Rabu, 16 Januari 2008

What is Crane?

A crane is a mechanical lifting device equipped with a winder, wire ropes and sheaves that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of a human. Cranes are commonly employed in the transport industry for the loading and unloading of freight; in the construction industry for the movement of materials; and in the manufacturing industry for the assembling of heavy equipment.

Overview

The first cranes were invented by the Ancient Greeks and were powered by men or beasts-of-burden, such as donkeys. These cranes were used for the construction of tall buildings. Larger cranes were later developed, employing the use of human treadwheels, permitting the lifting of heavier weights. In the High Middle Ages, harbour cranes were introduced to load and unload ships and assist with their construction – some were built into stone towers for extra strength and stability. The earliest cranes were constructed from wood, but cast iron and steel took over with the coming of the Industrial Revolution.go away

For many centuries, power was supplied by the physical exertion of men or animals, although hoists in watermills and windmills could be driven by the harnessed natural power. The first 'mechanical' power was provided by steam engines, the earliest steam crane being introduced in the 18th or 19th century, with many remaining in use well into the late 20th century. Modern cranes usually use internal combustion engines or electric motors and hydraulic systems to provide a much greater lifting capability than was previously possible, although manual cranes are still utilised where the provision of power would be uneconomic.

Cranes exist in an enormous variety of forms – each tailored to a specific use. Sizes range from the smallest jib cranes, used inside workshops, to the tallest tower cranes, used for constructing high buildings, and the largest floating cranes, used to build oil rigs and salvage sunken ships.

This article also covers lifting machines that do not strictly fit the above definition of a crane, but are generally known as cranes, such as stacker cranes and loader cranes.

History of cranes

Ancient Greek cranes

The crane for lifting heavy loads was invented by the ancient Greeks in the late 6th century BC.[1] 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.[1]

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 weighing less than 15-20 tons. Also, the practice of erecting large monolithic columns was practically abandoned in favour of using several column drums.[2]

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.[2]

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.[3]


Ancient Roman cranes

Reconstruction of a 10.4m high Roman Polyspastos at Bonn, Germany (I)
Reconstruction of a 10.4m high Roman Polyspastos at Bonn, Germany (I)
Reconstruction of a 10.4m high Roman Polyspastos at Bonn, Germany (II)
Reconstruction of a 10.4m high Roman Polyspastos at Bonn, Germany (II)

The heyday of 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 higher (3000 kg per person).[4]

However, numerous extant Roman buildings which feature much heavier stone blocks than those handled by the Polyspastos indicate that the overall lifting capability of the Romans went far beyond that of any single crane. 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.[3] 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.[5]

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).[6] 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).[7] 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.[8] Lifting such heavy weights in a concerted action required a great amount of coordination between the work groups applying the force to the capstans.

Medieval cranes

Small-scale reconstruction of the medieval gantry crane at Brugge harbor
Small-scale reconstruction of the medieval gantry crane at Brugge harbor
The Kraanplein ("Crane Square") at Brugge is glimpsed in this portrait of 1551 by Pieter Pourbus
The Kraanplein ("Crane Square") at Brugge is glimpsed in this portrait of 1551 by Pieter Pourbus
Medieval port crane with building overhanging in the former Hanse town of Danzig.
Medieval port crane with building overhanging in the former Hanse town of Danzig.

During the High Middle Ages the treadwheel crane was reintroduced on a large scale after the technology had fallen into disuse in western Europe with the demise of the Western Roman Empire.[9] The earliest reference to a treadwheel (magna rota) reappears in archival literature in France about 1225,[10] followed by an illuminated depiction in a manuscript of probably also French origin dating to 1240.[9] In navigation, the earliest uses of harbor cranes are documented for Utrecht in 1244, Antwerp in 1263, Brugge in 1288 and Hamburg in 1291,[11] while in England the treadwheel is not recorded before 1331.[12]

Generally, vertical transport was done safer and cheaper by cranes than by customary methods. Typical areas of application were harbors, mines, and, in particular, building sites where the treadwheel crane played a pivotal role in the construction of the lofty Gothic cathedrals. Nevertheless, both archival and pictorial sources of the time suggest that newly introduced machines like treadwheels or wheelbarrows did not completely replace more labor-intensive methods like ladders, hods and handbarrows. Rather, old and new machinery continued to coexist on medieval construction sites[13] and harbors.[11]

Apart from treadwheels, medieval depictions also show cranes to be powered manually by windlasses with radiating spokes, cranks and by the 15th century also by windlasses shaped like a ship's wheel. To smooth out irregularities of impulse and get over 'dead-spots' in the lifting process flywheels are known to be in use as early as 1123.[14]

Origins

The exact process by which the treadwheel crane was reintroduced is not recorded,[10] although its return to construction sites has undoubtedly to be viewed in close connection with the simultaneous rise of Gothic architecture. The reappearance of the treadwheel crane may have resulted from a technological development of the windlass from which the treadwheel structurally and mechanically evolved. Alternatively, the medieval treadwheel may represent a deliberate reinvention of its Roman counterpart drawn from Vitruvius' De architectura which was available in many monastic libraries. Its reintroduction may have been inspired, as well, by the observation of the labor-saving qualities of the waterwheel with which early treadwheels shared many structural similarities.[12]

Structure and placement

The medieval treadwheel was a large wooden wheel turning around a central shaft with a treadway wide enough for two workers walking side by side. While the earlier 'compass-arm' wheel had spokes directly driven into the central shaft, the more advanced 'clasp-arm' type featured arms arranged as chords to the wheel rim,[15] giving the possibility of using a thinner shaft and providing thus a greater mechanical advantage.[16]

Contrary to a popularly held belief, cranes on medieval building sites were neither placed on the extremely lightweight scaffolding used at the time nor on the thin walls of the Gothic churches which were incapable of supporting the weight of both hoisting machine and load. Rather, cranes were placed in the initial stages of construction on the ground, often within the building. When a new floor was completed, and massive tie beams of the roof connected the walls, the crane was dismantled and reassembled on the roof beams from where it was moved from bay to bay during construction of the vaults.[17] Thus, the crane ‘grew’ and ‘wandered’ with the building with the result that today all extant construction cranes in England are found in church towers above the vaulting and below the roof, where they remained after building construction for bringing material for repairs aloft.[18]

Less frequently, medieval illuminations also show cranes mounted on the outside of walls with the stand of the machine secured to putlogs.[19]

Mechanics and operation

Tower crane at the inland harbour of Trier from 1413.
Tower crane at the inland harbour of Trier from 1413.

In contrast to modern cranes, medieval cranes and hoists - much like their counterparts in Greece and Rome[20] - were primarily capable of a vertical lift, and not used to move loads for a considerable distance horizontally as well.[17] Accordingly, lifting work was organized at the workplace in a different way than today. In building construction, for example, it is assumed that the crane lifted the stone blocks either from the bottom directly into place,[17] or from a place opposite the centre of the wall from where it could deliver the blocks for two teams working at each end of the wall.[20] Additionally, the crane master who usually gave orders at the treadwheel workers from outside the crane was able to manipulate the movement laterally by a small rope attached to the load.[21] Slewing cranes which allowed a rotation of the load and were thus particularly suited for dockside work appeared as early as 1340.[22] While ashlar blocks were directly lifted by sling, lewis or devil's clamp (German Teufelskralle), other objects were placed before in containers like pallets, baskets, wooden boxes or barrels.[23]

It is noteworthy that medieval cranes rarely featured ratchets or brakes to forestall the load from running backward.[24] This curious absence is explained by the high friction force exercised by medieval treadwheels which normally prevented the wheel from accelerating beyond control.[21]

Harbor cranes

Beyond the modern warship stands a crane constructed in 1742, used for mounting masts to large sailing vessels. Copenhagen, Denmark
Beyond the modern warship stands a crane constructed in 1742, used for mounting masts to large sailing vessels. Copenhagen, Denmark

According to the “present state of knowledge” unknown in antiquity, stationary harbor cranes are considered a new development of the Middle Ages.[11] The typical harbor crane was a pivoting structure equipped with double treadwheels. These cranes were placed docksides for the loading and unloading of cargo where they replaced or complemented older lifting methods like see-saws, winches and yards.[11]

Two different types of harbor cranes can be identified with a varying geographical distribution: While gantry cranes which pivoted on a central vertical axle were commonly found at the Flemish and Dutch coastside, German sea and inland harbors typically featured tower cranes where the windlass and treadwheels were situated in a solid tower with only jib arm and roof rotating.[25] Interestingly, dockside cranes were not adopted in the Mediterranean region and the highly developed Italian ports where authorities continued to rely on the more labor-intensive method of unloading goods by ramps beyond the Middle Ages.[26]

Unlike construction cranes where the work speed was determined by the relatively slow progress of the masons, harbor cranes usually featured double treadwheels to speed up loading. The two treadwheels whose diameter is estimated to be 4 m or larger were attached to each side of the axle and rotated together.[11] Today, according to one survey, fifteen treadwheel harbor cranes from pre-industrial times are still extant throughout Europe.[27] Beside these stationary cranes, floating cranes which could be flexibly deployed in the whole port basin came into use by the 14th century.[25]

Mechanical principles

Cranes helping to construct a tower block in Melbourne, Australia
Cranes helping to construct a tower block in Melbourne, Australia

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.
Tower Crane constructing a building in Kansas City
Tower Crane constructing a building in Kansas City

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 into 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 moments about any point such as the base of the crane must equate to zero. In practice, the magnitude of load that is permitted to be lifted (called the "rated load" in the US) is some value less than the load that will cause the crane to tip.

Under US standards for mobile cranes, the stability-limited rated load for a crawler crane is 75% of the tipping load. The stability-limited rated load for a mobile crane supported on outriggers is 85% of the tipping load.

For stationary pedistal or kingpost mounted cranes, the moment created by the boom, jib, and load is resisted by the pedistal base or kingpost. Stress within the base must be less than the yield stress of the material or the crane will fail.

2 komentar:

Overhead Cranes mengatakan...

Excellent sharing about a crane introduction, I really like this so much. When the crane and hoist is actually lifting equipments, the operator should never leave it hanging for any reason at all. Thanks for sharing this.
Jib crane

Sesilia mengatakan...

Very awesome work about cranes,cranes and forklifts are both lifting objects, you introduced their working quit nicely,by the way we have some used and new Forklifts for sale you may check them out!