Section 5 - Hydraulics; Cylinders - Theory Of Operation; Systems Incorporating Double Acting Cylinders; Systems Incorporating Holding Valves - JLG 600SJ Service And Maintenance Manual

5.1 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: Slave Level,
Master Level, Lift, Telescope, Articulating Jib Boom Lift,
Axle Lockout and Steer. A double acting cylinder is one
that requires oil flow to operate the cylinder rod in both
directions. Directing oil (by actuating the corresponding
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 Lift, Telescope, Lockout,
Slave Level and Articulating Jib Boom Lift circuits to pre-
vent retraction of the cylinder rod should a hydraulic line
rupture or a leak develop between the cylinder and its
related control valve.

5.2 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.
3121202
SECTION 5. HYDRAULICS
4. If cylinder retract port leakage is less than 6-8 drops
5. With cylinder fully retracted, shut down engine and
6. Activate engine and retract cylinder. Check extend
7. If extend port leakage is less than 6-8 drops per min-

Cylinders With Single Counterbalance Valve

(Upper Lift Cylinder)
OPERATE ALL FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
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
3. There will be initial weeping of hydraulic fluid, which
– JLG Lift –

SECTION 5 - HYDRAULICS

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.
carefully disconnect hydraulic hose from cylinder
extend port.
port for leakage.
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.
hydraulic system.
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.
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.
5-1