Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/596

 576 LIFTS between b and a, will be decreased 7 inches. The pressure per square inch on a would decrease lio 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 10. 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. Tims 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 b-d. , :- + 1 . b-d a 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 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 setting 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 either breaking, or being wholly relieved of tension, so that the above catches may not come into action. This may be prevented by a self-acting clutch on the shaft, which prevents the barrel rotating unless the clutch is specially released. The most perfect and 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. On pulling the sheave d in one direction all the parts are frictionally Fig. 3. 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 circum ference of the barrel to the pitch of the helix. As there are two frictional sur faces, the whole fric tion is double this axial thrust multi plied by the coefficient of friction, and this friction must act at such a mean radius from the shaft as to have a moment greater : fpTi than that of the load. a I _j i If this is so for one load, it is so also for all others, as the fric tion is proportional to the load. To get suf ficient friction for heavy loads with a diminished axial thrust, the very in genious design shown in. fig. 5 is adopted. Here the shaft a is driven by power, and Via:. 4. is keyed to the boss d with a helix cut on one end. This helix abuts against a similar helix on the pinion c, 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, fb*- fri |. ^ three disks of hard d wood, P, P.P. These ( Tl. - -^ -*, ..*; disks can slide axial- ly along the interior of the drum, but are prevented from turn ing except along with the drum. In tervening between these wood disks are two iron disks, 0,0, which may slide axially along the boss of the pinion c, 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 c. 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 is 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 back wards and the load thus lowered. The weight of the load keeps a 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 ,_ ffiStSH ,b uncoupled and al lowed to lower the load by only a slight #1 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 nt 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. (B. H. S.*)