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| Illustration 1 | g01383530 |
Right Side of Machine (1) Generator pump (2) Pressure compensator spool (3) Flow compensator spool | |
Machines with a hydraulically driven generator are equipped with generator pump (1). The generator pump is a variable displacement, axial piston pump. The pump is load sensing and pressure compensated. The generator pump is located inside the pump compartment. The generator pump is on the right side of the machine.
The ports on the generator pump are labelled for identification. The following describes each port on the generator pump:
- Port "S" (pump inlet)
- Port "L2" (pump case drain)
- Port "X" (load sensing port)
- Port "B" (pump outlet)
The pump is equipped with a compensator valve. The pump compensator valve maintains the pump's pressure and flow at a level which is adequate to fulfill the system load and flow needs. The compensator valve contains flow compensator spool (3) and pressure compensator spool (2).
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| Illustration 2 | g01383542 |
Cross Section of the Generator Pump (2) Pressure compensator spool (3) Flow compensator spool (4) Compensator valve (5) Load sensing signal (6) Swashplate (7) Slipper retainer (8) Input shaft (9) Slipper (10) Piston (11) Bias piston (12) Bias spring (13) Line from hydraulic tank (14) Valve plate (15) Pump output (16) Barrel assembly (17) Displacement piston | |
When the input shaft is rotated, the barrel assembly turns. The pistons, which are installed in the barrel assembly, also turn with the barrel assembly. One slipper is attached to each piston. The slippers are held against the swashplate by a slipper retainer.
The slippers allow the pistons to follow the angle of the swashplate as the barrel assembly rotates. As the slippers move down the ramp of the swashplate, the pistons which are located on the downhill side of the swashplate are pulled out of the barrel assembly. This action draws oil through the valve plate and into the piston bore in the barrel assembly. When a slipper reaches the bottom of the swashplate ramp, rotation of the input shaft forces the slipper to move up the ramp. At this point, the angled swashplate pushes the piston back into the barrel assembly. This action pushes oil out of the piston bore, through the valve plate, and into the output port.
Inside the compensator valve, system pressure acts against one side of the flow compensator spool. The combined force from the flow compensator spring and the load sensing signal acts against the opposite side of the spool. Since the pump does not receive a load sensing signal when the engine is being started, the flow compensator spring provides the only force which acts against the pump supply pressure.
The angle of the swashplate determines the amount of oil that is drawn into each piston bore. Therefore, the angle determines the amount of oil that is pumped out of each piston bore per revolution of the input shaft. The greater the swashplate angle, the greater the amount of oil which is pulled into the pump. Also, the greater the angle, the greater the amount of oil which is discharged through the valve plate.
Before the engine is started, the bias spring forces the swashplate to the maximum angle. As the engine is started, the pump immediately generates flow.
The generator pump draws oil from the tank through a suction screen. The pump delivers oil to the pressure compensator spool, the flow compensator spool, and the generator valve.
The pump receives a load sensing signal from the generator valve. This signal is sent to the pump through load sensing port (X). The load sensing signal acts in conjunction with the flow compensator spring.
When the pump supply pressure rises above the value of the flow compensator spring, the flow compensator spool shifts. This shift opens a passage which allows pump supply oil to flow into the displacement piston chamber. The pressure in the displacement piston chamber moves the swashplate toward a minimum angle. As a result, flow is reduced.
When the pump flow is reduced, supply pressure is also reduced. In this situation, the flow compensator spring causes the flow compensator spool to shift against the force which is created by the pump supply pressure. This shift decreases the pressure in the displacement piston chamber, and the angle of the swashplate increases.
Eventually, the forces on either side of the flow compensator spool reach equilibrium. At that point, the pump is at low pressure standby. When the pump is in this position, the pump produces just enough flow that is needed to make up for system leakage.
Note: The time that is required for the forces on either side of the flow compensator spool to reach equilibrium is too short to detect with standard pressure gauges.
When the generator control switch is moved to the ON position, a load sensing signal is sent to load sensing port (X) on the compensator valve. Inside the compensator valve, signal pressure fills the cavity which contains the flow compensator spring. Signal pressure and the force from the flow compensator spring act against one side of the flow compensator spool. These two forces act against the pump supply pressure, which acts against the opposite end of the flow compensator spool.
The combined force from signal pressure and the flow compensator spring moves the flow compensator spool. This movement opens a passage which directs oil from the displacement piston cavity into the pump case drain. The force from the bias spring moves the swashplate. This movement increases the angle of the swashplate, and the pump generates additional flow.
As pump flow increases, pump supply pressure acting against the flow control spool becomes greater than the combined force from the signal pressure and the flow compensator spring. This causes the flow compensator spool to shift. This shift directs oil into the displacement piston chamber. When the pressure in the displacement piston chamber overcomes the force of the bias spring, the angle of the swashplate decreases. As a result, the pump output is reduced.
When the pump output is reduced, supply pressure is also reduced, and the flow compensator spring causes the flow compensator spool to shift against the force which is created by the pump supply pressure. This shift decreases the pressure in the displacement piston chamber, and the angle of the swashplate increases.
The slight cyclical movement of the flow compensator spool is called "metering". Metering maintains equal pressure on both ends of the flow compensator spool. The force that is created by the flow compensator spring causes pump supply pressure to be slightly greater than signal pressure. The difference between pump supply pressure and signal pressure is referred to as "margin pressure". Margin pressure is set at 1665 ± 70 kPa (240 ± 10 psi).
Note: Low pressure standby is higher than margin pressure, unless the system has excessive internal leakage. The flow compensator adjustment screw controls both margin pressure and low pressure standby. Both pressures increase or both pressures decrease simultaneously when the screw is adjusted. However, the two pressures do not necessarily change by the same amount.