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| Illustration 1 | g00531699 |
(1) Load sensing line (2) Adjustment screw for margin pressure (3) Destroke pressure adjustment screw | |
The main hydraulic pump of the M320 is similar to the main pump on the smaller M300 Excavators. This pump is larger and the pump has increased flow. The pump regulator on the M320 is the same pump regulator that is on the 375 Excavator. The load sensing line (1) is a larger line than the load sensing line on the smaller machines. The larger line serves as an accumulator to dampen pressure surges in the load sensing signal pressure which is used to control the main pump.
The adjustment screw for margin pressure (2) is used to adjust the margin pressure setting. The destroke pressure adjustment screw (3) is used to adjust the destroking pressure point of the main pump.
Main Pump Regulator in Standby Position
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| Illustration 2 | g00813093 |
When all the controls are in NEUTRAL position, the main pump is in the standby position. No load sensing pressure signal is directed from the main control valve to the pump regulator. As the pump pressure increases, the load sensing spool is pushed to the right against spring force. Since no load sensing pressure signal is present, the pump pressure increases to only the margin pressure setting.
As the load sensing spool moves to the right, pump output pressure oil is directed around the load sensing spool, through the slow return check valve, around the horsepower control spool and to the destroke servo piston. The destroke servo piston is larger than the upstroke servo piston. The pressure oil in the destroke servo piston moves the swashplate toward the minimum angle in order to decrease flow.
As the pump destrokes, flow decreases and the load sensing spool reaches a balance point, or a metering position. Then, the pump pressure oil is metered to the destroke servo piston. The pump swashplate reaches a balance point for standby flow.
Main Pump Regulator in Upstroke Position
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| Illustration 3 | g00813097 |
When a circuit is activated, the pump upstrokes.
The following conditions can cause an increase in flow from the main pump:
- An increase in the load sensing signal
- An increase in power shift pressure from the proportional reducing valve
- A decrease in pump output pressure
The load pressure signal for that circuit is directed through the load sensing signal line to the spring chamber on the right end of the load sensing spool. The increase in signal pressure pushes the load sensing spool to the left. Oil in the destroke servo piston is directed around the horsepower control spool, through the slow return check valve orifice, around the load sensing spool and to the tank drain. Pump pressure oil on the upstroke servo piston moves the swashplate toward the maximum angle.
The slow return check valve decreases the upstroking speed of the pump in order to prevent sudden loads on the engine. The slow return check valve allows the pump to destroke at a faster speed than upstroking.
Main Pump Regulator in Destroke Position
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| Illustration 4 | g00813110 |
When the pump output pressure increases to the destroke pressure, the pump starts to destroke.
The following conditions can cause the pump to destroke:
- A decrease in the load sensing signal pressure
- A decrease in power shift pressure from the proportional reducing valve
- An increase in pump output pressure
As the system pressure increases, the pressure oil pushes on the bottom of the actuator piston. The actuator piston raises the lever in order to move the horsepower control spool to the right. System pressure oil is directed around the horsepower spool to the destroke servo piston. The oil moves the swashplate toward the minimum angle.
As the swashplate moves toward the minimum angle, the actuator piston rides along the bottom of the lever. As the actuator piston moves closer to the pivot point of the lever, the mechanical advantage changes. An increase in system pressure is required to destroke the pump farther.
P-Q Characteristic Curve
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| Illustration 5 | g00531785 |
The following pressures affect the output of the main pump:
- Pump output circuit pressure
- Power shift pressure
After a pump starts to operate, each pump has a set of P-Q characteristic curves. The P-Q curve represents a set of flow rates for different pump circuit pressures. Each point on the horsepower characteristic curve represents the respective flow rate and the pressure that is required to maintain constant pump output horsepower.
The operator selects the power level with the power mode switch on the console. The controller establishes the correct power mode electronically by modulating a pilot signal through the PRV solenoid valves. The pilot signal is directed to the pump regulator. In Mode III, the system operates with maximum hydraulic horsepower. This is shown by the upper P-Q curve. Mode II provides medium hydraulic horsepower. This is shown by the middle P-Q curve. Mode I provides low hydraulic horsepower. This is shown by the lower P-Q curve.
The main pump also contains load sensing controls that allow the pump to operate at a flow rate that is less than the constant horsepower control range. The load sensing region on the P-Q curve is illustrated by the area that is below the curve.
While the pump flow is regulated by the load sensing control, the pump flow is controlled in direct relation to the amount of demand. Load sensing control is accomplished through the load signal that is generated in the main control valve.
The load signal that is generated in the control valves is directed to the load sensing control valve. The load sensing control valve uses the load signal pressure to regulate the pump output pressure to 2000 to 2200 kPa (290 to 320 psi) more than the load signal pressure. The difference between load signal pressure and system pressure is known as the margin pressure.
The flow rate from the pump is determined by the movement of the control valve provided that the system pressure is less than the constant horsepower regulation pressure.