橋式起重機(jī)橋架結(jié)構(gòu)設(shè)計(jì)
橋式起重機(jī)橋架結(jié)構(gòu)設(shè)計(jì),橋式起重機(jī),結(jié)構(gòu)設(shè)計(jì)
****學(xué)院****屆畢業(yè)設(shè)計(jì)中英文翻譯
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.
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 weightshandled, 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， 853a32-853bl3) 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.
Ancient Rome
The heyday of the crane in ancient limes 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.
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 = 30(H) 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 TrajarTs Column).
Middle Ages
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. The earliest reference to a treadwheel (magna rota) reappears in archival literature in France about 1225, followed by an illuminated depiction in a manuscript of probably also French origin dating to 1240. In navigation, the earliest uses of harbor cranes are documented for Utrecht in 1244，Antwerp in 1263，Brugge in 1288 and Hamburg in 1291, while in England the treadwheel is not recorded before 1331.
Generally, vertical transport could be done more safely and inexpensively 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 and harbors.
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.
The exact process by which the treadwheel crane was reintroduced is not recorded, 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.
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-arnV type featured arms arranged as chords to the wheel rim, giving the possibility of using a thinner shaft and providing thus a greater mechanical advantage.
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. 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.
Harbor usage
According to the "present state of knowledge” unknown in antiquity, stationary harbor cranes are considered a new development of the Middle Ages. 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.
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. 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.
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. Today, according to one survey, fifteen treadwheel harbor cranes from pre-industrial times are still extant throughout Europe.[28] Beside these stationary cranes, floating cranes which could be flexibly deployed in the whole port basin came into use by the 14th century. Renaissance
Mechanical principles
There are two major considerations 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 ami, 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 orindirectly 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 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
For stability, 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 (providing a safety margin).
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. These requirements, along with additional safety-related aspects of crane design, are established by the American Society of Mechanical Engineers in the volume ASME B30.5-2007 Mobile and Locomotive Cranes.
Standards for cranes mounted on ships or offshore platforms are somewhat stricter because of the dynamic load on the crane due to vessel motion. Additionally, the stability of the vessel or platform must be considered.
For stationary pedestal or kingpost mounted cranes, the moment created by the boom, jib，and load is resisted by the pedestal base or kingpost. Stress within the base must be less than the yield stress of the material or the crane
will fail.
The kinds of crane
Mobile
Main article: Mobile crane
The most basic type of mobile crane consists of a truss or telescopic boom mounted on a mobile platform - be it on road, rail or water.
Fixed
Exchanging mobility for the ability to carry greater loads and reach greater heights due to increased stability, these types of cranes are characterized that they, or at least their main structure does not move during the period of use. However，many can still be assembled and disassembled.
3. Overhead Cranes
Use
The most common overhead crane use is in the steel industry. Every step of steel, until it leaves a factory as a finished product, the steel is handled by an overhead crane. Raw materials are poured into a furnace by crane, hot steel is stored for cooling by an overhead crane, the finished coils are lifted and loaded onto trucks and trains by overhead crane, and the fabricator or stamper uses an overhead crane to handle the steel in his factory. The automobile industry uses overhead cranes for handling of raw materials. Smaller workstation cranes handle lighter loads in a work-area, such as CNC mill or saw.
History
Alton Shaw, of the Shaw Crane Company, is credited with the first overhead crane, in 1874. Alliance Machine, now defunct, holds an AISE citation for one of the earliest cranes as well. This crane was in service until approximately 1980，and is now in a museum in Birmingham, Alabama. Over the years important innovations, such as the Weston load brake (which is now rare) and the wire rope hoist (which is still popular)，have come and gone. The original hoist contained components mated together in what is now called the built-up style hoist. These built up hoists are used for heavy-duty applications such as steel coil handling and for users desiring long life and better durability. They also provide for easier maintenance. Now many hoists are package hoists, built as one unit in a single housing, generally designed for ten-year life or less.
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起重機(jī)的用途與歷史
每當(dāng)我們看到一臺(tái)正在運(yùn)作的起重機(jī)，我們都會(huì)驚訝不已，這些機(jī)器有時(shí)碩大無(wú)比，能把成噸的貨物提升到空中?？吹竭@些龐然大物的時(shí) 候我們心理都帶著一種驚愕，有時(shí)甚至是有一點(diǎn)恐懼的心情，我們會(huì)去想如果吊著著的東西掉下來(lái)了或者起重機(jī)吊錯(cuò)了位置會(huì)發(fā)生什么樣恐怖的情形。起重機(jī)的確是一種令人著迷的機(jī)械系統(tǒng)，無(wú)論是成人或者是孩子無(wú)不為止驚嘆。起重機(jī)的種類五花八門，并且歷史悠久。起重機(jī)是用一個(gè)或者幾個(gè)簡(jiǎn)單的機(jī)器來(lái)組成一個(gè)機(jī)械結(jié)構(gòu)并用于運(yùn)送那些人無(wú)法搬動(dòng)的物品。一般來(lái)說(shuō)，起重機(jī)由一個(gè)卷筒、一束金屬繩或是一條金屬鏈組成來(lái)同時(shí)提升、放置成者足水平移動(dòng)貨物。起重機(jī)的.工作領(lǐng)域 一般處在需要裝卸貨物的運(yùn)輸業(yè)、需要搬運(yùn)建材的建筑業(yè)和需要組裝重型設(shè)備的制造業(yè)。
1.概況
第一臺(tái)具有機(jī)械結(jié)構(gòu)的起重機(jī)是由古希臘人發(fā)明的，并且由人或者牲畜比如驢，作為動(dòng)力源。這種起重機(jī)被用于建筑的建造。這種起重機(jī)后來(lái)發(fā)展成了采用人力踏板驅(qū)動(dòng)的更人性的起重機(jī)，用來(lái)提升更重 的物料。中世紀(jì)時(shí)港口起重機(jī)被叫來(lái)裝卸船上的貨物，有的港口起重機(jī) 為求更大的起重重量和更好的穩(wěn)定性被造在了石塔里。最早的起重機(jī)是用木頭制造的，工業(yè)革命之后，鑄鐵和鋼材就代替了木頭用于制造起重機(jī)。
盡管水磨機(jī)和風(fēng)車都可以利用自然的能源來(lái)驅(qū)動(dòng)，但是幾個(gè)世紀(jì)以來(lái)，起重機(jī)的動(dòng)力源一直是人力或者畜力。第一臺(tái)真正釆用機(jī)械能量的起重機(jī)用的是蒸汽機(jī)，最早的蒸汽起重機(jī)出現(xiàn)于18到19世紀(jì)，有一些甚至到了 20世紀(jì)末仍能很好地使用。雖然由于能源的供應(yīng)仍不可及， 到現(xiàn)在有一些人力起重機(jī)還在使用，但是現(xiàn)代的起重機(jī)一般采用的內(nèi)燃機(jī)、電動(dòng)馬達(dá)、液壓系統(tǒng)能為起重機(jī)提供比之前大得多的提升力。
2.歷史
2.1古希臘時(shí)期
用來(lái)提升重型貨物的起重機(jī)是希臘人在公元前六世紀(jì)晚期發(fā)明的。 考古記錄顯示最早在公元前515年提升夾具和鐵制的吊楔開(kāi)始出現(xiàn)在古希臘人石塊結(jié)構(gòu)的神殿里。由干這些是起重設(shè)備的核心裝置、也由于他 們?cè)谑瘔K的重心的中央或者趟在離重心上一點(diǎn)距離相等的兩頭被發(fā)現(xiàn)，他們被考古學(xué)家認(rèn)為是起重機(jī)當(dāng)時(shí)就存在的確鑿證據(jù)。
絞盤與滑輪的的引入導(dǎo)致了人類之前用斜坡來(lái)向高處運(yùn)送貨物的方法被廣泛替代。在接下來(lái)的兩百年中，希臘的建筑都采了這樣新型的提升物料的技術(shù)，它利用了一些小型的石塊來(lái)來(lái)代替人塊的石頭，這樣更具實(shí)用性。與更早先的古希臘人神殿的建筑材料的尺寸不斷變得越來(lái)越大趨勢(shì)相比較，希臘古典廟宇比如帕臺(tái)農(nóng)神廟的石塊重景都小于15- 20噸。而且，要把巨型的石柱豎立起來(lái)的作業(yè)使希臘人實(shí)際上更喜歡用好幾塊像鼓一樣的圓柱石塊堆疊而成。
盡管確切何時(shí)從斜坡運(yùn)輸進(jìn)入起重機(jī)提升技術(shù)時(shí)代的時(shí)間還不是很淸楚。但是當(dāng)時(shí)古希臘不穩(wěn)定的社會(huì)周勢(shì)、和政治情況使得建造神殿更適合雇傭小觀的、更加專業(yè)的建筑團(tuán)隊(duì)而不是像埃及和亞述那樣大量使用的沒(méi)有技術(shù)的分動(dòng)力。這樣的情況使得起重機(jī)更像希臘城邦發(fā)明 的而非釆用純究動(dòng)力斜坡運(yùn)送貨物的埃及或是亞述那樣的獨(dú)裁國(guó)家。
文學(xué)上第一次的明確的記載滑輪組的復(fù)合系統(tǒng)是出現(xiàn)在亞里士多德的機(jī)械難題中，但清楚組成文字可能還要稍晚一些。與此同時(shí)，用于建造希臘神廟的石塊尺寸再一次開(kāi)始趕上他們的古代前輩了，這標(biāo)志著當(dāng) 時(shí)更多的久經(jīng)考驗(yàn)的的滑輪組在希臘建筑史上找到了它們的一席之地。
2.2古羅馬時(shí)期
起重機(jī)械在古代的全盛時(shí)期卻足在古羅馬帝國(guó)展幵的。當(dāng)時(shí)建筑物的數(shù)景激增，而且這些建筑都達(dá)到了巨型的尺寸。羅馬人采用了希臘人的起重機(jī)并將其發(fā)揚(yáng)光大。多虧了那些維特獸程師們撰寫(xiě)的相當(dāng)冗長(zhǎng) 的文獻(xiàn)和亞歷山大帝的蒼鷺的巢，我們才得以如此詳細(xì)地了解到了它們的其中技術(shù)。
三餅滑車是古羅馬最簡(jiǎn)單的一種起重機(jī)，它是由一個(gè)單梁吊臂、一個(gè) 絞盤、一條繩子和一個(gè)三個(gè)滑輪組成的滑輪組組成的。經(jīng)計(jì)算，假設(shè)一個(gè)人用盡力氣能夠長(zhǎng)時(shí)間地提起相當(dāng)于重 50千克的物體那么通過(guò)這樣的起重機(jī)械他以提升約150千克的物體(3 個(gè)滑輪X50千克= 150千克）。更加重型的起重機(jī)就擁有五個(gè)滑輪（五餅 滑車），最大的起重機(jī)會(huì)在兩根、三根甚至是四根桅桿上面裝上三餅和 五餅的復(fù)合滑輪組（復(fù)滑車），這足由最大的負(fù)載載荷決定的。復(fù)滑車工作的時(shí)候兩邊需要4個(gè)人：兩邊各站兩個(gè)已經(jīng)可以提起重約3000千克的 物體（3條繩子X(jué)5個(gè)滑輪X4個(gè)人X50千克= 3000千克）。如果用踏車來(lái)代替絞盤的話，最大的起重載荷可以在人工減半的情況下達(dá)到兩倍一 6000千克，因?yàn)樘ぼ囉懈蟮闹睆侥軌蛱峁┒鄠€(gè)人的力矩。這意味著，和建造埃及金字塔時(shí)50個(gè)人才能通過(guò)斜坡搬動(dòng)2.5噸的石塊（50 千克每人）的情況相比，羅馬的復(fù)滑車的提升能力把工作的效率提高60 倍（3000千克每人）。
然而，大量現(xiàn)存的古羅馬建筑中那些石塊的重量比復(fù)滑車所能操作的負(fù)載耍重得多。這表明古羅馬人全面的起重的能力要遠(yuǎn)遠(yuǎn)大于任何簡(jiǎn)單的起重機(jī)。以Baalbek的Jupiter神廟為例，那些楣梁的石塊每塊都重達(dá)60 噸以上，每個(gè)檐口的石塊甚至達(dá)到了 100噸以上，所有這些石料都被提 升到了 19m的半空中。在羅馬Trajan之柱的主要石塊重達(dá)53. 3噸，而這些石塊必須被提升到34m的高度。（見(jiàn)Trajian之柱）
2.3中世紀(jì)時(shí)期
在中世紀(jì)時(shí)，隨著西羅馬帝國(guó)的滅亡，歐洲的科技技術(shù)水平一落千丈。這時(shí)踏車式的起重機(jī)再次被大范圍地使用。最早的提到踏車式是大約1225年法國(guó)的一部檔案文學(xué)作品，它在一份手稿上也說(shuō)明敘述了直到 1240年法國(guó)人的血統(tǒng)起源。在航海方面，最大的港口起重機(jī)是在1244 年的 Utrecht、1263 年的 Antwerp、1288 年的 Brugge 和 1291 年的 Hamburg, 而在英格蘭踏車式的起重機(jī)直到1331年才有所記錄。
一般來(lái)說(shuō)，釆用起重機(jī)來(lái)垂直運(yùn)輸比傳統(tǒng)的方法更加的安全和經(jīng)濟(jì)。典型的應(yīng)用領(lǐng)域就包括港口、礦井。值得一提的是在哥特式人教堂的建 造過(guò)程中，踏車式的起重機(jī)起到了一個(gè)不可成缺的重要作用。但是，檔 案和圖畫(huà)都顯示了當(dāng)時(shí)新引進(jìn)的機(jī)械系統(tǒng)如踏車、獨(dú)輪手推車等卻沒(méi)有完全替代那些樓梯、木桶、手推車等依賴勞動(dòng)力的生產(chǎn)方法。這樣，舊式的和新式的機(jī)械在繼續(xù)在中世紀(jì)的建筑和港口共存。
除了踏車，中世紀(jì)的文獻(xiàn)中也記載了由手動(dòng)驅(qū)動(dòng)帶幅輪和曲柄的絞盤的起重機(jī)，在15 世紀(jì)時(shí)也是由卷?yè)P(yáng)機(jī)發(fā)展成為了類似船輪的系統(tǒng)。為了緩沖這些不規(guī)則的沖擊力和解決提升過(guò)程中的死點(diǎn)問(wèn)題，調(diào)速輪最早在1123年開(kāi)始投入使用。
踏車式起覓機(jī)具體以何種方式再次被釆用的已經(jīng)無(wú)從考證，盡管它多次被使用在建筑領(lǐng)域，被毋庸置疑地認(rèn)為和哥特式建筑的崛起有相當(dāng)密切的關(guān)系。踏車式起重機(jī)的再次出現(xiàn)可能導(dǎo)致了卷?yè)P(yáng)機(jī)的技術(shù)發(fā)展，因 為卷?yè)P(yáng)機(jī)在踏車式起重機(jī)的結(jié)構(gòu)和機(jī)械方面都有所發(fā)展。中世紀(jì)的踏車可以看作是羅馬Vitruvius’ De 工程師設(shè)計(jì)品的一個(gè)精心改造品，它們可以在很多寺廟館藏中看到。
3.結(jié)構(gòu)與用途
中世紀(jì)的踏車結(jié)構(gòu)是由一個(gè)木輪
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