Excavator Valve
Precision Control (Servo)
(continued)
(Machines from January 2003)
Load Sense - Pressure Compensator Valves
Operation
No service selected (all spools in neutral)
With the spools in the neutral position oil from the pump
P
flows to each service spool
S
and is dead ended. This
generates sufficient pressure (about 15bar) to overcome the
spring
E1
and oil flows across the flushing valve spool
assembly
E
to tank
T
. Any load acting on the service ram
R
is held by the service spool
S
.
Oil in the load sense gallery
LS
is vented to tank across the
flow regulator valve
F
.
The pressure compensator valve
PC
and load hold check
valves
CV
are closed.
8 - 19
Section E
9803/3280
Section E
8 - 19
Issue 1
Circuit Descriptions
Operating a service
The service spool
S
is moved by servo pressure connecting
the oil from the pump
P
to the gallery
P1
. The pressure in
this chamber opens the pressure compensator valve
PC
allowing oil into the load sense gallery
LS
. Oil pressure builds
in the gallery
LS
and the flushing valve spool
E
begins to
close under the combined action of the oil pressure and
spring force, causing the pump pressure to rise as follows:
Pump pressure = Pressure in load sense gallery
LS
+
pressure from spring
E1
.
When the pump pressure has risen sufficiently i.e. to a
pressure equal to the load + spring pressure (15 bar) the load
hold check valve
CV
moves off its seat and allows oil to flow
into the service gallery
A
, initiating movement of the service
ram
R
. Exhaust oil from the other side of the service ram is
directed by the service spool
S
to tank.
The maximum load sense pressure and hence pump
pressure is set by the load sense relief valve
G
.