Excavator Valve
Precision Control (Servo) (continued)
(Machines up to serial no. 931159)
In the interest of machine efficiency and consistent backhoe
operation the excavator valve is of a closed centre design
and incorporates load sensing valves D, for each service.
These features enable the valve to operate as follows:
No service is 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
bypass compensator spool assembly E and returns to tank
from the outlet port B.
Servo Spool Actuation
When a service is operated servo pressure from the joystick
control valve acts on the end of the relevant service spool
Eg. 4A and displaces the spool proportionally against the
force of the centering spring C, 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.
Note that spools 4G, 4D and 4C are manually actuated.
When a Service is Selected
When one or more services are selected the load sense
valves detect the service generating the highest pressure.
This causes the bypass compensator valve E to close and
allow pump flow to the spools and service ports at sufficient
pressure to move the load. The load sense valves D also
incorporate check valves. These valves 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 load sense valve operation
see Load Sense (Isolator Compensator) Valve Operation
on the next page.
8 - 14
Section E
Hydraulics
9803/3280
Section E
8 - 14
Issue 2*
Circuit Descriptions
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