Hydraulics
9803/3280 Issue 1
6 - 1 6 - 1
Circuit Descriptions
Section E Section E
Loader Valve - Neutral Circuit
Component Key:
B Parallel gallery
C Gallery
D Drilling
T Tank Port
1A Pump Inlet (section P1)
1B High Pressure Carry Over
1C Pump Inlet (section P2)
3B Main Relief Valve
3C LIft Ram Spool
3D Shovel Ram Spool
3E Auxiliary Spool
3L Unloader Valve Spool
3M Unloader Pilot Valve
3N Check Valve
3P Spring Cavity
3Q Solenoid Valve
The loader valve is mounted on the chassis frame, right
hand side (when viewed from the rear).
It includes the lift ram spool 3C, shovel ram spool 3D and
auxiliary spool 3E. Linkage rods connect the spool ends to
the control levers.
In neutral circuit hydraulic oil from the pump section P2
enters the loader valve at 1C via the steering priority valve.
On entering the valve block, oil flows around the waisted
section of the unloader valve 3L, past check valve 3N and
joins the flow from pump section P1.
Oil from pump section P1 enters the loader valve at 1A.
Combined oil flow from P1 and P2 passes the main relief
valve (MRV) 3B and fills the parallel gallery B. From the
parallel gallery the oil flows around the waisted central
portions of spools 3C, 3D and 3E (all in neutral position) and
flows on to feed the excavator valve via high pressure carry
over line 1B.
Pressure in inlet gallery C is sensed by pilot valve 3M via the
bore of spool 3L. At pressures below the setting of the
unloader valve, both pilot valve 3M and spool 3L remain
closed.
Loader Valve - Unloader Operation
Unloader spool operation protects the engine from being
overloaded if a service is being worked particularly hard, for
example when using the excavator to tear out. It does this
by dumping the oil from the pump section P2 to tank,
allowing engine power to be applied fully to the main pump
section P1.
If the pressure in the inlet gallery C rises to the setting of the
pilot valve 3M, this valve will open, allowing oil in spring
cavity 3P to escape more quickly than it can be replaced by
oil entering through the small drilling D.
This creates a pressure differential between the spring cavity
3P and gallery C. Higher pressure in gallery C acts on the
face of spool 3L causing the spool to be moved off its seat.
Oil entering the valve block from pump section P2 now flow
directly to tank T.
High pressure in gallery C also holds check valve 3N firmly
closed, preventing oil from pump section P1 from also being
dumped.
When pressure in inlet gallery C falls, for example if the
excavator has stopped tearing out, pilot valve 3M will close.
This means oil in spring cavity 3P will be at the same
pressure as oil in gallery C, spring pressure will move spool
3L back onto its seat, closing pump section P2 connection
to tank.
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