JLG 600A_AJ Service Manual - Manual (Page 213)

SECTION 5 - HYDRAULICS
3121201 – JLG Lift – 5-3
5.2 CYLINDERS - THEORY OF OPERATION
Systems Incorporating Double Acting
Cylinders
Cylinders are of the double acting type. Systems incorpo-
rating double acting cylinders are as follows: - Lower Lift,
Tower Telescope, Slave Level/Main Level, Upper Lift,
Upper Telescope, Master Level/Upright Level, Articulating
Jib Boom Lift, Steer and Axle lockout. A double acting cyl-
inder is one that requires oil flow to operate the cylinder
rod in both directions. Directing oil (by actuating the corre-
sponding control valve to the piston side of the cylinder)
forces the piston to travel toward the rod end of the barrel,
extending the cylinder rod (piston attached to rod). When
the oil flow is stopped, movement of rod will stop. By
directing oil to the rod side of the cylinder, the piston will
be forced in the opposite direction and the cylinder rod
will retract.
Systems Incorporating Holding Valves
Holding valves are used in the - Lower Lift, Tower Tele-
scope, Upright Level, Lockout, Articulating Jib Boom Lift,
Upper Lift/Slave Level and Upper Telescope circuits to
prevent retraction of the cylinder rod should a hydraulic
line rupture or a leak develop between the cylinder and its
related control valve.
5.3 CYLINDER CHECKING PROCEDURE
NOTE: Cylinder check must be performed anytime a system
component is replaced or when improper system
operation is suspected.
Cylinders Without Counterbalance Valves -
Master Cylinder and Steer Cylinder
1. Using all applicable safety precautions, activate
engine and fully extend cylinder to be checked. Shut
down engine.
2. Carefully disconnect hydraulic hoses from retract
port of cylinder. There will be some initial weeping of
hydraulic fluid which can be caught in a suitable
container. After the initial discharge, there should be
no further drainage from the retract port.
3. Activate engine and extend cylinder.
4. If cylinder retract port leakage is less than 6-8 drops
per minute, carefully reconnect hose to port and
retract cylinder. If leakage continues at a rate of 6-8
drops per minute or more, cylinder repair must be
made.
5. With cylinder fully retracted, shut down engine and
carefully disconnect hydraulic hose from cylinder
extend port.
6. Activate engine and retract cylinder. Check extend
port for leakage.
7. If extend port leakage is less than 6-8 drops per min-
ute, carefully reconnect hose to extend port, than
activate cylinder through one complete cycle and
check for leaks. If leakage continues at a rate of 6-8
drops per minute or more, cylinder repairs must be
made.
Cylinders With Single Counterbalance Valve
Upper Lift Cylinder.
OPERATE ALL FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
hydraulic system.
WHEN WORKING ON THE MAIN LIFT CYLINDER, RAISE THE
BOOM TO HORIZONTAL AND PLACE A BOOM PROP APPROXI-
MATELY 1 INCH (2.54 CM) BELOW THE MAIN BOOM. DO NOT
WORK ON THE CYLINDER WITHOUT A SUITABLE PROP IN PLACE.
2. Shut down hydraulic system and allow machine to
sit for 10-15 minutes. If machine is equipped with
bang-bang or proportional control valves, turn IGNI-
TION SWITCH to ON, move control switch or lever
for applicable cylinder in each direction, then turn
IGNITION SWITCH to OFF. If machine is equipped
with hydraulic control valves, move control lever for
applicable cylinder in each direction. This is done to
relieve pressure in the hydraulic lines. Carefully
remove hydraulic hoses from appropriate cylinder
port block.
3. There will be initial weeping of hydraulic fluid, which
can be caught in a suitable container. After the initial
discharge, there should be no further leakage from
the ports. If leakage continues at a rate of 6-8 drops
per minute or more, the counterbalance valve is
defective and must be replaced.
4. To check piston seals, carefully remove the counter-
balance valve from the retract port. After initial dis-
charge, there should be no further leakage from the