JLG 800A_AJ Service Manual - Manual (Page 286)

SECTION 5 - HYDRAULICS
5-4 – JLG Lift – 31208858
5. Check the Tower lift up holding valve function as fol-
lows:
a. Fully retract and fully lower the main boom and
tower boom assemblies.
b. Install a 5000 psi (345 bar) pressure gauge to
the pressure tap connection installed on port #7
or port MX7 of the main control valve block. This
pressure test connection was installed in earlier
steps.
c. Hold the tower boom lift up function between 2
and 5 seconds, and then release the function.
d. Verify that the gauge reads, and maintains, pres-
sure above 1000 psi (68.95 bar) for one minute.
NOTE: If pressure does not remain above the stated pres-
sure for one minute, replace the tower lift check valve
(#7017474).
e. Activate tower lift down to release any trapped
pressure and remove pressure gauge from the
test port.
6. Load the platform with the rated capacity and cycle
all functions a minimum of five (5) times to confirm
safe and proper operational characteristics.
7. The machine may be returned to service once
proper operation is confirmed.
5.4 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: - Lower Lift,
Tower Telescope, Slave Level/Main Level, Upper Lift,
Upper Telescope, Master Level/Upright Level, Articulating
Jib Boom Lift, Steer and Axle lockout. A double acting cyl-
inder is one that requires oil flow to operate the cylinder
rod in both directions. Directing oil (by actuating the corre-
sponding 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 cylinder, 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 - Lower Lift, Tower Tele-
scope, Upright Level, Lockout, Articulating Jib Boom Lift,
Upper Lift/Slave Level and Upper Telescope circuits to
prevent retraction of the cylinder rod should a hydraulic
line rupture or a leak develop between the cylinder and its
related control valve.