86
The left side of the front dash panel shows the low fuel pressure condition. The illuminated indicator is enabled by an output from the Engine ECM over the Cat Data Link, driven by the Implement ECM.
87
Implement Hydraulic System - HOLD
The "H" series Medium Wheel Loaders are now equipped with a Proportional Priority, Pressure Compensated (3PC) implement hydraulic system. The 3PC hydraulic system is load sensing with a signal duplication valve, a signal relief valve, pressure compensator valves, a pressure differential relief valve, a pressure reducing valve, and a resolver network. Also, the 3PC valve has antidrift solenoid valves for the lift and tilt functions.
The implement control valve is a closed-center valve. The 3PC hydraulic system will sense a demand for a change in flow and the implement pump will upstroke or destroke to provide the flow.
The machine may also be equipped with an optional auxiliary function. The auxiliary section is installed between the ride control valve and the cover manifold.
HOLD Lift Ant idrift
Valve
When the engine is started and the implement control levers are in the HOLD position, the implement pump supplies standby oil flow to the 3PC valve group.
The oil flows into the inlet manifold and is divided into two paths. The supply path for the implements flows through the inlet manifold into the tilt section where the flow path is divided again. One path flows to the tilt spool and is blocked. The second path flows to the lift control section, to the ride control section, to the optional auxiliary section, and to the cover manifold.
Within the cover manifold, the oil flows to both the pilot pressure reducing valve (PRV) and the differential pressure relief valve. The differential pressure relief valve maintains a difference between the load sensing pressure and the pump supply oil pressure of 3000 kPa (435 psi). When all of the control valves are in the HOLD position, the implement pump is at low pressure standby. The differential pressure relief valve maintains the minimum pressure for low pressure standby. The standby pressure is directed to the pilot pressure reducing valve (PRV). The pilot pressure reducing valve provides a regulated pilot oil pressure to activate the control valves as needed.
The pilot oil flows from the PRV through the check valve to the pilot accumulator and the hydraulic lockout solenoid valve. If the wheel loader is equipped with the optional Command Control Steering (CCS), the oil flow will be shared by the implement pilot system and the steering pilot system.
The hydraulic lockout solenoid valve is in the CLOSED position until the hydraulic lockout switch in the cab is activated. When the solenoid valve is energized, the solenoid valve opens and pilot oil flows to the various implement function solenoid valves.
The second path of oil in the inlet manifold flows through the screen to the signal duplication valve. The signal duplication valve uses highest work port pressure in the resolver network to supply an identical pressure to the pump control valve.
When all implement control valves are in the HOLD position, there is no load sensing signal in the resolver network. With no load sensing pressure present, the implement pump supplies low pressure standby.
88
Tilt Control Valve - HOLD
With the tilt control valve in the HOLD position, the springs on each end of the control spool keep the spool centered. The control spool blocks the flow of pump supply oil to the pressure compensator valve.
The bridge passage is open to tank through the internal passage in the control spool and there is no oil flow to the resolver valve. With no oil flow to the resolver network, there is no signal to the signal duplication valve and the signal pressure to the pump control valve is matching the oil pressure in the resolver network. The implement pump is at low pressure standby.
From Signal Duplication Valve
To / From Rod End
To Signal Duplication Valve From Resolver Valves
89
Implement Hydraulic System - DUMP
When the tilt lever is moved to the DUMP position, the Implement ECM energizes the dump proportional solenoid and the tilt antidrift valve. As the control valve initially shifts to the left, and there is pressure in the rod end of the tilt cylinder, the pressure goes around the control spool to the bridge passage. The pressure in the bridge passage goes to the resolver network and to the signal duplication valve to upstroke the implement pump. The pressure also goes to the spring chamber in the center of the pressure compensator valve. The lower half of the pressure compensator valve shifts down to block the oil flow from the bridge passage to the feeder passage.
As the tilt control spool continues shifting to the left, pump supply oil flows around the center land on the control spool to the feeder passage. Oil pressure in the feeder passage lifts the pressure compensator valve up. Pump supply oil goes through the orifices in the lower end of the compensator valve to the bridge passage. From the bridge passage, the pump flow goes around the control spool into the work port to the rod end of the tilt cylinder.
Returning oil from the head end of the tilt cylinder, flows around the tilt antidrift valve to the tank port.
From Signal Duplication Valve
To Rod End
To Signal Duplication Valve From Resolver Network
"H" SERIES MEDIUM WHEEL LOADER TILT CONTROL VALVE
DUMP
90
Pressure Compensator Valve - HOLD
When the control spool is in the HOLD position, the load check spool and the pressure compensator valve are held down by the spring force on top of the load sense spool. Pump supply oil in the supply passage is blocked by the control spool. No supply oil from the supply passage flows to the feeder passage. Therefore, no load sensing pressure is directed to the implement pump control valve. The implement pump is at low pressure standby.
From Signal Duplication Valve
To Rod End
of Cylinder Load Check
Spool
To Signal Duplication Valve From Resolver Network
91
Load Check Operation
This illustration shows the pressure compensator and load check valve in the load check operation. When the control spool is initially shifted to the left, work port pressure from the rod end of the cylinder (if any) flows around the right end of the control spool into the bridge passage. The pressure goes through the holes between the pressure compensator valve and the load check spool. The pressure moves the pressure compensator spool down and the load check spool up. With the pressure compensator valve shifted down, no pressure can go from the bridge passage to the feeder passage. The pressure compensator valve serves as a load check valve to hold the load up to prevent it from dropping.
The pressure in the bridge passage is directed through the resolver network to the signal duplication valve. The signal duplication valve generates a load sensing signal pressure equal to the work port pressure. The load sensing signal pressure is directed to the top of the spring chamber on top of the load sense spool. The load sensing signal pressure is also directed to the pump control valve to upstroke the pump (not shown).
From Signal Duplication Valve
To Rod End
To Signal Duplication Valve From Resolver Network
PRESSURE COMPENSATOR AND LOAD CHECK VALVE
LOAD CHECK OPERATION
Pressure Compensator
Valve
92
Pressure Compensator Operation
When the control spool is shifted farther to the left, the pump supply oil in the supply passage is directed around the center land of the control spool to the feeder passage. When pressure in the feeder passage increases to more than the pressure on top of the load sense spool plus the spring force, the pressure compensator valve and the load sense spool shift up. Pump flow in the feeder passage goes through the orifices in the bottom of the pressure compensator valve to the bridge passage. From the bridge passage, the flow goes around the control spool to the passage to the rod end of the cylinder.
The tilt antidrift valve is activated allowing flow from the head end of the cylinder to be directed around the left end of the control spool to the tank return passage.
During a stall condition, the signal limiter valve in the load sensing signal valve and the margin spool maintains pump discharge pressure approximately 2100 kPa (300 psi) higher than the work port pressure. The pressure compensator valve can direct full pump flow to the bridge passage if demand for flow is great enough.
From Signal Duplication Valve
To Rod End
To Signal Duplication Valve From Resolver Network
93
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 keeps the resolver valve closed.
The circuit with the highest work port pressure regulates the oil flow through all activated control valves. The highest work port pressure is directed through the ball resolver network to the pump control valve as the load sensing pressure.
When a high pressure circuit is activated as shown for valve "C", the control spool is shifted and pump supply oil enters the feeder passage below the pressure compensator valve. Pressure increases and the pressure compensator valve moves up. When the valve moves up, supply oil enters the bridged passage of the control valve. Supply oil in the bridged passage enters the signal network sending the work port pressure to the signal duplication valve.
Pump Supply
Spool Signal
Valve A Valve B Valve C
HOLD LOW PRESSURE HIGH PRESSURE
To Pump and Signal Limiter
Feeder Passage
PROPORTIONAL PRIORITY, PRESSURE COMPENSATOR
OPERATION
a shock absorber. The valve uses the work port pressure signal and the pump system pressure signal to generate a stabilized load sensing signal which is sent to the pump control valve. The pump control valve directs a pressure signal to the actuator piston to UPSTROKE the pump.
The increased flow lifts the pressure compensator spool up. The flow goes through the bridge passage, around the control spool, and out to the work port.
The signal oil also flows to the chamber above the compensator. The signal oil on the top of the pressure compensator valve works against the forces working below the pressure
compensator. When the forces are in balance, the supply oil is metered through the cross-drilled holes in the pressure compensator to provide work port oil. The pressure of the signal oil is limited by the signal relief valve.
When more than one circuit is activated at the same time, the highest work port pressure is directed to the signal duplication valve. The signal duplication valve sends the signal oil to the chamber at the top of all pressure compensators valves.
With the same circuit pressure working on all pressure compensators, the pressure differential across all shifted control stems 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 stems will be the same value whether the pump can supply 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 is approximately 2100 kPa (300 psi) regardless of the circuit pressure. With multiple valves activated, the individual 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 and pressure for each circuit. The pressure differential will be less than described in the example, but the pressure differential will be the same for all spools.
Valve "B" pressure compensator shows what occurs when an additional circuit is activated with a lower circuit pressure than the first activated valve.
The pressure compensator valve will respond to changes in the circuit pressure by opening and closing off the passage between the feeder passage and the bridged passage to maintain a constant flow rate for a given control stem 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. The pressure differential across the main control spool is the same for all activated main control spools.
The load signal from the 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 up. The pressure compensator valve does not move up enough to open the resolver valve to the signal network due to the higher forces working on the resolver valve.
The pressure compensator valve will respond to changes in the circuit pressure by opening and closing off the passage between the feeder passage and the bridged 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 is the same for all activated main control spools.
94
Implement Hydraulic System - DUMP
When the tilt control lever is moved into the DUMP position, a load sensing signal pressure equal to work port pressure is directed to the resolver network. The signal oil goes through the resolver to the top of the signal duplication valve. The signal duplication valve shifts down.
Pump flow goes through the signal duplication valve to the bottom of the duplication valve and the orifice. The duplication valve and the orifice stabilizes the load sensing signal pressure to the pump control, to the spring chamber on each compensator valve, and to the margin relief valve.
The load sensing pressure acts on the bottom of the margin relief valve. During the upstroking of the implement pump, the margin relief valve maintains an implement pressure equal to the load sensing pressure and the value of the spring. When the control lever is released, the load sensing pressure goes to approximately zero pressure. The margin relief valve opens to relieve supply oil pressure eliminating pressure spikes in the closed center system. Once the
implement pump is destroked, the margin relief valve maintains sufficient oil pressure for the pilot system. Lift Ant idrift
Valve
The pilot pressure reducing valve limits maximum pilot pressure to 3450 ± 170 kPa (500 ± 25 psi). The implement pump oil flows through the cover manifold, regulated pilot oil is directed to the various implement function solenoid valves.
The load sensing signal moves the pump load sensing spool in the pump control valve to upstroke the pump. The signal circuit is also equipped with a choke check valve. The valve will allow free flow to the pump control valve. Also, the choke check valve will slow the signal flow return back to the hydraulic tank.
With the control lever moved to DUMP direction, the current proportional to the movement of the control lever is directed to the coil on the dump pilot solenoid valve. The solenoid valve sends a proportional amount of pilot oil to the dump end of the tilt spool. The tilt spool begins to shift upward. The lower end of the tilt spool is active.
Supply oil flowing through the inlet manifold is directed around the lands of the tilt spool to the load check valve. The check valve unseats. Supply oil flows to the tilt pressure compensator valve. The oil flow through the compensator valve is blocked. As the pressure at the top of the compensator valve increases, the oil pressure shifts the compensator spool downward. The supply oil flows through the compensator valve and back around the tilt spool to the rod end of the tilt cylinder.
The supply oil is directed to rod end of the tilt cylinder. Also, as the tilt lever is moved, the solenoid for the tilt antidrift valve is energized. The oil from the head end of the tilt cylinder flows around the load check valve, through the tilt spool and back to tank.
Oil directed to the rod end of the tilt cylinder through the bridge passage is also directed to the tilt ball resolver in the resolver network. When the work port pressure increases the pressure in the resolver network, the resolver ball shifts and blocks oil from any other revolvers in the network. The oil pressure at the tilt ball resolver is directed to the top of the signal duplication valve or load sensing pressure.
The dump operation is also equipped with a makeup and a line relief valve. The line relief valve regulates the pressure spikes caused by outside forces acting on the work tool. This allows the pressure spike to return to the hydraulic tank. This will prevent high pressure from damaging any components in the work tool or actuator. The line relief valve acts like a makeup valve when the pump can not supply the amount of oil needed to fill the void in the cylinder.
When the negative pressure occurs in the tilt cylinder, the valves move off the seat and tank oil flows around the valve to fill the void in the cylinder.
NOTE: The pilot line used on the optional Command Control Steering has been removed form the illustration.
95
Implement Hydraulic System - RAISE
When the lift control lever is moved to the RAISE position, the lift lever position sensor sends a proportional electronic signal to the Implement ECM. The Implement ECM sends a
corresponding proportional signal to the raise pilot solenoid valve. A proportional amount of pilot oil is directed from the raise pilot solenoid valve to the top of the lift spool. The lift spool shifts downward.
Initially, as the lift spool begins to shift, any work port pressure will enter the control valve and is directed around the spool to the feeder passage. The work port oil pressure goes through the holes in the pressure compensator valve to the area between the compensator valve and the load check spool. The oil pressure helps the spring force hold the pressure compensator valve down to function as a load check valve.
As the control spool shifts down, supply oil flows through the throttling slots into the supply passage. The pressure compensator valve will move up to the load check spool as the pump
As the control spool shifts down, supply oil flows through the throttling slots into the supply passage. The pressure compensator valve will move up to the load check spool as the pump