起重机的用途与历史-中文英文-11000字

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河南理工大学万方科技学院本科毕业论文
The Use and History of Crane
Every time we see a crane in action we remains without words, these
machines are sometimes really huge, taking up tons of material hundreds of
meters in height. We watch with amazement and a bit of terror, thinking about
what would happen if the load comes off or if the movement of the crane was
wrong. It is a really fascinating system, surprising both adults and children.
These are especially tower cranes, but in reality there are plenty of types and
they are in use for centuries. The cranes are formed by one or more machines
used to create a mechanical advantage and thus move large loads. Cranes are
equipped with a winder, a wire rope or chain 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.
1. Overview
The first construction 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, harbor 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.
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 utilized where the
provision of power would be uneconomic.
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河南理工大学万方科技学院本科毕业论文
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. For a while, mini -
cranes are also used for constructing high buildings, in order to facilitate
constructions by reaching tight spaces. Finally, we can find larger floating
cranes, generally 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.
2. History
Ancient Greece
The crane for lifting heavy loads was invented by the Ancient Greeks in
the late 6th century BC. 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 center of
gravity of the block, or in pairs equidistant from a point over the center of
gravity, they are regarded by archaeologists as the positive evidence required
for the existence of the crane.
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 favor of using several column drums.
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 labor,
making the crane more preferable to the Greek polis than the more labor-
intensive ramp which had been the norm in the autocratic societies of Egypt
or Assyria.
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
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河南理工大学万方科技学院本科毕业论文
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.
Ancient Rome
The heyday of the crane in ancient times came during 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, 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).
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 instance, the architrave blocks weigh up
to 60 tons each, and one corner cornice block even over 100 tons, all of them
raised to a height of about 19 m. In Rome, the capital block of Trajan's
Column weighs 53.3 tons, which had to be lifted to a height of about 34 m
(see construction of Trajan's Column).
It is assumed that Roman engineers lifted these extraordinary weights by
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河南理工大学万方科技学院本科毕业论文TheUseandHistoryofCraneEverytimeweseeacraneinactionweremainswithoutwords,thesemachinesaresometimesreallyhuge,takinguptonsofmaterialhundredsofmetersinheight.Wewatchwithamazementandabitofterror,thinkingaboutwhatwouldhappeniftheloadcomesofforifthemovementofthecranewaswrong.Itisareally...

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作者:闻远设计 分类:课程设计课件资料 价格:10光币 属性:17 页 大小:80.5KB 格式:DOC 时间:2022-10-21

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