Crane (machine)
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A crane is a tower or derrick equipped with cables and pulleys that can be used both to lift and lower materials and to shift them horizontally. Cranes are commonly employed 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.
[edit] Types of cranes
[edit] Ancient Greek cranes
The crane for lifting heavy loads was invented by the ancient Greeks in the late 6th century BC.<ref>J. J. Coulton, p.7</ref> The archaeological record shows that no later than c.515 BC distinctive cuttings for both lifting tongs and lewis irons begin to appear on stone blocks of Greek temples. Since these holes point at the use of a lifting device, and since they are to be found either above the centre of gravity of the block, or in pairs equidistant from a point over the centre of gravity, they are regarded by archaeologists as the positive evidence required for the existence of the crane.<ref>J. J. Coulton, p.7</ref>
The introduction of the winch and pulley hoist soon lead to a widespread replacement of ramps as the main means of vertical motion. For the next two hundred years, Greek building sites witnessed a sharp drop in the weights handled, as the new lifting technique made the use of several smaller stones more practical than of fewer larger ones. In contrast to the archaic period with its tendency to ever-increasing block sizes, Greek temples of the classical age like the Parthenon invariably featured stone blocks weighting less than 15-20 tons, and the practice of erecting large monolithic columns was practically abandoned in favour of using several column drums.<ref>J. J. Coulton, p.14f.</ref>
Although the exact circumstances of the shift from the ramp to the crane technology remain unclear, it has been argued that the volatile social and political conditions of Greece were more suitable to the employment of small, professional construction teams than of large bodies of unskilled labour, making the crane more preferable to the Greek polis than the more labour-intensive ramp which had been the norm in the autocratic societies of Egypt or Assyria.<ref>J. J. Coulton, p.14f.</ref>
The first unequivocal literary evidence for the existence of the compound pulley system appears in the Mechanical Problems (Mech. 18, 853a32-853b13) attributed to Aristotle (384-322 BC), but perhaps composed at a slightly later date. Around the same time, block sizes at Greek temples began to match their archaic predecessors again, indicating that the more sophisticated compound pulley must have found its way to Greek construction sites by then.<ref>J. J. Coulton, p.16</ref>
[edit] Ancient Roman cranes
The heyday of the crane in ancient times came under the Roman Empire, when construction activity soared and buildings reached enormous dimensions. The Romans adopted the Greek crane and developed it further. We are relatively well informed about their lifting techniques thanks to rather lengthy accounts by the engineers Vitruvius (De Architectura 10.2, 1-10) and Heron of Alexandria (Mechanica 3.2-5). There are also two surviving reliefs of Roman treadwheel cranes offering pictorial evidence, with the Haterii tombstone from the late first century AD being particularly detailed.
The simplest Roman crane, the Trispastos, consisted of a single-beam jib, a winch, a rope, and a block containing three pulleys. Having thus a mechanical advantage of 3:1, it has been calculated that a single man working the winch could raise 150 kg (3 pulleys x 50 kg = 150), assuming that 50 kg represent the maximum effort a man can exert over a longer time period. Heavier crane types featured five pulleys (Pentaspastos) or, in case of the largest one, a set of three by five pulleys (Polyspastos) and came with two, three or four masts, depending on the maximum load. The Polyspastos, when worked by four men at both sides of the winch, could already lift 3000 kg (3 ropes x 5 pulleys x 4 men x 50 kg = 3000 kg). In case the winch was replaced by a treadwheel, the maximum load even doubled to 6000 kg at only half the crew, since the treadwheel possesses a much bigger mechanical advantage due to its larger diameter. This meant that, in comparison to the construction of the Egyptian Pyramids, where about 50 men were needed to move a 2.5 ton stone block up the ramp (50 kg per person), the lifting capability of the Roman Polyspastos proved to be 60 times more efficient (3000 kg per person).<ref>All data from: Hans-Liudger Dienel, Wolfgang Meighörner, p.13</ref>
Evidence from Roman architecture, however, indicates that the overall lifting capability of the Romans went far beyond that of any single crane, as numerous extant Roman buildings feature much heavier stone blocks than those handled by the Polyspastos. At the temple of Jupiter at Baalbek, for incidence, the architraves blocks weigh up to 60 tons each, and the corner cornices blocks even over 100 tons, all of them raised to a height of ca. 19 m above the ground.<ref>J. J. Coulton, p.16</ref> In Rome, the capital block of Trajan's Column weighs 53.3 tons which had to be lifted at a height of ca. 34 m.<ref>Lynne Lancaster, p.426</ref>
It is assumed that Roman engineers accomplished lifting these extraordinary weights by two measures: First, as suggested by Heron, a lifting tower was set up, whose four masts were arranged in the shape of a quadrangle with parallel sides, not unlike a siege tower, but with the column in the middle of the structure (Mechanica 3.5).<ref>Lynne Lancaster, p.427ff.</ref> Second, a multitude of capstans were placed on the ground around the tower, for, although having a lower leverage ratio than treadwheels, capstans could be set up in higher numbers and run by more men (and, moreover, by draught animals).<ref>Lynne Lancaster, p.434ff.</ref> This use of multiple capstans is also described by Ammianus Marcellinus (17.4.15) in connection with the lifting of the Lateranense obelisk in the Circus Maximus (ca. 357 AD). The maximum lifting capability of a single capstan can be established by the number of lewis iron holes bored into the monolith. In case of the Baalbek architrave blocks, which weigh between 55 and 60 tons, eight extant holes suggest an allowance of 7.5 ton per lewis iron, that is per capstan.<ref>Lynne Lancaster, p.436</ref> Lifting such heavy weights in a concerted action required a great amount of coordination between the work groups applying the force to the capstans.
[edit] Medieval cranes
[1] 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.
[edit] Railroad cranes
A railroad crane is a crane that is mounted on a railroad car or on a flatcar.
[edit] 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, PPM, Terex, Favelle Favco, Link Belt, Lima, Bantom and Spierings.
[edit] 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. is used on many constuction projects
[edit] 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 center 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 either sits in a cabin at the top of the tower or controls the crane by radio remote control from the ground, usually standing near the load. In the first case 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.
[edit] 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.
[edit] 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.
[edit] Crawler crane
A crawler is a crane mounted on an undercarriage with a set of tracks that provide for the stability and mobility of the crane. Crawler cranes have both advantages and disadvantages depending on their intended use. The main advantage of a crawler is that they can move on site and perform lifts with very little set-up, as the crane is stable on its tracks with no outriggers. In addition, a crawler crane is capable of moving with a load. The main disadvantage of a crawler crane is that they are very heavy, and cannot easily be moved from one job site to the next without significant expense. Typically, a large crawler must be disassembled or moved by barge in order to be transported.
[edit] 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].
[edit] Gantry crane
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A Gantry crane has a hoist in a trolley which runs horizontally along gantry rails, usually fitted underneath a beam spanning between uprights which themselves have wheels so that the whole crane can move at right angles to the direction of the gantry rails. These cranes come in all sizes, and some which are extremely large for use in shipyards or industrial installations can move very heavy loads. A special version is the Portainer crane for loading and unloading ship-borne containers of freight.
[edit] Overhead crane
Also known as a "suspended crane", this type of crane works in the same way as a gantry crane but without uprights. 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 tracks, often mounted along the side walls of an assembly area in a factory. Some of them can lift very heavy loads.
[edit] 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.
[edit] Floating crane
Image:SSCVThialf.jpg 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.
[edit] 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.
[edit] Mechanical principles
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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.
[edit] 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).
[edit] Stability of crane
In order for a crane to be stable the sum of all moments 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 ten times.
[edit] Cranes of special interest
[edit] Literature
- Hans-Liudger Dienel, Wolfgang Meighörner, “Der Tretradkran,“ Publication of the Deutsches Museum (Technikgeschichte Series), 2nd ed., München 1997
- J. J. Coulton, “Lifting in Early Greek Architecture,” The Journal of Hellenic Studies, Vol. 94. (1974), pp. 1-19
- Lynne Lancaster, “Building Trajan's Column,” American Journal of Archaeology, Vol. 103, No. 3. (Jul., 1999), pp. 419-439
[edit] See also
- Banksman
- Block and tackle
- Capstan
- Erickson S-64 Aircrane helicopter
- Tow truck
- Tripod
- Winch
- steam shovel
[edit] External links
- Knots, hitches and bends
- VRML Simulation of an Excavator, Tower Crane, and Dumptruck
- A medieval crane at work - Three different VR Panoramas - at the top of a tower on Charles Bridge, Prague, Czech Republic.
[edit] References
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