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
Hydraulics
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.
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