Crane (machine)

A worker telecommanding a crane from the ground

A crane is a tower or derrick equipped with cables and pulleys 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 or, in larger installations, radio systems; an experienced crew can position loads with great precision using only these signals. The largest revolving cranes in the world can be found on crane vessels

Types of cranes

Ancient cranes

A carved stone relief [1] 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 cathedrals. 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 Medieval ports and shipyards e.g. Żuraw in Gdańsk, Poland.

Railroad cranes

A railroad crane is a crane that is mounted on a railroad car or on a flatcar.

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 tracks. 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 pulleys. The cables are operated by whatever prime movers the designers have available, operating through a variety of transmissions. Steam engines, electric motors and internal combustion engines (IC) have all been used. Older cranes' transmissions tended to be clutches. 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 converters. 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, Lima, Bantom and Spierings.

Six tower cranes constructing buildings at Canons Marsh, 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 below 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 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 telescopic 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

Almost invariably called a "Hiab" by its operators, this is a hydraulically-powered articulated arm fitted to a trailer, used to move goods onto or off of the trailer. Unlike most cranes the operator must move around to be able to view his load; hence he will have a portable cabled or radio linked control system. The numerous jointed sections can be folded into a small space when the crane is not in use. One or more of the sections may be telescopic. Often the crane will have a degree of automation and be able to unload or stow itself without an operator's instruction. Manufacturers of loader cranes include the Swedish company Hiab (Hydrauliska Industri AB) and the Danish company HMF [2].

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.

Stacker crane

A crane with a forklift type mechanism used in automated (computer controlled) warehouses (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 for a baby.

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 barges with a lifting capacity exceeding 10,000 tonnes and have been used to transport entire bridge sections. Floating cranes have also been used to salvage sunken ships.

Crane vessels are often used in offshore construction. The largest revolving cranes can be found on SSCV Thialf, which has two cranes with a capacity of 7100 metric tons each.

Aerial crane

Aerial cranes usually extend from helicopters to lift large loads. Helicopters are able to travel to and lift in areas that are more difficult to reach by a conventional crane. Aerial helicopter cranes are most commonly used to lift units/loads onto shopping centers, multi-story buildings, highrises, etc. However, they can lift basically anything within their lifting capacity, (i.e. cars, boats, swimming pools, etc.). They also work as disaster relief after natural disasters for clean-up, and during wild-fires they are able to carry huge buckets of water over fires to put them out.

Mechanical principles

There are two major considerations that are taken into account in the design of cranes. The first is that the crane must be able to lift a load of a specified weight and the second is that the crane must remain stable and not topple over when the load is lifted and moved to another location.

Lifting capacity

Cranes illustrate the use of one or more simple machines to create mechanical advantage.

The lever. A balance crane contains a horizontal beam (the lever) pivoted about a point called the fulcrum. The principle of the lever allows a heavy load attached to the shorter end of the beam to be lifted by a smaller force applied in the opposite direction to the longer end of the beam. The ratio of the load's weight to the applied force is equal to the ratio of the lengths of the longer arm and the shorter arm, and is called the mechanical advantage.
The pulley. A jib crane contains a tilted strut (the jib) that supports a fixed pulley block. Cables are wrapped multiple times round the fixed block and round another block attached to the load. When the free end of the cable is pulled by hand or by a winding machine, the pulley system delivers a force to the load that is equal to the applied force multiplied by the number of lengths of cable passing between the two blocks. This number is the mechanical advantage.
The hydraulic cylinder. This can be used directly to lift the load (as with a HIAB), or indirectly to move the jib or beam that carries another lifting device.

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).

Stability of crane

In order for a crane to be stable the sum of all movements about any point such as the base of the crane must equate to zero. In practice the magnitude and combination of anticipated loads is increased so that a crane should have a factor of safety against toppling of about five million times.

Cranes of special interest

Kockumskranen

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