Very few machines exist in as wide a variety of designs as cranes. Before the crane is constructed, the manufacturer must consider the site where it will be used and the weight it will need to lift. In addition, cranes are often modified to suit the needs of the user. For this reason, it is not much of an exaggeration to say that no two cranes are exactly alike.
Cranes used for industrial purposes are generally designed to remain permanently in one location. These cranes often perform repetitive tasks that can be automated. An important type of industrial crane is the bridge crane. Traveling on tracks attached to two horizontal beams, known as a bridge, a trolley enables the movement of the bridge crane. Usually, the bridge itself can be moved along a pair of parallel rails, allowing the crane to reach a large, rectangular area. A bridge crane may also be designed so that one end of the bridge is supported by a central pivot while the other end moves on a circular rail, allowing a large, round area to be reached. An overhead traveling crane is a kind of bridge crane in which the rails are located high above the ground. Usually supported from the ceiling of a building, an overhead traveling crane has the advantage of causing no obstruction in the work area.
Cranes used in construction often perform a variety of tasks and must be controlled by highly skilled operators. Construction cranes are divided into mobile cranes and tower cranes:
Mobile cranes:
Mounted on trucks or crawlers in order to travel from place to place. An articulating crane is a mobile crane in which there is a joint between two sections of the boom, allowing it to move in a way similar to a knuckle in a human finger. Articulating cranes are generally used to lift objects located a relatively short distance away, but with a wide range of motion. A telescoping crane is a mobile crane in which two or more sections of the boom can extend and retract, changing the length of the boom. Telescoping cranes are less versatile than articulating cranes, but are usually able to lift heavier objects located a greater distance away.
Tower Cranes:
Used in the construction of tall buildings. They are installed when construction begins and dismantled when the building is completed. An external tower crane is installed outside the building. As the building increases in height, the crane is raised by lifting the upper part of the crane and adding a new section of tower beneath it. An internal tower crane is installed within the building. As the building increases in height, the crane is raised by lifting the base of the crane to a higher level within the building..