1902 Encyclopedia > Lifts

Lifts




LIFTS may properly be held to include all sorts of ap-paratus whose object is the lifting of weights. When the apparatus consists of comparatively small, separate, and portable pieces, it is called lifting tackle. When the lifting apparatus reaches that degree of size and compli-cation that entitles it to be called machinery, there seems to be no general technical term that will include all kinds, but for the different classes of lifting machines there are such special names as cranes, hoists, elevators, lifts, wind-ing engines, and lift pumps.
There is very little distinction made between hoists, 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 " cage" 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 " barrel" or " sheave " 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 i3 lined in the 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 the 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 B> 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 60 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 lb for every ton of load, or nearly -i-th 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 be-low by 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 lb 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 lb. This would give a difference of support-ing pressure of 285 lb for every ton of total load in a lift of 60 feet, —that is, about gth of the total load. More commonly the weight of the cage and the ram is balanced by counterpoises on chains fastened to the top of the cage and passing over a pulley overhead, while the water pressure is used to overcome only the friction, and the additional load of passengers or goods. In this case again, owing to the passage of the chains over the pulleys, the balance is disturbed in a rise of 60 feet, by about Ath the weight of the cage and the ram, while the upward water pressure on the ram is in the same rise diminished by s th. The former disturbance of balance is a decrease of the load resting on the base of the ram, while the latter is a decrease of the supporting pressure on the same base. If these were made equal, the cage and load would be perfectly balanced in every position. To make them equal, it would be necessary simply to adjust the ratio of the part of the load borne by the counter-poise to the part borne by the water. Let the former part be Wx and the latter W2, the total load being Wj + W2. Then for a water pressure of 200 lb per square inch, it would be necessary to have

iVi-lW,, ar.Wx-IW.-ACWi+W,). For a pressure of 400 lb per square inch, the equation would be
Wx-AW.-JWWi+W,), For 100 lb per square inch it would be

W1-fW1-«W1+Wi). This adjustment would necessitate a large unnecessary consump-tion of water, because the weight of cage and ram always bears a much greater ratio to the extra weight of passengers or goods than any of the above fractions -fa, f, or even f, The adjustment being attainable by other means, this waste can in no ease be desirable.

If a second cylinder stand beside the lift-well and be connected by a pipe to the cylinder directly underneath the cage, so that there is a continually open passage between the two cylinders, then the supporting rod underneath the cage, together with the column of water leading from its base through the pipes to the second cylinder, is the exact counterpart in compression of the overhead ropes in tension in the other class of machine ; and, as counterweights are hung upon these ropes, a balancing weight may be laid on the surface
, of the water in the second cylinder. The balance weight, equal-ling that of cage and ram, rests on a plunger or piston fitting this cylinder, and the rod is extended upwards into a third smaller cylinder, on the plunger of which is admitted, by means of the valve worked from the cage or landing-platforms, an extra amount of water pressure sufficient to elevate the extra load of passengers or goods. This is the arrangement in Tomassi's hydraulic balanced lift. The column of water which takes the place of the rope in the over-head arrangement passes from one cylinder to the other, and vice versa, in the same way as the rope passes from the cage side to the counterweight side of the overhead pulley. Thus the balance, which may be made correct for one position of the lift, becomes dis-turbed for other positions by a similar amount to that already investigated. A perfect balance of" the constant part of the total load, namely, that of the cage and ram, is, however, obtained for all positions of the cage by the arrangement shown in figs. 1 and 2. This is the design of Mr Edward Ellington, described by him in a paper read before the Institution of Mechanical Engineers in January 1882. The whole load is borne by the rod a underneath the cage, which enters as a ram into the vertical cylinder A. This rod is made solid in orderto reduce the size of the cylinder as much as possible, and, therefore, also the size of the well that has to be bored in tire ground to contain this cylinder. This class of lifts is especially expensive on account of this boring, and the objection tc them on the score of expense is lessened by making the well small. The rod is made only just strong enough to safely bear the load on it. Its section should be designed with reference to the height of lift, because the longer the free length of the supporting pillar the greater is its tendency to buckle under a given load. If k be the stress per square inch calculated to be admissible on its section, and if W be the weight of cage and ram together and W that of passengers

or goods to be raised, the section is made equal to ^"t . Since

this same load has to be supported by the water pressure on the lower end of this rod, that water pressure is made also equal to k. This cylinder A is kept always in open communication with the lower end of the cylinder B. In this moves a piston b fastened to the top of the thick piston rod d. This passes downwards into the third cylinder C, and to its lower extremity is fastened the large piston c. These pistons have a common stroke, which is much shorter than the lift of the cage. The cylinders are correspondingly shorter than A, and they stand above ground. The ratio of the strokes may range from 5 to 8, and is the same as the ratio of the annular area of the under side of the piston b to that of the rod a. If b and d be the areas of the piston and piston rod, and a that of the rod supporting the cage, the ratio of the strokes is b - d: as. Suppose the piston b to be at the top of its stroke and the cage to be consequently at the bottom of the lift-well, then, if in this position the piston b be at a height above the lower end of a equal to ft inches, and if w be the weight of a cubic inch of water, then, the pressure per square inch on a being k, that on the lower side of piston b is k - hw. The whole upward pressure on this piston is therefore (k - hw)(b — d) ; and a downward pressure equal to or rather more than this must be exerted on this piston in order to lift the cage. This is supplied by admitting the water from the main, or from thehydraulic accumulator if force-pumps are employed to provide water-pressure, into the upper ends of the cylinders B and G. The lower end of C is always kept in open communication with the atmosphere. The water is continually admitted to B, and the water pressure on the top surface of piston b is designed to balance the constant load of cage and ram when this piston is at the top of its stroke. During the ascent of the cage, the water is admitted into C by a valve moved from the cage or platforms by means of the rope t, and the water pressure on the annular top surface of piston c, when that piston is at its highest position, is designed to balance the extra load of passengers or goods. During the descent, the cage being empty, the connexion between C and the accumulator is shut by the valve actuated from the cage, and the water is allowed to escape freely to the drains, so that the pressure on c becomes equal to atmospheric pressure. If p be the pressure per square inch of the working water at the level of the piston b at its highest position, and c be the area of the cross-section of cylinder C, and if ft' be the length of plunger d, then in this position the whole downward force that is borne by the water underneath the piston b, and distributed over its area (b - d), is pb + (p+h'w)(c- d), when the pressure is on piston c, and simply pb when this pressure is cut off from.C. (To this should be added the weight of b, d, and c, but for our present purpose of explanation only this may be left out of account.) The former quantity has to equal {k - hw){b - d), and the latter should equal between b and a, will be decreased 7 inches. The pressure per square inch on a would decrease 7w if that on the under side of b kept constant. But, as the upper side of b also sinks 1 inch, the pressure per square inch on it will increase by w. If now the ratio of this upper area of b to its lower area be made 7, this increase of w on the top face will cause an increase of 7w on the lower face, and thus just neutralize the diminution of pressure on a due to the combined rise of the cage and fall of the lower side of b. Thus the unloaded cage will be in perfect balance, at whatever height it stands, if the areas b and b-d are given the ratio

b-d
_b_ b-c

The ratio b - d: a of the two strokes having been already chosen, this equation gives b directly. From the other two equations c and the necessary pressure p are found. This pressure p may be obtained by hydraulic pumps and an accumulator loaded to the right amount. If, however, the water from the mains is to be used, the ratio of the strokes or the size of a may be modified so as to suit the avail-able working pressure p. If c be proportioned for the extra load at a given height, it will not be correct for all other heights, but this is of little consequence, because the extra, load itself is variable from 0 upwards, so that no adjustment of c except to its maximum desired amount is possible. An excess of pressure on c above that needed for any given load has the effect simply of accelerating the speed of ascent, and this is modified roughly by partially closing the valve admitting water to C.





We have chosen this lift for description as the latest improve-ment in the design of hydraulic lifts. In it no water is wasted in raising or lowering the constant load.

When the hydraulic power is applied to the cage through a chain or rope passing over an overhead pulley, the hydraulic cylinder is usually laid horizontally for facility of sotting and examination. Of course this arrangement involves much greater frictional resistance to the motion of the apparatus, but in it all the severely stressed parts may be in tension. There is greater security when they are so than when they are in compression. Tangye Brothers' hydraulic lift is arranged in this way.

Accidents to lifts occur in two ways. First, the suspending chain or rope may break, or, in those supported from below, the ram may break, or the cylinder or pipes enclosing the water may burst. To lessen the risk of such breakages the only method is to insist on good design in the details, good materials (which should be subjected to test before being used), and good workmanship. The connexion at both ends of the rope or chain to the load sus-pended from it, or the jointing of the different sections of the ram to each other and to the cage, is a point especially important. If such a breakage does actually occur, however, the cage is usually kept from falling by an automatic catch which grips it in whatever position it hap-pens to occupy when the accident occurs. Tangye Brothers have for this purpose at each corner of the cage a toothed cam. The suspending rope sustains the cage through levers as shown in fig. 3. So long as there is a considerable pull on the rope, the levers keep the cams in the position shown. If the strain on the rope is relieved by accident to it, powerful spiral springs immediately force the cams outwards and the teeth become buried in the wooden guide-posts. A toothed rack is sometimes bolted to the vertical posts and tooth-shaped prongs are forced forward by springs to engage with the rack when the rope breaks. Similar arrangements are not placed between the top of the ram and the cage of direct-acting hydraulic lifts, but it is a mistaken idea that they are not as necessary in this case as in the other. Such appliances should be examined and tested at regular frequent intervals. They are apt to get out of working order through disuse. A double rope is a greater safe-guard against accident.

In chain or rope lifts the gearing or other machinery may break, and in consequence the cage might run down with dangerous rapidity without the rope cither breaking, or being wholly relieved of tension, so that the above catches may not come into action. This may bo prevented by a self-acting clutch on the shaft, which prevents the barrel rotating unless the clutch is specially released. The most perfect ana mechanically beautiful of the many devices that have been invented for this purpose is Weston's frictional automatic coupling. Fig. 4 shows it as applied to a hand sack-hoist. To the shaft a is keyed a ratchet wheel b. A pawl gearing in this prevents the shaft from ever rotating except in one direction. The plate c is also keyed to the shaft. The hauling rope sheave d and the wind-ing barrel e both run loose on the shaft. Their opposing end sur-faces are cut helically, so that, according to the relative angular positions of d and e, they are either jammed against each other and between c and b, or are loose and free to rotate round the shaft. Ou pulling the sheave d in one direction all the parts are frictionally coupled together, and the barrel hauls up the load. The axial pressure producing friction between c and e and between d and b is greater than the load being hauled up in the ratio of the circumference of the barrel to the pitch of the helix. As there are two frictional surfaces, the whole friction is double this axial thrust multiplied by the coefficient of friction, and this friction must act at such a mean radius from the shaft as to have amoment greater than that of the load. °\ If this is so for one load, it is so also for all others, as the friction is proportional to the load. To get sufficient friction for heavy loads with a diminished axial thrust, the very ingenious design shown in fig. 5 is adoyrted. Here the shaft a is driven by power, and is keyed to the boss d with a helix cut on one end. This helix abuts against a similar helix on the pinion e, which drives the hoisting barrel on a second shaft. The ratchet wheel b abuts against the collar/ on the shaft a ; b runs loose on the shaft and is cast on the end of a hollow drum containing three disks of hard wood, P,P,P. These disks can slide axial- ct ly along the interior of the drum, but are prevented from turning except along with the drum. Intervening between these wood disks are two iron disks, 0,0, which may slide axially along the boss of the pinion e, but are prevented from rotating except along with this pinion. The axial pressure is transmitted from d to/, through the surfaces of the disks P and 0, and, there being six pairs of surfaces between which this pressure is exerted, a very slight axial thrust produces sufficient friction at these surfaces to couple the ratchet wheel b to the pinion e. So long as this is exerted all the parts are jammed together, and the pawl engaging in b prevents the load lowering. When, however, the shaft a ia rotated backwards, the helices disengage and the friction no longer binds e with b, so that c along with d and a can be rotated backwards and the load thus lowered. The weight of the load keeps e following d closely in its backward motion, and as soon as the operator or machine ceases to turn the shaft backwards the whole apparatus becomes once more frictionally bound together, and the ratchet wheel prevents further lowering. Fig. 6 shows another arrangement where- ~—

by the pinion e is —&-

uncoupled and al-lowed to lower the load by only a slight backward motion of the shaft a, it being unnecessary to ro-tate the shaft back-ward continuously. This last is obvious-ly the most handy arrangement, and when worked carefully is as absolutely safe as the other. This device in a modified form is used in Tangye's lifts.

Thomas & Sons, of Cardiff, have a similar patent safety shaft coup-ling, which, although it has a very different form, is constructed on exactly the same principle as that of fig. 4. Steam has been used as a motive power in long cylinders similar to those in hydraulic lifts. It has the great advantage of having very little weight, so that the difference of head occasioned by the rise of the piston is practically nil. The disadvantage is that the steam rapidly condenses, and thus the load could not be held up at any desired height for a length of time, without a continual fresh supply of steam to the cylinders. It is not likely to come into general use for passenger lifts, but may be used advantageously for goods lifts and heavy cranes. (R. H. S.*)



The above article was written by R. H. Smith, Professor of Engineering, Mason Science, College, Birmingham.



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