Komatsu Utility PC12R-8 MISTRAL, Shop Manual

Page: 66 / 324

1.Unload valve
FUNCTION
1) In the case of fixed pump system, the Unload valve has functions included variable pump and LS valve of varia-
ble pump system.
. When the control valve is at HOLD, pump discharge amount Q is released to the tank circuit.
. When the control valve is operated, the flow from pump is sent to the actuator circuit in proportion to the opening
areas of valve spool.
. The LS differential pressure D P
LS
is become variable according to engine speed.
(Engine speed sensing function).
OPERATION
When control valve is at HOLD
. At the left end of spool (1), pump pressure P
P1
is
acting on area S1 , and at the right end of spool
(1), pump pressure P
P2
and spring force of spring
(2) is acting on area S2 , and LS pressure P
LS
is ac-
ting on area S3 .
. The reason of difference between pump pressure
P
P1
and P
P2
comes from pressure loss P
P
, when
the pump discharge amount passes through the
orifice (3) of spool (1). ( P
P1
= P
P2
+ D P
P
).
. When the control valve is at HOLD, LS pressure
P
LS
is not generated, so the pump pressure P
P1
,
P
P2
, and spring force of spring (2) is acting on spool
(1).
. As pump discharge pressure P
P1
rises and reaches
the P
P2
x S1 = P
P1
x S2 + spring force of spring (2),
spool (1) is moved to the right. Pump circuit P
P1
,
P
P2
are then connected to tank circuit T through
the drill hole.
. In this way, the differential pressure ( LS differential
pressure) between pump discharge pressure P
P1
and LS pressure P
LS
is set to 19.6
9.8
bar (20
10
kg/cm
2
).
P
P
=Pump circuit
P
LS
=Load Sensing circuit
T-TS =Tank circuit
A =To control valve spool
B =To LS valve
10-48
PC12R-8 PC15R-8
To actuator
RKP00381
A
B
1
P
P1
S
3
T
S
S
1
2
P
LS
P
P2
T
3
S
2