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Published 10/19/2017, Control # 618-00
GRT880 SERVICE MANUAL BOOM
TELESCOPE CIRCUIT
Description
The boom telescope circuit consists of the telescope
hydraulic remote controller, telescope directional control
valve, Telescope Stage Selector Valve, Telescope Fill Tube
Control Valve, holding valve, and the two telescope
cylinders.
NOTE: If the crane is equipped with an auxiliary hoist, the
telescope function is controlled by a foot pedal
instead of a controller.
Refer to Valves, page 2-39 for a complete description of the
hydraulic remote controller, the Telescope Stage Selector
and Fill Tube Control Valve Manifold.
Theory of Operation
Boom can be operated using an automatic or manual mode.
The boom is operated in a synchronized sequence using a
two telescope cylinders. Tele 1 is controlled by the lower
cylinder and Tele 2 is controlled by the upper cylinder. Tele 3
is controlled by cables fixed to Tele 2.
The cylinder is controlled using several proportionally control
solenoid operated valves.
Pressure in the telescope cylinders are monitored by three
pressure transducers, TR1,TR2, and tele extend. TR1
pressure transducer monitors the pressure in the lower
telescope cylinder fill tube. TR2 pressure transducer
monitors the pressure in the upper telescope cylinder.The
tele extend pressure transducer monitors the pressure in the
extend circuit when either the upper or lower telescope
cylinder is extending. The three pressure transducers
communicate via CAN messages to the boom control
system. Pressure readings are monitored by the boom
control system and are controlled to maintain
synchronization of the boom in Auto and Manual Modes.
Boom Control Modes
The boom control offers two methods of telescoping the
boom: Auto Mode B and Manual Mode.
Auto B Mode
In the auto B mode, the boom extends to full length by
extending Tele 1 first then Tele 2. The crane decides how
and when to stop extending Tele 1 and start extending Tele
2. This mode is used for normal crane work. The boom
control system uses length feedback to ramp the telescoping
speed of the boom sections when approaching the fully
retracted or fully extended positions, and near the transition
point. This mode has the best structural performance.
Manual Mode
In the manual mode, either Tele 1 or Tele 2 extends based on
the operator’s selection made within the crane control
system. Once the selected section is extended the operator
must then select the other boom section within the crane
control system. This mode should not be used for lifting. It is
to be used for boom recovery only. The load chart for is not
published for manual mode and is greatly reduced. All
maintenance and lubrication of the boom is to be done in
Auto B mode.
When the operator telescopes Tele 1, the boom control
system will maintain the relative position (percent extended)
of Tele 2and 3. When the operator telescopes Tele 2 and 3,
the relative position of Tele 1 is maintained.
Manual mode does not ramp the speed of the boom near
transition points. The boom speed is controlled by the
operator.
Reference Only
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