Komatsu PC200-8M0 Shop Manual page 192

SEN06120-00
Variable back pressure valve
Function
q This valve creates back pressure in the drain
circuit to prevent cavitation caused by negative
pressure in each actuator circuit (motors, cylin-
ders, etc.).
q In order to reduce output loss, this valve
reduces the back pressure in the drain circuit
when any operation other than the travel and
swing is performed.
Operation
When engine is stopped
q Pressurized oil is not supplied from the main
pump to self-pressure reducing valve (1).
No oil pressure is transmitted from variable
q
back pressure solenoid valve (3) to spring
chamber (C), and valve (6) is pressed to the
right by the reaction force of spring (5).
Drain circuit (A) of the control valve is intercon-
q
nected to port (T) through orifice (b) of valve
(6).
50
10-402
402 Hydraulic system, Part 2
Functions and operation by valve
When engine is running (during swing or travel
operation)
The pressurized oil (PR) from self-pressure
q
reducing valve (1) flows to variable back pres-
sure solenoid valve (3).
q Since drive signal (IS3) of variable back pres-
sure solenoid valve (3) is "OFF", pressurized
oil (PR) is flown to spring chamber (C) of vari-
able back pressure valve (4).
Pressurized oil (PR) transmitted to spring
q
chamber (C) exerts a force on the left end face
of valve (6) (surface of fd), and it pushes valve
(6) to the right as a combined force with the
reaction force of spring (5).
q Pressure (PA) of drain circuit (A) of the control
valve acts on the right end face of valve (6)
(surface of fd1) to push valve (6) to the left.
q Valve (6) balances at a position where it cre-
ates back pressure (PA) determined as fol-
lows; PA = (Area of surface fd) x Pressure
(PR) + Reaction force of spring (5) / Area of
surface fd1
q Accordingly, the oil flow rate from drain circuit
(A) of the control valve to port (T) is limited. In
other words, the set pressure of variable back
pressure valve (4) is adjusted.
PC200, 200LC, 220, 220LC-8M0