Lift/Lower Cylinder - JLG sky trak mmv Service Manual

IMPORTANT: When sliding the rod (5) and piston
assembly in the tube, prevent the threaded end of the
tube from damaging the piston (7). Keep the rod cen-
tered within the tube to help prevent binding. Protect the
finish on the rod at all times. Damage to the surface of
the rod can cause seal failure.
13. Carefully insert the rod (5) with all attachments
straight into the tube (2).
14. Using a pin spanner wrench, thread the head gland
(6) into the tube (2). Torque the head gland to
300-400 lb/ft (407-542 Nm).
15. Lubricate the counterbalance valve (3) and the pilot-
operated check valve (4) with clean, filtered
hydraulic oil. Using new oiled o-rings, thread the
counterbalance valve and the pilot-operated check
valve into the trunnion end of the cylinder. Torque the
valves to 45-50 lb/ft (61-68 Nm).
IMPORTANT: DO NOT tamper with or attempt to reset
the counterbalance valve cartridge. If adjustment or
replacement is necessary, replace a counterbalance
valve with a new part.
16. Test the cylinder at low operating pressure
(100 psi or 7 bar). Verify that the piston and the rod
move freely in both directions.
17. Increase the operating pressure to the maximum
amount for the cylinder (4000 psi or 275 bar) and
again check for free movement in both directions.
18. Retract the rod fully into the tube and cap and plug
the hydraulic hose ports.
f. Extend/Retract Cylinder Installation
Installation procedures are covered in Section 4.3.2, In-
ner Boom Replacement .
g. Extend/Retract Cylinder Pressure Checking and
Circuit Bleeding
1. Attach a 5000 psi (345 bar) gauge to the main
control valve at the extend/retract base port relief to
test the extend/retract cylinder hydraulic system
circuit pressure. The unit is equipped with test
fittings from the factory. Pressure readings should be
3000 ±100 psi (206,8 ±7 bar) on the extend and
3750 psi (259 bar) on the retract. Refer to Section
9.7, Hydraulic System Testing .
2. With no accessory installed on the quick attach, start
with the lift/lower and attachment tilt cylinders fully
retracted.
3. Fully extend the extend/retract cylinder. Hold the
joystick at full extend while fully raising the lift/lower
cylinder.
Model MMV Rev. 12/04
4. Fully retract the attachment tilt cylinder. Hold the
attachment tilt joystick at full retract while fully
retracting the lift/lower cylinder.
5. Repeat Steps 3-4 five times.
9.14.7

Lift/Lower Cylinder

The lift/lower cylinder (26) is anchored to the underside of
the outer boom and to the vehicle frame.
26
a. Lift/Lower Cylinder Removal
WARNING:
raised boom unless the boom is blocked up.
Always block the boom before doing any servic-
ing that requires the boom to be up. Unexpected
lowering of the boom may cause death or serious
injury.
1. Park the vehicle on a firm, level surface, fully retract
all hydraulic cylinders, place the travel select lever in
[N] NEUTRAL, place the neutral lock lever in the [N]
NEUTRAL LOCK position, engage the park brake
switch, raise and support the boom to allow removal
of the lift/lower cylinder and shut the engine OFF.
2. Place an Accident Prevention Tag on both the
ignition switch and the steering wheel, stating that
the vehicle should not be operated. Refer to Section
1.5, Accident Prevention Tag Usage .
WARNING:
cause severe burns. Wait for hydraulic fluid to
cool before servicing any hydraulic component.
3. Unlatch and open the engine cover. Allow the
hydraulic fluid to cool.
Hydraulic System
DO NOT get under a
Hot hydraulic fluid can
MM3790
9.155