When pump controller is normal

(1) When the load on the actuator is small and pump discharge pressures (PP1) and (PP2) are low

Action of PC-EPC valve solenoid (1)

q Command current (X) is being sent to PC-EPC valve solenoid (1) from the pump controller.

q This command current acts on PC-EPC valve to output the signal pressure in order to modify the force pushing piston (2).

q Spool (3) stops at a position where the com-bined spool-pushing force is balanced by the setting force of springs (4) and (6) as well as the pump pressures (PP1) (self-pressure) and (PP2) (another pump's pressure).

q The pressure [port (C) pressure] output from PC valve is changed depending on the above position.

q The size of command current (X) is determined by the nature of the operation (lever opera-tion), the selected working mode, and the set value and actual value of the engine speed.

a Other pump's pressure denotes the pressure of the pump situated on the opposite side.

For the front pump pressure, the other pump's pressure is that of the rear pump.

And for the rear pump pressure, the other pump's pressure is that of the front pump.

10 Structure, function and maintenance standard SEN01993-00

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Action of spring

q Load of springs (4) and (6) on the PC valve is determined by the swash plate position.

q As servo piston (9) moves to right, spring (6) is retracted.

q If the servo piston moves further, it will be con-tacted again seat (5) and spring (6) will be fixed.

q After that, spring (4) alone will operate.

q The spring load is changed by servo piston (9) as it extends or compresses springs (4) and (6).

q If the command current (X) to PC-EPC valve solenoid (1) changes, so does the force push-ing piston (2).

q Spring load of springs (4) and (6) is also affected by the command current (X) to PC-EPC valve solenoid.

q Port (C) of the PC valve is connected to port (E) of the LS valve.

q Self pressure (PP1) enters port (B) and the small diameter end of servo piston (9), and other pump pressure (PP2) enters port (A).

q When pump pressures (PP1) and (PP2) are small, spool (3) will be positioned in the left side.

q Port (C) and (D) are connected, and the pres-sure entering the LS valve becomes drain pressure (PT).

q If port (E) and port (G) of the LS valve are con-nected, the pressure entering the large diame-ter end of the piston from port (J) becomes drain pressure (PT), and servo piston (9) moves to the left side.

q The pump delivery will be set to the increasing trend.

q Accompanied with move of servo piston (9), springs (4) and (6) will be expanded and the spring force becomes weaker.

q As the spring force is weakened, spool (3) moves to the right, the connecting between port (C) and port (D) is shut off and the pump discharge pressure ports (B) and (C) are con-nected.

10 Structure, function and maintenance standard SEN01993-00

q As a result, the pressure on port (C) rises and the pressure on the large diameter end of the piston also rises. Thus, the leftward move of servo piston (9) is stopped.

q Servo piston (9) stop position (= Pump deliv-ery) is decided by the position where the push-ing force generated from the pressures (PP1) and (PP2) applied to spool (3), the pushing force of the solenoid in PC-EPC valve gener-ates and the pushing force of springs (4) and (6) are balanced.

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(2) When load on actuator is large and pump discharge pressure is high

Outline

q When the load is large and pump discharge pressures (PP1) and (PP2) are high, the force pushing spool (3) to the right becomes larger and spool (3) will be moved to the position shown in above figure.

q Part of the pressure to be conducted from port (C) to LS valve flows from port (B) to port (C) and (D) via LS valve. At the end this flow, level of this pressure becomes approximately half of the main pump pressure (PP2).

Operation

q When port (E) and port (G) of the LS valve are connected, this pressure from port (J) enters the large diameter end of servo piston (9), stopping servo piston (9).

q If main pump pressure (PP2) increases further and spool (3) moves further to the right, main pump pressure (PP1) flows to port (C) and acts to make the pump delivery the minimum.

q When servo piston (9) moves to the right, springs (4) and (6) are compressed and push back spool (3).

q When spool (3) moves to the left, the opening of port (C) and port (D) becomes larger.

q As a result, the pressure on port (C) (= J) is decreased and the rightward move servo pis-ton (9) is stopped.

q The position in which servo piston (9) stops at this time is further to the right than the position when pump pressures (PP1) and (PP2) are low.

10 Structure, function and maintenance standard SEN01993-00

q The relationship between the average pump pressure (PP1 + PP2)/2 and servo piston (9) in terms of their positions can be represented by the broken line in the figure springs (4) and (6) form the double springs.

q The relationship between the average pump pressure (PP1 + PP2)/2 and average pump delivery (Q) becomes as shown below.

q If command voltage (X) sent to PC-EPC valve solenoid (1) increases further, the relationship between average pump pressure (PP1 + PP2)/

2, and pump delivery (Q) is proportional to the force of the PC-EPC valve solenoid and moves in parallel.

q Namely, the force of PC-EPC valve solenoid (1) is added to the pushing force to the right because of the pump pressure applied to the spool (3), so the relationship between the aver-age pump pressure (PP1 + PP2)/2 and the pump delivery (Q) moves from (A) to (B) as the command current (X) is increased.

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2) As the emergency pump drive switch is turned on due to failure on the pump controller