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| Illustration 1 | g02454376 |
Main Pump Components (Constant Flow) (1) Soft stop for destroke (2) Barrel (3) Stop screw for maximum angle (4) Shaft (5) Plug (6) Swashplate (7) Drain plug (8) Stop screw for minimum angle (9) Hydraulic horsepower regulator (10) Spline of shaft and barrel (11) Rotary group piston (12) Main pump control (13) Actuator piston (14) Maximum angle piston (15) Input from tank (16) Pressurized flow to main control valve (17) Minimum angle piston (18) Plug (control pressure) | |
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.
Mounted beneath the main pump is the main pump control valve (12). The main pump control valve consists of two spools. One, which senses power shift pressure signal from the main pump PRV in order to regulate hydraulic horsepower output. The other spool senses load or signal pressure from the main control valve in order to control the pump swashplate (6). Three control pressures are used by the main pump in order to regulate pump flow. These pressures are: load sensing signal pressure, pump supply pressure and power shift pressure.
The pump generates flow when shaft (4) splines with barrel (10). The spline causes the pistons (11) in the rotary group to rotate while riding on the inclined surface of swashplate (6). Maximum angle stop screw (3) determines the maximum pump flow. Minimum angle stop screw (8) 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 maximum angle piston (14) .
When maximum angle piston (14) is moved to the left, swashplate (6) approaches the minimum displacement angle. As swashplate (6) approaches the minimum displacement angle the destroke pressure begins to purge through the passage into the pump case. Progressively bleeding off the destroke pressure softly stops swashplate (6) before the swashplate contacts minimum angle stop screw (8) .
Minimum angle stop screw (8) is used to determine the minimum swash plate angle and flow
The minimum displacement is defined as 22.5 cubic centimeters per revolution (45 L/min at 2000 RPM). The minimum displacement is adjusted by the minimum angle stop screw (8). The margin pressure is specified to be between 2100 kPa (305 psi) and 2300 kPa (334 psi).
The output flow is proportional to the rpm and to the swashplate angle. The pump control determines the amount of pump flow by regulating the oil pressure that is supplied to maximum angle piston (14) .
Component and Location
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| Illustration 2 | g02456598 |
M322D Pump Compartment (12) Main pump control (19) Swing pump regulator (20) Main pump (21) Fan drive pump (22) Charge pump (23) Swing pump | |
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| Illustration 3 | g02456637 |
M316D - M318D Pump Compartment (12) Main pump control (19) Swing pump regulator (20) Main pump (21) Fan drive pump (22) Charge pump (23) Swing pump | |
The main pump (20) and the swing pump (23) are mounted in tandem. The nearest pump to the engine is main pump (20). Main pump (20) supplies flow to all of the implement and travel circuits.
The main pump control (12) consists of the following separate sections:
- Load sensing regulator
- Hydraulic 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 main hydraulic system. The load sensing regulator uses the load sensing signal in order to establish the correct rate of pump flow.
Hydraulic horsepower regulator works with the load sensing regulator in order to control the pump swashplate angle. Hydraulic 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) in the pilot manifold. The PRV destrokes the main pump displacement according to specific characteristics. The signal is generated electronically and considers engine speed and engine rpm drop difference to nominal engine speed.
The pump output pressure is constantly pushing maximum angle piston (14) in order to upstroke the pump to prevent underspeeding caused by too high of a hydraulic power demand. To regulate the pump flow, horsepower regulator (8) directs a signal pressure to maximum angle piston (14). Maximum angle piston (14) is connected to swashplate (6) with a rod. Maximum angle piston (14) is also connected to hydraulic horsepower regulator with actuator piston (13) and a lever. The movement of maximum angle piston (14) moves swashplate (6). The lever follows hydraulic horsepower regulator upward.
Flow Control
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| Illustration 4 | g02453881 |
M300D Power Mode (Flow Versus Pressure) (A) Travel mode (B) Power mode (C) Economy mode | |
The main pump output flow control is performed by the operation of main pump control (12). Main pump control (12) has the following functions:
- Maintain Constant Pump Output Horsepower.
When the operator requires the full hydraulic power of the main pump, main pump control (12) 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 signal for power shift pressure is varied:
- Travel mode (mode T)
- Power mode (mode P)
- Economy mode (mode E)
Travel mode is automatically activated when the travel pedal is depressed.
Machines with a six cylinder engine have an additional characteristic when the machine is traveling in second gear. The engine automatically operates at 1600 RPM. Optimum engine power and main pump efficiency is available at this engine speed. The monitor will display "ECO". This display indicates that the machine is in "Travel Economic Mode". This mode is different from Economy mode (mode E).
- Travel mode (mode T)
- The pump output flow is regulated by 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.