JLG 80SL Service Manual - Manual (Page 81)

SECTION 4 - HYDRAULICS
3121325 – JLG Lift – 4-1
SECTION 4. HYDRAULICS
4.1 CYLINDERS - THEORY OF OPERATION
Deck Extension Cylinder:
The deck is extended and retracted by means of a double
acting cylinder controlled by the first section in the main
control valve (see Figure 4-37., Main Valve Block). The
service line relief valve in the control valve is set at 1160
psi ( 80 bar) for extend and 3046 psi (210 bar) for retract.
Lift Cylinder:
The lift cylinder is a single acting cylinder incorporating
dual solenoid holding valves and a pilot operated check
valve for emergency lowering. The lifting function is con-
trolled by the 3
rd section of the main control valve. Lift
speed is limited by flow from section 1 in the pump.
Lowering is operated by energizing the 2 solenoid holding
valves together with the 3
rd section of the main control
valve. Descent speed is controlled by an adjustable line
mounted flow control valve.
To enable lowering at full speed when the engine is
stopped (i.e. there is no hydraulic power available to act
as a pilot supply for the electrical actuator), a pilot line is
connected to the auxiliary port "M" on the control valve via
a check valve. This provides the hydraulic power neces-
sary to shift the lower valve section.
Outrigger Cylinders:
The automatic self leveling system is controlled by 4 dou-
ble acting cylinders mounted at each corner of the
machine chassis. These cylinders incorporate pilot oper-
ated holding valves on the piston side. The cracking pres-
sure of these valves is 15 psi (1 bar) for the rear and 116
psi (8 bar) for the front.
Steer Cylinder:
Steering is controlled by a double acting cylinder con-
nected to the 4
th section of the main control valve.
4.2 VALVES - THEORY OF OPERATION
Solenoid Control Valves (Bang-Bang)
Control valves used are four-way three-position solenoid
valves of the sliding spool design. When a circuit is acti-
vated and the control valve solenoid energizes, the spool
is shifted and the corresponding work port opens to per-
mit oil flow to the component in the selected circuit, with
the opposite work port opening to reservoir. Once the cir-
cuit is deactivated (control returned to neutral), the valve
spool returns to neutral (center) and oil flow is then
directed through the valve body and returns to reservoir. A
typical control valve consists of the valve body, sliding
spool, and two solenoid assemblies. The spool is
machine fitted in the bore of the valve body. Lands on the
spool divide the bore into various chambers, which, when
the spool is shifted, align with corresponding ports in the
valve body open to common flow. At the same time other
ports would be blocked to flow. The spool is spring-
loaded to center position, therefore when the control is
released, the spool automatically returns to neutral, pro-
hibiting any flow through the circuit.
Relief Valves
Main relief valves are installed at various points within the
hydraulic system to protect associated systems and com-
ponents against excessive pressure. Excessive pressure
can be developed when a cylinder reaches its limit of
travel and the flow of pressurized fluid continues from the
system control. The relief valve provides an alternate path
for the continuing flow from the pump, thus preventing
rupture of the cylinder, hydraulic line or fitting. Complete
failure of the system pump is also avoided by relieving cir-
cuit pressure. The relief valve is installed in the circuit
between the pump outlet (pressure line) and the cylinder
of the circuit, generally as an integral part of the system
valve bank. Relief pressures are set slightly higher than
the load requirement, with the valve diverting excess
pump delivery back to the reservoir when operating pres-
sure of the component is reached.
Crossover Relief Valves
Crossover relief valves are used in circuits where the actu-
ator requires an operating pressure lower than that sup-
plied to the system. When the circuit is activated and the
required pressure at the actuator is developed, the cross-
over relief diverts excess pump flow to the reservoir. Indi-
vidual, integral relief’s are provided for each side of the
circuit.
Proportional Valve
Flow is proportional to the amount of voltage supplied to
the valve coil. Voltage is gained by the machine controller
and determined by the position of the joystick.