Lull 944E-42 Service Manual - Manual (Page 403)

8.151 Model 644E-42/944E-42 Rev. 6/04
Hydraulic System
8.8.54 Boom Lift/Lower Circuit Description
(Single Joystick)
Hydraulic flow is applied in the Lift/Lower Circuit (see
page 8.84 and page 8.88) 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 the “PIL” Port, to the Single Joystick
Pilot Select Valve “P” Port and to the Single Joystick “P”
Port. The pressure is regulated by the position of the
joystick.
a. Boom Lift Circuit (See page 8.84)
With the Joystick in the boom LIFT position, pilot
pressure from the Joystick Port “3” flows to the
Single Joystick Pilot Select Valve Port “P3” and
exits Port “HB”. Then flows to and shifts the Lift/
Lower Spool in the Main Control Valve to direct system
pressure to the Lift/Lower Cylinder. Oil enters 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 flows through the Pilot-Operated
Check Valve out the “Retract” to the Lift/Lower Spool in
the Main Control Valve out the “Outlet” Port, through the
Oil Cooler, to 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
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 8.88)
With the Joystick in the boom LOWER position,
pilot pressure from the Joystick Port “4” flows to
the Single Joystick Pilot Select Valve Port “P4”
and exits Port “HA.” Then flows to and shifts the
Lift/Lower Spool in the Main Control Valve to direct
system pressure to the Lift/Lower Cylinder. Oil enters the
“Retract” Port and passes the Pilot-Operated Check
Valve out the “Extend” Port at the base end of the Lift/
Lower cylinder. Return oil from the rod end of the cylinder
flows through the Counterbalance Valve out the “Extend”
Port to the Lift/Lower Spool in the Main Control Valve,
through the Oil Cooler, to 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