flow to the circuit. If
more than one circuit is used at a time, the circuit with the highest workport pressure is sensed
by the signal duplication valve. The signal duplication valve, duplicates this signal using pump
supply oil and sends this new signal to all of the compensator valves to regulate the flow
through each control valve.
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Text Reference
Part of the pressure compensator valve acts as a load check valve to prevent implement drift
when the individual spool is initially shifted or whenever the pump supply pressure drops
below the circuit pressure.
Some of the oil in each bridge passage of each circuit activated is directed to a resolver. The
resolver is used to compare the workport pressure between two control valves. The resolvers
are part of the signal network. The signal network then sends the highest load signal to the
signal duplication valve.
NOTE: The fourth circuit does not have a ball in the resolver. This feature prevents
the signal pressure from being trapped when the control valve is returned to HOLD.
The signal duplication valve senses the load pressure and converts some of the pump supply oil
to a load signal pressure, which is sent through an orifice to the pump control valve, signal
limiter valve, and also back to all of the pressure compensators. The pump supply oil is used to
create the signal to prevent drifting of the load when the signal limiter valve opens.
Flow through the orifice between the signal duplication valve and the signal limiter valve
(when it is activated) creates a pressure drop that is approximately the same as the margin
pressure. The maximum load signal is limited by the signal limiter valve. The signal limiter
valve works with the margin spring in the pump control valve to control the maximum system
pressure.
NOTE: Without a signal limiter valve in a PPPC system, if a single circuit is stalled,
no other circuit would work due to the pressure compensators in the individual control
valves sensing the same high pressure signal. System pressure and the signal pressure
would be the same. With the signal limiter there will always be a difference in the load
signal and the supply pressure.
Depending on the circuit needs, the individual control valves may be equipped with line relief
valves, makeup valves, or pilot operated lock (load check) valves. The lock valves are used to
reduce cylinder drift when the respective circuit is in HOLD.
NOTE: The pilot operated lock valves may be controlled by the ECM on control valves
that are solenoid controlled.
The pressure reducing valve is used to provide pilot oil for the pilot circuit. The valve also
limits or controls the maximum pilot pressure.
The pilot shutoff valve is used to direct or block pilot oil to the pilot control valves or the
solenoid valves. When the pilot shutoff valve is shifted to the blocked position, the valve
drains the pilot oil in the passage to the pilot control valves or the solenoid valves to the tank.
This action will prevent inadvertent movement of the implements in this position.
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Text Reference
The pilot accumulator will permit the implements to be lowered if the engine is stopped and
helps to maintain a constant pilot supply pressure whenever multiple pilot valves are being
operated at the same time.
The spools in each control valve will be shifted by pilot oil from either hydro-mechanical pilot
control valves or solenoid valves. The solenoid valves may be either on/off type or
proportional. In most cases proportional solenoid valves will be used.
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Text Reference
PILOT OPERATED CONTROL VALVE
HOLD
Signal Duplication
Valve
Signal Duplication
Valve
Bridge
Passage
Port A
Port B
Line
Relief Valve
Pressure
Compensator
Valve
Previous
Resolver
Load Check
Spool
Makeup
Valve
Control
Spool
Lock
Valve
Pilot Control
Valve
Feeder
Passage
Pilot Control
Valve
Internal
Passage
5
Control Valve Operation
In HOLD, the centering springs center the spool in the valve body. The valve may feature line
relief valves and makeup valves. The pressure setting of each line relief valve is adjustable.
The pilot operated lock valve prevents implement drift whenever the control spool is in HOLD.
The pressure compensator maintains a controlled pressure differential across the spool to
control flow to the circuit. If more than one circuit is used at a time, the circuit with the
highest workport pressure is used by the signal duplication valve to create a load signal, which
is sent to all pressure compensator valves. This signal is used to regulate the flow through each
control valve.
The load check portion of the pressure compensator prevents implement drift when actuating a
function which initially has circuit pressure higher than the system pressure. The load check
valve enables the pressure compensator valve to close. This action is accomplished by
connecting the pressure in the bridge passage to the area between the load check valve and the
pressure compensator when the control spool is initially shifted.
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Text Reference
Oil in the bridge passage with the highest workport pressure flows from the bridge passage into
the signal network. The signal network consists of resolver valves, internal passages, and
signal lines. The highest workport pressure is directed through the signal network to the signal
duplication valve.
The signal duplication valve uses supply oil to create a load signal for the main control valve
group. The pressure compensator valves receive a load signal from the signal duplication valve
to proportionally control the oil flow in each control valve.
The internal passage inside the control spools allows the oil in the bridge passage to drain to
tank in HOLD.
NOTE: Typically the springs used with the compensators in a PPPC system are weak
and the springs primary purpose is to push the compensator down against its seat when
the control valves returns to HOLD.
For the valve cut-away illustrations shown, the pilot control valve could be a
hydro-mechanical pilot control valve, solenoid valves mounted directly to the valve
body, or remote mounted solenoid valves.
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Text Reference
PRESSURE COMPENSATOR VALVE OPERATION
From Signal
Duplication Valve
Signal
Pump Control
Valve and
Signal Relief Valve
Signal
Duplication
Valve
Resolver
Valve A
Valve B
Valve C
Pressure
Compensator
Valve
Load
Check Spool
Spool
Pump
Supply
HOLD
Bridge
Passage
Pump
Supply
Feeder
Passage
LOW PRESSURE
Pump
Supply
HIGH PRESSURE
6
Three compensators are shown in various states in this illustration. The pressure compensator
valve for valve "A" is in HOLD. The circuit with the highest workport pressure, valve "C", is
sent to the signal duplication valve.
The signal from the duplication valve regulates the flow through all activated control valves.
When a single hydraulic circuit is activated from the HOLD position to the position as shown
for valve "C" from a pump STANDBY condition, the control spool is shifted and the bridge
passage no longer is open to the tank. The bridge passage will be connected to either Port A or
Port B (see previous visual).
Pump supply oil, at standby pressure, enters the feeder passage below the pressure compensator
valve and the load check valve. If the implement is on the ground or is being lowered, the
standby pressure moves the pressure compensator valve and load check valve up. When the
valves move up, supply oil enters the bridge passage of the control valve. Supply oil in the
bridge passage enters the signal network sending the workport pressure to the signal duplication
valve.
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Text Reference
NOTE: Once the load check valve opens, the load check valve and pressure
compensator will move together as if they were one solid valve.
The signal duplication valve (not shown) converts some of the pump supply oil to a load signal
and sends the signal to the pump control valve and back to all of the pressure compensators.
The pump control valve reacts to the change in flow demand and the pump UPSTROKES to
increase flow. The increased flow increases the pressure in the feeder passage below the
pressure compensator and load check valve. The pressure continues to increase rapidly until
the implement starts to move.
The signal oil (from the signal duplication valve) acting on the top of the pressure compensator
valve works against the forces working below the pressure compensator and load check valve.
When the forces are in balance, the supply oil is metered through the cross-drilled holes in the
pressure compensator to provide workport oil. The maximum pressure of the signal oil is
limited by the signal limiter valve.
If the implement is off of the ground and the operator decides to further raise the implement,
the pressure in the bridge passage will initially be higher than the pressure in the feeder
passage.
Some of the oil in the bridge passage will be used to stroke the pump prior to the pressure
compensator and load check valve moving up. Once system pressure increases above the
workport pressure, then the pressure compensator and load check valve move up.
When more than one circuit is activated at the same time, the highest workport pressure is
directed to the signal duplication valve. The signal duplication valve converts the supply oil to
duplication signal oil and sends the signal oil to the chamber at the top of all pressure
compensators valves.
With the same signal duplication pressure working on all pressure compensators, the pressure
differential across all shifted control spools is the same, as shown in the illustration for the
pressure compensator for valve "C" and for valve "B." The pressure differential across the
control spools will be the same value whether the pump can satisfy the flow demand for all
activated circuits or not.
For example, if the margin pressure is 2100 kPa (300 psi) the pressure differential between the
pump supply passage and the feeder passage will be approximately 2100 kPa (300 psi)
regardless of what the circuit pressure is. With multiple valves activated the circuit pressures
will vary.
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Text Reference
When the pump cannot meet the flow needs of all activated circuits, the pressure compensators
will move down to proportion the pump flow in relation to the amount of control spool travel
for each circuit. The pressure differential will be less than described in the example, but the
pressure differential across all spools will be the same.
Valve "B" pressure compensator shows what occurs when an additional circuit is activated with
a lower circuit pressure than the first activated valve.
The load signal from valve "C" pressure compensator is directed to the top of the valve "B"
pressure compensator valve with the lower circuit pressure. When the control spool is moved,
pressure oil in the feeder passage moves the pressure compensator valve and load check valve
up allowing oil into the bridge passage. Since the workport pressure is lower than the workport
pressure in valve "C", the resolver valve for valve "B" transmit the valve "C" workport pressure
to the valve "A" resolver.
The pressure compensator valve for valve "B" will respond to changes in the circuit pressure by
opening and closing off the passage between the feeder passage and the bridge passage to
maintain a constant flow rate for a given control spool displacement. As the compensator
opens and closes, the pressure differential across the compensator will vary in order to maintain
a constant flow rate to the implement, while the pressure differential across the main control
spool remains the same as all other activated main control spools.
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Text Reference
PILOT OPERATED CONTROL VALVE
LOAD CHECK VALVE OPERATION
Signal
Duplication Valve
Signal Duplication Valve
Previous Resolver
Rod
End
Head
End
Pressure
Compensator
Valve
Load Check
Valve
Line Relief
Valve
Bridge
Passage
Feeder
Passage
Pilot Control
Valve
Pilot Control
Valve
Pump
Supply
Control
Spool
7
The above illustration shows the load check valve and the compensator when they are
separated. The example above would occur when raising or racking back an implement when
the pump is in a standby state or supply pressure is lower than the work port pressure.
This separation occurs after machine startup and when the operator moves the joystick partially.
When the workport pressure of the circuit is greater than the pump pressure in the feeder
passage and before the pump supply pressure can increase, the workport pressure will enter
between the load check and the compensator causing them to split.
The load check moves down blocking the pump pressure in the bridge passage from entering
the circuit. This action prevents unwanted or unexpected movement of the implement by not
allowing the circuit pressure to flow back through the compensator and into the feeder and
pump supply passages.
As soon as pump supply pressure is greater than the circuit pressure, the load check and the
compensator will move back together and function as one unit.
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Text Reference
PILOT OPERATED CONTROL VALVE
HIGHEST CIRCUIT LOAD SIGNAL
Next
Resolver
Signal Duplication
Valve
Bridge
Passage
Previous
Resolver
Port A
Port B
Pressure
Compensator
Valve
Load Check
Spool
Feeder
Passage
Lock
Valve
Control
Spool
Pilot Control
Valve
Pilot Control
Valve
8
When a single hydraulic circuit is activated from the HOLD position from a pump STANDBY
condition, the control spool is shifted by pilot pressure.
Pump supply oil enters the feeder passage below the pressure compensator valve and the load
check valve. Pressure increases and the pressure compensator valve and the load check valve
move up. When the valves moves up, supply oil in the feeder passage flows into the bridge
passage and becomes circuit oil.
Some of the oil is directed to the resolver valve and is sent to the signal network. The oil in the
signal network acts on the signal duplication valve. The signal duplication valve senses the oil
in the signal network. The signal duplication valve uses some of the pump supply oil to create
a load signal. The signal duplication valve directs this load signal to the chamber above the
pressure compensator and to the margin spool in the pump control valve.
Operation of the pump control, the pressure compensator, and the load check is the same as
previously discussed.
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Text Reference
When supply pressure in the bridge passage increases to more than the circuit pressure, the lock
valve shifts to the left and pump flow in the bridge passage goes past the lock valve into the
circuit port.
Return oil flows from the other port, flows around the main spool, and is directed to the tank.
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Text Reference
PILOT OPERATED CONTROL VALVE
LOWER CIRCUIT LOAD SIGNAL
Next
Resolver
Signal Duplication
Valve
Bridge
Passage
Previous
Resolver
Port A
Port B
Pressure
Compensator
Valve
Feeder
Passage
Check
Valve
Control
Spool
Piston
Pilot Control
Valve
Pilot Control
Valve
9
When more than one circuit is activated at a time, the highest circuit pressure is directed to the
signal duplication valve through the signal network. The signal duplication valve then creates a
load signal as previously discussed. The signal is sent to to the top of all the pressure
compensator valves.
With the duplicated signal pressure working on all pressure compensators, the pressure
differential across all shifted spools is the same.
When the pump cannot meet the flow needs of all activated circuits, the pressure compensators
will move down to proportion the pump flow in relation to the amount of control spool travel
for each circuit. The pressure differential will be less than when the flow needs can be met, but
the lower pressure differential will be the same for all spools.
This illustration shows what occurs when an additional circuit is activated with a lower circuit
pressure than the first activated valve,which has a higher circuit pressure. Operation of the
pressure compensator for a lower circuit pressure is as previously discussed.
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Text Reference
When the control spool is shifted to the right, pilot oil is directed to the lock valve. Pilot oil
enters the chamber above the piston and moves the piston to the right. The piston then unseats
the check ball allowing the blocked oil in the chamber to flow to the tank. The oil returning
from the workport is now able to open the check valve and the return oil flows to tank.
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Text Reference
3PC MAIN CONTROL VALVE GROUP
ACTIVE
Pilot
Accumulator
Lift
Cylinders
Tilt
Cylinder
Third
Function
Fourth
Function
Pilot Shutoff
Solenoid
Lock
Valve
Line Relief
Valve
Pilot
Supply
Makeup
Valve
Tank
Pressure
Reducing
Valve
Pump
Supply
Pressure
Compensator
Valve
Control
Spool
Signal
Duplication
Valve
Bridge
Passage
Resolver
Orifice
Load
Signal
to Pump
Signal
Relief
Valve
Tank
Pilot Control
Valve or
Solenoid Valves
Pilot Control
Valve or
Solenoid Valves
Pilot Control
Valve or
Solenoid Valves
Pilot Control
Valve or
Solenoid Valves
10
The illustration above shows two valves being activated. One has a higher workport pressure
than the other. The higher workport pressure is transmitted through the signal network to the
signal duplication valve.
The pressure differential across both activated spools is the same due to the same signal
working on both of the pressure compensators.
NOTE: Due to the orifice in the upper pressure compensator envelope, there would be
a pressure drop across the pressure compensator, even if the compensator did not use a
spring.
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Text Reference
CONTROLS FOR THE CONTROL VALVES
Function
PILOT CONTROLLED
SOLENOID CONTROLLED
Lock
Valve
Pilot Oil
Pump Supply
Next Resolver
Pump Supply
Signal
Duplication
Valve
Orifice
Pilot Oil
ECM
Next Resolver
Signal Duplication
Valve
Pilot Oil
Proportional
Solenoid
Pilot Control
Valve
Position
Sensor
11
CONTROL OF THE CONTROL SPOOL
The spools of each circuit are controlled by pilot oil. The pilot oil is directed to the spools by
either hydro-mechanical pilot control valves or solenoid valves. Typically, proportional
solenoid valves will be used.
An ECM is typically used to control the solenoid valves. The ECM will have flashable
software, which will provide additional control flexibility.
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Text Reference
PILOT CONTROL
VALVE
Lever
Detent Plunger
HOLD
Plate
Adjustment
Pin
Spring
Retainer
Plunger
Detent Coil
Spring
Spool
Spring
Orifice
Pilot Supply
Spring
Chamber
Control
Valve
Control
Valve
12
Pilot Control Valves
The pilot control valves can be a lever type or a joystick type. A typical joystick-type pilot
control valve is shown above.
In HOLD, the springs hold the plungers and spools up. Pilot oil is blocked by the spools. The
pilot lines from the control valve are open to tank around each spool.
Some of the circuits may use detent coils to hold the lever for certain functions, such as FLOAT
or for the bucket kickouts. The detent plungers provide feedback to the operator, as to starting
to enter a detent coil position. The adjustment pin is used to adjust the point at which the
plunger contact begins when the lever is shifted.
The orifice in the lower end of the spool dampens the downward spool travel when the pilot
valve is shifted. Oil trapped in the spring chamber is forced up through the orifice as the spool
moves down.
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Text Reference
PILOT CONTROL
VALVE
Lever
SHIFT
Detent Plunger
Adjustment Pin
Plate
Retainer
Plunger Spring
Plunger
Detent Coil
Spool Spring
Spool
Orifice
Pilot Supply
Control
Valve
Control
Valve
13
When the lever is shifted, the adjustment pin contacts the plunger and pushes it down against
its spring. The plunger will contact the spool and move it down against its spring. Depending
on how far the lever is shifted, determines how far the spool moves.
As the spool moves down, the spool will close off the drain passage for the oil to the control
valve and meter pilot oil to the control valve to cause the control spool (not shown) to shift.
The greater the pilot oil flow to the control spool the greater the control spool travel.
As pressure increases in the pilot line to the control valve, the pressure works on the spool to
move the spool up to a balance position against the spool and the plunger springs to maintain
the pilot pressure in the pilot line. This action will maintain the position of the control spool in
the control valve.
In summary, once the pilot lever is shifted, the pilot valve becomes a pressure reducing valve
which maintains a downstream pressure equal to the spring forces above the spool.
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Text Reference
PILOT CONTROL
VALVE
Lever
IN DETENT
Detent Plunger
Adjustment Pin
Plate
Retainer
Plunger
Detent Coil
Spool
Orifice
Pilot Supply
Control
Valve
Control
Valve
14
When the key start switch is in the ON position, the detent coils are energized.
As the operator shifts the lever further, the detent plunger begins to provide feedback
resistance.
The retainer contacts the detent coil. The retainer and lever is then held by the detent coil until
the operator moves the lever out of detent or power to the detent coil is stopped.
Power to the detent coil may be stopped by a "kickout" switch mounted on the cylinders or to
the loader linkage.
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Text Reference
SOLENOID OPERATED CONTROL VALVE
HOLD
Next Resolver
Signal Duplication Valve
Previous
Resolver
Port A
Port B
Lock
Valve
Pressure
Compensator Valve
Load Check
Spool
Bridge Passage
Line Relief
and Makeup
Valve
Feeder Passage
Internal
Drain
Passage
Control Spool
Pilot Oil
Proportional
Solenoid
Joystick
Control
Joystick
Control
Tank
Pump
Supply
Tank
15
Solenoid Control Valves
Solenoid control valves can also be used to shift the control spool. The solenoid valves may be
on/off or proportional as shown above. The solenoid valves are installed into the main control
valve group.
NOTE: The solenoid valves could also be remotely mounted if needed, perhaps due to
a serviceability issue.
Pilot oil is supplied to the proportional solenoids by the pressure reducing valve in the inlet
manifold. The solenoid valve blocks the pilot oil until a function is activated. At the same
time the solenoid valve will drain the passage to the end of the control spool.
SERV1862
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Text Reference
SOLENOID OPERATED CONTROL VALVE
CIRCUIT ACTIVATED
Next Resolver
Signal Duplication Valve
Previous
Resolver
Port A
Port B
Lock
Valve
Pressure
Compensator
Valve
Load Check
Spool
Bridge
Passage
Control
Spool
Feeder
Passage
Pilot
Oil
Proportional
Solenoid
Joystick
Control
Pump
Supply
Internal
Passage
Joystick
Control
16
When a circuit is activated, a Pulse Width Modulated (PWM) signal sent from the joystick is
sent to the ECM. The ECM sends a current to the proportional solenoid. The proportional
solenoid moves in relation to the amount of current that is sent from the ECM. When the
proportional solenoid moves the valve, pilot oil shifts the control spool.
The control spool shifts allowing supply oil to enter the feeder passage. The resolvers, the
signal duplication valve, the pump control, and the pressure compensator valve and the load
check will operate as previously discussed.
When the lock valve is energized, the check valve (not shown) inside the valve unseats to allow
return oil from Port A to flow to the control spool and then back to the tank.
SERV1862
06/08
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Text Reference
930G WHEEL LOADER PILOT OPERATED HYDRAULIC SYSTEM
HOLD
Quick Coupler
and
Auxiliary Control
Valves
Manual Lowering
Valve
Combination
Valve
Pilot
Accumulator
Steering
Steering
Manifold
Return
Restrictor
Inlet
Manifold
Priority
Valve
Tilt
Cylinder
Third
Function
Fourth
Function
Lift
Cylinders
Pilot
Shutoff
Valve
Pressure
Reducing
Valve
Tilt
Control
Valve
Lift
Control
Valve
Auxiliary
Control
Valve
Auxiliary
Control
Valve
Pump
Control
Valve
Signal
Duplication
Valve
Hydraulic Piston
Pump
Signal
Relief
Valve
Signal
Drain
Valve
Tank
Drain
Valve
Signal From
Quick Coupler
And Aux 5 and 6
Wired To
Controller
Flow
Control
Valve
Tilt Pilot
Valve
Lift Pilot
Valve
Auxiliary
Pilot
Valve
Auxiliary
Pilot
Valve
17
REPRESENTATIVE APPLICATIONS
The following illustrations will cover the PPPC control valve used in two different applications.
Only the differences from what has been already discussed will be covered.
930G Wheel Loader Hydraulic System
The above illustration is for the 930G Wheel Loader. All of the PPPC control valves are
controlled by pilot valves. For the 930G system, a steering priority valve is provided to
prioritize the flow to the steering circuit before the oil is made available to the implement
circuits. This strategy has been used on other products before such as Caterpillar Backhoe
Loaders and Telehandlers. The steering priority valve is only pressure compensated versus
being PPPC.
Oil from the pump has to flow through the priority valve before the flow is available to the
implements. Whenever steering is activated, the priority valve shifts to the right. Flow is
restricted to the implements in order to meet the flow needs of the steering circuit first. Once
steering flow needs are met the priority valve will move to a metering position. Any additional
pump flow can be used to operate the implements.
SERV1862
06/08
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Text Reference
A separate resolver between the priority valve on the main control valve group is used to
compare the steering signal to the highest implement circuit signal. The resolver sends the
highest signal to the pump control valve.
The flow control valve for one of the auxiliary circuits allows the auxiliary pilot valve to be put
into detent and then the operator can vary the amount of pilot oil to the auxiliary control spool
in order to control the speed of the work tool. When the engine is started the solenoid on the
valve is fully energized to allow full pilot oil to the auxiliary spool. The current to the solenoid
can then be varied by a switch in the operator's compartment.
The return restrictor is used to slow the return oil from going to the tank when am implement is
lowered. The restrictor will help to open the makeup valves in the individual circuits to reduce
cylinder cavitation.
The resolver, next to the signal duplication valve, allows an external load signal to be shared
with the main control valve group. The resolver provides flow sharing between the main
control valve group and an external valve (auxiliary and coupler as used on the 930G) by
allowing the external valve load signal to act on the compensators when the external valve is
higher than the pressure created by the signal duplicating valve.
The signal drain valve sends the signal pressure from an external valve to the tank when the
engine is off or the external valve is returned to HOLD. The signal drain valve is not required
for the valves in the main control valve group.
The combination valve for the tilt circuit on the 930G provides a head end line relief function
for back dragging. The combination valve in the tilt circuit permits a higher circuit pressure
when back dragging than in non-back dragging condition.
The manual lowering valve permits the lift cylinders to lower the loader arms in case the
engine stops and it is necessary to lower the bucket or work tool to the ground.
The drain valve is used to drain the hydraulic system.
SERV1862
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Text Reference
"H" SERIES MEDIUM WHEEL LOADER IMPLEMENT HYDRAULIC SYSTEM
HOLD / RIDE CONTROL AUTO / 9.7 KM/H (6 MPH) AND ABOVE
Tilt
Cylinder
Ride
Control
Accumulator
Lift
Cylinders
Load Check
Valve
Manual
Lower
Valve
Line
Relief
Valves
Rod End
Solenoid Valve
Load Check
Valve
Rackback Pilot
Solenoid Valve
Lift
Spool
Choke Check
Valve
Case
Drain
Filter
Tank
Dump Pilot
Solenoid Valve
Pump Control
Valve
Pilot
Accumulator
Balance
Solenoid
Valve
Pressure
Compensator
Valve
Signal
Relief
Valve
Pilot Shutoff
Valve
Ride Control
Relief Valve
Pilot
Pressure
Reducing
Valve
Balance
Valve
Resolver
Signal
Duplication
Valve
Head End
Solenoid Valve
Raise Pilot
Solenoid Valve
Tilt Spool
Screen
Auxiliary
Function
Lower / Float
Pilot
Solenoid Valve
Steering
Pilot
Supply
(Command
Control
Steering
only)
Margin Relief
Valve
Orifice
Min
Pump
18
"H" Series Medium Wheel Loader Hydraulic System
The implement control spools for the "H" Series Medium Wheel Loaders are controlled by
proportional solenoid valves, which are mounted on the individual valve sections. Operation of
the implement control valves with proportional solenoid valves is as discussed previously.
This system is not equipped with a signal drain valve. Besides the pressure compensator in the
pump control valve the system features a margin relief valve in the end cover.
The margin relief valve limits the maximum differential pressure between the pump supply
pressure and the signal pressure. The valve is set higher than the margin pressure setting of the
pump. This valve provides better control of pressure spikes when flow is reduced, such as
cylinder reaching end of stroke or when quickly returning valves to HOLD or NEUTRAL.
A choke check valve is used to control the de-stroking speed of the pump.
SERV1862
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Text Reference
The "H" Series Medium Wheel Loader system is equipped with Ride Control. Ride Control
cushions the load during traveling conditions to provide smoother machine travel. In the
illustration, Ride Control is ON.
When the Ride Control system is activated by the transmission ECM, the oil at the head ends
of the lift cylinders is connected to the Ride Control accumulator.
A floating piston in the Ride Control accumulator separates the oil from the nitrogen gas.
Since nitrogen gas is compressible, the gas serves as a spring. Any downward force on the lift
arms is transferred through the oil at the head end of the lift cylinders to the accumulator. The
force in the oil is transmitted to the accumulator piston, which compresses the nitrogen gas.
Compressing the nitrogen gas absorbs the pressure spike and the oil displacement caused by the
downward force on the lift arms.
This operation results in less ground induced shocks on structures and components, reduced tire
flexing, and a greater payload retention.
The Ride Control system is turned on or off by the three-position Ride Control switch located
in the operator's station. The operator can select three modes: AUTO, OFF, or ON.
The function of the Ride Control balance valve is to balance the pressure in the accumulator
and the head end of the lift cylinders. Oil pressure from the head end of the lift cylinders
always acts against the left end of the Ride Control balance valve.
Pressure in the accumulator works on the right end of the balance whenever the balance
solenoid valve is energized. When the accumulator is charging, the balance valve is shifted to
the right. As pressure increases in the accumulator, the balance valve will move back to the left
to maintain the pressure in the accumulator.
With the system ON, both the head end and rod end solenoid valves are energized to allow
flow to and from the lift cylinders.
SERV1862
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Text Reference
963D IMPLEMENT HYDRAULIC SCHEMATICS
HOLD
Accumulator
Charge
Pump
Lift Cylinders
Lower
Raise
MP Bucket Cylinders
Tilt Cylinder
Dump
Manual Lower
Valve
Relief
Valve
Rackback
Two Stage
Relief Valve
Implement
Lockout
Valve
Ripper Cylinders
Open
Raise
Lower
Close
Anti-drift
Valve
EH Pilot
Valve
Signal
Duplication
Valve
Resolver
Signal
Relief
Valve
Compensator
EH Pilot
Valve
Margin
Relief
Valve
Machine ECM
Flow Compensator
Tank
Pressure Cutoff
Large
Actuator
Fan
Pump
Small
Actuator
Joystick
Implement
Pump
19
963D Track-Type Loader Hydraulic System
The above illustration shows the 963D implement hydraulic system with all controls in the
HOLD position. A signal relief valve limits the maximum load sensing pressure for the system.
The main control valve group features a parallel feeder path, closed-center hydraulic system.
The parallel feeder path in the main control valve group allows two or more functions to be
operated at the same time. The control valves use pressure compensating valves and a resolver
network. The resolver network is used to direct the highest workport pressure to the signal
duplication valve.
Each individual control valve have a pressure compensator valve. The signal duplication valve,
duplicates this signal using pump supply oil and sends this new signal to all of the compensator
valves to regulate the flow through each control valve.
Part of the pressure compensator valve acts as a load check valve to prevent implement drift
when the individual spool is initially shifted or whenever the pump supply pressure drops
below the circuit pressure.
SERV1862
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Text Reference
The oil pressure in each bridge passage of each activated circuit is directed to a resolver. The
resolver is used to compare the workport pressure between two control valves. The resolvers
are part of the signal network. The signal network then sends the highest load signal to the
signal duplication valve.
NOTE: The ripper circuit does not have a ball in the resolver. This feature prevents
the signal pressure from being trapped when the control valve is returned to HOLD.
The signal duplication valve senses the load pressure and converts some of the pump supply oil
to a load signal pressure, which is sent through an orifice to the pump control valve, to the
signal limiter valve, and also to the spring chamber of all of the pressure compensators. The
pump supply oil is used to create the signal to prevent drifting of the load when the signal relief
valve opens.
Flow through the orifice between the signal duplication valve and the signal relief valve (when
it is activated) creates a pressure drop that is approximately the same as the margin pressure.
The maximum load signal is limited by the signal relief valve. The signal relief valve works
with the margin spring in the pump control valve to control the maximum system pressure.
NOTE: Without a signal relief valve in a PPPC system, if a single circuit is stalled, no
other circuit would work due to the pressure compensators in the individual control
valves sensing the same high pressure signal. System pressure and the signal pressure
would be the same. With the signal relief there will always be a difference in the load
signal and the supply pressure.
Depending on the circuit needs, the individual control valves may be equipped with line relief
valves, makeup valves, or pilot operated lock (load check) valves. The lock valves are used to
reduce cylinder drift when the respective circuit is in HOLD.
The pressure reducing valve is used to provide pilot oil for the pilot circuit. The valve also
limits or controls the maximum pilot pressure.
The implement lockout or pilot shutoff valve is used to direct or block pilot oil to the pilot
control valves or the solenoid valves. When the valve is shifted to the blocked position, the
valve drains the pilot oil in the passage to the pilot control valves or the solenoid valves to the
tank. This action will prevent inadvertent movement of the implements in this position.
A pressure reducing valve creates pilot supply oil. This oil is sensed at all of the PWM
solenoids. When a solenoid is energized, pilot oil is directed to shift a control spool. The pilot
accumulator will permit the implements to be lowered if the engine is stopped and helps to
maintain a constant pilot supply pressure whenever multiple pilot valves are being operated at
the same time.
The margin relief is used to unload the pump flow when one of the control valves is rapidly
shifted for less flow. Standby pressure is typically higher on systems with a margin relief valve
due to the additional flow being relieved to tank.
SERV1862
06/08
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Text Reference
20
CONCLUSION
This presentation has discussed the component locations and hydraulic system operation for the
3PC Control Valve.
When used in conjunction with the service manual, the information in this package should
permit the technician to do a thorough job of analyzing a problem in these systems.
Always refer to the service manual for the latest service information and specifications when
servicing, testing and adjusting, or making repairs.
SERV1862
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VISUAL LIST
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Model view
PPPC hydraulic system
Implement pump
Main control valve group
Control valve operation
Pressure compensator valve operation
Pilot operated control valve - load check
valve operation
Pilot operated control valve - highest
circuit load signal
Pilot operated control valve - lower
circuit load signal
3PC main control valve group - active
Controls for the control valves
Pilot control valve - hold
Pilot control valve - shift
Pilot control valve - in detent
Solenoid operated control valve - hold
Solenoid operated control valve - circuit
activated
930G wheel loader pilot operated
hydraulic system - hold
"H" series medium wheel loader
implement hydraulic system
963D track-type loader hydraulic system
Conclusion
Text Reference
SERV1862
06/08
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Text Reference
HYDRAULIC SCHEMATIC COLOR CODE
Black - Mechanical Connection. Seal
Red - High Pressure Oil
Dark Gray - Cutaway Section
Red / White Stripes - 1st Pressure Reduction
Light Gray - Surface Color
Red Crosshatch - 2nd Reduction in Pressure
White - Atmosphere or Air (No Pressure)
Pink - 3rd Reduction in Pressure
Purple - Pneumatic Pressure
Red / Pink Stripes - Secondary Source Oil Pressure
Yellow - Moving or Activated Components
Orange - Pilot, Charge or Torque Converter Oil
Cat Yellow - (Restricted Usage)
Identification of Components
within a Moving Group
Orange / White Stripes - Reduced Pilot, Charge, or
TC Oil Pressure
Brown - Lubricating Oil
Orange / Crosshatch - 2nd Reduction in Pilot,
Charge, or TC Oil Pressure
Green - Tank, Sump, or Return Oil
Blue - Trapped Oil
Green / White Stripes Scavenge / Suction Oil or Hydraulic Void
HYDRAULIC SCHEMATIC COLOR CODE
This illustration identifies the meanings of the colors used in the hydraulic schematics and
cross-sectional views shown throughout this presentation.
Green / White Stripes Scavenge / Suction Oil or Hydraulic Void
Blue - Trapped Oil
- 38 -
Green - Tank, Sump, or Return Oil
Orange / Crosshatch - 2nd Reduction in Pilot,
Charge, or TC Oil Pressure
Orange / White Stripes - Reduced Pilot, Charge, or
TC Oil Pressure
Cat Yellow - (Restricted Usage)
Identification of Components
within a Moving Group
Brown - Lubricating Oil
Orange - Pilot, Charge or Torque Converter Oil
Red / Pink Stripes - Secondary Source Oil Pressure
Pink - 3rd Reduction in Pressure
Yellow - Moving or Activated Components
Purple - Pneumatic Pressure
White - Atmosphere or Air (No Pressure)
Red Crosshatch - 2nd Reduction in Pressure
Red / White Stripes - 1st Pressure Reduction
Dark Gray - Cutaway Section
Light Gray - Surface Color
Red - High Pressure Oil
Black - Mechanical Connection. Seal
HYDRAULIC SCHEMATIC COLOR CODE
SERV1862
06/08
Handout No. 1
SERV1862
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Handout No. 2
POSTTEST
Directions: Modified True/False. If the question is false, circle the word or words that make
the statement incorrect and replace with the word(s) to make the statement correct.
1. The signal duplication valve uses pilot oil to create a load sense signal.
2. The pressure compensators all receive the same load sense signal when control
valves are activated
3. The control spools can only be controlled by proportional solenoids.
4. All circuits are equipped with lock valves.
5. The pressure drop across all activated control spools is the same.
6. The pressure drop across all activated pressure compensators is the same.
7. When two or more circuits are activated, the circuit with the highest work port
pressure is sensed at the signal duplication valve.
8. The load check valve opens whenever the work port pressure is higher than the pump
supply pressure.
9. Whenever the flow demand exceeds the capability of the pump, the compensators
will shift to proportionally reduce the flow to all activated circuits.
10. Whenever the flow demand exceeds the capability of the pump, the pressure
differential across all activated control spools will be higher.
SERV1862
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Handout No. 3
POSTTEST ANSWERS
Directions: Modified True/False. If the question is false, circle the word or words that make
the statement incorrect and replace with the word(s) to make the statement correct.
F
1. The signal duplication valve uses pilot pump supply oil to create a load sense
signal.
T
2. The pressure compensators all receive the same load sense signal when control
valves are activated
F
3. The control spools can only be controlled by proportional solenoids, pilot control
valves, on/off solenoid valve and even mechanical linkage
F
4. All circuits are NOT equipped with lock valves.
T
5. The pressure drop across all activated control spools is the same.
F
6. The pressure drop across all activated pressure compensators is NOT the same.
T
7. When two or more circuits are activated, the circuit with the highest work port
pressure is sensed at the signal duplication valve.
F
8. The load check valve opens whenever the work port pressure is higher less than the
pump supply pressure.
T
9. Whenever the flow demand exceeds the capability of the pump, the compensators
will shift to proportionally reduce the flow to all activated circuits.
F 10. Whenever the flow demand exceeds the capability of the pump, the pressure
differential across all activated control spools will be higher lower.