Lull 944E-42 Service Manual - Manual (Page 402)

Hydraulic System
8.150 Model 644E-42/944E-42 Rev. 6/04
8.8.53 Boom Lift/Lower Circuit Description
(Dual Joystick)
Hydraulic flow is applied in the Lift/Lower circuit (see
page 8.82 and page 8.86) by the secondary section of
the Hydraulic Gear Pump through the Main Control
Valve. The primary section of the pump provides
hydraulic flow to the “P” Port through the Accumulator
Charge/Secondary Function Valve out the “EF” Port to
the Main Control Valve. System pressure is directed to
either side of the Lift/Lower Cylinder by the shifting of the
spool in the Main Control Valve. The spool is shifted by
pilot pressure provided from the Accumulator Charge/
Secondary Function Valve out of the “PIL” Port, to the
Rear Joystick “P” Port. The pressure is regulated by the
position of the joystick.
a. Boom Lift Circuit (See page 8.82)
With the Joystick in the boom LIFT position, pilot
pressure from the Joystick Port “3” shifts the Lift/
Lower Spool in the Main Control Valve to direct
system pressure to the “Extend” Port of the Lift/
Lower Cylinder. Oil enters the cylinder through the
“Extend” Port and passes the Counterbalance Valve at
the base end of the Lift/Lower Cylinder. Return oil from
the rod end of the cylinder exits through the Pilot-
Operated Check Valve out the “Retract” Port to the Main
Control Valve through the Lift/Lower Spool, to the Oil
Cooler, through the Return Filter and then to the
Hydraulic Reservoir.
If system pressure exceeds 3400 psi (234 bar), the main
relief will open, allowing oil to return to the reservoir. If the
return filter becomes plugged the indicator light will come
ON, indicating that return hydraulic oil will bypass the
filter when pressure reaches 25 psi (1,7 bar) and return to
the reservoir unfiltered.
b. Boom Lower Circuit (See page 8.86)
With the Joystick in the boom LOWER position,
pilot pressure from the Joystick Port “4” shifts the
Lift/Lower Spool in the Main Control Valve to
direct system pressure to the Lift/Lower
Cylinder. Oil enters the cylinder through the “Retract” Port
and passes the Pilot-Operated Check Valve at the base
end of the Lift/Lower Cylinder. Return oil from the rod end
of the cylinder exits through the Counterbalance Valve
out the “Extend” Port to the Main Control Valve through
the Lift/Lower Spool, to the Oil Cooler, through the Return
Filter and then to the Hydraulic Reservoir.
If system pressure exceeds 3400 psi (234 bar), the Main
Relief Valve will open allowing oil to return to the
Hydraulic Reservoir. If the return filter becomes plugged
the indicator light will come ON, indicating that return
hydraulic oil will bypass the filter when pressure reaches
25 psi (1,7 bar) and return to the reservoir unfiltered.
The Counterbalance and Pilot-Operated Check Valves
serve another main function for safety purposes. This
function can be described as follows:
Should any of the hydraulic lines routed to the Lift/Lower
cylinder 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, closing 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 the Owner/Operators Manual that came
with your vehicle.
IMPORTANT: DO NOT attempt to reset the
counterbalance valve cartridge. In the event that the
counterbalance valve cartridge was disabled for
emergency boom lowering, or if any unauthorized
adjustments are ever made to the cartridge, remove and
replace the cartridge with a new part. Failure to replace
the cartridge with a new part may alter the holding
characteristics of the counterbalance valve, creating an
unsafe condition for vehicle operation.
MT3820
MT3830