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| Illustration 1 | g03717794 |
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Bottom view of main pump group (1) Displacement solenoid and EPC spool (2) Pump regulator (3) Adjustment for power-shift piston (4) Power-shift spool | |
The main pump control group consists of two separate spools: EPC spool (1) and power-shift spool (4).
Displacement solenoid and EPC spool (1) regulates the margin pressure. The margin pressure is the difference between the pump supply pressure and the load sensing pressure. In some systems, EPC spool (1) may be referred to as the margin spool or the flow compensator.
Power shift-spool (4) limits the maximum hydraulic horsepower output of the main pump for the selected power mode.
The power-shift spool regulates pump flow in relation to pump supply pressure and power shift pressure. Power-shift pressure is a reduced amount of pilot system pressure. The main pump torque limit solenoid valve at the pilot manifold receives a proportional electrical signal from the machine ECM. This electrical signal controls the power-shift pressure and will vary depending on the power mode selected and the engine speed. Power-shift pressure limits hydraulic horsepower demands on the engine.
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| Illustration 2 | g03717810 |
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Main pump hydraulic schematic (1) EPC spool (4) Power-shift spool (5) Main control valve (6) Unloading valve (7) Signal limiting valve (8) Main relief valve (9) Signal drain valve (10) Main pump group (11) Maximum angle piston (12) Power shift piston (13) Lever (14) Actuator piston (15) Minimum angle piston (16) Port - inlet (17) Main pump pressure sensor (18) Load sense pressure sensor | |
When the engine is not running, the spring in maximum angle piston (11) moves the pump swashplate to the UPSTROKED position. This action allows the pump to provide flow as the engine is started.
Power shift spool (4) is held to the right and EPC spool (1) is held to the left by the respective springs when the engine is not running. The spring on the right end of power-shift piston (12) holds the power shift piston to the left.
Signal drain valve (9) is used to regulate the signal flow and to allow the signal oil to flow to the tank when implements are returned to NEUTRAL. The extra flow provides faster response when an implement is initially activated.
Because minimum output is near zero, the system is in STANDBY. When the system is in standby, margin pressure is lower than the opening point of the margin valve.
The force created by actuator piston (14) working on lever (13) changes with the following:
- Position of maximum angle piston (11)
- Value of the pump supply pressure
- Value of the power shift pressure
Actuator piston (14) rides along lever (13) as the pump UPSTROKES and DESTROKES. As the actuator piston moves closer to the pivot pin, the mechanical advantage decreases. As the piston moves away from the pivot pin, the mechanical advantage increases. If the mechanical advantage increases to the point that the lever is able to move power-shift piston (12) to the right to overcome a given power-shift pressure, spring force will then move power-shift spool (4) to the right. This action will DESTROKE the pump.
Signal limiting valve (7) limits the maximum load sensing signal pressure. The signal limiter valve regulates pressure within the load sense circuit. Load sense pressure sensor (18) will be monitored by the machine ECM in order to control EPC spool (1) to control the maximum system pressure. The signal limiter pressure will increase when the heavy lift solenoid is energized.
Main relief valve (8) serves as a backup to the signal relief valve. The main relief valve set point is above the signal relief valve set point.
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| Illustration 3 | g03717972 |
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Main pump hydraulic schematic - UPSTROKE (1) EPC spool (4) Power-shift spool (5) Main control valve (6) Unloading valve (7) Signal limiting valve (8) Main relief valve (9) Signal drain valve (10) Main pump group (11) Maximum angle piston (12) Power shift piston (13) Lever (14) Actuator piston (15) Minimum angle piston (16) Port - inlet (17) Main pump pressure sensor (18) Load sense pressure sensor | |
The following are some of the conditions that cause the pump to upstroke:
- Pump is being controlled by EPC spool (1) and the pump flow is not on the horsepower curve with an increased demand for flow
- Power shift pressure increases when the pump flow is being controlled by power-shift spool (4)
- System pressure decreases when the pump flow is being controlled by power-shift spool (4)
Illustration 3 shows the pump in the UPSTROKE position when flow increases due to a change in the position of EPC spool (1).
The pump is upstroked because of an imbalance in the pressures that are read by the ECM. The machine ECM will control the solenoid on EPC spool (1). This imbalance causes EPC spool (1) to shift to the right. Minimum angle piston (15) is open to the tank through power-shift spool (4) and EPC spool (1).
Supply pressure and the spring in maximum angle piston (11) move the swashplate toward the maximum angle in order to increase pump flow.
The force that is working on each end of EPC spool (1) are no longer balanced when the following conditions occur:
- A hydraulic circuit is activated when the pump is at minimum displacement
- Additional circuits are operated
- A single circuit is shifted and the circuit requires more flow
On the right end of EPC spool (1), the margin spring works against the main pump displacement solenoid that is on the left end of the load sensing spool. When more flow is required, the displacement solenoid forces the spool to the right and overcomes the spring pressure.
This action opens the minimum angle piston chamber to the tank around power-shift spool (4) and EPC spool (1). Pump supply pressure and spring force on the maximum angle piston (11) move the swashplate toward maximum angle, and pump flow increases. When the pump UPSTROKES due to an increase in flow requirements, unloading valve (6) will close.
As the pump UPSTROKES, actuator piston (14) rides along lever (13) moving farther away from the pivot pin increasing the mechanical advantage. As long as the increased force is not able to overcome the power shift pressure, the change in flow demand will have no impact on the horsepower control.
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| Illustration 4 | g03718004 |
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Main pump hydraulic schematic - DESTROKE (1) EPC spool (4) Power-shift spool (5) Main control valve (6) Unloading valve (7) Signal limiting valve (8) Main relief valve (9) Signal drain valve (10) Main pump group (11) Maximum angle piston (12) Power shift piston (13) Lever (14) Actuator piston (15) Minimum angle piston (16) Port - inlet (17) Main pump pressure sensor (18) Load sense pressure sensor | |
The following are some of the conditions that cause the pump to destroke:
- Flow demand is decreased when the pump flow is being controlled by EPC spool (1)
- Power-shift pressure is decreased when the pump flow is being controlled by power shift spool (4)
- System pressure is increased when the pump flow is being controlled by power shift spool (4)
Illustration 4 shows the pump in the DESTROKE position when flow decreases due to a change in the position of EPC spool (1).
With power-shift pressure spool (4) shifted left, EPC spool (1) is shifted to the left and some of the pump supply oil is directed to minimum angle piston (15). Minimum angle piston (15) moves the swashplate toward the minimum angle in order to meet the new lower flow requirements.
The pump destrokes in order to provide less flow under any of the following conditions:
- A single hydraulic circuit is returned to the NEUTRAL position and the pump goes to a standby condition
- An additional circuit or circuits are returned to the NEUTRAL position
- A single circuit is shifted and less flow is required
On the right end of EPC spool (1), the margin spring work against the DEACTIVATED main pump displacement solenoid on the left end of EPC spool (1). When less flow is required, the force on the right end of the spool is greater than on the left end, and EPC spool (1) moves to the left.
This action directs pump supply pressure around power shift spool (4) to minimum angle piston (15) and moves the swashplate toward the minimum angle decreasing pump flow. With less flow, the system pressure will decrease.
As the pump destrokes, actuator piston (14) rides along lever (12) and moves closer to the pivot pin. The mechanical advantage is reduced.