JLG sky trak mmv Service Manual page 455

3. Slowly remove the counterbalance valve (3) and
pilot-operated check valve (4) from the eyelet end of
the tube (2). If necessary, remove the grease
fitting (5) from the tapped hole in the eyelet end of
the tube.
IMPORTANT: DO NOT tamper with or attempt to reset
the counterbalance valve cartridge. If adjustment or
replacement is necessary, replace the counterbalance
valve with a new part.
4. If the self-aligning bearing (6) requires replacement,
carefully press the bearing out of the tube eyelet.
5. If necessary, remove the grease fitting (7) from the
tapped hole in the eyelet end of the rod (8).
6. If the self-aligning bearing (9) requires replacement,
support the rod (8) in a soft-jawed vise or other
suitable holding device. Carefully press the bearing
out of the rod eyelet.
7. Extend the rod (8) as required to allow access to the
base of the cylinder.
IMPORTANT: Protect the finish on the rod at all times.
Damage to the surface of the rod can cause seal failure.
8. Using a pin spanner wrench, unscrew the head
gland (10) from the tube (2). The head gland was
originally torqued to 500-600 lb/ft (678-814 Nm), so a
considerable amount of force is required. Carefully
slide the head gland down along the rod toward the
eyelet end, away from the cylinder tube (2).
IMPORTANT: When sliding the rod and piston assembly
in the tube, prevent the threaded end of the tube from
damaging the piston (11). Keep the rod centered within
the tube to help prevent binding.
9. Carefully pull the rod (8) with all attachments straight
out of the tube (2).
10. Fasten the eyelet end of the rod in a soft-jawed vise
and put a padded support below and near the threaded
end of the rod to help prevent damage to the rod.
11. Remove both locking inserts (12) from their
installation holes in the head gland threads. Pry or
drill out the inserts as required. DO NOT damage the
head gland threads. New locking inserts will be
required for reassembly.
12. Remove the two setscrews (13) from the piston (11).
New setscrews will be required for reassembly.
13. Remove the piston (11) from the rod (8). The piston
was installed with Loctite #271 (red) and torqued to
500-600 lb/ft (678-814 Nm).
Model MMV Rev. 12/04
Note: It may be necessary to apply heat to break the
bond of the sealant between piston (11) and the rod (8)
before the piston can be removed. Some parts of cylin-
ders are sealed with a special organic sealant and lock-
ing compound.
Before attempting to disassemble these parts, remove
any accessible seals from the area of the bonded joint.
Wipe off any hydraulic oil and heat the joint uniformly to
break the bond. A temperature of 300-400° F (149-204° C)
will destroy the bond. Avoid overheating or the parts may
become distorted or damaged. Apply sufficient torque or
pressure for removal while the parts are still hot.
Breakdown of sealant will leave a white, powdery resi-
due on threads and parts. Clean the residue away with a
soft brass wire brush prior to reassembly, and wipe with
Loctite "T" cleaner before reinstallation.
14. Remove the o-ring (14) and back-up rings (15) from
the piston (11).
15. Remove both precision wearbands (16) and the
capped T-seal (17) from the piston (11).
Note: The T-seal actually consists of four components; a
wide, flexible inner band, the flexible T-seal band itself,
and two supportive split caps that mount on either side of
the "T" itself.
16. Remove the small o-ring (18) from within the piston
(11).
17. Remove the rod spacer (19) from the rod (8).
18. Remove the o-ring (20), back-up ring (21) and o-ring
(22) from the head gland (10). Slide the rings off the
rod (8).
19. Remove the precision wearbands (23), deep Z-seal
with back-up (24) and sealed outside diameter
heavy-duty rod wiper (25) from the head gland (10).
c. Lift/Lower Cylinder Cleaning
1. Remove all hydraulic fluid, dirt and debris from the
inner surface of the cylinder tube (2).
2. Discard all seals, back-up rings, o-rings, etc. Replace
with new items from a complete seal kit (26) to help
ensure proper cylinder function.
3. Clean all metal parts with an approved cleaning
solvent such as trichlorethylene. Carefully clean
cavities, grooves, threads, etc.
Note: If a white, powdery residue is present on threads
and parts, it can be removed. Clean the residue away
with a soft brass wire brush prior to reassembly, and
wipe with Loctite "T" cleaner before reinstallation.
Hydraulic System
9.157