JLG 120-SXJ Service Manual - Manual (Page 71)

SECTION 5 - HYDRAULICS
3121810 – JLG Lift – 5-1
SECTION 5. HYDRAULICS
5.1 CYLINDERS - THEORY OF OPERATION
1. Cylinders are of the double acting type. Systems
incorporating double acting cylinders are as follows:
Lift, Telescope, Steer, Master Level, Slave Level, Axle
Extend, Frame Lift (if equipped) and Jib (if
equipped). A double acting cylinder 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 cylin-
der) 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 the 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.
2. Holding valves are used in the Lift, Slave Level, Tele-
scope, and Jib 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.
5.2 CYLINDER CHECKING PROCEDURES
NOTE: Cylinder checks must be performed any time a cylin-
der component is replaced or when improper system
operation is suspected.
Cylinder w/o Counterbalance Valves - Steer
Cylinders, Master Level Cylinder, Frame
Jack Cylinders
OPERATE FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
engine and fully extend cylinder to be checked. Shut
down engine.
2. Carefully disconnect hydraulic hose from retract port
of cylinder. 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 retract port.
3. Activate engine and activate cylinder extend func-
tion. Check retract port for leakage.
4. If cylinder leakage is 6-8 drops per minute or more,
piston seals are defective and must be replaced. If
cylinder retract port leakage is less than 6-8 drops
per minute, carefully reconnect hose to retract port
and retract cylinder.
5. With cylinder fully retracted, shut down engine and
carefully disconnect hydraulic hose from cylinder
extend port.
6. Activate engine and activate cylinder retract func-
tion. Check extend port for leakage.
7. If cylinder leakage is 6-8 drops per minute or more,
piston seals are defective and must be replaced. If
extend port leakage is less than 6-8 drops per
minute, carefully reconnect hose to extend port,
then activate cylinder through one complete cycle
and check for leaks.
Cylinders w/Single Counterbalance Valves -
Lift Cylinder, Telescope Cylinder, Jib
Cylinder
OPERATE ALL FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
hydraulic system.
WHEN WORKING ON THE LIFT CYLINDER, RAISE THE BOOM TO
HORIZONTAL AND SUPPORT THE BOOM USING A SUITABLE
BOOM PROP OR OVERHEAD LIFTING DEVICE.
2. If working on the lift cylinder, raise boom to horizon-
tal and place a suitable boom prop approximately 1
inch (2.5 cm) below the boom. If working on the tele-
scope cylinder, raise the boom above horizontal and
extend the fly boom approximately 30.5 cm (1 foot).
3. Shut down hydraulic system and allow machine to
sit for 10-15 minutes. Turn ignition switch to ON,
move control switch or lever for applicable cylinder
in each direction, then turn ignition switch to OFF.
This is done to relieve excess pressure in the
hydraulic lines. Carefully remove hydraulic hoses
from appropriate cylinder port block.
4. There will be initial weeping of hydraulic fluid, which
can be caught in a suitable container. After the initial
discharge, there should not be any further leakage
from the ports. If leakage continues at a rate of 6-8
drops per minute or more, cylinder repairs must be
made. If the retract port is leaking, the piston seals
are defective and must be replaced. If the extend