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起重机的用途与历史有时当我们看到一台正在运作的起重机，都会非常惊讶，这些机器有时非常大，能把成吨的货物提升到半空中。看到这些庞然大物的时候我们心理都带着一种惊愕，有时甚至是有一点恐惧的心情，我们会去想如果吊着着的东西掉下来了或者是起重机吊错了位置会发生什么样恐怖的事情。起重机的确是一种令人着迷的机械系统，无论是成人或者是孩子无不为止惊叹。起重机的种类五花八门，并且历史悠久。起重机是用一个或者几个简单的机器来组成一个机械结构并用于运送那些人无法搬动的物品。一般来说，起重机由一个卷筒、一束金属绳或者是一条金属链组成用来同时提升、放置或者是水平移动货物。起重机的工作领域一般是在需要装卸货物的运输业、需要搬运建材的建筑业和需要组装重型设备的制造业。1.概况第一台具有机械结构的起重机是由古希腊人发明的，并且由人或者是牲畜比如驴，作为动力源。这种起重机被用于大型建筑的建造。这种起重机后来发展成了采用人力踏板驱动的更大型的起重机，用于提升更重的物料。中世纪时港口起重机被用来装卸船上的货物，有的港口起重机为求更大的起重重量和更好的稳定性被造在了石塔里。最早的起重机是用木头制造的，但是工业革命之后，铸铁和钢材就代替了木头用于制造起重机。尽管水磨机和风车都可以利用自然的能源来驱动，但是几个世纪以来，起重机的动力源一直是人力或者是畜力。第一台真正采用机械能量的起重机用的是蒸汽机，最早的蒸汽起重机出现于18到19世纪，有一些甚至到了20世纪末仍能很好地使用。虽然由于能源的供应仍不可及，到现在有一些人力起重机还在使用，但是现代的起重机一般采用的内燃机、电动马达、液压系统能为起重机提供比之前大得多的提升力。起重机的类型多种多样每一种都是量身定做。尺寸由最小的在车间里使用的臂式起重机到用于建造高楼的最高的塔式起重机应有尽有。然而，小型的起重机也被用来建造摩天大楼，目的是为了在高楼中狭小的空间内使用使建造更加方便。最后，我们来看看更加巨型的浮船式起重机，一般用来建造石油钻探平台和打捞沉没的船只。这篇文章也会涉及到之前没有提到，但是也非常常见的的起重机械，比如说堆垛起重机和装卸起重机。2.历史古希腊时期用来提升重型货物的其中节是古希腊人在公元前六世纪晚期发明的。考古记录显示最早在公元前515年提升夹具和铁制的吊楔开始出现在古希腊人石块结构的神殿里。由于这些是起重设备的核心装置、也由于他们是在石块的重心的中央或者是在离重心上一点距离相等的两头被发现，他们被考古学家认为是起重机当时就存在的确凿证据。绞盘与滑轮的的引入导致了人类之前用斜坡来向高处运送货物的方法被广泛替代。在接下来的两百年中，希腊的建筑都采用了这样新型的提升物料的技术，它利用了一些小型的石块来来代替大块的石头，这样更具实用性。与更早先的古希腊人神殿的建筑材料的尺寸不断变得越来越大趋势相比较，希腊古典庙宇比如帕台农神庙的石块重量都小于1520吨。而且，要把巨型的石柱竖立起来的作业古希腊人实际上更喜欢用好几块像鼓一样的圆柱石块堆叠而成。尽管确切是何时从斜坡运输进入起重机提升技术时代的时间还不是很清楚。但是当时古希腊不稳定的社会局势、和政治情况使得建造神殿更适合雇佣小型的、更加专业的建筑团队而不是像埃及和亚述那样大量使用的没有技术的劳动力。这样的情况使得起重机更像是希腊城邦发明的而非是采用纯劳动力斜坡运送货物的埃及或是亚述那样的独裁国家。 文学上第一次的明确的记载滑轮组的复合系统是出现在亚里士多德的机械难题中，但是组成文字可能还要稍晚一些。与此同时，用于建造希腊神庙的石块尺寸再一次开始赶上他们的古代前辈了，这标志着当时更多的久经考验的的滑轮组一定在希腊建筑史上找到了它们的一席之地。古罗马时期起重机械在古代的全盛时期却是在古罗马帝国展开的。当时建筑物的数量激增，而且这些建筑都达到了巨型的尺寸。罗马人采用了希腊人的起重机并将其发扬光大。多亏了那些维特鲁威工程师们撰写的相当冗长的文献和亚历山大大帝的苍鹭的巢，我们才得以如此详细地了解到了它们的其中技术。目前与Haterii的墓碑一起现存于世还有两座公元一世纪晚期、雕刻精细的古罗马脚踏式起重机的浮雕作品。三饼滑车是古罗马最简单的一种起重机，它是由一个单梁吊臂、一个绞盘、一条绳子和一个三个滑轮组成的滑轮组组成的。这样就有能够省下3倍的力。经计算，假设一个人用尽力气能够长时间地提起相当于重50千克的物体那么通过这样的起重机械他可以提升约150千克的物体（3个滑轮X50千克150千克）。更加重型的起重机就拥有五个滑轮（五饼滑车），最大型的起重机会在两根、三根甚至是四根桅杆上面装上三饼和五饼的复合滑轮组（复滑车），这是由最大的负载载荷决定的。复滑车工作的时候两边需要4个人：两边各站两个已经可以提起重约3000千克的物体（3条绳子X5个滑轮X4个人X50千克3000千克）。如果用踏车来代替绞盘的话，最大的起重载荷可以在人工减半的情况下达到两倍6000千克，因为踏车有更大的直径能够提供一个大得多的力矩。这意味着，和建造埃及金字塔时50个人才能通过斜坡搬动2.5吨的石块（50千克每人）的情况相比，罗马的复滑车的提升能力把工作的效率提高60倍（3000千克每人）。然而，大量现存的古罗马建筑中那些石块的重量比复滑车所能操作的负载要重得多。这表明古罗马人全面的起重的能力要远远任何简单的起重机。以Baalbek的Jupiter神庙为例，那些楣梁的石块每块都重达60吨以上，每个檐口的石块甚至达到了100吨以上，所有这些石料都被提升到了19m的半空中。在罗马Trajan之柱的主要石块重达53.3吨，而这些石块必须被提升到34m的高度。（见Trajian之柱）假定古罗马的建筑师们是用两种方法把这么巨型石块提起来的：第一种方法是由苍鹭之巢的暗示得来，首先一座起重塔矗立了起来，它四个桅杆以两条平行的边各一个的方式形成了一个方形的形状，不像一个围起来的塔，而是塔的中间有圆柱体。然后，大量的绞盘被放置在塔周围的地面上，因为虽然绞盘的杠杆比比踏车要低，但是绞盘可以安装在更高的地方由更多的人来驱动（此外还可以用牲畜）。这种大量绞盘的使用也被Ammianus Marcellinus记录和在Circus Maximus起升的Lateranense 方尖塔联系了起来。单个绞盘的最大起重量由在那些大石块上钻的抓取孔的数量决定。就拿Baalbek楣梁上那些重量在55到60吨的石块来说，八个明显的抓取孔表明了每个吊爪允许承受7.5吨的重量，这也是每一个绞盘所要承受的重量。以既定的动作来提升如此重量的物体需要各个施力于绞盘上的各个工作组之间有大力的协调和配合。中世纪时期在中世纪时，随着西罗马帝国的灭亡，西欧洲的科技技术水平一落千丈。这时踏车式的起重机再次被大范围地使用。最早的提到踏车式是大约1225年法国的一部档案文学作品，它在一份手稿上也说明叙述了直到1240年法国人的血统起源。在航海方面，最早使用港口起重机是在1244年的Utrecht、1263年的Antwerp、1288年的Brugge和1291年的Hamburg，而在英格兰踏车式的起重机直到1331年才有所记录。一般来说，采用起重机来垂直运输比传统的方法更加的安全和经济。典型的应用领域就包括港口、矿井。值得一提的是在哥特式大教堂的建造过程中，踏车式的起重机起到了一个不可或缺的重要作用。但是，档案和图画都显示了当时新引进的机械系统如踏车、独轮手推车等却没有完全替代那些楼梯、木桶、手推车等依赖劳动力的生产方法。这样，旧式的和新式的机械在继续在中世纪的建筑和港口共存。除了踏车，中世纪的文献中也记载了由手动驱动带幅轮和曲柄的绞盘的起重机，在15世纪时也是由卷扬机发展成为了类似船轮的系统。为了缓冲这些不规则的冲击力和解决提升过程中的死点问题，调速轮最早于1123年开始投入使用。踏车式起重机具体以何种方式再次被采用的已经无从考证，尽管它再次被使用在建筑领域是被毋庸置疑地认为和哥特式建筑的崛起有相当密切的关系。踏车式起重机的再次出现可能导致了卷扬机的技术发展，因为卷扬机在踏车式起重机的结构和机械方面都有所发展。中世纪的踏车可以看作是罗马Vitruvius De工程师设计品的一个精心改造品，它们可以在很多寺庙馆藏中看到。结构与使用地点中世纪的踏车结构是由一个木轮围绕在一根中心轴上，中心轴的两旁有足够宽的踏板以供两旁的工人踩踏。虽然以前的圆盘臂有轮辐可以直接用来驱动中心轴，但是更为先进的钩状臂更适合作为轮子边缘的弦来使用，这样可以用一个更细的轴来以供一个更大的机械利益。与常理相悖的是，中世纪建筑使用的起重机既不是安装在当时相当不可靠的脚手架上，也不是安装在哥特式教堂那纤细的墙上，那种墙不足以支撑起重机械和载荷的重量。当时的起重机是被安装在建筑物最初的底台上的，经常是在建筑物的内部。每当新的一层建成后，屋顶厚重的横梁和墙连在了一起，起重机被拆卸然后在那些在拱顶建造期间一根一根被搬运上来的顶梁上。这样，起重机就跟着建筑物一起升高和移动，这也是英格兰现存的建筑用起重机都是在保存在教堂的圆顶之上屋顶之下的原因，因为它们是建造完工后保留下来用来起吊屋顶维修的材料的。技巧与操作与现代的起重机相比，中世纪的起重机更像它们在希腊和罗马的祖先，它们主要的用途仅是用于垂直地起吊物体而不同时用于水平地长距离地移动物体。因此 起吊的工作也在一个与现在不同的地方进行。比如说在建造建筑时，起重机只能要么直接从底部把石块吊到指定地点，要么是从它放置石块的墙对面的另一边起吊。因为两个工作组是在墙的两头。另外，那个经常给踏车工人发号施令的工头可以用附加在物体上的一个小绳子来操纵物体侧向的移动。可以允许货物旋转的快速定向起重机特别适合港口的装卸工作，它出现在1340年。虽然方石堆的石块是直接由吊索、吊钩或者是恶魔夹（德语 Teufelskralle）起吊的，但是其他的货物都是事先就装在例如货盘、篮子、木箱或是木桶内的。值得注意的是，中世纪的起重机很少用棘齿或者是刹车闸来阻止货物后滑的。这个古怪的缺点可以这样来解释，踏车工作时所产生的巨大摩擦力一般可以防止轮子的加速而不受控制。港口使用由于对于古代义务还不甚了解，固定的港口起重机被认为是中世纪的一个新兴产物。典型的港口起重机是装配了两台踏车同时运转的。这些起重机在港口码头被用来了装卸船上的货物，它们取代或者是完善了老式的起重方法，例如秋千，绞盘和桅杆。两种不同种类的港口起重机可以用不用的地理区域来划分：佛兰德人和荷兰人的海港是使用一种可以绕着垂直轴转动的构台起重机。而德国和内陆的港口使用的是典型的塔形起重机，它的绞盘和踏车都被安装在一个坚固的塔里，只有它的吊臂和屋顶能够旋转。有趣的是港口起重机却不被地中海区域和高度发达的意大利港口所采用，那些地方的统治者直到中世纪还是继续在使用那些依赖劳动力的利用斜坡来起升货物的方法。建筑用起重机的工作速度是由相关泥瓦匠的工作进程决定的，与之不同的是港口起重机一般都采用两台踏车来提升装卸货物的速度。这两个踏车的的直径估计在4m或者更大，两边都装有轮轴并且同时转动。据调查，现今欧洲还现存有15台工业革命前的踏车式港口起重机。除了这种固定的起重机，那些可以在港口内随处移动的移动式起重机在14世纪开始投入使用。文艺复兴时期1586年一名叫做Domenico Fontana的建筑师在把一块重达361吨的梵蒂冈方尖碑重新放置到罗马的过程中用到了一台类似于古罗马人使用的塔式起重机。在他自己的记录中清楚地记录到：如此大量的拖拉工作组之间的通力配合需要十分的团结和严明的纪律，因为如果拉力不是均匀地施加的话，那些多余力会使得绳子断裂。近现代在17、18世纪起重机的使用日趋家庭化，烟囱和壁炉起重机被用来把罐子和茶壶放置到火焰上面，它的提升高度由一个闸门调节的。3.机械原理设计起重机时最重要的是要考虑到两点。第一：起重机必须能够提升起指定重量的物体，第二：起重机必须保持其稳定性并且在起升并移动物体到另一个位置的过程中不能发生倾倒。起升能力 机械优势起重机是利用一个或者是几个简单机器来创造一个机械优势杠杆。一台杠杆平衡起重机包含了一个水平的横梁（杠杆）围绕着一个支点旋转。杠杆法则使得一个横梁近段的重量载荷可以被一个施加在横梁远端的相反方向的较小的力提升起来。货物的重量所提供的重力之比等于长横梁和短横梁的长度之比。这就是所谓的机械优势。绞盘。一台悬臂起重机包含了一个伸出的压杆（悬臂），它可以支撑一个固定的滑轮组。缆绳多次围绕着固定的滑轮组和一个安装在负载上的滑轮穿过。当缆绳活动的那一头被人力或者是风力驱动的机器拖拉时，滑轮组就向负载施加了一个力，这个力的大小等于所提供的力乘以两个滑轮组之间缆绳长度的数值。这个结果就是所谓的机械优势。液压缸。这可以用于直接提升货物或者是间接地移动那些带有其他起升装置的吊臂或是横梁正如所有机器一样，起重机也遵循能量守恒定律。这就意味着施加于载荷物体的能量不可能超越输入机器的能量。比如说一个滑轮系统乘以所用的力是10，那么载荷物体只运动了相当于施加力十分之一的距离。因为能量是等于力乘以距离，输出的能量大致上等于输出的能量（实际上会略小一些，因为一些能量会因为摩擦力和其他方式损失）。稳定性为了达到稳定性，起重机在各个点上例如起重机的基座上的力矩之和必须等于零。在实际中，允许起重量（在美国叫额定载荷）比会导致起重机倾倒的重量（一个给定的安全极限）要小一些。按照移动起重机的美国标准，履带式起重机的额定载荷是倾倒载荷的75。而舷外支架式起重机的额定载荷是倾倒载荷的85。这些在起重机设计时额外的安全要求是由美国机械工程师协会在ASME B30.52007 移动与运动起重机标准中制定的。针对安装在船只或者是海上平台的起重机标准要更加严格一些，因为由于船体的晃动会造成动态载荷。另外，船只和平台的稳定性也是需要考虑的。对于固定底座或者是安装在主梁上的起重机来说，由横梁和起重机的臂所产生的力矩和载荷是由基座或者是主梁来承受的。基座内部的压力必须小于物料产生的压力，否则起重机就会倾倒。4.起重机的种类移动式起重机移动式起重机是由安装在移动平台上的一个捆式或者是伸缩式的起重臂构成的，它可以在公路、铁路、或者是水面上进行操作。固定式起重机固定的起重机牺牲了可移动性，而由此提升了它的稳定性能从而有能力运输更大的载荷和更重的重量。这种起重机的特性是它们，或者说是它们的主体结构在使用的时候是不可移动的。然而，它们还是能够被组装和拆卸的。5.悬挂式起重机用途大多数普通的悬挂式起重机应用在钢铁行业。钢铁制造的每一阶段，直到离开工厂成为成品前，钢铁都通过悬挂式起重机操控。例如把原料料注入熔炉、铁水的贮藏冷却、将成品钢卷板提升并运送到卡车或者火车上、工人和模压工在工厂里起吊移动钢铁，这些都需要使用悬挂式的起重机。汽车行业使用悬挂式起重机处理原料。小一些工作站的起重机在工作区域处理较轻的装载物，例如数控加工中的磨削和锯削工艺历史第一台悬挂式起重机是由Shaw 起重机公司的Alton Shaw在1874年发明的，现已解散的联合机械也保存了一台由钢铁工程师协会仿制的最早的起重机。这台起重机直到大约1980年时还在运作，现存于阿拉巴马州伯明翰的一家博物馆里。经过多年来重要的创新和更新换代，中间经历了Weston 负载闸（现在很少见了）和钢丝绳起重机（现在仍普遍使用的）。最初起重机是由成对的部件组合而成，现在被称作组合型起重机。这些组合型起重机应用于重型负载的工作，例如钢铁卷绕处理，对于使用者来说，希望机器经久耐用。他们的维护工作较容易。现在许多起重机是整合性起重机，采用一个机架内一个单元的制造模式，一般设计寿命为10年或少于10年。近代著名起重机目录1874年：Alton shaw 发明了第一架悬挂式起重机1938年：Yale引入Cable-King起重机1944年：在新墨西哥洲 Shepard-Niles 提供一个起重机起吊原子弹进行测试1969年：国际电子能源机构，对悬挂式起重机变速驱动进行了开发1983年：来自 Baredlla 公司的世界最大的悬挂式起重机开始其在巴西 Itaibu dam水利发电厂的工作1997年：工业巨头P&H 宣布 Chapter eleven破产。后重新命名Morris物料运输公司但仍然使用P&H这个名字，他们再次破产。1998年：Dearborn起重机提供芝加哥Verson Press公司两台500吨载荷的悬挂式起重机。由于Verson的破产这两台起重机不再被使用了。The Use and History of CraneEvery 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. OverviewThe 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.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. HistoryAncient GreeceThe 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 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 RomeThe 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 Trajans Column weighs 53.3 tons, which had to be lifted to a height of about 34 m (see construction of Trajans Column). It is assumed that Roman engineers lifted these extraordinary weights by two measures (see picture below for comparable Renaissance technique): 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). 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). 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. Lifting such heavy weights in a concerted action required a great amount of coordination between the work groups applying the force to the capstans.Middle AgesDuring 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 ships 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 placementThe 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, 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. Less frequently, medieval illuminations also show cranes mounted on the outside of walls with the stand of the machine secured to putlogs.Mechanics and operationIn contrast to modern cranes, medieval cranes and hoists - much like their counterparts in Greece and Rome - were primarily capable of a vertical lift, and not used to move loads for a considerable distance horizontally as well. Accordingly, lifting work was organized at the workplace in a different way than today. In building construction, for example, it is assumed that
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