Service Manual User Manual

Section 9
9-26
9.8.2 Boom Hoist/Lower Circuit
a. Hoist Circuit
With the joystick in the boom lift position, pilot pressure
shifts the hoist/lower spool valve (Fig. 9– 15) in the direc-
tional control valve to direct system pressure to ports A-D
through E-B and through the check valve to the base end
of the hoist cylinder.
Return oil from the rod end of the cylinder flows through
ports C-F of the spool valve to the return filter and then to
the hydraulic fluid reservoir (Fig. 9– 16).
• If system pressure exceeds 3500 psi (241,5 bar),
the main relief will open, allowing oil to return to the
reservoir.
• If the return filter becomes plugged, return hydraulic
oil will bypass the filter when pressure reaches 25
psi (1,725 bar) and return to the reservoir unfiltered.
Figure 9– 15. Hoist/lower spool valve schematic.
b. Lower Circuit
When the joystick is in the boom lowering position, pilot
pressure shifts the hoist/lower spool valve (Fig. 9– 15) in
the directional control valve to direct system pressure
through ports A-D, through the flow compensator. Oil is
then directed through ports F-B to the rod end of the
hoist cylinder. System pressure also pilots open the
counterbalance valve in the hoist cylinder.
The open counterbalance valve allows return oil from the
base of the cylinder to flow to ports C-E of the directional
control valve, to the return filter and then to the hydraulic
fluid reservoir (Fig. 9– 16).
• If system pressure exceeds 3500 psi (241,5 bar),
the main relief will open allowing oil to return to the
reservoir.
• If the return filter becomes plugged, return hydraulic
oil will bypass the filter when pressure reaches 25
psi (1,725 bar) and return to the reservoir unfiltered.
The counterbalance valve in the boom hoist cylinder
serves another main function for safety purposes. This
function can be described as follows:
Should any of the hydraulic lines routed to the hoist cylin-
der fail, there will be a loss of hydraulic system pressure
to the cylinder. For example, if a hydraulic line failure
occurred in the pressurized lowering line during lowering,
the counterbalance valve would lose pilot pressure, clos-
ing off flow returning to the reservoir. The oil in the base
end of the cylinder would then be trapped, which would
immediately stop boom lowering and prevent an elevated
load from falling to the ground uncontrolled.
The load can be lowered safely to the ground by following
the
Emergency Boom Lowering
procedures in
Section 3
Boom
of this manual.
IMPORTANT: DO NOT attempt to reset the counterbal-
ance valve cartridges. In the event that the counterbal-
ance valve cartridges were disabled for emergency boom
lowering, or if any unauthorized adjustments are ever
made to these cartridges, remove and replace both car-
tridges with new parts. Failure to replace these cartridges
with new parts may alter the holding characteristics of the
counterbalance valves, creating an unsafe condition for
vehicle operation.
HOIST/LOWER SPOOL VALVE
(Part of Main Control Valve)
BASE END
ROD END
INLET
RESERVOIR
LOAD SENSE OUT
PILOT SELECT MANIFOLD
XX
4000
PSI
HOIST/LOWER CYLINDER
C
MS1890
BA
FED
CBACBA
FED
FED
Model 3606 • Origin 10/99