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| Illustration 1 | g00812947 |
Illustration 1 shows the internal components of the main pump. The main pump is a variable displacement piston pump with a swashplate that is directly mounted to the engine flywheel housing. The pump is driven by a flexible coupling. The pump is self-priming. The pump generates flow when the pistons in the rotary group ride on the inclined surface of the swashplate. The maximum angle stop screw determines the maximum pump flow. The minimum angle stop screw only keeps the pump flow from going over center during minimum operating angle actuation. The minimum pump flow is fixed by the length of the destroke rod and the position of the opening to the passage inside the destroke piston. When the destroke piston is moved to the right, the swashplate approaches the minimum displacement angle. As the swashplate approaches the minimum displacement angle the destroke pressure begins to progressively bleed off through the passage into the pump case. Progressively bleeding off the destroke pressure softly stops the swashplate before the swashplate makes contact with the minimum angle stop screw. The minimum angle stop screw prevents excessive destroking of the pump, but the minimum angle stop screw does not determine the minimum displacement angle. The output flow is proportional to the rpm and to the swashplate angle. The pump regulator determines the amount of pump flow by regulating the oil pressure that is supplied to the destroking piston.
Note: The pump on the M320 Excavator is similar in operation to the 375 Excavator pump.
Component and Location
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| Illustration 2 | g00527543 |
(1) Swing pump regulator (2) Main pump (3) Charging pump (4) Swing pump (5) Main pump regulator | |
The two hydraulic pumps are mounted in tandem. The nearest pump to the engine (front pump) is the main pump. The main pump supplies flow to all of the implement and travel circuits.
The main pump regulator consists of the following separate sections:
- Load sensing regulator
- Constant horsepower regulator
The load sensing regulator uses the load sensing signal pressure from the main control valve in order to regulate the pump flow. The pump output is held at 2000 to 2200 kPa (290 to 320 psi) above the load sensing signal pressure. The pressure difference between the pump output and the load sensing signal pressure is called the margin pressure. The load sensing signal is supplied by the main control valve. The load sensing signal pressure is the highest implement pressure in the entire hydraulic system. The load sensing regulator uses the load sensing signal in order to establish the correct rate of pump flow.
The horsepower regulator works with the load sensing regulator in order to control the pump swashplate angle. The horsepower regulator uses the load sensing signal pressure, the pump output pressure and the power shift pressure to regulate the pump swashplate angle. The power shift pressure is controlled by a proportional reducing valve (PRV) that is identical to the PRV on the tracked excavators.
The pump output pressure is constantly pushing on the angle piston in order to upstroke the pump. To regulate the pump flow, the horsepower regulator directs a signal pressure to the destroking piston. The destroking piston is connected to the swashplate with a rod. The upstroking piston is also connected to the horsepower control sleeve with an actuator piston and a lever. The movement of the upstroke piston moves the swashplate. The lever follows the horsepower sleeve upward.
Flow Control
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| Illustration 3 | g00527548 |
P-Q Characteristic Curve (X) Delivery pressure ("P") (Y) Flow ("Q") | |
The main pump output flow control is performed by the operation of the main pump regulator. The pump regulator has the following functions:
- Maintain Constant Pump Output Horsepower.
When the operator requires the full hydraulic power of the main pump, the main pump regulator limits the pump output horsepower. The horsepower limitation is based on the combination of the power shift pressure and the load sensing signal pressure.
Three different horsepower curves are produced when the power shift pressure signal is varied:
- Travel mode (mode III)
- Medium mode (mode II)
- Minimum mode (mode I)
During the constant pump output horsepower control with the implement joysticks in the FULL STROKE position, the pump has a characteristic curve. The characteristic curve for the pump is shown in figure 3. The P-Q curve represents flow rates for different pump output pressures. Each point on the P-Q curve represents the available pump flow at a given pump output pressure when pump output horsepower is kept constant. For example, when the delivery pressure decreases from P2 to P1, the available output flow increases from Q2 to Q1.
- Travel mode (mode III)
- The pump output flow is regulated on the basis of the load sensing signal pressure.
When full hydraulic horsepower of the main pump is not required, the regulator operates in the following way:
- When both of the implement joysticks are in the NEUTRAL position, there is no hydraulic demand. The load sensing signal pressure is at a minimum. Therefore, the pump output flow is kept at a minimum.
- When the implement joysticks are partially moved, the regulator maintains the pressure difference between pump output and load sensing signal. The constant pressure difference is called the margin pressure. The margin pressure ensures that the flow to the different implements is proportional to the position of the respective implement joysticks.
When the pump flow is controlled by the load sensing signal pressure and the implement joysticks are partially actuated, the pump output flow is below the constant horsepower characteristic curve. For example, the pump is operating at a point of (P1, Q3) with a delivery pressure P1. When the implement joystick is partially moved to the NEUTRAL position, the output flow decreases. When the implement joystick is moved further away from the NEUTRAL position, the output flow increases to a maximum of Q1. The pump output flow is directly proportional to the stroke of the implement joystick when the flow demand remains below Q1.