390D Excavator Hydraulic System Proportional Priority Pressure Compensation System Caterpillar


Proportional Priority Pressure Compensation System
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Illustration 1g02160325

Stick control spool and boom control spool with boom at higher system flow and pressure

(1) Stick flow compensator piston

(2) Signal duplication valve

(3) Oil flow to stick cylinder

(4) Resolver valve

(5) Boom flow compensator

(6) Oil flow to boom cylinder

(7) Load check valve for the stick

(8) Load check valve for the boom

(9) Stick control spool

(10) Boom control spool

(11) Main pumps

Note: The shuttle valve in this system is more commonly known as a resolver valve.

The hydraulic system on this machine is referred to as a proportional priority pressure compensated system (PPPC). Oil flows from the main pumps to individual implement circuits in proportion to the amount of movement of the implement joystick. The implement circuit or the travel circuit with the highest working pressure provides the true load signal pressure during multiple implement functions and travel functions.

A common example of a combined function is movement of the boom and the stick at the same time. The examples given below are for the boom and stick system. The conditions listed below will be described so that the boom system is moving at a higher speed than the stick system. Other combinations of multiple functions are optional and perform similarly.

Pump oil flow in the supply passages is directed around boom control spool (10) to the feeder passage. The feeder passage is located below boom load check valve (8). The pump flow into the feeder passage pushes boom load check valve (8) and boom flow compensator piston (5) upward. The pump oil flows through the openings in the bottom of boom load check valve (8) to the bridge passage. From the bridge passage, pump flow is directed around boom control spool (10) to the boom cylinder.

Note: Stick control spool (9) operates in the same manner as boom control spool (10) .

Stick control spool (9) controls flow from the pump supply passage to the feeder passage. Stick flow compensator piston (1) and stick load check valve (7) controls the pump oil flow from the feeder passage to the stick bridge passage. The remaining pump oil flow flows to the boom circuit.

The boom circuit pressure in the bridge passage flows to resolver valve (4). The pressure in the bridge passage is the true load signal pressure of the boom circuit. The pressure of the stick circuit is also directed to resolver valve (4). This pressure is the true load signal pressure of the stick circuit. The true load signal pressure of the boom circuit is higher than the true load signal pressure of the stick circuit.

The true load signal pressure of the boom circuit is sent to signal duplication valve (2). The true load signal pressure of the boom circuit works with signal duplication valve (2) in order to generate duplicate load signal pressure. The duplicate load signal pressure is directed to the differential relief valves. The signal relief valves limit the pressure in the duplicate load signal system. During implement-only operation, the duplicate load signal pressure is sent to the pump regulators. The pumps upstroke when the duplicate load sensing signal pressure increases. The pumps destroke when the duplicate load signal pressure decreases.

The duplicate load signal pressure is also directed to the spring chambers on top of the flow compensator pistons. Each implement control valve spool has a load check valve and flow compensator piston. The force of the duplicate load signal pressure and the compensator spring must be lower than the bridge passage pressure for implements to be moved.

During the combined function, the stick control valve is shifted less than the boom control valve. Boom flow compensator piston (5) will remain fully open while stick flow compensator piston (1) is partially open. Hydraulic oil flow to the boom circuit is higher than the flow to the stick circuit. Less oil flows through load check valve (7) for the stick circuit than load check valve (8) for the boom circuit. Pump oil flows through the orifices in stick load check valve (7) to the bridge passage and to the stick cylinders.

As pump oil flow passes through the orifices in the bottom of the partially open stick load check valve, a pressure drop occurs. The pressure drop occurs between the stick feeder passage and the stick bridge passage. The pump oil in the stick bridge passage is a reduced pressure. This reduced pressure is also the true load signal pressure. The reduced true load signal pressure from the stick circuit flows to resolver valve (4). Because the true load signal pressure from the stick circuit is lower than the boom circuit, resolver valve (4) blocks the stick true load signal pressure.

Note: The boom joystick can be moved less than the stick joystick but have a higher pressure than the stick circuit. When the pressure in the boom circuit is higher than the pressure in the stick circuit, the boom true load signal pressure is used. The true load signal from the boom circuit will control the pump oil flow.

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