BOOM RT9130E-2 SERVICE MANUAL
4-24 Published 11-22-2016, Control # 345-12
TELESCOPE CIRCUIT
Description
The boom telescope circuit consists of the telescope
hydraulic remote controller, telescope directional control
valve, holding valve, hose reel, and the upper and lower
telescope cylinders.
The telescope control valve is the closed spool type and is
described under Valves, page 2-31.
Refer to Valves, page 2-31 for a complete description of the
hydraulic remote controller.
The boom telescope system has a lower and an upper
telescope cylinder. The lower telescope cylinder is a two
stage cylinder. The first stage has a 7.50 in (190.5 mm) bore
and the second stage is a 9.25 in (235 mm) bore. The upper
telescope cylinder has a 7.50 in (190.5 mm) bore. Foreign
material is prevented from entering the cylinder by a wiper
seal during rod retraction. O-ring seals prevent internal and
external leakage. Refer to Temperature Effects on Hydraulic
Cylinders, page 2-67 for a complete description of the
telescope cylinder.
A holding valve is threaded into a port block on the rod end of
the upper telescope cylinder and the first stage rod end on
the lower telescope cylinder. The holding valve for the
second stage of the lower telescope cylinder is mounted into
the port block on the barrel end of the second stage barrel.
The holding valves function during the retraction, extension,
or holding operation. When holding the boom section at a
given length, oil is trapped in the cylinder by the holding
valve. Refer to Valves, page 2-31 for a complete description
of the holding valve.
Cranes equipped with a telescope cylinder charge system
will have an additional valve manifold in the telescope circuit
to prevent boom retraction due to thermal contraction under
the following operating conditions: If the crane is operated
with a long boom length and low boom angle, and the
hydraulic oil in the telescope cylinders cools and contracts,
the friction forces in the extended boom will prevent the
boom from retracting. Then, if the operator begins to elevate
the boom without telescoping out slightly, the crane may
reach a boom elevation angle where the weight of the boom
sections and load overcomes the friction forces in the boom
sections, causing the boom to retract until the column of oil
inside the cylinders once again supports the boom sections.
The telescope cylinder charge system will prevent the boom
from retracting under these conditions by automatically
replenishing the volume of hydraulic oil lost in the telescope
cylinders due to thermal contraction.
Additionally, the system utilizes a pressure switch plumbed
into the telescope extend circuit, which will turn on the
telescope cylinder charge indicator and sound the warning
buzzer in the operator’s cab if the charge system is not
functioning properly (see Telescope Cylinder Charge
Indicator in Section 3 of the applicable Operator’s Manual).
Refer to Valves, page 2-31 for a complete description of the
telescope cylinder charge valve manifold. Refer to the sub-
section titled Boom Drift Chart (Cylinder length change in
millimeters), page 2-68 for an electrical schematic of this
system.
Theory Of Operation
Flow from the pump travels to the telescope directional
control valve. Movement of the telescope hydraulic remote
controller (foot pedal, when equipped with optional auxiliary
hoist) for telescope functions from neutral sends a pilot
pressure signal to the directional control valve to shift the
spool in the directional control valve. This aligns the
appropriate passages in the control valve to route oil to the
telescope cylinders.
For more information, refer to Theory Of Operation, page 4-
2.
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