TECHNICAL PRESENTATION
3PC CONTROL VALVE
PROPORTIONAL PRIORITY, PRESSURE
COMPENSATED (PPPC) HYDRAULIC SYSTEM
OPERATION
Service Training Meeting Guide
(STMG)
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 SESV1595 STMG 767 "924G and 924Gz Wheel Loaders Steering, Brake, Fan, and
Implement Hydraulic Systems" SERV1767
STMG 760 "M313C, M315C, M316C, M318C, and M322C Hydraulic
Excavators - Systems Operation" SERV1760
TIM "Telehandler 'B' Series Implement and Steering Systems" SERV2741
Estimated Time: 1 Hour Illustrations: 20
Handouts: 3 Form: SERV1862 Date: June 2008
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
1
3PC CONTROL VALVE
PROPORTIONAL PRIORITY, PRESSURE
COMPENSATED (PPPC) HYDRAULIC SYSTEM
OPERATION
© 2008 Caterpillar Inc.
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.
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. Manifold Tank Wired to Controller Lift Cylinders Tilt Cylinder Third
Function FunctionFourth Pilot Accumulator Auxiliary Solenoid Valves Lift Pilot Valve Tilt Pilot Valve Auxiliary Solenoid Valves Auxiliary Control Valve Auxiliary Control Valve Lift Control Valve Tilt Control Valve Inlet Manifold Hydraulic Piston Pump Pump Control Valve
3PC HYDRAULIC SYSTEM BLOCK DIAGRAM HOLD
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.
Signal Relief Valve
STEERING AND IMPLEMENT PUMP
CONSTANT FLOW Control Valve Group Bias Spring Actuator Piston Swashplate Stability Orifice Signal Pressure Margin Spring Pressure Cutoff Spool Margin Spool Piston and Barrel Assembly
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.
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.
Lift Cylinders Tilt
Cylinder
Third
Function FunctionFourth
Pump Supply Tank Pilot Supply Load Signal to Pump Tank Pressure Reducing Valve Signal Duplication Valve Pilot Control Valve or Solenoid Valves Signal Relief Valve Resolver Pressure Compensator Valve Line Relief Valve Lock Valve Control Spool Pilot Shutoff Solenoid Pilot Control Valve or Solenoid Valves Pilot Control Valve or Solenoid Valves Pilot Control Valve or Solenoid Valves Makeup Valve Pilot Accumulator Bridge Passage Orifice
3PC MAIN CONTROL VALVE GROUP HOLD
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.
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
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.
Signal Duplication Valve Previous Resolver Port B Pilot Control Valve Pilot Control Valve Line Relief Valve Pressure Compensator Valve Makeup Valve Bridge Passage Lock Valve Feeder Passage Internal Passage Control Spool Load Check Spool
PILOT OPERATED CONTROL VALVE
HOLD
Port A
Signal Duplication Valve
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.
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.
Pump Supply
Spool Signal
Valve A Valve B Valve C
HOLD LOW PRESSURE HIGH PRESSURE
Pump Control Valve and Signal Relief Valve
Feeder Passage Bridge
Passage Pump
Supply SupplyPump
Signal Duplication Valve Resolver Load Check Spool Pressure Compensator Valve From Signal Duplication Valve
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.
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.
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.
Rod
End Head End
Line Relief Valve Pressure Compensator Valve Bridge Passage Feeder Passage Control Spool Load Check Valve Signal Duplication Valve Pump Supply
Signal Duplication Valve Previous Resolver
PILOT OPERATED CONTROL VALVE
LOAD CHECK VALVE OPERATION
Pilot Control Valve Pilot Control
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. Signal Duplication Valve Pilot Control Valve Pressure Compensator Valve Bridge Passage Feeder Passage Control Spool Load Check Spool Port B
PILOT OPERATED CONTROL VALVE
HIGHEST CIRCUIT LOAD SIGNAL
Port A Pilot Control Valve Lock Valve Previous Resolver Next Resolver
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.
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.
Signal Duplication Valve Previous Resolver Pilot Control Valve Pilot Control Valve Pressure Compensator Valve Bridge Passage Feeder Passage Control Spool Port A Port B Next Resolver
PILOT OPERATED CONTROL VALVE
LOWER CIRCUIT LOAD SIGNAL
Piston Check
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.
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. Load Signal to Pump Tank Resolver Pressure Compensator Valve Bridge Passage Lift Cylinders Tilt Cylinder Third
Function FunctionFourth
Pump Supply Tank Pilot Supply Pressure Reducing Valve Signal Duplication Valve Pilot Control Valve or Solenoid Valves Signal Relief Valve Line Relief Valve Lock Valve Control Spool Pilot Shutoff Solenoid Pilot Control Valve or Solenoid Valves Pilot Control Valve or Solenoid Valves Pilot Control Valve or Solenoid Valves Makeup Valve Pilot Accumulator Orifice
3PC MAIN CONTROL VALVE GROUP ACTIVE
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.
CONTROLS FOR THE CONTROL VALVES
Function Pilot Control Valve ECM Position Sensor Proportional Solenoid Lock Valve Orifice Signal Duplication Valve Next Resolver Pump Supply Pilot Oil Next Resolver Pump Supply Pilot Oil Pilot Oil Signal Duplication 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.
PILOT CONTROL
VALVE
HOLD
Control
Valve Control Valve
Pilot Supply Spool Orifice Detent Plunger Plunger Lever Plate Detent Coil Retainer Adjustment Pin Spring Spring Spring Chamber Spring
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.
PILOT CONTROL
VALVE
SHIFT
Control
Valve Control Valve
Pilot Supply Spool Orifice Detent Plunger Plunger Lever Plate Detent Coil Retainer Adjustment Pin Plunger Spring Spool Spring
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.
PILOT CONTROL
VALVE
IN DETENT
Control
Valve Control Valve
Pilot Supply Spool Orifice Detent Plunger Plunger Lever Plate Detent Coil Retainer Adjustment Pin
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.
Next Resolver Signal Duplication Valve
Port A Pressure Compensator Valve Bridge Passage Lock Valve Feeder Passage Internal Drain Passage Control Spool Line Relief and Makeup Valve Pump Supply Joystick Control Joystick Control Previous Resolver Proportional Solenoid Load Check Spool Port B Tank Tank Pilot Oil
SOLENOID OPERATED CONTROL VALVE HOLD
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.
Signal Duplication Valve
Port B Pressure Compensator Valve Bridge Passage Lock Valve Feeder Passage Internal Passage Control Spool Pump Supply Joystick Control Joystick Control Next Resolver Previous Resolver Proportional Solenoid Load Check Spool Port A Pilot Oil
SOLENOID OPERATED CONTROL 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.
Manifold Return Restrictor Steering Tank Wired To Controller Lift Cylinders Tilt Cylinder Third
Function FunctionFourth Steering Manual Lowering Valve Pilot Accumulator Combination Valve Flow Control Valve Auxiliary Pilot Valve Lift Pilot Valve Tilt Pilot Valve Auxiliary Pilot Valve Auxiliary Control Valve Auxiliary Control Valve Lift Control Valve Tilt Control Valve Inlet Manifold Priority Valve Quick Coupler and Auxiliary Control Valves Pump Control Valve Hydraulic Piston Pump Drain Valve Signal Relief Valve Signal Drain Valve Signal Duplication Valve Pressure Reducing Valve Pilot Shutoff Valve Signal From Quick Coupler And Aux 5 and 6
930G WHEEL LOADER PILOT OPERATED HYDRAULIC SYSTEM
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.
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.
Lift Cylinders Tilt
Cylinder Ride
Control
Accumulator Auxiliary Function
Tank Pilot Shutoff Valve Steering Pilot Supply (Command Control Steering only) Margin Relief Valve Pilot Accumulator Pilot Pressure Reducing Valve Signal Duplication Valve Signal Relief Valve Manual Lower Valve Load Check Valve Case Drain Filter Balance Valve Ride Control Relief Valve Load Check Valve Choke Check Valve Dump Pilot Solenoid Valve Lower / Float Pilot Solenoid Valve Raise Pilot Solenoid Valve Rackback Pilot Solenoid Valve Line Relief Valves Screen Tilt Spool Lift Spool Pressure Compensator Valve Resolver Balance Solenoid Valve Rod End Solenoid Valve Head End Solenoid Valve Pump Min Pump Control Valve Orifice
"H" SERIES MEDIUM WHEEL LOADER IMPLEMENT HYDRAULIC SYSTEM
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.
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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.
Tilt Cylinder Dump Rackback Lift Cylinders
Lower
Raise Manual Lower Valve
Tank Machine ECM Joystick Implement Pump Margin Relief Valve Anti-drift Valve Relief Valve Implement Lockout Valve Signal Duplication Valve Signal Relief Valve Charge Pump
Accumulator MP Bucket Cylinders Ripper Cylinders Raise Lower Open Close EH Pilot Valve Flow Compensator Pressure Cutoff Small Actuator Large Actuator EH Pilot Valve Compensator Resolver
963D IMPLEMENT HYDRAULIC SCHEMATICS HOLD
Fan Pump
Two Stage Relief Valve
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.
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.
1. Model view
2. PPPC hydraulic system 3. Implement pump
4. Main control valve group 5. Control valve operation
6. Pressure compensator valve operation 7. Pilot operated control valve - load check
valve operation
8. Pilot operated control valve - highest circuit load signal
9. Pilot operated control valve - lower circuit load signal
10. 3PC main control valve group - active 11. Controls for the control valves
12. Pilot control valve - hold 13. Pilot control valve - shift 14. Pilot control valve - in detent
15. Solenoid operated control valve - hold 16. Solenoid operated control valve - circuit
activated
17. 930G wheel loader pilot operated hydraulic system - hold
18. "H" series medium wheel loader implement hydraulic system
19. 963D track-type loader hydraulic system 20. Conclusion
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.
Red - High Pressure Oil
Red / White Stripes - 1st Pressure Reduction
Pink - 3rd Reduction in Pressure
Red / Pink Stripes - Secondary Source Oil Pressure Orange - Pilot, Charge or Torque Converter Oil
Blue - Trapped Oil Brown - Lubricating Oil
Cat Yellow - (Restricted Usage)
Green / White Stripes -
Scavenge / Suction Oil or Hydraulic Void Identification of Components within a Moving Group Black - Mechanical Connection. Seal Dark Gray - Cutaway Section Light Gray - Surface Color
White - Atmosphere or Air (No Pressure) Purple - Pneumatic Pressure
Yellow - Moving or Activated Components
Orange / Crosshatch - 2nd Reduction in Pilot, Charge, or TC Oil Pressure
Orange / White Stripes - Reduced Pilot, Charge, or TC Oil Pressure
Red Crosshatch - 2nd Reduction in Pressure
Green - Tank, Sump, or Return Oil
Red - High Pressure Oil Red / White Stripes - 1st Pressure Reduction Pink - 3rd Reduction in Pressure Red / Pink Stripes - Secondary Source Oil Pressure Orange - Pilot, Charge or Torque Converter Oil Blue - Trapped Oil
Brown - Lubricating Oil
Cat Yellow - (Restricted Usage) Green / White Stripes - Scavenge / Suction Oil or Hydraulic Void Identification of Components within a Moving Group
Black - Mechanical Connection. Seal Dark Gray - Cutaway Section Light Gray - Surface Color White - Atmosphere or Air (No Pressure) Purple - Pneumatic Pressure Yellow - Moving or Activated Components
Orange / Crosshatch - 2nd Reduction in Pilot, Charge, or TC Oil Pressure Orange / White Stripes - Reduced Pilot, Charge, or TC Oil Pressure
Red Crosshatch - 2nd Reduction in Pressure
Green - Tank, Sump, or Return Oil
HYDRAULIC SCHEMA
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.
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.
