45 - 8
Section D
9803/3280
Section D
45 - 8
Issue 1
Precision Control (Servo)
A404750
Hydraulic symbol
PV
P2
T
P1
Basic System Operation
Servo Pressure Supply Valve - Description
The servo pressure supply valve is fitted with a pressure
regulating valve
1
, a pressure relief valve
2
and two solenoid
valves
3
for servo pressure isolation.
The servo system is supplied from a single line tapped into
the hydraulic pump feed at the excavator valve block. The
pressure regulating valve
1
maintains servo pressure by
regulating the incoming supply pressure. The valve is non-
adjustable and is factory set with shims.
The pressure relief valve
2
is adjustable and limits the
maximum pressure, protecting the valve block from over-
pressurisation by opening and allowing excess pressure to
vent to tank.
Down stream of the pressure regulating valve is located the
servo accumulator
4
with its associated non return valve.
The accumulator traps and stores servo pressure to enable
the excavator spools to be operated for a limited period with
a stopped engine. This allows the excavator end to be
lowered safely to the ground should the engine stop.
The servo isolator solenoid valves
3
are energised by
proximity switches on the drivers seat. When the seat arm
rest is raised forward, the proximity switch closes and
energises the relevant isolator solenoid, directing servo
pressure to the particular joystick controller. When the arm
rest is folded back the proximity switch opens and the
isolation solenoid is de-energised, the isolation valve spool is
spring returned to the closed position isolating the supply to
the joystick controller.
1
4
3
2
3
