JLG 740AJ, Service And Maintenance Manual

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SECTION 5 - BASIC HYDRAULIC INFOR M ATION AND SCHE M ATICS
5-4 3121160
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, pressure
above 1000 psi (68.95 bar) for one minute.
NOTE: If pressure does not remain above the stated pressure 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 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 incorporating
double acting cylinders are as follows: - Tower Lift, Slave Level/
Main Level, Main Lift, Main Telescope, Master Level/Upright
Level, Articulating Jib Boom Lift, Steer and Axle lockout. 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 the piston side of the cylin-
der) forces the piston to travel toward the rod end of the bar-
rel, 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 - Tower lift, Tower Telescope,
Upright Level, Lockout, Articulating Jib Boom Lift, Main Lift/
Slave Level and Main 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.