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1、 Machines for Lifting The strongest athletes can lift close to 455kg just off the ground , but few ordinary people can lift more than about 68kg . Early man soon developed machines for lifting large stones and tree trunks .A simple device is a single pulley wheel arranged as a hoist .But a rope arou
2、nd one pulley merely changes the direction of the pull and friction in the pulleys bearing in fact makes this simple machine less efficient than a straight pull .If the rope is wound around a wheel or cylinder to form a windlass ,a mechanical advantage is gained ,by means of which a man can easily l
3、ift more than his own weight . A small windlass can be driven by a hand crank -first used in the ninth century -and many early cranes and hoists used this principle.Screws and Pulleys Many basic inventions such as the screw and the pulley , cannot be credited to any one man .The Greeks were probably
4、 using screws by about 400 BC and by the time of Archimedes the screw certainly had various applications .Archimedes himself invented a type of pump consisting of a long helix in an upward-sloping tube; by turning a handle at the upper end the operator could screw water from the lower end of aspiral
5、to its upper end until it flowed out of the Nile Valley. In Roman and medieval times,screw presses were used for crushing olives and grapes.In the thirteenth century the French monk Villard de Honneecourt made a machine that used ascrew for lifting instead of pressing downward .Today known as the sc
6、rew jack,the device has many applications from lifting s car to change a wheel to jacking up whole buildings while a story is slipped underneath.Pulleys were also known also known to the ancient Greeks who used them to lower a religious dreama.By the time of Christ,Roman engineers were desiging and
7、making multiple pulley blocks for lifting and making multiple pulley blocks for lifting heavy loads.A 200t Egyptian granite obelisk,similar to the so called Cleopatras needles now standing in New York and London,was erected in ancient Rome using many pulley blocks and treams of slaves to provide the
8、 muscle power,Todays compact hoistsuse exactly the same principles.Hoists and Cranes Machines for lifting can also be made using gears to obtain mechanical advantage.With only horse power,sixteenth-century miners hauled loads of ore and other minerals.Later hoists using steam enginesand even modern
9、ones with electric motorsuse similar principles.Early cranes were merely rope-and-pulley hoists rigged between two or three wooden legs straddling the object to be lifted.Power was provided through a winglass or,for heavy loads,by a treadmill.The building projects and dock installations of the Middl
10、e A ges depended on such machines for lifting huge blocks of stone.Modern cranes are of teo main typesbridge cranes and jib cranes.both use a windlass with steel wire rope wrapped around a powered drum.A bridge crane gas a box-girder beam (called a gantry) running on long elevated tracks at each of
11、its ends . The hoisting system is carried in a trolley,which moves along the gantry bean.Bridge cranes are commonly set up above a working area to handle such loads as tree trunks and steel beans.A jib crane has a long boom that can swing horizontally to move the load side ways.Many such cranes can
12、also luffto control the reach of the crane by anging the boom more or less to the horizontal. A cantilever crane is a typical example .A cantitlever or tower crane is used in construcing high rise-buildings. Anchored to the ground or to the building itself,the crane is extended upward as the work pr
13、oceeds. Standard sections form the tower,which supports a horzontal bom whose weight, and that of the load,is counter-balanced by a block of concrete.Thehoist is in a trolley that can travel along the boom.Lifting PeopleSkyscrapers and high-rise apartments would have been impossible without elevator
14、s interconnecting the floors and giving access to the ground. In 1857 the American inventor Elisha Otis(18111861)installed a steam-powered elevator in a New York department store.Early elevators used the screw-jack principle,soon to be replaced in the 1870s by elevators using hydraulic pressure.Wate
15、r,oil,or other gluid is pumped to provide pressure against a piston,which is turu raissesthe load.Many building use a combination of hydraulics and pulleys which allows the elevator to go up higher.In the twentieth century-rise buildings need passenger elevators using electic motors that travel at m
16、ore than 365m perminute,or about two floors each second.In subway stations and large stores,there is a more or less continuous flow of people between various levels,Here the people-lifting problem is solved by using escalators,which are continuosly moving staircases basded on the conveyor belt princ
17、iple with an endless belt of steps.The original patents of 1891 were obtained and improved by the Oitis Elevator Company that, together with Westinghouse Elevators,developed the modern escalator in the 1930s.An escalator 1.25m wide moving at 27m per minute can carry about 8,000people an hour.Movings
18、 heavy loads Ordinary cranes, used in the construction industry or for loading ships,can lift weights of up to 200t.But consider the following problems: a prefabricated 1,500t section of s ship(such as the whole superstructure or the front part of the bow) has to be placed in its final position; a 6
19、,000t rocker has to be moved 5km to its launching site; a7,000tsection of a stadium has to be placed in a heavy load,and each has been solved.What are heavy loads?The abitity to move heavy loads is increasingly important to the engineering industries because the cost-saving of bulding assemblies on
20、a specific site before moving them to their final places is now accepted. But preafabricated structures are becoming larger and heavier.As new load-moving techniques have been developed, other industries have adopted them The word heavy is arbitrary, but for these purpose it includes loads ranging f
21、rom hundreds of tons to tens of thousands of tons.moving heavy loads has presented engineers with problems for thousands of years.Many suggestions have been put forward as to how stone was moved in the building of the pyraminds and Stonehenge.Certainly a method using tree trunks as rollers would hav
22、e been known then,and animal or human power could have provided the moving force.Man started with the lever and soon discovered the arrangement of the moving force ,the load ,and the fulcrum (pivot)that would be most useful in particular applications . Archimedes is reputed to have claimed , “give m
23、e a firm place on which to stand and I will move the earth .” He knew that given a lever long enough to gain the necessary mechanical advantage a small movement of the heaviest load could be obtained with a sufficiently large movement of even a small applied force.The Problems InvolvedMoving heavy l
24、oads involves reducing the friction underneath the load and providing sufficient force to overcome the friction remaining once the load is moving . To reduce friction ,rolling logs were used and later wheels of various types. Grease was also applied to ease the movement of the load, particularly in
25、the shipbuilding industry. More recently various slippery plastic coatings , such as (PTFE),have been used ,as well as air and water cushions that operate like hovercraft.There are two kinds of friction involved in moving anything . Static ,or stationary , friction has to be overcome to start someth
26、ing moving , and dynamic ,or moving ,friction opposes its continued movement .The coefficient of friction between two materials is defined as the ration of the force required to move the load to the weight of the load .Static and dynamic coefficient have wide ranges . These maximum values drop betwe
27、en the traditional slippery slopes of steel on greased steel (used for ship launching) from 0.25 and 0.17,down to 0.10 and 0.05 for steel on PTFE plastic . The values fall to 0.01 for air-bearing systems. The use of friction-reducing system can introduce further difficulties . Loads of thousands of
28、tons , once started moving ,also have to be stopped . For this reason , when coefficients of friction are low , suitable braking systems must be incorporated . After a ship is launched , strong chains and cables are needed to stop it .Any conventional system can be used for moving power , providing
29、that it can overcome the frictional forces that remain . When large cranes ,such as portal cranes ,are used to move a load from one point to another ,they are merely lifting the load to reduce the coefficient of friction between it and the ground .This means that stresses are then put into the groun
30、d at chosen points ,which have previously been strengthened to bear the loads .Strengthening the Surface A third difficulty concerns the amount of preparation required for the surface on which the load is to be moved . This involves a calculation of the maximum permissible ground loading . For many
31、materials this quantity is known-for example ,compacted gravel will with stand a ground loading of 3.3 tons per square foot and wet sand 0.55 tons per square foot .At greater loadings the gravel or sand collapses and the load sinks . The loading is a pressure -a weight on a given area-and for a give
32、n load increasing the area of contact obviously lowers the pressure . This is why snowshoes support a man on loose snow whereas in ordinary boots he would sink . In most heavy load-moving techniques there must be a way of spreading point loads . But even this is not a complete answer to ground loadi
33、ng limitations . A hovercraft or air-cushion system that can carry loads over water has been known slowly to bury itself in dry sand as the sand is blown out from under the load .It rapidly becomes clear that there is no universal way of moving a variety of load over various of loads over various su
34、rfaces for different applications . The answer to the problems illustrated here are merely a few modern solutions-a portal crane to lift a ship structure , caterpillar crawlers to move the stadium stands.The air-cushion technique uses high air pressure underneath a load to lift the load slightly so
35、that air can escape . This continually escaping air acts as a bearing for the load and very low coefficients of friction can be obtained . The loading is spread over the area of the cushion . The Oahu Stadium in Hawaii was designed so that the stands could be moved from a rectangular shape for footb
36、all to a diamond shape for baseball. The air-cushion system was chosen because it reduces friction . Problem on the smooth surface have arisen because of loading and slopes that could cause the loads to run away .Movable friction grippers are used for braking the stands .The experiences of tower cra
37、ne construction in High buildingWith tower crane, it can accelerate construction speed and get good benifit in high building construction as well-known, but in keep with ones wish to make it practical, need to solve properly a little bit good related problem: Select suitable tower crane and arrange
38、reasonably under construction is the key factors.The factors that affect tower crane option are：The factors that affect tower crane option；Plane and the design of build buildings；Number of construct plies and the height of layer and building degree of height overall; Building project material object
39、 ；Construct component, product and materials, and equipment carry quantity; Construct the arrangement of construction progress as well as the partition of project time limit, construction rhythm and the construction section of running water; Construct base and environments construction ambient condi
40、tions; Unit resource conditions (whether financial resources purchase large scale equipment , whether the people that know management and use large scale equipment ); At that time tower type crane supply conditions as well as the requirement for the index of economic benefits. Therefore make best ch
41、oice need to expound and prove further.For certain high-rise building or exceed high-rise building comprehensive body projects, through main parameter for score I, type selecting and the calculation of class productivity are studied, with certain kind length hang arm to adhere type from liter tower
42、machine possibility is the option with appropriate comparison. But choose what length actually to hang arm bend type from liter tower machine? Below is 3 kinds of different length is 1, the schoolwork area of 55 m that hangs arm is 1.2, and is of 60 m to hang the schoolwork area of arm for 1.44. See
43、 from cost, in type identical, biggest range lifting capacity match approximately, under hook high equal condition, if with 50 m, hanging the schoolwork area of arm for 1.2 the cost of 60 m that hangs arm tower machine is 1.19. Thus it can be seen, so long as increasing the expense of 6%, can make t
44、he schoolwork area of tower crane increase 20%. If pay out the price of 19% , can make tower machine schoolwork area raise 44%. Experience shows that if tower machine hangs arm proper increase somewhat, the favorable level of the building capital on-the-spot is carried, alleviate the construction in
45、ternal level on-the-spot to carry miscellaneous levels, can at the same time still solve the contradiction of different construction paragraph volume disequilibria. Therefore increase expense properly to purchase hang arm some longer tower machines is worthwhile.Pass for range 50 60 the length of m
46、hang arm tower machine cost analysis, can still find out , in biggest range lifting capacity match approximately, under hook high equal condition the cost of the crane of rail mounted, than corresponding stationary from liter tower machine go back to 12 one hundred and forty thousands Yuan. As being
47、 implementing course , as being under construction progress pass , tower machine in climb continuously upward liter per separate 15 25 m need and construct to carry out once pull knot tight firmly, therefore it has no necessity that expense one-hundred thousands remains Yuan to purchase a tower mach
48、ine of rail mounted.As for 50 60 m length hang arm the cost from liter type tower machine make further analysis, can still know: The cost of base stationary tower machine than the cost high 2 without base stationary tower machine thirty thousands Yuan. Since seeing superficially, for oneness investm
49、ent purchase base stationary tower confidential than purchase same parameter do not have some base stationary tower machine colored money belong to not economic decisions probably. the principle of Tower crane chooseMain parametersThe chief principles of Tower crane choose are the main parameters that must meet needs, indicates before parameter to include range, load-lifting height and lifting capacity, with load moments. Parameter ag
