Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/594

 574 elevators, and lifts. The word hoist refers more particu larly to machines used in warehouses and factories for raising goods from one story to another. They are worked by hand or by power, and are for comparatively light loads. Elevator is used in two different senses. It refers to apparatus for lifting passengers to the upper stories of buildings. It also refers to the very different sort of apparatus used in grain-mills and storehouses for trans ferring the grain from one floor to another. The grain is drawn along channels or pipes, which are sometimes vertical and more often inclined, by means of a rotating archimedean screw, or of a strap continuously travelling upwards through the interior of the channel and carrying, fastened to it, a series of small buckets. Occasionally, if the inclination to the horizontal be small, a broad strap of the same width as the bottom of the channel runs along that bottom, and carries the grain with it simply lying on its upper surface. This latter method of transportation is more efficient, however, as a horizontal carrier or distributor than as a means of lifting. Grain might also easily be blown up a pipe by an air-blast, but the writer does not know any instance of this method having been used. Lifts are constructed either for raising passengers in buildings or for heavier loads, such as freighted trucks and waggons, or the superstructure of bridges and large roofs during their erection. In lifts or elevators, the working force is either hand, steam, or hydraulic power. Gas-engines are unsuitable as direct sources of power for lifts, but they may be advantageously used to store hydraulic power in an ac cumulator from which water is supplied to work an hydraulic lift. Electricity has quite recently been used, but has not yet been tried sufficiently to allow of any valuable opinion being formed of its ultimate practical success. The lift consists of (1) a box or &quot; cage&quot; to contain the persons or material to be raised ; (2) a vertical square well or shaft, to the walls of which are attached guides to prevent the cage swinging to and fro ; (3) a rope or chain by which to haul the cage upwards from above, or else a long rod or pillar by which to push it up from below ; (4) a &quot;barrel&quot; or &quot;sheave&quot; over which to wind the chain or rope, and which is mounted on a shaft lying in bearings firmly supported by the building, or else a cylinder to contain water or steam to actuate the lifting rod ; (5) mechanism through which the working power is transmitted to the barrel, or else water or steam piping connecting the cylinder above mentioned with the source of power; and (6) the driving engine or other source of power. Most accidents happen to lifts through the hauling chain or rope breaking. For the sake of safety, therefore, particular care should be exercised in the choice of material for this part, and an appliance should always be attached to the cage whereby, if the rope breaks, the cage is caught immediately in whatever position it may be at the time of the breakage. For light loads hempen ropes are sufficient and more convenient than chains, because they are noiseless in their action. If of the best quality (Manila) they are quite as reliable as ordinary chains, and an advantage claimed for them is that their gradual destruction by wear becomes easily apparent, and gives timely warning before they become dangerous, whereas the failure of a chain may take place without any easily visible previous sign having been given. For very heavy loads, however, chains or wire ropes should be used in preference to hempen ropes. Wire ropes may be made stronger for a given weight per foot of length than chains are, but unfortunately as commonly manufactured their quality cannot be certainly relied on. Like hempen ropes, they are almost noiseless. To insure smoothness and noiselessness in passenger lifts, the sheave over which the rope passes is lined in thd groove with leather. For the sake of safety, the rope by which the cage hangs is often duplicated. Sometimes even three or four are used. In order that these should give additional safety, each rope must be capable of supporting the load by itself. Generally the load is lifted by one or other kind of power, and descends by the weight of the cage itself. This weight is always much more than sufficient for the purpose, and therefore counterpoises are intro duced to balance the greater part of it, thus lessening the work to be done during ascent by an amount equal to ths product of the balance weight and the height of the lift. In the commonest arrangement, the balance weights are hung on the same rope as that by which the cage is sus pended. This passes over a pulley whose diameter is half the width of the well, so that the cage end of the rope rises vertically from the centre of the roof of the cage. This pulley is keyed on a horizontal shaft, which is driven by power from below, either directly by means of a rope or chain passing over another pulley, or else through inter mediate spur gearing. The actual working rope is in this case not attached to the cage. Less frequently the rope from the engine forms one of the suspenders of the cage, the balance weights being attached by separate ropes. The rope or chain by which the load hangs has to be so strong that its own weight is very considerable. A large excess of strength being more in demand in this kind of machinery than in other kinds, a greater stress than about 1 ton per square inch cannot be put upon the chain or rope (supposed to be of iron). This would make the rope weigh 3 4 Ib per foot of length for every ton of load carried. If the height of lift were, for example, 60 feet, then, comparing the top and bottom positions of the cage, there would be in the former CO feet less of rope on the cage side of the pulley, and 60 feet more on the counter poise side, than in the latter position, so that if the counter-weight just balanced the load when the cage was at the bottom, it, along with the rope, would outweigh the cage in its highest position by the weight of 120 feet of rope, that is 408 K) for every ton of load, or nearly -1th of the whole load. Since the whole load that is, that of cage, ropes, and passengers or goods is three or four and sometimes five or six times as great as the net load, this is a very serious increase on the unavoidable loss of balance resulting from the fact that the cage is alternately loaded and unloaded. The difficulty can be got over by extend ing the rope downwards from the balance weight to pass underneath a grooved pulley at the bottom of the well, and up from this to the under side of the cage, where it is attached. There will then be an equal length of rope always hanging on each side of the top bearing pulley; but an extra amount of friction occurs at the bearing jour nals due to the weight of the extra rope. The lower half of the rope may be of cheap inferior material, since there is very little stress upon it. A precisely similar difficulty occurs if the cage be lifted from he- low hy an hydraulic ram or piston-rod. Occasionally the weight of the cage and ram is left unbalanced. In this case the water pressure on the ram or piston has to support the whole load. Sup pose the pressure in the reservoir from which the water is drawn to remain steady during the ascent, then evidently at the top of its stroke the water pressure on the ram is less than at the bottom of its stroke, by the weight of a column of water, of section equal to that of the ram, and height equal to the lift. Suppose, for example, that the water pressure at the level of the face of the ram in its highest position is 200 lt&amp;gt; per square inch. Then for every ton of total load there must be provided about 11 square inches of piston area. A column of water of this horizontal section and 1 foot high weighs about 4 75 It). This would give a difference of support ing pressure of 285 K&amp;gt; for every ton of total load in a lift of 60 feet, that is, about Jth of the total load. More commonly the weight