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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 around one pulley merely changes the direction of the pull and friction in the pulley`s 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 lift 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 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 aspiralto 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 screw 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 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 Cleopatra`s needles now standing in New York and London,was erected in ancient Rome using many pulley blocks and treams of slaves to provide the muscle power,Today`s 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 engines——and even modern ones with electric motors——use 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 Middle A ges depended on such machines for lifting huge blocks of stone.
Modern cranes are of teo main types——bridge 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 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 also ‘luff’to 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 proceeds. 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 People
Skyscrapers and high-rise apartments would have been impossible without elevators interconnecting the floors and giving access to the ground. In 1857 the American inventor Elisha Otis(1811～1861)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.Water,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 more 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 principle 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 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 6,000t 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 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 from 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 have 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 me 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 Involved
Moving heavy loads 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 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 something 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 between 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 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 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 ground 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 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 given 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 loading 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 surfaces 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 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 football 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 crane construction in High building
With tower crane, it can accelerate construction speed and get good benifit in high building construction as well-known, but in keep with one's wish to make it practical, need to solve properly a little bit good related problem: Select suitable tower crane and arrange 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 ；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 conditions; 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 choice 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 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 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 the 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 internal 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 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 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 machine 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 investment 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 choose
Main parameters
The 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 again divides into ranges: Minimum range and the range in biggest range and biggest lifting capacity. Meet the range of construction needs parameters should pass according to the body size in construction building and make picture determine
The lifting capacity claimed denotes to take the sum of heavy weight, horizontal dams (50kg), sling and vessel weight that hangs. When the parameter of lifting capacity again divides into biggest range, load lifting capacity (Q0) with biggest lifting capacity ( Qmax ), the former denotes hook pulley to locate in the lifting capacity in arm head, then hook pulley the biggest load lifting capacity when works with multiple rate ( 3 the rope, 4 ropes, 6 ropes or 8 ropes )
For reinforced concrete, it is high and exceeds high building, the load lifting capacity in biggest range is crucial. If completely assembly type big board is constructed, the biggest range lifting capacity should be basis with biggest outside wallboard weight. If cast-in-place reinforced concrete is constructed , it should determine the biggest range needed according to biggest concrete hopper capacity lifting capacity, normally 1.5 ～ 2.5 t. For steel structure, it is in high and exceeds high building. The biggest lifting capacity of tower type crane is crucial parameter .It should be accurate with the weight of the heaviest component.
Load moment is the product of lifting capacity and corresponding working range. Load moments usually expresses with the curve map of load moment that abscissa is range (L, with m as unit), ordinate is lifting capacity ( Q, with t as unit ). It has the curve of load moments frequently from tower crane; express hanging arm respectively different combination length conditions the change condition of load moment. For reinforced concrete, it is important is biggest range to take weight satisfied in construction requirement in high and exceed high building. For steel structure, in high and exceed high building, it is important that the load moment of biggest lifting capacity must accord with needs.
Hook altitude is from rail end face or from basic end face the vertical distance to the center of hook. The load-lifting height of tower type crane not only depends on rigidity and the strength of tower structures, and depends on the time rate of hoisting mechanism of the reel silk rope capacity and hook pulley group of key link. Load-lifting height is a crucial main parameter. How to handle regardless of other tower type crane parameters, specifications how to be superior, does not join needs as load-lifting height, still has no way of completing construction task. The same range and load-lifting height that tower type crane needs for carrying out hoisting construction can determine through making picture and calculating as parameter.
Tower type crane production efficiency should satisfy construction progress requirement
First the structural high-rise building standard layers of reinforced concrete the in average each contract needs of square meters of floor area 1.1 ～ 1.6 hang. According to the building layer estimation of floor area hang N always estimate always.
Secondly, tower crane in average each class asks can complete 50 ～ 70 hang. Can calculate tower type crane according to the tower type crane quantity and every day schoolwork class number used for planning to match it can be complete and total to hang N to total.
If N totals》 N total, think that the production efficiency of tower type crane can satisfy the requirement of construction progress.
Suitable types of tower crane
According to recent years domestic every place, construct the high construction experience of exceeding high building, choose length to hang arm adhere type from liter tower type crane carry out the high-rise favorable condition of exceeding high building construction is: 1 can meet the needs of building build and layer high change better; 2 does not affect building internal construction arrangement; 3 arrangements dismantle comparison convenience; 4 do not hinder driver line of vision, convenient driver is operating with the raising of tower type crane productivity. Adopt to adhere type the unfavorable point from liter tower machine is: 1 influences are local to decorate project; 2 needs more tower standard section and is a certain quantity of to bend dressing to set up, increase high cost and class cost.
According to in recent years domestic every place, construct the high-rise construction experience of exceeding high-rise building, choose length to hang arm adhere type from liter tower type crane carry out the high-rise favorable condition of exceeding high-rise building construction is: 1 can choose better climb the unfavorable point of type tower type crane is: The layer structure of 1 floor need to install often to climb liter hole move hole and is going to mend properly strong, affect the construction arrangement in building; 2 take apart to pack trouble, it is greater to take apart t