Manitowoc Published 10-01-2012, Control # 045-08 1-47
777 SERVICE MANUAL INTRODUCTION
1
The weight of the load on the front drum will attempt to drive
the motor faster than return oil is available to the pump. Front
hoist charge pump maintains the oil supply in the closed loop
circuit at a positive pressure to front hoist motor.
The position of the pump swashplate restricts the returning
oil flow, and pressure builds on the return side of the closed-
loop circuit, acting as a brake against the front hoist motor to
control the lowering speed.
The PC governs the lowering speed of the load by varying
the voltage to the pump EDC in proportion to movement of
the control handle. The angle of the pump swashplate
increases as the control handle is moved farther forward;
more oil is allowed to return to the pump, more oil is pumped
to the motor, and the front drum lowers the load faster.
As control handle approaches the full handle command
position, and if lifting conditions permit, the PC instructs the
front hoist motor PCP valve, to shift and redirect oil flow to
motor servo cylinder. This allows the servo mechanism to
shift the motor to minimum displacement for maximum motor
speed and less operating torque.
As control handle is returned to the off position, the PC
commands the front hoist pump EDC to decrease the angle
of the pump swashplate, causing a reduction in oil flow
output. The PC also instructs the front hoist motor PCP
valve, to shift front hoist motor to maximum displacement in
proportion to return handle position for slower output speed
to slow the drum rotation. When the control handle is fully off,
the PC memorizes the pressure required to support the load
and de-energizes solenoid HS16 to apply the band brake
after the control handle neutral switch opens and about 5
seconds later solenoid HS12 de-energizes to apply the drum
brakes.
Clamshell Operation: Clamshell Mode
See Figure 1-33 for following procedures.
The load drum hydraulic systems for clamshell operation in
full power mode are identical to those illustrated for liftcrane
operation in full power mode.
When CLAMSHELL excavator mode is selected and
confirmed at crane mode selector, front hoist control handle
operates the front drum for opening and closing the bucket,
and rear hoist control handle operates the front and rear
drums for raising and lowering the bucket.
Programming
The PC controls the speed of the drums in proportion to
movement of the control handles and the weight of the
bucket. The more the bucket weighs and/or the farther the
handle is moved in the forward direction, the faster the
bucket will lower.
When an open or closed bucket is lowered, the PC monitors
both front and rear drum speed senders for variance in
rotational speed of the drums. If a speed difference is
determined, the PC calculates a speed correction factor
which is then used to increase or decrease the displacement
of each load drum pump as needed to maintain equal
rotational speed of the drums.
To ensure the bucket remains closed when raising, the PC
monitors the high pressure side of both load drum systems
for variance in working pressures. If pressure differences are
detected, the PC calculates a pressure correction factor
which is then used to increase or decrease the displacement
of each load drum pump as needed to maintain equal
working pressures.
When the bucket comes in contact with an object that offers
resistance, such as striking a pile of excavatable material,
the hoist lines relax and system pressure decreases. Front
and rear hoist pressure senders supply the PC with data
obtained from the high pressure side of both load drum
hydraulic systems. The PC monitors this data and uses it to
destroke the load drum pumps when pressure decreases
and automatically stops the load drums from turning.
Off
See Figure 1-33 for following procedures.
When front and rear hoist control handles are in the off
position, the following conditions exist:
• Front and rear hoist pump EDC circuits are opened, and
the pumps do not stroke.
• Front and rear drum park brake solenoids HS12, HS8
and front and rear drum band brake solenoids HS16 and
HS17 are de-energized, causing front and rear drum
park brakes to spring-apply and prevent drums from
turning.
Close Bucket
See Figure 1-33 for following procedures.
When front hoist control handle is pulled back, the PC closes
a regulated voltage circuit to the front hoist pump EDC. The
EDC tilts the pump swashplate in the hoisting direction,
causing the bucket to close with the closing line. The PC
governs the closing speed by varying the current to the EDC
in conjunction with movement of the front hoist control
handle.
The PC energizes front drum park brake solenoid HS12 and
front drum band brake solenoid HS16. As the solenoids shift,
hydraulic system pressure is supplied to release the front
drum hydraulic park brakes. The brakes then release,
permitting the front hoist motor to rotate the front drum in the
hoisting direction and close the bucket.
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