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A crane is a Tower or Derrick equipped with Cable s and Pulley s that is used to lift and lower materials. Cranes are commonly used in the Construction industry and in manufacturing Heavy Equipment . Construction cranes are usually temporary structures, either fixed to the ground or mounted on a purpose-built vehicle. Cranes may either be controlled from an operator in a cab that travels with the crane, by a pushbutton pendant control station, or by Infrared or Radio Control . Where a cab operator is employed, workers on the ground will communicate with the operator through a system of standardised hand-signals; an experienced crew can position loads with great precision using only these signals. TYPES OF CRANES Ancient cranes A carved stone relief {Link without Title} from the First or Second Century found in the tomb of the Haterii family in Rome shows a crane being used to build a monument. Medieval cranes Cranes in the Middle Ages were used to build Europe 's Cathedral s. The crane would be fixed on top of a wall as it was being constructed and was powered by men running inside two large wheels on each side. Also cranes were used in mediaeval ports and shipyards e.g. Żuraw in Gdańsk , Poland . , Mediaeval port crane in Gdańsk , Poland ]] Mobile crane The most basic type of crane consists of a Steel Truss or telescopic boom mounted on a mobile platform, which may be rail, wheeled (including " Truck " carriers) or Caterpillar Track s. The boom is hinged at the bottom, and can be raised and lowered by cables or by hydraulic cylinders. A hook is suspended from the top of the boom by cables and Pulley s. The cables are operated by whatever Prime Mover s the designers have available, operating through a variety of Transmission s. Steam Engine s, Electric Motor s and Internal Combustion Engine s (IC) have all been used. Older cranes' transmissions tended to be Clutch es. This was later modified when using IC engines to match the steam engines "max torque at zero speed" characteristic by the addition of a hydrokinetic element culminating in controlled Torque Converter s. The operational advantages of this arrangement can now be achieved by electronic control of hydrostatic drives, which for size and other considerations is becoming standard. Some examples of this type of crane can be converted to a Demolition crane by adding a demolition ball, or to an Earthmover by adding a clamshell bucket or a Dragline and scoop, although design details can limit their effectiveness. Manufacturers include: Koehring , Manitowoc , American Hoist And Derrick , NCK-Rapier , Bucyrus-Erie , Ruston-Bucyrus , Jones, Sumitomo, Hitachi , Mannesman Dematic (Demag), Liebherr , Sennebogen , Northwest , Lorain ,Grove, P&H, Terex, Link Belt, Bantom and Spierings. , Bristol , England .]] Telescopic crane A type of crane whose boom consists of a number of tubes fitted one inside the other. A Hydraulic mechanism extends or retracts the tubes to increase or decrease the length of the boom. Tower crane The tower crane is a modern form of balance crane. Fixed to the ground, tower cranes often give the best combination of height and lifting capacity and are used in the construction of tall buildings. To save space and to provide stability the vertical part of the crane is often braced onto the completed structure which is normally the concrete lift shaft in the centre of the building. 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. The operator's cabin is located at the top of the tower just above the horizontal boom. The boom is mounted on a Slewing Bearing and is rotated by means of a Slewing Motor . The Lifting hook is operated by a system of Pulleys . A tower crane is usually assembled by a telescopic crane of smaller lifting capacity but greater height, and in the case of tower cranes that have risen while constructing very tall skyscrapers a smaller crane will sometimes be lifted to the roof of the completed tower to dismantle the tower crane afterward. 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. Truck-mounted crane A crane mounted on a rubber tired Truck carrier which provides the mobility for the crane. Outriggers that extend horizontally and vertically are used to level and stabilize the crane for hoisting. Rough terrain crane A crane mounted on an undercarriage with four rubber tires that is designed for pick-and-carry operations and for off-road and "rough terrain" applications. Outriggers that extend horizontally and vertically are used to level and stabilize the crane for hoisting. These {Link without Title} cranes are single-engine machines where the same engine is used for powering the undercarriage as is used for powering the crane, similar to a crawler crane. However, in a rough terrain crane, the engine is usually mounted in the undercarriage rather than in the upper, like the crawler crane. Crawler crane A crane mounted on an undercarriage with a set of tracks that provide for the stability and mobility of the crane. Loader crane This is a 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. Manufacturers of loader cranes include the Swedish company Hiab (Hydrauliska Industri AB) and the Danish company HMF {Link without Title} . 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. A special version is the Portainer crane for loading and unloading ship-borne containers of freight. Overhead crane Also known as a "suspended crane", this type of crane is usually in a Factory , and some of them can lift very heavy loads. The Hoist is on a Trolley which moves in one direction along one or two beams, which move at right angles to that direction along elevated or ground-level tracks, often mounted along the side of an assembly area. Altavista search for images of overhead cranes Stacker crane A crane with a Forklift type mechanism used in automated (computer controlled) Warehouse s (known as an automated storage and retrieval system (AS/RS)). The crane moves on a track in an aisle of the warehouse. The fork can be raised or lowered to any of the levels of a storage rack and can be extended into the rack to store and retrieve product. The product can in some cases be as large as an Automobile . Stacker cranes are often used in the large freezer warehouses of frozen food manufacturers. This automation avoids requiring forklift drivers to work in below freezing temperatures every day. Floating crane Floating cranes are used mainly in Bridge building and Port construction, but they are also used for occasional loading and unloading of especially heavy or awkward loads on and off ships. Some floating cranes are mounted on a Pontoon , others are specialized crane Barge s with a lifting capacity exceeding 10,000 Tonne s and have been used to transport entire bridge sections. Floating cranes have also been used to salvage sunken Ship s. MECHANICAL PRINCIPLES Cranes illustrate the use of one or more Simple Machine s to create Mechanical Advantage .
Cranes, like all machines, obey the principle of Conservation Of Energy . This means that the Energy delivered to the load cannot exceed the energy put in to the machine. For example, if a pulley system multiplies the applied force by ten, then the load moves only one tenth as far as the applied force. Since energy is proportional to force multiplied by distance, the output energy is kept roughly equal to the input energy (in practice slightly less, because some energy is lost to Friction and other Inefficiencies ). CRANES OF SPECIAL INTEREST SEE ALSO
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