Otis's elevators were steam powered; the elevator car was moved by a system that used a belt-driven winding drum. The first electrically driven elevator was installed in a New York City office building in 1889, presaging the skyscraper era. Improvements have come with smooth rides, even while accelerating and decelerating; increased speed; and automatic leveling at all floor landings. Stringent laws assure maximum safety of riders and equipment.
High-speed elevators for tall residential or office buildings range in speed from about 140 to 550 m (460 to 1,800 ft) per minute. The higher the building, the greater the speed. Elevator speed determines the type of machinery and the complexity of controls. For speeds greater than 45 m (150 ft) per minute, cable-operated elevators are used. A number of heavy steel cables pass over a cylindrical drum above an open shaft running the occupied height of the building. The elevator car, an enclosed cab, is fastened at one end of the cable. At the opposite end is a counterweight made of cast-iron blocks. Both cab and counterweight are guided within carefully machined and fitted vertical rails.
The cab door matches doors in the shaft wall at every level at which the elevator is programmed to stop. Push-button controls select the destination level and summon the elevator from outside the shaft. In large buildings, computer software controls the complex system of elevator responses to passenger calls.
Safety features prevent elevator operation in case any shaft door or the cab door is open, in case of excessive speed, or in case of overtravel above or below the designed limits. A fail-safe, magnetically operated brake clamps the drum whenever the car is stopped, either in routine operation or in an emergency. Finally, in the event of cable breakage or other serious aberration, automatic clamps on the cab itself engage the guide rails and prevent the car from falling. In the lower speed ranges of cable-driven elevators, the driving motor may rotate the drum through gears. At the higher speeds gear noise would prove objectionable; therefore, gearless direct drive is used.
For buildings that are not very tall, and where elevator speeds of less than 45 m (150 ft) per minute are judged efficient, a hydraulic rather than a cable-and-drum mechanism is used. The cab is supported on top of a steel piston enclosed in a cylinder that is sunk into the ground to a depth equal to the rise of the elevator. To raise the car, a pump forces oil into the cylinder, which moves the piston upward. When the car descends, valves control the release of oil from the cylinder.
Ever-higher skyscraper construction will require new technologies to achieve even higher speeds. Some buildings now operate double-deck elevator cars, where two or more cars occupy the same shaft, move in the same programmed direction, and serve different floors simultaneously. Pressurized sky lobbies every 75 stories can relieve the ear-pressure problems caused by too-rapid ascents and descents. They also allow passengers to switch from express to local cars. The future may hold the development of the ropeless elevator and of laterally as well as vertically moving cabs.
