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Telescopic boom cranes are typically called hydraulic cranes. The booms are composed of a series of rectangular, trapezoidal, or other shape of symmetrically cross-sectional segments, fitting into each other. The largest segment, at the bottom of the boom, is called the base section or boom butt. The smallest section, at the top of the boom, is called the tip section or boom tip. In between there can be one or more sections called the first, second, and so forth, sections. With the boom fully retracted, the telescopic boom crane is highly maneuverable and easy to transport to jobsites. Telescopic boom crane nomenclature is shown in figure 12-27.

Figure 12-27.-Telescopic boom crane nomenclature.


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 operator's 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 operator's 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 operator's cab and counterweight attached to the revolving unit.

OPERATOR'S 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 operator's 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.

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