Manitowoc Published 10-01-2012, Control # 045-08 1-27
777 SERVICE MANUAL INTRODUCTION
1
modulating valve. The flow passes through system solenoid
control valves which are held open when in free fall mode
and open and close by the drum control handle when in
standard operating mode. The pressurized fluid supply
enters the band brake cylinder causing the cylinder piston to
rise or retract in proportion to applied spring force and
system regulated pressure.
Pressure switch provides information to the PC concerning
the position of the working brake when entering free fall
mode. If the PC is able to detect system pressure (working
brake is unlatched), the PC will not release the clutch and
permit free fall. The working brake pedal must be latched
(pressure switch is unable to detect pressure) when entering
free fall.
Load Drum Clutch Release System (with free
fall)
The clutch assemblies of the front and rear load drums are
hydraulic actuated. Each assembly is spring-applied-
hydraulic released. The PC automatically controls the
clutches in conjunction with movement of the selected load
drum control handle when hoisting a load in free fall.
When in free fall mode with the control handle in the neutral
position, the PC closes the circuit to HS20 and the oil flows to
drum clutch. The clutch application spring is compressed so
the clutch disconnects the drum from the motor drive
assembly.
When the control handle is moved into the hoisting position
while in the free fall mode, the PC opens the circuit to rear/
luffing drum or front drum clutch valve solenoid HS20.
Manifold pressure is blocked from entering the valve and oil
is exhausted from the cylinder as the clutch spring applies.
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