Sections
Telescopic booms may be a pinned boom,
full-powered boom, or a combination of both. A
pinned boom means sections are pinned in the
extended or retracted position. A full-powered boom
means sections extend or retract hydraulically. Some
models have a full-powered main boom with a pinned
boom tip section. Read the operators manual for the
proper operation of the type of boom that is equipped on
the crane you are assigned to operate.
On a full-powered boom, the sections are extended
and retracted (except for the base section) by hydraulic
cylinders, called extension cylinders. The cylinders are
mounted parallel to the boom center line within each
section.
The boom extension cylinders on most
telescopic booms have sequencing valves that allow the
sections to extend (telescope) by equal amounts. These
cranes usually have a single telescope control lever in
the cab.
However, on cranes not equipped with
sequencing valves, the operator will have to extend each
section equally. (The crane will have two or three boom
telescope control levers in the cab, each controlling only
a single boom section.) If the boom sections are
extended unequally, the most fully extended section of
boom could bend to uneven stresses. Additionally, the
load chart will be invalidated for determining rated
capacity of the crane. Boom sections that are marked
off in equal increments, as shown on the boom in figure
12-27, make it easier for the operator or signalman to
make sure each section is extended equally.
When a load is placed on a telescopic boom, the load
weight on the boom causes the hydraulic rams within
the boom to stiffen up and slightly curve. As the load is
removed from the boom, the rams return straight.
Because of this, do not extend the boom while it is under
load. Read the operators manual for boom extension
information.
Hoisting Mechanism
The hoisting mechanism for a telescopic crane is a
hydraulically powered hoist drum. The hoist drum is
mounted behind the boom on the crane house or
revolving turntable.
Some hydraulic cranes are
equipped with two hoist drums: one for the main hoist
and the second for the auxiliary or whip line.
House Assembly
The house assembly is a revolving unit that supports
the boom.
Some small hydraulic cranes have the
operators cab and counterweight attached to the
revolving unit.
OPERATORS CAB. The telescopic crane will
have hoist, swing, and boom control levers similar to
the cab of the lattice boom crane. Control lever(s)
is/are also provided to extend and retract the boom.
The hoist system does not require foot-controlled
brakes. When the hoist control lever is returned to the
neutral position, the hydraulic system holds the load
in place.
POWER SOURCE. The power for a telescopic
crane comes from hydraulic fluid. In most cases, the
main carrier engine drives the hydraulic pump that
supplies the hydraulic fluid to hydraulically controlled
components. Power is diverted to hydraulic motors or
cylinders by the valve body at the operators control
station. The hydraulic power provides positive control
of all crane functions.
COUNTERWEIGHT. The counterweight on a
telescopic crane provides greater stability when lifting
loads. When you are performing near-capacity lifts at
high boom angles using a telescopic crane, about 60
percent of load weight is placed on the outriggers away
from the load. When you are performing the same lift
with a lattice boom crane, about 60 percent of the load
is placed on outriggers close to the load.
CRANE ATTACHMENTS
The crane is a versatile piece of equipment that
can be equipped with various attachments to
perform a number of different operations. These
attachments include a hook block, a clamshell, and
a dragline.
HOOK BLOCK
A crane that is rigged with a hook block is the
primary unit for lifting an objector load, transferring it
to a new place by swinging or traveling and then placing
the load. Figure 12-28 shows an eight-part line rigged
hook block.
The number of parts of a line rigged on the hook
block is important for figuring the capacity of the crane.
Most crane load charts show the rated capacity of the
crane for different parts of the line; for example, a crane
that is capable of being rigged with a eight-part line is
rigged with a six-part line. The eight-part line gives the
crane a greater lifting capacity; therefore, you must
check the load chart for the six-part line capacity to
avoid overloading the crane.
12-16