Tall buildings and underground railways have brought about the rapid development of two highly efficient systems of vertical transport, the safety and reliability of which are accepted without question by millions of people every day
THREE OF THE LONGEST PASSENGER ESCALATORS IN THE WORLD - at Leicester Square Station, London, on the Piccadilly Line. Each of these escalators has a length of 160 feet and a vertical rise of 80 feet. They are inclined at 30 degrees to the horizontal. Two of the three are run in the direction of maximum traffic, all three being reversible.
STARK against the stars the skyscraper rears its beaconed head - a man-made mountain of concrete and steel. Floodlighted from without, ablaze with a myriad lights within, the vast building towers above the busy streets. Economic necessity serves to explain the accommodation of nearly 20,000 people in one building 1,000 feet high. One essential to the skyscraper is the elevator, or lift. Without the elevator the upper stories of the huge building would develop into an uninhabited desert. No one can climb to a height of 1,000 feet by stairway and arrive in a fit condition either for work or for play. New York, Chicago, San Francisco and the other great cities of the United States would be unable to exist without the elevator. This is true also of cities and towns composed mainly of tall buildings in any part of the world. The counterpart of the elevator, the escalator, plays an equally important part in the lives of millions of people. In London, with its teeming population, served by the greatest system of underground railways in the world, the escalator is vitally important. The London Passenger Transport Board uses more, larger and faster escalators than any other concern in the world.
London’s escalators and the elevators of the United States, however, represent only two aspects of what may well be described as vertical transport. Lifts exist in almost infinite variety, simple and complicated, large and small, fast for the carrying of passengers, slow for the handling of goods. Goods lifts trespass on the sphere of the crane or hoist, and some are of so specialized a nature that they defy classification. They range from lifts that handle planes from deck to hangar in the naval aircraft carrier, to the moving platforms that serve ascending angels or disappearing demons on the stages of theatreland. Modern express passenger lifts are nearly always electrically driven and controlled, but there are thousands still in use, especially for goods and the special purposes outlined above, which are operated by hydraulic power.
Passenger lifts are not confined to hotels and other large buildings. Often they are found in other settings. At seaside holiday resorts the cliff-top is frequently linked with the beach and sands by lift, sometimes arranged vertically, more often inclined at an angle to the face of the cliff. In the chapter “Giant of the Ether” are described the lifts in the giant aerial masts at Rugby Radio Station. Most tall towers are equipped with one or more lifts to permit sightseeing from the topmost level, and this is true of towers that rise from buildings used for widely different purposes. Westminster Cathedral, with its tower overlooking the whole of Greater London, is provided with a lift. The lofty North Tower of the Crystal Palace also has a lift to the great circular gallery that surmounts that gigantic glass cylinder.
IN A LARGE HYDRAULIC LIFT for transporting naval aircraft between the flight deck and the hangar of H.M.S. Courageous. Some of the aircraft carriers of the British Navy were originally built as cruisers, and the hydraulic equipment installed for the working of heavy guns was retained for operating lifts.
We can expect to find lifts wherever man has overcome height or depth. Some lifts, if we include, mine cages as lifts, descend to depths of more than 8,000 feet, and not always in a straight vertical line. In some of the South African gold mines the cage drops straight down for a few thousand feet, then runs on rails for a few more hundred feet down a slope, and may even finish the journey down a second vertical shaft some distance from the first.
The United States are generally regarded as the original home of the passenger elevator, and to-day that country boasts the loftiest lift shafts in the world. The first elevator is accredited to Henry Waterman, who in 1850 installed a “platform hoist” operating between two floors, in New York. Lifts of the Waterman type were manufactured about that time by George H. Fox and Company, of Boston, for use in all parts of the States. In 1852 Elisha Graves Otis began the manufacture of elevators at Yonkers, in New York State. Two years later he introduced the first safety device capable of stopping the lift in the event of the rope breaking. The first passenger elevator was installed by Otis in New York in 1857. A remarkable type of lift was introduced two years later in New York by Otis Tufts. This lift was known as the Vertical Railway and it was operated by a huge vertical screw (described on page 500), reminiscent of the screw height-adjustment on an old-fashioned music stool. Screw lifts are used to-day for disappearing organ consoles in cinemas.
First Electric Elevator
These early elevators were operated by steam engines, and the first steam-driven passenger elevator installed in an office building made its appearance in New York in 1869. The governor device, controlling the speed of lifts automatically, was invented by Charles R. Otis in 1878.
The steam-driven lift held an undisputed position until the early ’seventies of last century, when hydraulic power was first developed for lift operation. For nearly twenty years the hydraulic lift, in its turn, knew no rival. But increasing knowledge of electricity and its possibilities were to bring further changes and the era of the modern electric lift began.
The first electric elevator was installed in 1887 by William Baxter, Junior, in Baltimore, but the honour of installing the first of New York’s highly efficient elevators goes to Otis Brothers and Company, with an elevator which, installed in 1889, was destined to give thirty-five years’ satisfactory service. Improvements followed in rapid succession. Steam-operated lifts are things of the past, although for the winding of mine cages the steam engine is still in common use. The field of lift engineering is thus shared by hydraulic and electrical power.
Hydraulic power is derived from water under pressure, although some hydraulic lifts make use of the, weight of water alone. Among such lifts are many of those to be found at seaside resorts. These lifts are generally arranged in pairs and run on steeply inclined parallel tracks secured to the face of the cliffs. The weight of one lift counterbalances that of the other. Beneath either lift is a large tank to hold water. When one lift is at the cliff top, a pipe discharges water into its tank, while the water under the lower lift is allowed to flow away into the sea. When the brakes are released the water - ballasted lift descends the cliff and hauls up the other lift with its empty tank. The weight of the passengers ascend-i n g is approximately balanced by that of the people going down. The weight of water in the tank of the descending lift is, however, more than sufficient to haul up a full lift even should there be no passengers making the downward journey. Modern hydraulic lifts make use of water under pressure and are of two main types. In the building of one type, using the direct-acting system, a shaft is sunk into the ground, and in this is fixed a long cast-iron cylinder. Within the cylinder is a large steel rod or ram. To the upper end of the ram is attached the platform or floor of the lift, which runs between vertical guide rails fixed inside the lift shaft or well. The upper end of the cylinder is fitted with a gland and gun-metal bushes that surround the ram and prevent any leakage of water. A control valve is fitted to the cylinder to regulate the flow of water, and this is operated by a rope or rod, arranged vertically in the lift shaft. Openings are provided in the roof and floor of the lift through which the rope passes.
AN ELECTRIC PASSENGER LIFT of the geared unit multi-voltage type. The power unit (top) is an electric motor which drives the winding drum through the medium of suitable gearing. The ropes, of which there are several to ensure safety, pass round the drum and are directly connected to the lift at one end and to the balance weight at the other. The first electric lift was installed in 1887.
To send the lift on its upward journey the operator pulls on the rope, the valve admits water at high pressure to the cylinder and the ram with its load is forced upwards. On the downward journey the rope is pulled in the opposite direction and the valve closes to the water supply and opens to the waste pipe. The weight of the descending lift forces the water out of the cylinder, which is then ready for another ascent. The operation of the valve is so arranged that it is necessary only to grip the rope, when travelling either up or down, to stop the lift. The rope is also fitted with “bobbins” (or tappets, if a rod is used) to stop the lift automatically at the limits of its travel.
Hydraulic Power Mains
The other hydraulic lift system in common use employs a similar cylinder and ram, but the ram is not attached directly to the lift. Instead it is fixed to a framework containing a number of pulleys. A similar pulley frame is attached to the roof of the lift shaft, and both systems of pulleys are joined by an endless steel cable passing over sheaves leading down to the roof of the lift. This is the system that is used also for the operation of hydraulic cranes. Hydraulic lifts are counterbalanced by heavy iron weights that slide up and down in the lift shaft and are attached to the lift roof by a cable passing over an overhead pulley.
Water from the ordinary street mains is not obtainable at a sufficiently high pressure to operate lifts or any other machinery. Lifts (and cranes) are worked by water at a pressure of 750 to 1,000 lb. a square inch, supplied by special hydraulic mains. Many of the world’s large cities are equipped with hydraulic power mains. In London alone there are more than 200 miles of hydraulic mains. Pressure is maintained by an ingenious device known as an accumulator, which must not be confused with the electric accumulator or storage battery of the motor car or radio set.
The hydraulic accumulator comprises an enormous cylinder containing a ram, and resembles in some points the smaller cylinder and ram used for lifts. Attached to the top of the ram is the upper end of a huge hollow iron cylinder which encircles the ram cylinder in the same manner as a collar. Concrete, pig iron, or some other form of heavy weight is packed into the accumulator cylinder between its central tube and the outer walls. Water is pumped into the ram cylinder by steam or internal combustion engines and gradually the great “collar” rises and exerts tremendous pressure on the ram. The water supply from the pumps is fed into the cylinder through a non-return valve and pumping ceases automatically when the accumulator is fully loaded with the ram at the top of its stroke. In practice the engines in the accumulator house are at work continuously and the columns of water in the hundreds of hydraulic mains transmit the stored power to lifts, cranes, or other machinery.
The pumping of water into the accumulator-ram cylinder is sometimes accomplished by electric pumps and the use of electricity is applied in other ways to the operation of hydraulic lifts. In some types the operating valve is worked by a system of electric push buttons and in most hydraulic lifts the lighting is electric from power supplied by a trailing cable suspended under the floor. Electrically operated safety grips are sometimes fitted to hydraulic lifts. These interesting devices generally comprise a spring clamp that grips the valve rope. Release of the rope for starting the lift is permitted only when a sliding gate has been shut, thus completing an electrical circuit that releases the clamp.
One of the most remarkable instances of electrically controlled hydraulic power is to be found at the Savoy Hotel, London. The installation is not strictly a lift. Its function is to raise a whole dance floor. The hotel management gives cabaret shows in the restaurant, and because this form of entertainment appeals to the eye as well as to the ear, people in the corners of the vast room were wont to jump on chairs, or even tables, to obtain a better view. The management decided that feasting could not be interrupted in this manner and that all should have an equal opportunity of watching the cabaret in comfort. It was decided that a section of the restaurant dance floor, 40 feet by 20 feet, should be made capable of being raised rapidly to a height of 2 ft. 3 in. above the main floor and as speedily lowered for dancing.
The engineers decided that here was a task for the hydraulic ram, but they were faced at the outset by one big difficulty. Immediately below the restaurant was the marvellously decorated ceiling of the Banqueting Hall and this could not be touched. There was no possibility of building an immense girder platform operated by one huge hydraulic ram. The alternative was the use of eight smaller short-lift rams spaced conveniently under the steel platform of the dance floor. The cast steel ram cylinders were secured in as low a position as possible to the girderwork of the restaurant main floor. The level of the main floor was then built up to conform to the lowest position of the moving platform.
The ram heads were attached to the platform framework by flexible joints and, because no guiding system was permissible, the stability of the whole structure was entirely dependent on the rigidity of the rams. This dependence on the rams for stability brought about the second big difficulty that beset the engineers at the Savoy. It was impossible to operate these rams directly from the hydraulic main. With a perfectly balanced platform and equal pressure in each of the ram cylinders a satisfactory action would have been possible. But with a grand piano and big bass drum at one end of the platform and only a fairy-footed dancer at the other end the weight would be distributed unevenly on the rams. The speed of the lightly loaded rams would have exceeded that of those heavily stressed, with disastrous consequences to the platform.
The difficulty was overcome by installing, three floors down from the restaurant, a special hydraulic machine. This machine comprises a group of eight rams, the cylinders of which are respectively connected by some hundreds of feet of high-pressure piping to the eight ram cylinders of the dance floor platform. The eight machine rams are all attached to the massive head of a huge main ram which is worked by the hydraulic main. Consequently when the main ram valve is opened the great crosshead forces the eight machine rams downwards and their simultaneous action is transmitted to the rams of the platform. The hydraulic valve of the main ram is worked by an electric motor controlled by a push button from the restaurant. Once the “up” or “down” button has been pushed the dance platform moves automatically to the limits of its travel, but it can be stopped at any intermediate position by the operation of a third push button. When the platform is raised the exposed undersides are protected by shutters that fold away under the floor when the structure is lowered.
CONTROLLER FOR A DIRECT CURRENT LIFT, here operated by a car switch which is connected to the controller by a trailing cable hanging in the lift shaft under the car. Electro-magnetic relays govern the direction and speed of the motor at the will of the attendant in the car.
Large hydraulic lifts, however, have other uses and those in modern aircraft carriers provide an interesting contrast. Some of the aircraft carriers in the British Navy were originally intended for service as cruisers but, to conform to the provisions of the Washington Naval Treaty, were not completed as designed. The hydraulic equipment installed for the working of heavy guns was retained for operating the lifts that feed the flight decks with aircraft for service with the Fleet.
The operation of electric lifts involves the use of many ingenious devices that have been evolved by the electrical engineer. It is not possible to describe all the types and combinations of types of lift or elevator in use to-day, but the general principles of their operation are similar in most instances. The arrangement of shafts, with landing doors, guide rails and counterbalance weights, is the same for electric as for hydraulic lifts. There are two main types of electric power units for lift operation and both can be run on either direct or alternating current. All lifts were formerly operated by direct current, but the use of alternating current was introduced for the purpose late in the nineteenth century.
One type of power unit comprises an electric motor that drives a winding drum through the medium of gearing. The other type, introduced in 1904, is gearless, consisting of an electric motor, on the armature of which a brake drum and a winding drum are secured directly.
One of the most important components of the power unit is the main brake. This generally comprises a pair of C-shaped shoes which are normally held together by a powerful spring with the brake drum between, nutcracker-fashion. Attached to one of the shoes is an electro-magnet which, when energized, forces the brake shoes apart by the action of a plunger. In the event of current failure the brake is applied automatically. In addition to the main brake, a magnetic brake is fitted for the purpose of slowing down the lift at landings. The magnetic brake consists of a rotor revolving in a strong magnetic field. Eddy currents are set up in the rotor, which is thus retarded. This feature is used only on alternating-current equipments, when the motor is not capable of a reduction in speed before stopping. Dynamic braking is generally used with direct-current machines, except in the smaller sizes. An important attribute of the modern lift is its ability to stop with the floor dead-level at all landings, and this is accomplished in a number of ways. In the modern lift the injunction to “mind the step” is unnecessary. Levelling is done automatically and is not dependent on the skill of the attendant. In one system, applicable to lifts of almost any type, a vertical ramp is fixed in the lift shaft below each ceiling. On the roof of the lift is fixed a levelling switch operated by rollers that make contact with the ramps. The switch is connected with the power unit by a cable attached to the lift.
An ingenious feature of the design provides that the rollers shall come into play only on approaching the landing at which the lift is intended to stop. The release of the attendant’s control handle, for the purpose of stopping, automatically brings the rollers into action. With this system there is no clicking of rollers as the lift speeds past intermediate floors.
There is, however, a levelling system worked on the “induction” principle. Attached to the lift is an “inducer box”, comprising a series of electric coils, projecting in hat-rack fashion over the side of the roof.» In place of the lift-shaft ramps are some staggered iron plates that pass in turn between the inducer coils. Some of the coils are energized by alternating current which induces current in neighbouring coils. When a coil passes an iron plate (it does not touch it), the induced current is cut off and the magnetic relays of the main controller come into operation to slow down the lift and stop it at the floor required. The induced current from the coils is passed through an amplifier, somewhat resembling those used for radio work, to the main controller. Safety gear is essential in all lifts, to prevent the car from falling to the bottom of the shaft in the event of derangement of the ropes, gear, brake or controller. One type of safety gear consists of a framework, attached to the underside of the lift, containing two screw-operated levers which are capable of being pressed in contact with the lift guides. A centrifugal governor is fixed in the top of the lift shaft.
The governor is driven by a steel wire rope attached to the lift. As long as the speed of the lift is within predetermined limits the rope continues to drive the governor and all is well. Immediately the speed of the lift becomes excessive, the governor operates a trip that causes its own driving rope to be held fast. Instantly a strain is imposed by the falling lift on the governor rope which pulls on the safety gear, wedges the gripping levers against the guides and screws them tightly outwards, thus bringing the lift to rest.
SECTIONAL VIEW OF AN ESCALATOR, showing the way in which the treads are arranged as a continuous chain, the formation of which is changed by the guide rails running beneath the stairway. The driving apparatus is housed in a machine room below the floor near the top of the escalator.
Other systems are adopted to provide safeguards for the lift gates and landing doors. Gates are provided with electro-mechanical locks that make it impossible for a lift containing a passenger to move unless its own gate and all landing doors leading into the shaft are securely closed. Landing doors also are provided with electrical locks that make it impossible to open any door except that opposite which the lift has stopped.
The doors of automatic lifts, requiring no attendant and worked by electric push buttons, are sometimes operated electrically or by compressed air. To guard against any possibility of a passenger becoming trapped between closing doors, a ray of light is sometimes used as a safeguard. An electric light contained in a metal tube is attached to one side of the lift doorway at about shoulder height. The light is focused across the doorway on to a prism, whence the beam is reflected downwards to another prism at knee height. The second prism reflects the beam of light back across the doorway to a photoelectric unit. As long as the beam of light illuminates the light-sensitive cell in this unit, a current (amplified by a thermionic valve) flows and permits normal operation of the door controls. Any obstruction in the path of the beam, however, causes the doors to remain open or, if they are in the act of closing, will cause them to reopen.
Modern lifts are operated either with or without an attendant. The various lift control systems that have been involved since the introduction of push buttons appear to work on the principle that the simpler the array of push buttons, the more amazingly complicated is the control apparatus. The control gear, generally placed near the power unit, consists of a wonderful collection of relays, contactors and switches which link the push buttons in the lifts and on the various landings with the levelling switches on the lift roofs and the control switches in the winding motor room.
The following system of lift control may be regarded as typical of the most up-to-date practice, but it is cap able of considerable modification. The power unit consists of a geared or a gearless direct-current motor supplied by current from a generator that also supplies current at a low voltage to work the relays of the control system. The generator is driven by a motor taking its supply from the power mains. The function of the control gear is to govern the voltage supplied by the generator to the winding motor. This ingenious arrangement permits high speeds for the lift cars and provides rapid acceleration. Deceleration is independent of the load, and lift cars are brought rapidly to rest. The usual levelling switch and safety gear are fitted to each car.
When the lift is controlled by an attendant, a switch handle is moved to the “up” or “down” position and the car starts. When the handle is released the switch returns to its central position, and the lift automatically slows down and stops at the next floor level in the direction of its travel. The use of the inducer box in carrying out this operation has been explained above. The attendant stops the lift at various floors in accordance with signals given by an indicator in the car. This indicator shows at a glance the floors on which waiting passengers have pressed a button calling the lift.
Such lifts are often dual-equipped so that they can be worked by the passenger at times when the volume of traffic does not warrant the services of an attendant; these lifts are automatic. It is necessary only to press a button to be carried to the floor required. At the side of the lift door is a panel with a push button representing each floor of the building. A person wishing to travel to the sixth floor presses the button numbered 6. In a moment the lift, with its gate open, appears before the door, which it unlocks. If the door is not power-operated the passenger closes the door behind him. Having entered the lift, he has stepped on a floor switch that necessitates the closing of the lift gate - a precaution insisted on by the electrical interlocking gear only when there is a passenger in the car. The car starts on its upward journey, but on reaching, say, the third floor it stops to collect a passenger. Just after it has restarted, this passenger presses an “up” button, say No. 5. Automatically the lift has stopped for him and automatically it stops to enable him to alight at the fifth floor. If another passenger, on the fourth floor for instance, presses the “ground floor” button while the lift is going up, the lift takes no notice. It takes its passengers up to the sixth floor and then goes down, collecting the passenger from the fourth floor and landing him at the ground floor.
A LARGE SAFETY BRAKE of the type fitted to modern escalators. The huge shoes are held clear of the drum, which is attached to the main driving shaft, by electro-magnetic means. Should the current fail, powerful springs immediately apply the brakes, which are capable of stopping a fully-loaded escalator within a few inches.
Once inside the lift the passenger may change his mind as to his destination, because there is a duplicate set of push buttons inside the car. This “storing” and answering of passengers’ calls by the controller of a system of lifts is not the limit of its capacity. The amazing mechanism incorporates devices that interconnect one or more lifts so that they share the pressure of traffic during the rush hours. The control gear can also be arranged to return a lift car, after a predetermined interval from its last use, to any floor where its services are most likely to be required.
A modern refinement in the operation of lifts is known as standard signal control. Signal control has many advantages where the lift service is intensive enough to demand two or more lifts operated by attendants.
As each passenger enters the car, he tells the attendant the floor to which he wishes to travel. The attendant presses the corresponding button in the car and throws the operating lever to the “start” position. The lift then ascends, stopping at the first of the floors for which a call has been registered; the doors open automatically.
Only two buttons are provided on each landing, one for passengers going up and the other for passengers going down. When the button is pressed, the nearest car travelling in the required direction stops automatically to take up the waiting passenger.
Above each lift entrance are fixed “up” and “down” arrow indicators, arranged to light in advance of the car, so that the waiting passenger is informed which lift is arriving in answer to his call.
The importance of these refinements and of safety gear and high speeds will readily be appreciated when a lift system is considered such as that in the Empire State Building in New York City (see the chapter “The World’s Highest Buildings”). The vast skyscraper is 1,250 feet high. The elevators are called upon during rush periods to transport 15,000 people in one direction in thirty minutes. Some of these journeys are to the eighty-sixth story, a height of 1,045 feet. Above this other elevators go to the top of the secondary tower. The peak load is 2,400 people in five minutes. There are fifty-eight main passenger elevators running at 1,000 feet to 1,200 feet a minute. Two auxiliary elevators in the secondary tower and another in the mooring mast run at 500 feet a minute. There are also six freight elevators with speeds ranging from 250 feet to 700 feet a minute. The total length of the elevator shafts is seven miles and there are 1,239 entrances at the landings. About 120 miles of steel wire rope are used.
First Appearance in London
In striking contrast to the elevators of New York are the non-stop escalators of London. The escalator as well as the elevator originated in the United States. The first passenger escalator was built by the Otis Elevator Company and installed as a side show at the Paris Exposition of 1900. Some months later an escalator made its appearance in London, at the Crystal Palace, where all and sundry could try out the “wonderful moving staircase” at a penny a trip. The first escalators to be built in London for public service were installed at Earls Court Station in October 1911. These escalators were of the “step-off-sideways” type, and gave twenty-five years’ service.
THE MACHINERY CHAMBER for the automatic lifts at Goodge Street Station, London. This illustration gives an excellent impression of the complexity of the control gear, as well as of the massive proportions of the winding mechanism.
The modern escalator with cleated steps and a metal comb at the landing was finally evolved. The cleated escalator comprises a series of steps, each complete in itself. A step is a small truck or trolley consisting of a wedge-shaped framework. The wedge may be regarded as resting on one of its faces with the thin edge pointing forward. On the upper face are a number of wooden strips, pointing in a fore-and-aft direction. These strips form the tread, and they pass between the prongs of steel combs. Should a passenger neglect to step off the escalator on reaching the landing he is “combed” off without any difficulty or danger. The four lower corners of the stair frame are provided with wheels. Those at the back are set close to the frame, but the front wheels are spaced more widely apart. Front and rear wheels run on separate but parallel tracks, so that the treads are maintained in a level position. On approaching the landings the tracks on either side of the stairway are so arranged that the stair treads are brought level with one another for a short distance before entering the comb. Having passed beneath the comb the stairs continue their journey along the tracks, which are carried back beneath the escalator to form a continuous path. The system of stairs somewhat resembles a giant chain, carried on large wheels at the top and bottom of the escalator.
Flanking the stairway are two parallel balustrades, on the top of which run rubber handrails that move in conjunction with the stairs. The axles of the front wheels of the stairs are joined by an endless chain and this transmits the power from a main driving motor of 20 to 150 horse-power, according to the size of the escalator. The power motor is generally housed in a machinery room under the floor at the top landing.
Many safety devices are incorporated in the design of the escalator. A large brake, acting on a drum attached to the main driving shaft, is capable of stopping the escalator within a few inches when it is fully loaded and running at top speed. Another brake is fitted to the main driving gear. Both brakes are applied by powerful springs held off electrically so that the machine stops immediately should the current fail.
Other safety devices include a governor to control the speed, a “broken handrail detector” and a non-return device to ensure that the escalator shall run in its correct direction. Modern escalators can be run in either direction, a most important consideration in London with its complete reversal of rush-hour traffic. For this reason, too, it is usual to arrange escalators in groups of three machines, two for the rush traffic, up or down, and the third for the use of passengers travelling against the stream. All escalators are provided with hand-operated emergency stop switches resembling fire alarms.
Cruising Speed for Slack Periods
The longest passenger escalators in the world in regular use are three of those at Leicester Square Underground Station in London. Each has a vertical rise of 80 feet and a stairway length of 160 feet. London Transport escalators are inclined at an angle of 30 degrees with the horizontal.
Escalator speed is more a matter of psychology than engineering. The maximum speed of London’s escalators is 180 feet a minute, approximating to two miles an hour. It has been found, however, that if escalators are run at maximum speed people stand on them instead of walking. Passengers who walk increase an escalator’s capacity by as much as 50 per cent, so the transport authorities run certain escalators at less than maximum speed to encourage walking. At some of London’s stations, however, this rule does not apply. The passengers are mainly City business men, often carrying heavy brief bags. These people dump their bags on the stairs and stand and wait. Their escalators are run at full speed.
There is one other matter relative to speed that is worthy of mention. At stations where there are long slack periods between rush hours, it is economical to run the escalators at what is known as a cruising speed, about half normal speed, a crawl scarcely welcome to the stray passenger in a hurry. However, directly the passenger steps on to the escalator he breaks a beam of light similar to that used as a lift door safety device and described earlier in this chapter. Imperceptibly the rate of travel increases automatically to top speed. When the passenger steps off the escalator the machinery drops down to cruising speed again unless the light ray has been broken by another passenger, in which event top speed is maintained.
THE TREADS OF AN ESCALATOR at its turning point. The escalator illustrated is at Wood Green Station, London, and is of the modern “comb” type. When the treads have passed the comb on which the passengers are set down, the treads continue their journey along tracks which are carried back beneath the escalator to form a continuous path.