8 - 18
Section E
9803/3280
Section E
8 - 18
Issue 1
Circuit Descriptions
T
P
M
A
B
A
B
A
B
A
B
A
B
A
B
A
B
T1
LS
P
M
P
T
T3
C
A403680
Excavator Valve
Precision Control (Servo)
(continued)
(Machines from January 2003)
In the interest of machine efficiency and consistent backhoe
operation the excavator valve is of a closed centre design
and incorporates pressure compensating valves
F
, for each
service.
These features enable the valve to operate as follows:
No service selected (all spools in neutral)
Oil from the loader valve high pressure carry over enters the
excavator valve at the inlet port
A
. Oil flows across the
flushing valve spool assembly
B
and returns to tank from the
outlet port
C
.
Servo Spool Actuation
When a service is operated servo pressure from the joystick
control valve acts on the end of the relevant service spool
D
and displaces the spool proportionally against the force of
the centering spring
E
, allowing the oil to flow to move the
required service. When the servo pressure on the end of the
service spool is reduced the force of the centering spring
overcomes the servo pressure and the service spool will
move back proportionaly to the central neutral position.
When a Service is Selected
When one or more services are selected the pressure
compensator valves
F
detect the service generating the
highest pressure. This causes the flushing valve spool
B
to
close and allow pump flow to the spools and service ports at
sufficient pressure to move the load. Load hold check valves
G
prevent the load falling back if the load pressure is greater
than the pump pressure. The closed centre design ensures
consistent service operation regardless of load.
For a full description of spool and pressure compensator
valve operation see
Load Sense - Pressure Compensator
Valve Operation
.
LS
T
P
T
A
B
D
E
G
F
G
A405000
B
C
A
