written by: Finn Orfano•edited by: Lamar Stonecypher•updated: 3/8/2010
Construction cranes have a significant contribution in the growth of the construction industry. The construction cranes include tower cranes, mobile cranes, truck mounted cranes, and several other types. The selection of construction cranes is based on the loads to be lifted.
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Essentials of Construction Cranes
A construction crane is basically a machine that is used for the lifting of heavy objects. A crane has a drum for wire, and chains with sheaves used for lifting and lowering loads. The loads may also be moved in a horizontal direction. Mechanical advantage is created for lifting of heavy loads. Cranes are widely used in the construction industry for numerous applications. Cranes are also used in several other engineering applications. However, construction cranes have contributed significantly in the construction of huge tall structures. Several construction cranes are employed in the construction industry, including tower cranes, mobile cranes, and telescope cranes. The self-erecting tower crane is assembled at the construction site. Construction cranes can be fixed at the ground or located on a special vehicle that is designed for these applications. Construction cranes are categorized into a tower form, or a derrick equipped with suitable pulleys and cables.
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Tower cranes are one of the important construction cranes that are widely used in the construction field. Tower cranes are rigidly fixed with the ground, which facilitates the attainment of height and increase the lifting capability. Tower cranes are widely being employed for the erection of tall structures. The essential parts of a tower crane include a base, tower, and the slewing mechanism. The tower stand is secured to a huge concrete pad that is used for supporting the crane. The concrete pads are formed much before the crane erection to ensure a firm concrete foundation. The base is fixed with the mast that provides the height to the tower crane. The mast top is joined with the slewing unit that has motor and gear for the crane rotation. The major components of the slewing unit are the working arm or the extended horizontal jib, a short horizontal arm, and the operator cab. The load is carried by the long horizontal jib by a trolley that operates along the jib.
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Types of Tower Cranes
The main kinds of tower cranes that are generally used in the construction industry are the Self Erect and Assisted Erect tower crane. Both the types of these tower cranes are extensively used with numerous jib configurations. The details of these tower cranes are as under:
Tower Crane Self Erect
This type of tower crane is basically intended to be transported, simply and rapidly, for fast erection. Some of the self-erecting cranes are equipped with a generator, and are thus totally self-contained.
Tower Crane Assisted Erect
This kind of tower crane is generally bigger with an enhanced lifting capacity compared to the self-erecting cranes. It is designed to be built at the construction site, normally utilizing a mobile crane. Therefore, it is called an assisted crane.
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Tower Crane Theory of Operation
The tower crane functions by using the principle of moment or counterbalance. The force of counterbalance is created by the appropriate location of weights at the counter jib end. It is generally located opposite to the working jib. The counterweight is dependent upon the working jib length, and these details are provided by the crane manufacturer. Maximum tower crane stability is ensured by keeping the counterweights at a low level. Therefore, counterweights are normally located at the base of the tower cranes, and linked with the working jib through a wire rope.