Komatsu PC500LC-10R Shop Manual page 280

COMPONENT PARTS OF HYDRAULIC SYSTEM
VARIABLE BACK PRESSURE VALVE OF CONTROL VALVE
FUNCTION OF VARIABLE BACK PRESSURE VALVE OF CONTROL VALVE
• This valve applies back pressure to the drain circuit to prevent cavitation which is caused by negative pres-
sure of each actuator (motor, cylinder, etc.)
• Lower the back pressure of the drain circuit except for travel operations in order to reduce output loss.
OPERATION OF VARIABLE BACK PRESSURE VALVE OF CONTROL VALVE
When the engine is stopped
1.
No pressurized oil is supplied from the main pump to self-
pressure reducing valve (1).
2.
No hydraulic pressure is supplied from variable back pres-
sure solenoid valve (3) to spring chamber (C), and valve
(6) is pressed to the right by the reaction force of spring
(5).
3.
Drain circuit (A) of the control valve is interconnected with
port (T) through orifice (b) of valve (6).
When engine is in operation (swing or travel operation)
1.
Pressurized oil (PR) output from self-pressure reducing
valve (1) flows to variable back pressure solenoid valve
(3).
2.
Since drive signal (IS3) to variable back pressure solenoid
valve (3) is "OFF", pressurized oil (PR) flows to spring
chamber (C) of variable back pressure valve.
3.
Pressurized oil (PR) flowing to spring chamber (C) acts on
the left end surface (area of diameter "d") of valve (6) and
pushes valve (6) to the right together with the reaction
force of spring (5).
4.
Pressure (PA) in drain circuit (A) of the control valve acts
on the right end surface (area of diameter "d1") of valve (6)
and pushes valve (6) to the left.
5.
Valve (6) keeps balance so that the back pressure be-
comes (PA) in the following equation. PA = (Area of diame-
ter "d" x Pressure (PR) + Reaction force of spring (5) ) /
Area of diameter "d1"
6.
The oil flow from drain circuit (A) of the control valve to
port (T) is adjusted to the set pressure of variable back
pressure valve (4).
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10 STRUCTURE AND FUNCTION
PC500LC-10R
A EP2 16 1 3
A EP2 16 1 4