JLG 660SJ Service Manual - Manual (Page 239)

SECTION 5 - HYDRAULICS
3121202 – JLG Lift – 5-29
9. Align the control sleeve so its slot will engage the
swash plate feedback pin (slot positioned toward
the pump cover) and insert the sleeve into the hous-
ing. Carefully align the control spool with the sleeve
and install the control onto the pump housing. Install
the four mounting screws and torque to 10 to 11
ft.lbs. (13 to 14 Nm).
10. Install the four cover screws and torque to 18 to 24
in. lbs. (2.0 to 2.7 Nm).
11. Readjust the neutral position of the control. Refer to
the instructions in the Inspections and Adjustment.
5.6 VALVES - THEORY OF OPERATION
Solenoid Control Valve - Rexroth
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 consist 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, prohibiting any
flow through the circuit.
Relief Valves
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.