A crane is a tower or derrick equipped with cables and pulleys that is used to lift and lower materials. It is most commonly used in the construction industry. Cranes are usually temporary structures, either fixed to the ground or mounted on a purpose-built vehicle.
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Types of crane
Ancient cranes
Mobile crane
The most basic type of crane consists of a steel latticework boom mounted on a mobile platform. The boom is hinged at the bottom, and can be raised and lowered by cables. A hook is suspended from the top of the boom by cables and pulleys. The cables are operated by electric motors (although in old cranes they were operated by clutches driven by the crane's internal combustion engine). This type of crane is easily converted to a demolition crane by adding a demolition ball, or to an earthmover by adding a bucket.
Telescopic crane
This is a type of mobile crane whose boom consists of a number of square tubes fitted one inside the other. When the crane is deployed on site, an hydraulic mechanism extends the tubes to increase the working height.
Tower crane
Fixed to the ground, this type gives best combination of height and lifting capacity and is used in the construction of tall buildings. To save space, the vertical part of the crane is often built in a well in the centre of the building, which is then converted to a lift (elevator) shaft when the crane is dismantled. A horizontal boom is balanced asymmetrically across the top of the tower. Its short arm carries a counterweight of concrete blocks, and its long arm carries the lifting gear. The crane operator sits in a cabin at the top of the tower.
A tower crane is usually assembled by a telescopic crane of smaller lifting capacity but greater height. A self-assembling tower crane has been demonstrated, which lifts itself off the ground using jacks, allowing the next section of the tower to be inserted at ground level.
Loader crane
This is an hydraulically-powered articulated arm fitted to a trailer, used to load goods onto the trailer. The numerous jointed sections can be folded into a small space when the crane is not in use. One of the sections may be telescopic. A well-known manufacturer of loader cranes is the Swedish company Hiab (Hydrauliska Industri AB).
Gantry crane
This is a large installation used in container ports and rail freight terminals. The lifting mechanism is mounted on a cross-beam supported on vertical legs which run on rails. This crane can move very heavy loads.