is connected with a set of pulleys at each end of a frame (fig. 2).
The rope is made fast at one end, and its intermediate part is
carried round first one 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 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 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
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
short stroke and large diameter, is more satisfactory in design
and construction as well as in operation. Electricity may be
applied to elevators of this type by attaching the travelling
sheaves to a nut in which works a screwed shaft driven by an
electric motor. In other electric lifts the cables which support
the cage are wound on a drum which is turned by a motor, the
drum being connected to the motor-shaft either by a series of
pinions or by a worm-gear. The drum may also be worked by a
steam or gas engine. Where the 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 press a button
placed by the side of the lift-gate on the floor on which he happens
to be 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 control
of the switches or valves that govern the action of the apparatus
is electric.
The history of the elevator is chronologically extensive, but only since 1850 has rapid or important progress been effected. In that year George H. Fox & Co. built an elevator operated by the motion of a vertical 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 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 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 peculiar work. Whatever the class, the following are the essentials of design, Essentials of design, &c. construction and operation: the elevator must be safe, comfortable, speedy and convenient, must not be too expensive in either first cost or maintenance, and must be absolutely trustworthy. It must not be liable to fracture of any element of the hoisting gear that will permit either the fall of the cage or its projection by an overweighted balance upwards against the top of its shaft. It must be possible to stop it, whether in regular working or in emergency, or when accident occurs, with sufficient promptness, yet without endangering life or property, or even very seriously inconveniencing the passengers. 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 1000 to 5000 ℔ with speeds of from 80 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 Singer and Metropolitan buildings in New York, elevators have been installed having a maximum speed of 600 ft. a minute, with a rise of over 500 ft. Where electric motors are employed, their speed ranges from 600 and 700 revolutions per minute in the larger to 1000 and 1200 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 absolute safety. The electric elevators of the Central London railway are guaranteed to raise 17,000 ℔ 65 ft. in some of its shafts, in 30 secs. from start to stop. Over 100,000 ft. of 78 in. and 17,000 ft. of 34 in. 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 car and counter-weight, has a tenacity of 85 to 90 tons per sq. in. of section of wire. The maximum pull on each set of rope is assumed to be not over 9500 ℔, the remainder of the load being taken by the counterbalance. Oil “dash-pots” or buffers, into which enter plungers attached to the bottom of the cage, prevent too sudden a stop in case of accident, and safety-clutches with friction adjustments of ample power and fully tested before use give ample 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 time, was in these elevators found by test to be between 70 and 75%.
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| Fig. 3. Safety Air-Cushion. |
Safety devices constitute perhaps the most important of the later improvements in elevator construction where passengers are carried. The simplest and, where practicable, most certain of them is the “air-cushion”, a chamber into which the cage drops if detached or from any cause Safety devices. allowed to fall too rapidly to the bottom, compression of the air bringing it to rest without 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 depth of the elevator shaft; in high buildings it may be a well 20 or 30 ft. deep. The Empire building, in New York, is twenty storeys in height, the total travel of the cage is 287 ft., and the air-cushion is 50 ft. deep, extending from the floor of the third 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 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 strain is suddenly relieved, as by breakage of a cable or accident to the engine or motor, they instantly fly into 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 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 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
