The boom lift circuit consists of the lift hydraulic
The lift cylinder has a 7.5 in. (19.1 cm) bore and is
remote controller, lift directional control valve, holding
the double acting-type. Dirt and other foreign
valve, and the lift cylinder. These components
material are prevented from entering the cylinder and
enable the boom to be raised or lowered to various
causing internal damage by a wiper seal during rod
degrees of elevation ranging from -3 to 78 degrees
retraction. Oil seals on both the piston and cylinder
head prevent internal and external hydraulic oil
leakage. Refer to CYLINDERS in Section 8 for a
The lift directional control valve is the closed spool-
complete description of the lift cylinder.
type and is described under VALVES in Section 8 -
The holding valve is a balanced poppet-type
hydraulic valve. It is threaded into the port block
Refer to VALVES in Section 8 for a complete
which is an integral portion of the lift cylinder barrel.
description of the hydraulic remote controller.
The holding valve functions when booming up
(cylinder rod extended), booming down (cylinder rod
retracted), or holding (cylinder rod stationary).
THEORY OF OPERATION
The directional control valve bank housing the lift
When booming down, oil enters the retract port of
control valve is supplied by flow from the 39 gpm
the port block and flows to the cylinder rod side.
(147.6 lpm) middle section of the hydraulic pump.
When pilot pressure reaches a pre-determined
Flow from the pump travels through port 13 of the
valve, the main poppet unseats and oil flows from
swivel to the lift directional control valve.
the piston side of the cylinder to the reservoir.
When booming up, oil unseats the poppet (check)
All return flow from the control valve goes through
valve in the holding valve, letting oil flow to the piston
the swivel to the reservoir.
side of the cylinder. Pressure is applied to the
piston, forcing the rod to extend, raising the boom.