Lull 944E-42 Service Manual - Manual (Page 400)

Hydraulic System
8.148 Model 644E-42/944E-42 Rev. 6/04
8.8.51 Boom Extend/Retract Circuit
Description (Dual Joystick)
Hydraulic flow is applied in the Boom Extend/Retract
circuit (see page 8.74 and page 8.78) 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 Extend/Retract 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 Rear Joystick “P” Port. The pressure is regulated by
the position of the joystick.
a. Boom Extend Position (See page 8.74)
When the joystick is in the boom EXTEND
position, pilot pressure from the joystick Port “2”
is directed through Extend/Retract Spool Ports
on the Main Valve, to the Counterbalance Valve
at the base end of the Extend/Retract 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, to the Oil Cooler, through the Return Filter
and then to the Hydraulic Reservoir.
If system pressure exceeds 2900 psi (200 bar), the
Extend Port Relief will open and allow hydraulic 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 Retract Position (See page 8.78)
When the joystick is in the boom RETRACT
position, pilot pressure from Joystick Port “1”
directs system pressure through the Extend/
Retract Spool on the Main Valve, to the rod end
of the Extend/Retract Cylinder “Retract” Port. The oil
passes the Check Valve and enters the Extend/Retract
Cylinder and exits through the Counterbalance Valve and
“Retract” Port. The oil will pass from the cylinder through
the Extend/Retract Spool on the Main Valve, to the Oil
Cooler, through the Return Filter and then to the
Hydraulic Reservoir.
If system pressure reaches 3400 psi (234 bar), the Main
Relief Valve Opens, allowing hydraulic 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 Extend/Retract Cylinder includes a Counterbalance
Valve and a Pilot-Operated Check Valve. The valves
serve two main functions. One of the functions is to help
prevent component damage.
The other valve functions can be described as follows:
• If the vehicle is travelling and the boom runs into a
solid object, pressure will build up on the base end of
the of the cylinder. Without a counterbalance valve,
the cylinder rod would bend, damaging the cylinder.
With the counterbalance valve installed, the
pressure will build up until it reaches 4000 psi
(276 bar) and then the counterbalance valve will be
internally piloted open, allowing the pressure to
return to the reservoir. This condition will only exist if
the cylinder has been extended. If the cylinder is fully
retracted, the counterbalance valve will have no
effect and damage can result.
• Should any of the hydraulic lines going to the
Extend/Retract Cylinder fail, there will be a loss of
hydraulic system pressure to the cylinder. For
example, if there was a hydraulic line failure in the
pressurized retract line during retracting, 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 retraction,
preventing an elevated load from uncontrolled
retraction. 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 a
counterbalance valve cartridge. In the event that a
counterbalance valve cartridge was disabled for
emergency boom lowering or any unauthorized
adjustments are ever made to this cartridge, remove and
replace the cartridge with a new part. Failure to replace
this cartridge with a new part may alter the holding
characteristics of the counterbalance valve, creating an
unsafe condition for vehicle operation.
MT3800
MT3810