Section 4 - Hydraulics; Cylinders - Theory Of Operation; Valves - Theory Of Operation; Solenoid Control Valves (Bang-Bang) - JLG 1930ES Service Maintenance Manual

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4.1 CYLINDERS - THEORY OF OPERATION

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. Directing 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 cyl-

inder rod will retract.

NOTE: The lift cylinder is a single acting cylinder which

takes hydraulic pressure to extend and gravity to

retract.

A holding valve is used in the Lift circuit to prevent retrac-

tion of the cylinder rod should a hydraulic line rupture or a

leak develop between the cylinder and its related 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.

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SECTION 4. HYDRAULICS

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.

Manual Descent Valve

The manual descent valve is located on top of the holding

valve on the lift cylinder. The holding valve is a normally

closed solenoid valve, and holds the platform in place

when raised. When activated, the valve opens to permit lift

down. The holding valve is connected to the manual

descent valve, which is connected to a cable which, when

pulled, manually opens the lift down port of the valve and

allows the platform to be lowered in the event hydraulic

power is lost.

– JLG Lift –