JLG M4069 Service Manual - Manual (Page 75)

SECTION 4 - HYDRAULICS
3121639 – JLG Lift – 4-3
basis. However, filter elements must be changed
after the first 50 hours of operation and every 250
hours thereafter. If it is necessary to change the oil,
use only those oils meeting or exceeding the specifi-
cations appearing in this manual. If unable to obtain
the same type of oil supplied with the machine, con-
sult local supplier for assistance in selecting the
proper equivalent. Avoid mixing petroleum and syn-
thetic base oils. JLG Industries recommends chang-
ing the hydraulic oil annually.
2. Use every precaution to keep the hydraulic oil clean.
If the oil must be poured from the original container
into another, be sure to clean all possible contami-
nants from the service container. Always replace the
filter cartridge any time the system oil is changed.
3. While the unit is shut down, a good preventive main-
tenance measure is to make a thorough inspection
of all hydraulic components, lines, fittings, etc., as
well as a functional check of each system, before
placing the machine back in service.
Lubrication Specifications
Specified lubricants, as recommended by the component
manufacturers, are always the best choice, however,
multi-purpose greases usually have the qualities which
meet a variety of single purpose requirements. Should
any question arise regarding the use of greases in mainte-
nance stock, consult your local supplier for evaluation.
Refer to Section 1 for an explanation of the lubricant key
designations appearing in the Lubrication Chart.
4.3 CYLINDERS - THEORY OF OPERATION
1. Cylinders are of the double acting type. The Lift and
Steer systems incorporate double acting cylinders.
A double acting cylinder is one that requires oil flow
to operate the cylinder rod in both directions. Direct-
ing oil, by actuating the corresponding 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 the rod will stop. By
directing oil to the rod side of the cylinder, the piston
will be forced in the opposite direction and the cylin-
der rod will retract.
2. Holding valves are used in the lift circuits to prevent
retraction of the cylinder rod should a hydraulic line
rupture or leak develop between the cylinder and its
related control valve.
4.4 VALVES - THEORY OF OPERATION
Control Valves
The main control valve consists of cartridge type valves in
an aluminum manifold. The cartridge valves provide for
control of flow, volume of flow and pressure in the hydrau-
lic system.
The flow to the lift cylinder(s) is provided by a solenoid
operated 3 way spool valve.When flow is required to lift
the load the solenoid is energized in one direction. When
the solenoid is not energized the cylinder is held in place
by the holding valve mounted on the cylinder.
The directional control valve for steer is a solenoid oper-
ated, three position, 4-way sliding spool type valve. Ener-
gizing one of the electrical coils on a valve will divert the
supply of hydraulic oil to provide motion of that function in
one direction. Energizing the other coil will divert the oil for
motion in the other direction. When neither coil is ener-
gized, the supply of hydraulic oil is blocked.
A solenoid operated three way spool valve controls flow to
the brakes to release the brakes.When energized it allows
flow to the brake parts until the brake pressure switch is
satisfied.
Pressure relief valves limit the pressure in the steer circuit
and the rod end of each lift cylinder. When the pressure in
one of those circuits reaches the set point of the valve, the
valve opens, allowing enough flow to return to the reser-
voir so that the set pressure is not exceeded. An adjusting
screw on each cartridge relief valve allows the set pres-
sure to be adjusted.
The aluminum manifold provides the passages through
which the hydraulic oil is diverted to provide the desired
movement of the actuators. No moving parts of the valves
slide against the manifold and so it is not subject to wear.
4.5 COMPONENT FUNCTIONAL
DESCRIPTION
Hydraulic Pump
The single stage gear pump is attached to the electric
motor located in the left side tray of the machine.
Manual Descent
The manual descent cable handle is located at the rear of
the machine frame.
4.6 WEAR PADS
The original thickness of the sliding wear pads is 5 in (12.7
cm). Replace sliding pads when worn to 4.7 in (11.9 cm).