Elevator: device for vertical transportation of passengers or freight to different floors or levels, as in a building or a mine.
The term elevator generally denotes a unit with automatic safety devices; the very earliest units were called hoists. Elevators consist of a platform or car traveling in vertical guides in a shaft or hoistway, with related hoisting and lowering mechanisms and a source of power. The development of the modern elevator profoundly affected both architecture and the mode of development of cities by making many-storied buildings practical.
Although hoists and primitive elevators operated by human and animal power or by water wheels were in use as early as the 3rd century BC, the modern power elevator is largely a product of the 19th century. Most elevators of the 19th century were powered by steam engines, either directly or through some form of hydraulic drive.
In the early 19th century, hydraulic plunger elevators were used in some European factories. In this type, later used to some extent in the United States and more extensively elsewhere, the car is mounted on a hollow steel plunger that drops into a cylinder sunk into the ground. Water forced into the cylinder under pressure raises the plunger and car, which fall by gravity when water is released. In early installations the main valve controlling the flow of water was operated by hand by means of ropes running vertically through the car; lever control and pilot valves regulating acceleration and deceleration were later improvements.
A forerunner of the modern traction elevator was in use in Great Britain in 1835. In this case the hoisting rope passed over a belt-driven sheave, or pulley, to a counterweight traveling in guides. The downward pull of the two weights held the rope tight against its sheave, creating sufficient adhesive friction, or traction, between the two so that the turning sheave pulled the rope along.
The history of power elevators in the U.S. began in 1850, when a crude freight hoist operating between two adjacent floors was installed in a New York City building. In 1853, at the New York Crystal Palace exposition, the American inventor and manufacturer Elisha Otis exhibited an elevator equipped with a device called a safety to stop the fall of the car if the hoisting rope broke. In this event a spring would operate two pawls on the car, forcing them into engagement with racks at the sides of the shafts so as to support the car. This invention gave impetus to elevator construction.
Three years later the first passenger elevator in the U.S., designed by Otis, was installed in a New York City store. In these early elevators, a steam engine was connected by belt and gears to a revolving drum on which the hoisting rope was wound. In 1859 an elevator raised and lowered by a vertical screw was installed in the Fifth Avenue Hotel in York City. In the 1870s the rope-geared hydraulic elevator was introduced. The plunger was replaced in this type by a relatively short piston moving in a cylinder that was mounted, either vertically or horizontally, within the building; the effective length of the stroke of the piston was multiplied by a system of ropes and sheaves. Because of its smoother operation and greater efficiency, the hydraulic elevator generally replaced the type with a rope wound on a revolving drum.
The electric motor was introduced in elevator construction in 1880 by the German inventor Werner von Siemens. His car, carrying the motor below, climbed its shaft by means of revolving pinion gears that engaged racks at the sides of the shaft. An electric elevator was constructed in Baltimore, Maryland, in 1887, operated by an electric motor turning a revolving drum on which the hoisting rope was wound. Within the next 12 years, electric elevators with worm gearing connecting the motor and drum came into general use except in tall buildings. In the drum elevator the length of the hoisting rope, and therefore the height to which the car can rise, are limited by the size of the drum; space limitations and manufacturing difficulties prevented the use of the drum mechanism in skyscrapers. The advantages of the electric elevator, however, including efficiency, relatively low installation costs, and virtually constant speed regardless of the load, spurred inventors to search for a way of using electric motive power in skyscrapers. Counterweights creating traction on electrically driven sheaves solved the problem.
Since the introduction of electric motive power for elevators, various improvements have been made in motors and methods of control. At first, single-speed motors only were used. Because a second, lower speed was desirable to facilitate leveling the car with landings, low-speed auxiliary motors were introduced, but later several systems were devised for varying speed by varying the voltage supplied to the hoisting motor. In recent years devices for automatic leveling of cars at landings are commonly used. Originally the motor switch and the brakes were operated mechanically from the car by means of hand ropes. Soon electromagnets, controlled by operating switches in the car, were introduced to throw the motor switch and to release a spring brake. Push-button control was an early development, later supplemented by elaborate signal systems.
Safety devices have been highly developed. In 1878 Charles Otis (1835-1927), a son of the inventor of the original car safety, introduced a similar mechanism connected to a speed governor that applied the safety if the car was traveling at a dangerous speed, whether or not the rope broke. In later car safeties, clamps were used to grip the guide rails so as to bring the car to a stop gradually. Today so-called governors control a series of devices to slow down the car if it is speeding only slightly, to shut off the motor and apply an electromagnetic brake if the car continues to accelerate, and then to apply the mechanical safety if the speed becomes dangerous. Terminal switches independent of other controlling mechanisms stop the car at the upper and lower limits of travel. For low-speed cars, spring bumpers are provided at the bottom of the hoistway; high-speed cars are buffered by pistons fitting into oil-filled cylinders. Electric circuits, completed by contact points in hoistway doors on various floors and in car gates, permit operation only when gates and doors are closed.
The great advances in electronic systems during World War II resulted in many changes in elevator design and installation. Computing equipment was developed for compiling automatically information that vastly improved the operational efficiency of elevators in large buildings. The equipment, which became available in 1948, made possible the solution of such scheduling problems as morning and evening peak loads and traffic balance and the elimination of operators. The use of automatic programming equipment eventually eliminated the need for starters at the ground level of large commercial buildings, and thus the operation of elevators became completely automatic. Automatic elevators are now generally employed in all types of buildings. The World Trade Center in New York City, with its two 110-story towers, had 244 elevators with carrying capacities of up to 4536 kg (10,000 lb) and speeds of up to 488 m (1600 ft) per min. The 110-story Sears-Roebuck Building in Chicago has 109 elevators with speeds of up to 549 m (1800 ft) per min.
The next generation of elevators currently being tested involve fiber optics, artificial intelligence and self propelled elevators that contain their own motors.