Oshkosh JLG 400S, Service And Maintenance Manual

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SECTION 5 - HYDRAULICS
3120895 – JLG Lift – 5-1
SECTION 5. HYDRAULICS
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, Axle Lockout and Steer. A
double acting cylinder is one that requires oil flow to oper-
ate 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 cylin-
der, 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,
and Slave Level circuits to prevent retraction of the cylin-
der rod should a hydraulic line rupture or a leak develop
between the cylinder and its related control valve.
5.2 CYLINDER REPAIR
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 Dual Counterbalance Valves
Slave Level, Lift, and Telescope
OPERATE ALL FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
hydraulic system.
IF WORKING ON THE PLATFORM LEVEL CYLINDER, STROKE
PLATFORM LEVEL CYLINDER FORWARD UNTIL PLATFORM SITS
AT A 45 DEGREES ANGLE.
2. Shut down hydraulic system and allow machine to
sit for 10-15 minutes. Move control lever for applica-
ble 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
ports. If leakage occurs at a rate of 6-8 drops per
minute or more, the piston seals are defective and
must be replaced.
5. If no repairs are necessary or when repairs have
been made, replace counterbalance valve and care-
fully connect hydraulic hoses to cylinder port block.
6. If used, remove lifting device from upright or remove
prop from below main boom, activate hydraulic sys-
tem and run cylinder through one complete cycle to
check for leaks.