JLG 600SC_660SJC Service Manual - Manual (Page 147)

SECTION 5 - HYDRAULICS
3121607 – JLG Lift – 5-3
5.2 CYLINDERS - THEORY OF OPERATION
Systems With Double Acting Cylinders
Systems with double acting cylinders are the Slave Level, Mas-
ter Level, Lift, Telescope, Articulating Jib Boom Lift, Axle Lock-
out, 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 piston
side of cylinder) forces the piston to travel toward the rod end
of the barrel, extending the cylinder rod (piston attached to
rod). When oil flow is stopped, movement of rod stops. By
directing oil to the rod side of the cylinder, the piston is forced
in the opposite direction and the cylinder rod retracts.
Systems With Holding Valves
Holding valves are used in the Lift, Telescope, Lockout, Slave
Level, and Articulating Jib Boom Lift circuits to prevent retrac-
tion of the cylinder rod should a hydraulic line rupture or a
leak develop between the cylinder and its control valve.
5.3 CYLINDER CHECKING PROCEDURE
NOTE: Cylinder check must be performed anytime a system com-
ponent 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
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 cyl-
inder. If leakage continues at a rate of 6-8 drops per min-
ute or more, repair cylinder.
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 minute,
carefully reconnect hose to extend port, then activate
cylinder through one complete cycle and check for
leaks. If leakage continues at a rate of 6-8 drops per min-
ute or more, repair cylinder.
Figure 5-1. Boom Positioning and Support, Cylinder Repair
Boom at Horizontal