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ELEVATORS, LIFTS
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See also:ELEVATORS, LIFTS or HOISTS, See also:machines for raising or lowering loads, whether of See also:people or material, from one level to another. They are operated by See also:steam, See also:hydraulic or electric See also:power, or, when small and See also:light, by See also:hand. Their construction varies with the magnitude of the See also:work to be performed and the See also:character of the See also:motive power. In private houses, where only small weights, as See also:coal, See also:food, &c., have to be transferred from one See also:floor to another, they usually consist simply of a small See also:counter-balanced See also:platform suspended from the roof or an upper floor by a tackle, the See also:running See also:part of which hangs from See also:top to bottom and can be reached and operated at any level. In buildings where See also:great weights and See also:numbers of people have to be lifted, or a high See also:speed of See also:elevation is demanded, some See also:form of motor is necessary. This is usually, directly or indirectly, a steam-See also:engine or occasionally a See also:gas-engine; sometimes a See also:water-pressure engine is adopted, and it is becoming more and more See also:common to employ an electric motor deriving its See also:energy from the See also:general See also:distribution of the See also:city. Large establishments, hotels or business houses, commonly have their own source of energy, an electric or other power " plant," on the premises. The hydraulic elevator is the simplest in construction of elevators proper, sometimes consisting merely of a See also:long See also:pipe set deeply in the ground under the cage and containing construe- a correspondingly long plunger, which rises and falls See also:lion of elevators. as required and carries the elevator-cage on its upper end (fig. I). The " stroke " is thus necessarily equal to the height traversed by the cage, with some surplus to keep the plunger steady within its guiding-pipe. The pipe or See also:pump chamber has a length exceeding the maximum rise and fall of the plunger, and must be strong enough to sustain safely the heavy hydraulic pressures needed to raise plunger and cage with load. The power is usually supplied by a steam pump (occasion-ally by a hydraulic motor), which forces water into the chamber of the great pipe as the elevator rises, a See also:waste-See also:cock See also:drawing off the liquid in the See also:process of lowering the cage. A single handle within the cage generally serves to apply the pressure when raising, and to reduce it when lowering the load. The most common form of hydraulic elevator, for important work and under usual conditions of operation, as in cities, consists of a suspended cage, carried by a tackle, the running part of which 2.-The See also:Otis See also:Standard Hydraulic Passenger Lift, with See also:Pilot See also:Valve and See also:Lever-operating See also:Device. are employed, their speed ranges from 600 and 700 revolutions per See also:minute in the larger to r000 and 'zoo in the smaller sizes, corresponding to from 20 down to 4 or 5 h.p. Two or more counter-weights are employed, and from four to six suspension cables ensure as nearly as possible See also:absolute safety. The electric elevators of the Central See also:London railway are guaranteed to raise 17,000 lb 65 ft. in See also:soma of its shafts, in 30 secs. from start to stop. Over 'oo,000 ft. of fin. and 17,000 ft. of 4-in. See also:steel rope are required for its 24 shafts, and each rope can carry from 16 to 22 tons without breaking. The steel used in the cables, of which there are four to six for each See also:car and counter-See also:weight, has a tenacity of 85 to 90 tons per sq. in. of See also:section of See also:wire. The maximum pull on each set of rope is assumed to be not over 9500 lb, the See also:remainder of the load being taken by the counter-See also:balance. Oil " dash-pots " or buffers, into which enter plungers attached to the bottom of the cage, prevent too sudden a stop in See also:case of See also:accident, and safety-clutches with See also:friction adjustments of ample power and fully tested before use give ample See also:insurance against a fall even if all the cables should yield at once—an almost inconceivable contingency. The efficiency, i.e. the ratio of work performed to power expended in the same See also:time, was in these elevators found by test to be between 70 and 75 %• Safety devices constitute perhaps the most important of the later improvements in elevator construction where passengers are carried. The simplest and, where practicable, is connected with a set of pulleys at each end of a See also:frame (fig. 2). The rope is made fast at one end, and its intermediate part is carried See also:round first one See also:pulley at the farther end of the frame and then round another at the nearer end, and so on as often as is found advisable in the particular case. The two pulley shafts carrying these two sets of pulleys are made to See also:traverse the frame in such a way as, by their separation, to haul in on the running part, or, by their approximation, to permit the weight of the cage to haul out the rope. By this alternate hauling and " rendering " of the rope the cage is raised and lowered. The use of a number of parallel and See also:independent sets of pulleys and tackles assures safety in case of the breakage of any one, each being strong enough alone to hold the load. The See also:movement of the pair of pulley shafts is effected by a water-pressure engine, actuating the plunger of a pump which is similar to that used in the preceding apparatus, but being relatively of See also:short stroke and large See also:diameter, is more satisfactory in See also:design and construction as well as in operation. See also:Electricity may be applied to elevators of this type by attaching the travelling sheaves to a See also:nut in which See also:works a screwed See also:shaft driven by an electric motor. In other electric lifts the cables which support the cage are See also:wound on a See also:drum which is turned by a motor, the drum being connected to the motor-shaft either by a See also:series of pinions or by a See also:worm-See also:gear. The drum may also be worked by a steam or gas engine. Where the See also:traffic is not very heavy, a form of elevator that requires no attendant is convenient. In this any one wishing to use the lift has merely to See also:press a See also:button placed by the See also:side of the lift-See also:gate on the floor on which he happens to be See also:standing, when the car will come to him; and having entered it he can cause it to travel to any floor he desires by pressing another button inside the car. The motive power in such cases may be either electric or hydraulic, but the See also:control of the switches or valves that govern the See also:action of the apparatus is electric. The See also:history of the elevator is chronologically extensive, but only since 185o has rapid or important progress been effected. In that See also:year See also:George H. See also:Fox & Co. built an elevator operated by the See also:motion of a See also:vertical See also:screw, the nut on which carried the cage. This device was used in a number of instances, especially in hotels in the large cities, during the succeeding twenty years, and was then generally supplanted by the hydraulic lift of the See also:kind already described as the plunger-lift. With the increased demand for power, speed, safety, convenience of manipulation, and comfort in operation, the inventive ability of the engineer See also:developed the various systems more and more perfectly, and experience gradually showed to what service each type was best adapted and the best construction of each for its See also:peculiar work. Whatever the class, the following are the essentials of design, construction and operation: the elevator must be safe, comfortable, speedy and convenient, must not be too expensive in either first cost or See also:maintenance, and must be absolutely trustworthy. It must not be liable to fracture of any See also:element of the hoisting gear that will permit either the fall of the cage or its See also:projection by an over-weighted balance upwards against the top of its shaft. It must be possible to stop it, whether in See also:regular working or in emergency, or when accident occurs, with sufficient promptness, yet without endangering See also:life or See also:property, or even very seriously inconveniencing the passengers. See also:Acceleration and retardation in starting and stopping must be smooth and easy, the stop must be capable of being made precisely where and when intended, and no danger must be incurred by the passengers from contact with running parts of the mechanism or with the walls and doors of the elevator shaft. These requirements have been fully met in the later forms of elevator commonly employed for passenger service. Usual sizes range from loads of r000 to 5000 lb with speeds of from 8o to 250 ft. a minute unloaded, and 75 to 200 ft. loaded, and a height of travel of from 50 to 200 ft. In some very tall buildings, as the See also:Singer and See also:Metropolitan buildings in New See also:York, elevators have been installed having a maximum speed of 600 ft. a minute, with a rise of over 500 ft. Where electric motorsmost certain of them is the " See also:air-See also:cushion," a chamber devices. into which the cage drops if detached or from any cause allowed to fall too rapidly to the bottom, See also:compression of the air bringing it to See also:rest without See also:shock (fig. 3). This chamber must be perfectly air-tight, except in so far as a purposely arranged clearance around the sides, diminishing downwards and in well-established proportion, is adjusted to permit a " dash-pot " action and to prevent rebound. The air-cushion should be about one-tenth the See also:depth of the elevator shaft; in high buildings it may be a well 20 or 30 ft. deep. The See also:Empire See also:building, in New York, is twenty storeys in height, the See also:total travel of the cage is 287 ft., and the air-cushion is 50 ft. deep, extending from the floor of the third See also:storey to the bottom of the shaft. Sliding doors of great strength, and automatic in action, at the first and second floors, are the only openings. The shaft is tapered for some distance below the third floor, and then carried straight to the bottom. An inlet valve admits air freely as the cage rises, and an adjusted safety-valve provides against excess pressure. A " car," falling freely from the twentieth storey, was checked by this arrangement without injury to a See also:basket of eggs placed on its floor. Other safety devices consist of catches under the floor of the cage, so arranged that they are held out of engagement by the pull on the cables. But if the See also:strain is suddenly relieved, as by breakage of a See also:cable or accident to the engine or motor, they instantly See also:fly into See also:place and, engaging strong side-struts in the shaft, hold the car until it can be once more lifted by its cables. These operate well when the cables part at or near the car, but they are See also:apt to fail if the break occurs on the opposite side of the carrying sheaves at the top of the shaft, since the friction and inertia of the See also:mass of the cables may in that case be sufficient to hold the pawls out of gear either entirely or until the headway is so great as to cause the smashing of all resistances when they do engage. Another principle employed in safety arrangements is the Essentials of design, See also:ea Safety Air-Cushion. action of inertia of parts properly formed and attached. Any dangerous acceleration of the cage causes the inertia of these parts to produce a retardation relative to the car which throws into action a See also:brake or a catch, and thus controls the motion within safe limits or breaks the fall: The hydraulic brake has been used in this apparatus, as have See also:mechanical and pneumatic apparatus. This control of the speed of fall is most commonly secured by the employment of a centrifugal or other See also:governor or regulator. The governor may be on the top of the cage and driven by a stationary rope fixed between the top and bottom of the shafts, or it may be placed at the top of the shaft and driven by a rope travelling with the car. Its action is usually to trip into service a set of See also:spring grips or friction clutches, which, as a See also:rule, grasp the guides of the cage and by their immense pressure and great resultant friction bring the cage to rest within a safe limit of speed, time and distance. A coefficient of friction of about 15% is assumed in their design, and this estimate is confirmed by their operation. Pressures of to tons or more are sometimes provided in these grips to ensure the friction required. There are many different forms of safety device of these various classes, each maker having his own. The importance of absolute safety against a fall is so great that the best builders are not satisfied with any one form or principle, but combine provisions against every known danger, and often duplicate such precautions against the most common accidents. The " travelling See also:staircase," which may be classed among the passenger elevators, usually consists of a staircase so constructed that while the passenger is ascending it the whole structure is also ascending at a predetermined See also:rate, so that the progress made is the sum of the two rates of motion. The See also:system of " treads and risers " is carried on a long endless See also:band of See also:chain sustained by guides holding it in its desired See also:line, and rendering at either end over cylinders or sprockets. The junctions between the stairway and the upper or See also:lower floors are ingeniously arranged so as to avoid danger of injury to the passengers. See also:Freight elevators have the same general forms as the passenger elevators, but are often vastly larger and more powerful, and are not as a rule fitted up for such heights of lift, or constructed with such elaborate See also:provision for safety or with any See also:special finish. Elevators raising See also:grain, coal, See also:earth and similar materials, such as can be taken up by scooping into a bucket, or can be run into and out of the bucket by gravity, constitute a class by them-selves, and are described in the See also:article See also:CONVEYORS.
The See also:term " grain elevator " is often used to include buildings as well as machinery, and it is not unusual in See also:Europe to hear a See also:flour-See also: Additional information and CommentsThere are no comments yet for this article.
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