JLG M450 ANSI Service Manual - Manual (Page 156)

SECTION 5 - HYDRAULICS
5-4 – JLG Lift – 3121127
Cylinders With Dual Counterbalance Valve
(Lower Lift, Mid Lift, Telescope, and Slave Cylinders)
OPERATE ALL FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1.Using all applicable safety precautions, activate
hydraulic system.
WHEN WORKING ON THE UPPER BOOM LIFT CYLINDER RAISE
THE UPPER BOOM TO HORIZONTAL AND PLACE A BOOM PROP
APPROXIMATELY 1 INCH (2.54 CM) BELOW THE MAIN BOOM. IF
WORKING ON LOWER BOOM LIFT CYLINDER, RAISE LOWER
BOOM HALFWAY, FULLY ELEVATE UPPER BOOM AND ATTACH
OVERHEAD CRANE TO THE UPRIGHT FOR SUPPORT, LEAVING
APPROXIMATELY 1 INCH (2.54 CM) OF SLACK IN CHAIN OR
SLING FOR TEST PURPOSES.
2. When working on the platform slave cylinder, stroke
platform slave level cylinder forward until platform
sits at a 45 degree angle.
3. After completing the above, shut down hydraulic
system and allow machine to sit for 10-15 minutes.
This is done to relieve pressure in the hydraulic
lines. Carefully remove hydraulic hoses from appro-
priate 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, the following cylinder
repairs must be made. If the retract port is leaking,
the piston is leaking, the piston seals are defective
and must be replaced. If the extend port is leaking,
the counterbalance is defective and must be
replaced.
5. To check piston seals, carefully remove the counter-
balance valve from the retract port. After initial dis-
charge there should not be any further leakage from
the ports. If leakage occurs at a rate of 6-8 drops per
minute or more, the piston seals are defective and
must be replaced.
6. If no repairs are necessary or when repairs have
been made, carefully reconnect hydraulic hoses to
the appropriate ports.
7. Remove boom prop/overhead crane, activate
hydraulic system and run cylinder through complete
cycle to check for leaks and operation.
5.4 CYLINDER REPAIR
NOTE: The following are general procedures that apply to
all of the cylinders on this machine. Procedures that
apply to a specific cylinder will be so noted.
Disassembly
DISASSEMBLY OF THE CYLINDER SHOULD BE PERFORMED ON A
CLEAN WORK SURFACE IN A DIRT FREE WORK AREA.
1. Connect a suitable auxiliary hydraulic power source
to the cylinder port block fitting.
DO NOT FULLY EXTEND CYLINDER TO THE END OF STROKE.
RETRACT CYLINDER SLIGHTLY TO AVOID TRAPPING PRESSURE.
2. Operate the hydraulic power source and extend the
cylinder. Shut down and disconnect the power
source. Adequately support the cylinder rod, if appli-
cable.
3. If applicable, remove the cartridge-type holding
valve and fittings from the cylinder port block. Dis-
card o-rings.
4. Place the cylinder barrel into a suitable holding fix-
ture.
5. Mark cylinder head and barrel with a center punch
for easy realignment. Using an allen wrench, loosen
Figure 5-1. Cylinder Barrel Support