JLG 20VP (3120728) Service Manual - Manual (Page 42)

SECTION 2 - SERVICE PROCEDURES
2-24 – JLG Lift – 3120728
Lift Cylinder Component Inspection
Cylinder Rod
There should be no scratches or pits deep enough to
catch the fingernail. Pits that go to the base metal are
unacceptable. Scratches that catch the fingernail but are
not to the base metal, less than 0.5 inch long and primarily
in the circumferential direction are acceptable provided
they cannot cut the rod seal. Chrome should be present
over the entire surface of the rod and the lack thereof is
unacceptable. In the event that an unacceptable condition
occurs, the rod should be repaired or replaced.
Cylinder Head
Visually inspect the inside bore for scratches or polishing.
Deep scratches are unacceptable. Polishing indicates
uneven loading and when this occurs, the bore should be
checked for out-of-roundness. If out-of-roundness exceed
0.007", this is unacceptable. Check the condition of the
dynamic seals (wiper, rod seals) looking particulary for
metallic particles embedded in the seal surface. It is nor-
mal to cut the static seal on the retaining ring groove upon
disassembly. Remove the rod seal, static o-ring and
backup and rod wiper. Damage to the seal grooves, par-
ticularly on the sealing surfaces, is unacceptable. In the
event that an unacceptable condition occurs, the head
should be replaced.
Piston
Visually inspect the outside surface for scratches or pol-
ishing. Deep scratches are unacceptable. Polishing indi-
cates uneven loading and when this occurs, the diameter
should be checked for out-of-roundness. If out-of-round-
ness exceeds 0.007", this is unacceptable. Check the con-
dition of the dynamic seals and bearings looking
particularly for metallic particles embedded in the bearing
and in the piston seal surface. Remove the seals and
bearings. Damage to the seal grooves, particularly on the
sealing surfaces, is unacceptable. In the event that an
unacceptable condition occurs, the piston should be
replaced.
Tube Assembly
Visually inspect the inside bore for scratches and pits.
There should be no scratches or pits deep enough to
catch the fingernail. Scratches that catch the fingernail but
are less than 0.5 inch long and primarily in the circumfer-
ential direction are acceptable provided they cannot cut
the piston seal. The roughness of the bore should be
between 10 and 20 µ inches RMS. Significant variation
(greater than 8 µ inches difference) are unacceptable. In
the event that an unacceptable condition occurs, the tube
assembly should be repaired or replaced.
Cylinder Assembly
(See Figure 2-17.)
1. Rinse the inside of the tube with hydraulic fluid and
allow to drain. A high-pressure rinse followed by a
wipe with a lint-free rag is preferable. Clean all inter-
nal components of any foreign material.
2. Lubricate the head and all seals with hydraulic fluid
prior to installation. Install the seal, wiper, o-ring,
back-up ring, and retraining ring to the cylinder
head.
3. Lubricate the piston and all components with
hydraulic fluid. Install the seal and wear ring to the
piston.
NOTE: Re-check that seals are not twisted or pinched and
are properly seated.
4. Place the rod on a clean table. Install the static o-
ring seal into the groove on the piston end of the
rod.
5. Install the head followed by the piston onto the rod
noting the proper orientation of each component.
Torque the piston nut to 175-200 ft. lbs.
6. When the rod assembly is ready to be installed into
the tube, liberally apply an anti-seize lubricant to the
head outer surface, especially the static seal.
7. Next dip the entire rod assembly into hydraulic fluid
and stuff this assembly into the tube. Watch the
seals as they pass over the rod port (if visible) to be
sure they are not nicked or cut.
8. Install the head until the retaining ring seats in it’s
groove.
Cylinder Installation
To install the lift cylinder reverse the Lift Cylinder Removal
instructions at the start of this section, however perform
the following additional steps during re-assembly.
1. Apply Loctite #222 (purple) to the setscrew securing
the lift cylinder rod to the sheave wheel anchor block
at the top of mast section-2.
2. Torque the four (4) cylinder lift mounting bolts to 34
ft. lb.