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Elevator

Elevator

Background

An elevator is a platform, either open or enclosed, used for lifting people or freight to upper floors within a building. Elevators are a standard part of any tall commercial or residential building. In recent years, the introduction of the Federal Americans with Disabilities Act has required that many two-story and three-story buildings be retrofitted with elevators.

Manually operated elevators were used for lifting freight in warehouses and manufacturing plants as early as the 1600s. The modern elevator is a direct descendant of a design first shown by Elisha G. Otis at the New York World's Fair in 1853. A notable feature of the Otis elevator, and the principal reason for its popular acceptance, was a safety device that immediately engaged and held the elevator in the event the hoisting cables broke. The first elevators were operated by steam power to turn the cable drums. In 1871, the first hydraulic elevators were introduced using water pressure as the source of power. At first the hydraulic rams were one-piece, which meant a hole had to be dug under the elevator shaft as deep as the elevator was to be high. Later multiple-section, telescoping hydraulic rams allowed shallower holes. In many cities hydraulic power for these early elevators was supplied by power companies which installed and maintained networks of hydraulic piping throughout the city. The first commercially successful electric elevator was installed in 1889, and electricity quickly became the accepted source of power.

Electric-powered elevators offered two significant advantages. First, electric power was clearly becoming universally available, and any building likely to be equipped with an elevator would also have electric power. Second, hydraulic elevators were severely limited in the height to which they could rise, while electric elevators, using a simple cable and pulley system, had virtually no height limit. For many years, electric elevators used either direct current (DC) motors or alternating current (AC) motors. Today, almost all elevators use one of two types of AC motors: the most common are geared motors for elevators moving at speeds up to 500 feet per minute (153 m per minute), while direct-drive motors are used for elevators moving at higher speeds. Some modern high-speed elevators move at up to 2,000 feet per minute (610 m per minute).

Control systems on early elevators required human operators to regulate the speed of the lift and descent, to stop the elevator at each floor, and to open and close the doors. In the 1950s automatic pushbutton control systems replaced manual controls. In the 1970s electromechanical controls were gradually replaced with solid state electronic controls.

Riding in a small box hundreds of feet in the air would be a disconcerting experience if one were not convinced of its safety. Electric elevators are equipped with two primary safety mechanisms: a governor which controls the elevator's speed by controlling the speed of the cable pulleys, and the emergency brake which consists of jaws that grip the elevator guide rails in the event the cables break. Elevators also include electromechanical door interlocks to prevent the elevator from operating if the door is not completely closed and to protect passengers from being trapped by the closing door. The same door interlocks also prevent the outer doors on each floor from opening if the elevator is not present. Most elevators are equipped with a telephone, and sometimes a trap door in the ceiling, so that passengers can call for help or escape if an elevator becomes stuck between floors.

Design

Elevators themselves are simple devices, and the basic lifting systems have not changed much in over 50 years. The control systems, however, have changed sub-stantially to improve safety and speed of operation. Elevators are designed for a specific building, taking into account such factors as the height of the building, the number of people traveling to each floor, and the expected periods of high usage.

Most elevators use counterweights which equal the weight of the elevator plus 40% of its maximum rated load. This counter-weight reduces the weight the motor must lift and ensures that the elevator cannot fall out of control while the cable is intact. In a lifting drum installation, a hoist cable runs down from a drive drum attached to the hoist motor, around a large pulley on the top of the elevator, up to a second pulley hanging from the roof of the elevator shaft, and down again to the counterweight. In a traction drum installation, the cable runs from the elevator, up and once around a drive drum attached to the hoist motor, then back to the counterweight. The elevator, called the car, and the counterweight each run in their own sets of guide rails. A second governor cable runs from the car up to a governor pulley, then down to a tension pulley at the bottom of the elevator shaft, and up to the car again. This cable rotates the governor pulley at a speed directly proportional to the speed of the car. In the event of excessive car speed, the governor uses another cable to activate the emergency brake jaws which grip the guide rails and slow the car to a stop.

A ramped bar on the side of the elevator shaft activates a series of switches on the outside of the car to slow and stop the car at the proper floor. As the car approaches the desired floor, the ramp activates the slow-down switch, which signals the hoist motor to reduce speed. When the car is aligned with the outer door opening, the ramp activates a limit switch to stop the car. If the door interlock switches also sense that the car is in the proper location, the electric door opening motor is activated to open both the inner car door and the outer floor door.

Modern commercial buildings commonly have multiple elevators with a unified control system. The object of the control system is to minimize the average time any passenger spends from the time the elevator call button is pushed to the arrival of the first available elevator. Different systems use different levels of sophistication. The simplest systems use a single up and down button on each floor regardless of the number of elevators. When a passenger calls for an elevator, the controller sends the nearest elevator that is traveling in the desired direction. The approach of an elevator car is signaled by an illuminated arrow above the elevators doors pointing up or down.

In more sophisticated systems, the controller monitors the elevator call system for a set, or bank, of elevators operating side by side. The operation zone of these elevators is divided into sectors, with each sector being made up of adjacent floors. When a car has answered a call and completed the designated run, it becomes available to answer another call. At this point, depending on the controller's programming, the car may be returned to a designated "home" floor, or may be sent to the sector furthest from other operating or available cars to cover that sector. When a call is received, the controller automatically compares the location of all the cars in the bank and sends the nearest one.

Controllers can also be programmed to respond differently at different times of the day. For example, the elevator controller in a busy office building will receive a preponderance of calls from the ground floor in the morning, when workers are arriving and need to go to their workplaces on the upper floors. In that case, the controller will be programmed to send all unassigned cars to the ground floor, rather than have them return to a home floor in their sector. Later in the day, a different set of instructions can be used to send unassigned elevators to different sectors, since passengers leaving the building will be much more evenly distributed among the floors than in the morning.

All modern elevators also have special override controls that firefighters can activate with a key to make elevators go directly to a specific floor without intermediate stops.

Raw Materials

The elevator car itself is constructed with a steel framework for durability and strength. A set of steel beams above the car, called the crosshead, span the elevator shaft from side to side and hold the pulley for the hoist cable. A steel structure, called the sling, extends down the sides of the car from the crosshead and cradles the floor, or platform. The sides of a passenger elevator car are usually made from steel sheet and are trimmed on the inside with decorative paneling. The floor of the car may be tiled or carpeted. Handrails and other interior trim may be made from stainless steel for appearance and wearability. A suspended ceiling is usually hung below the actual top of the car and may contain fluorescent lighting above plastic diffuser panels. The elevator controls, alarm buttons, and emergency telephone are contained behind panels in the front of the car, next to the doors.

Steel guide rollers or guide shoes are attached to the top and bottom of the sling structure on each side to run along the guide rails. The guide rails are also steel and are attached to the interior walls of the elevator shaft which runs from the top of the building to the bottom. The emergency brake mechanism consists of two clamping faces which can be driven together by a wedge to squeeze on the guide rail. The wedge is activated by a screw turned by a drum attached to the emergency cable.

The elevator is one of those inventions whose "ripple effect" is often overlooked. Just think of the practicality of any building over eight or ten stories without an elevator. Then imagine a modern city without buildings over ten storiesl Along with structural steel and reinforced concrete, the elevator was essential to the development of the modern skyscraper and thus to the common form of the modern urban center.

The elevator's practical impact was almost matched by its symbolic impact. The 1880$ were years of immense urban growth, and the influx of newcomers to the cities included middle-class career people as well as factory workers. With property values skyrocketing in the cities, the middle-class families could not afford single family homes. Apartment building owners promoted apartment living with advertisements of "high-tech" amenities: hot and cold running water, telephone systems, central gas for cooking and lighting, fully equipped bathrooms, and elevators.

Moreover, with all these modern conveniences, apartment living captured the middle-class imagination as the embodiment of a new organization of domestic duties. Buildings came with centralized heating, ventilating, and plumbing systems; some had kitchens in the basement which would prepare food for individual apartment dwellers; some even had a centralized vacuum system with nozzles in each room connected to a pump in the basement.

The elevator was even extolled as a contributor to democracy. In an elevator-equipped building, it mode little difference which floor one lived on; every floor was equally accessible. By contrast, in Europe, wealthy families were generally found on the middle floors where they did not have to climb many flights. Poorer families were usually confined to the basement or the upper floors.

William S. Pretzer

Elevator hoisting cable usually consists of six or more strands, each of which consist of a number of separate steel wires. The strands may be twisted around a hemp center which serves as a cushion and also contains a lubricant.

The electric hoisting motors are specifically designed for elevator service and may drive the hoisting drum through a gearbox, both of which are purchased parts.

The Manufacturing
Process

  1. The elevator cars are built at the elevator manufacturer's plant using standard metal cutting, welding, and forming techniques. If the cars will be exposed to the weather during building construction, the interior trim may be installed after the building is finished.
  2. The rest of the elevator is assembled on the building site. The building design integrates the elevator shaft from the beginning, and the shaft grows as the building is erected. The walls of the shaft are poured concrete, and the shaft straightness and other dimensions are carefully monitored as each floor goes up.
  3. Guide rails, switch ramps, service ladders, and similar support equipment are bolted into the shaft after the shaft walls are complete, but before the shaft is roofed.
  4. While the shaft is still open at the top, a crane raises the counterweight to the top of the building and lowers it into the shaft along its rails.
  5. The crane then lifts the elevator car and inserts it partly into the shaft. The guide wheels connect the car to the guide rails, and the car is carefully lowered to the bottom of the shaft.
  6. The shaft is then roofed over, leaving a machine room above the shaft. The hoist motor, governor, controller, and other equipment are mounted in this room, with the motor located directly over the elevator car pulley.
  7. The elevator and governor cables are strung and attached, the electrical connections completed, and the controller programmed.

Quality Control

Each elevator installation in the United States must meet the safety standards of the American National Standards Institute and the American Society of Mechanical Engineers. These standards may be incorporated into local building codes, or the local codes may have their own safety standards. The state must inspect, rate, and certify each passenger elevator installation before it goes into operation and must reinspect on a regular basis thereafter.

The Future

Elevators have not changed substantially in many years and are unlikely to do so in the near future. Electronic controls will continue to improve in ways that are evolutionary and not very dramatic. Control systems are being developed that will learn from past traffic patterns and use this information to predict future needs in order to reduce waiting times. Laser controls are coming into use, both to gauge car speed and distance, as well as to scan building floors for potential passengers.

Where To Learn More

Books

Elevator Technology. Published for the International Association of Elevator Engineers by Ellis Horwood. Halsted Press, 1986.

Ford, Barbara. The Elevator. Walker and Company, 1982.

Periodicals

Evans, Barrie. "Fuzzy Logic for Smarter Lifts." Architect's Journal, May 18, 1994, pp. 24-25.

Richards, Kristen. "Design Files." Interiors, February 1991, pp. 22-23.

Joel Simon

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elevator (in machinery)

elevator, in machinery, device for transporting people or goods from one level to another. The term is applied to the enclosed structures as well as the open platforms used to provide vertical transportation in buildings, large ships, and mines; it is also applied to devices consisting of a continuous belt or chain with attached buckets for handling bulk materials. Simple hoists were used from ancient times. From about the middle of the 19th cent., power elevators, often steam-operated, were used for conveying materials in factories, mines, and warehouses. In 1853 the American inventor Elisha G. Otis demonstrated a freight elevator equipped with a safety device to prevent falling in case a supporting cable should break. This increased public confidence in such devices and served as an impetus to the industry. Otis established a company for manufacturing elevators and patented (1861) a steam elevator. After the introduction by Sir William Armstrong of the hydraulic crane (1846), the hydraulic principle was applied to the elevator, and in the early 1870s hydraulic machines began to replace the steam-powered elevator. The hydraulic elevator is supported by a heavy piston, moving in a cylinder and operated by the water (or oil) pressure produced by pumps. As improvement of design made increased speed of movement possible, various safety devices, such as speed governors, were developed. Toward the end of the 19th cent., electric elevators came into use, and operation by electric motor gradually became the chief method. Later improved safety devices were added, and automatic and partly automatic elevators were introduced. Increase in speed of operation and improvement in general design also characterize the more modern elevators.

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elevator

el·e·va·tor / ˈeləˌvātər/ • n. 1. a platform or compartment housed in a shaft for raising and lowering people or things to different floors or levels. ∎  a machine consisting of an endless belt with scoops attached, used typically for raising grain to be stored in an upper story: a grain elevator. ∎  a tall building used for storing large quantities of grain. 2. a hinged flap on the horizontal stabilizer of an aircraft, typically one of a pair, used to control the motion of the aircraft about its lateral axis. 3. a muscle whose contraction raises a part of the body: elevators of the upper lip. 4. (also el·e·va·tor shoe) a shoe with a raised insole designed to make the wearer appear taller.

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elevator

elevator. Means by which loads or people can be moved vertically in a chamber within a shaft from one floor of a building to another, called lift in the UK. Although primitive lifts were known in the early C19 (e.g. in the Bunker Hill Monument, Charlestown, MA (1824–42, by Willard), more sophisticated examples were in use in the USA by the 1850s, and Post's Equitable Life Assurance Building, NYC (1868–70—demolished) was one of the first office buildings to be equipped with elevators. Hydraulic power was used for a time, and by the 1880s electrically-powered lifts were evolving, so that by the end of C19 their use was widespread, facilitating the development of tall buildings. See also grain elevator.

Bibliography

C. Elliott (1992)

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elevator

elevator (el-i-vay-ter) n.
1. an instrument that is used to raise a depressed broken bone. periosteal e. an instrument used in orthopaedics to strip the fibrous tissue (periosteum) covering bone.

2. a lever-like instrument used to ease a tooth or root out of its socket during extraction.

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elevator (in aviation)

elevator, in aviation: see airplane.

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Elevator

Elevator

History

Modern elevators

Resources

An elevator is an enclosed car that moves in a vertical shaft between the multi-story floors of a building carrying passengers or freight. In England, it is called a lift. All elevators are based on the principle of the counterweight. Modern elevators also use geared, electric motors and a system of cables and pulleys to propel them. It is the worlds most often used means of mechanical transportation, and it is also the safest. The elevator has played a crucial role in the development of the high-rise or skyscraper. It is largely responsible for how cities look today. Sometimes also called vertical transport systems in the elevator industry, it has become an indispensable factor of modern urban life.

History

Lifting loads by mechanical means goes back at least to the Romans who used primitive hoists operated by human, animal, or water power during their ambitious building projects. An elevator employing a counterweight is said to have been built in the seventeenth century by a Frenchman named Velayer. It was also in that country that a passenger elevator was built in 1743 at the Versailles Palace for King Louis XV. By 1800, steam power was used to power such lift devices, and in 1830, several European factories were operating with hydraulic elevators that were pushed up and down by a plunger that worked in and out of a cylinder.

All of these lifting systems were based on the principle of the counterweight, by which the weight of one object is used to balance the weight of another object. For example, while it may be very difficult to pull up a heavy object using only a rope tied to it, this job can be made very easy if a weight is attached to the other end of the rope and hung over a pulley. This other weight, or counterweight, balances the first and makes it easy to pull up. Thus, an elevator, which uses the counterweight system, never has to pull up the total weight of its load, but only the difference between the load-weight and that of the counter-weight. Counterweights are also found inside the sash of old-style windows, in grandfather clocks, and in dumbwaiters.

Until the mid-nineteenth century, the prevailing elevator systems had two problems. The plunger system was very safe but also extremely slow, and it had obvious height limitations. If the plunger system was scrapped and the elevator car was hung from a rope to achieve higher speeds, the risk of the rope or cable breaking was an ever-present and very real danger. Safety was the main technical problem that American inventor Elisha Graves Otis (18111861) solved when he invented the first modern, fail-safe passenger elevator in 1853. In that year, Otis demonstrated his fail-safe mechanism at the Crystal Palace Exposition in London, England. In front of an astonished audience, he rode his invention high above the crowd and ordered that the cable holding the car be severed. When it was, instead of crashing to the ground, his fail-safe mechanism worked automatically and stopped the car.

The secret of Otiss success was a bow-shaped wagon spring device that would flex and jam its ends into the guide rails if tension on the rope or cable was released. What he had invented was a type of speed governor that translated an elevators downward motion into a sideways, braking action. On March 23, 1857, Otis installed the first commercial passenger elevator in the Haughwout Department Store (488 Broadway) in New York City, and the age of the skyscraper was begun. Until then, large city buildings were limited to five or six stories, which was the maximum number of stairs people were willing to climb. When architects developed the iron-frame building in the 1880s, the elevator was ready to service them. By then, electric power had replaced the old steam-driven elevator. German inventor Ernest Werner von Siemens (18161892) built the first electric elevator in 1880. The first commercial passenger elevator to be powered by electricity was installed in 1889 in the Desmarest Building in New York City. In 1904, a gearless feature was added to the electric motor, making elevator speed virtually limitless. By 1915, automatic leveling had been introduced and cars would now stop precisely where they should.

KEY TERMS

Centrifugal force The inertial reaction that causes a body to move away from a center about which it revolves.

Counterweight The principle in which the weight of one object is used to balance the weight of another object; for an elevator, it is a weight that counterbalances the weight of the elevator car plus approximately 40% of its capacity load.

Hydraulic elevator A power elevator where the energy is applied, by means of a liquid under pressure, in a cylinder equipped with a plunger or a piston; a direct-plunge elevator had the elevator car attached directly to the plunger or piston that went up and down a sunken shaft.

Microprocessor The central processing unit of a microcomputer that contains the silicon chip, which decodes instructions and controls operations.

Sheave A wheel mounted in bearings and having one or more grooves over which a rope or ropes may pass.

Speed governor A device that mechanically regulates the speed of a machine, preventing it from going any faster than a preset velocity.

Modern elevators

Todays passenger elevators are not fundamentally different from the Otis original. Practically all are electrically propelled and are lifted between two guide rails by steel cables that loop over a pulley device called a sheave at the top of the elevator shaft. They still employ the counterweight principle. The safety mechanism, called the overspeed governor, is an improved version of the Otis original. It uses centrifugal force that causes a system of weights to swing outward toward the rails should the cars speed exceed a certain limit. Although the travel system has changed little, its control system has been revolutionized. Speed and automation now characterize elevators, with microprocessors gradually replacing older electromechanical control systems. Speeds ranging up to 1, 800 ft (550 m) per minute can be attained. Separate outer and inner doors are another essential safety feature, and most now have electrical sensors that pull the doors open if they sense something between them. Most elevators also have telephones, alarm buttons, and emergency lighting. Escape hatches in their roofs serve both for maintenance and for emergency use.

Modern elevators can also be programmed to provide the fastest possible service with a minimum number of cars. They can further be set to sense the weight of a car and to bypass all landing calls when fully loaded. In addition to regular passenger or freight elevators, todays specialized lifts are used in ships, dams, and even on rocket launch pads. Todays elevators are safe, efficient, and an essential part of daily lives.

In 2006, the Otis Elevator Company is part of United Technologies Corporation. It is the largest manufacturer of elevators. Other manufacturers of elevators are Thyssen-Krupp, Kone, and Schindler.

See also Building design/architecture.

Resources

BOOKS

The First One Hundred Years. New York: The Otis Elevator Company, 1953.

Goetz, Alisa, ed. Up, Down, Across: Elevators, Escalator, and Moving Sidewalks. London, UK: Merrell, 2003.

PERIODICALS

Jackson, Donald Dale. Elevating Thoughts from Elisha Otis and Fellow Uplifters. Smithsonian (November 1989): 211+.

Leonard C. Bruno

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Elevator

Elevator

An elevator is an enclosed car that moves in a vertical shaft between the multi-story floors of a building carrying passengers or freight. All elevators are based on the principle of the counterweight, and modern elevators also use geared, electric motors and a system of cables and pulleys to propel them. The world's most often used means of mechanical transportation, it is also the safest. The elevator has played a crucial role in the development of the high-rise or skyscraper and is largely responsible for how our cities look today. It has become an indispensable factor of modern urban life.


History

Lifting loads by mechanical means goes back at least to the Romans who used primitive hoists operated by human, animal , or water power during their ambitious building projects. An elevator employing a counter-weight is said to have been built in the seventeenth century by a Frenchman named Velayer, and it was also in that country that a passenger elevator was built in 1743 at the Versailles Palace for King Louis XV. By 1800, steam power was used to power such lift devices, and in 1830, several European factories were operating with hydraulic elevators that were pushed up and down by a plunger that worked in and out of a cylinder.

All of these lifting systems were based on the principle of the counterweight, by which the weight of one object is used to balance the weight of another object. For example, while it may be very difficult to pull up a heavy object using only a rope tied to it, this job can be made very easy if a weight is attached to the other end of the rope and hung over a pulley. This other weight, or counterweight, balances the first and makes it easy to pull up. Thus an elevator, which uses the counterweight system, never has to pull up the total weight of its load, but only the difference between the load-weight and that of the counterweight. Counterweights are also found inside the sash of old-style windows, in grandfather clocks, and in dumbwaiters.

Until the mid-nineteenth century, the prevailing elevator systems had two problems. The plunger system was very safe but also extremely slow, and it had obvious height limitations. If the plunger system was scrapped and the elevator car was hung from a rope to achieve higher speeds, the risk of the rope or cable breaking was an ever-present and very real danger. Safety was the main technical problem that the American inventor, Elisha Graves Otis (1811–1861) solved when he invented the first modern, fail-safe passenger elevator in 1853. In that year, Otis demonstrated his fail-safe mechanism at the Crystal Palace Exposition in London. In front of an astonished audience, he rode his invention high above the crowd and ordered that the cable holding the car be severed. When it was, instead of crashing to the ground, his fail-safe mechanism worked automatically and stopped the car dead.

The secret of Otis's success was a bow-shaped wagon spring device that would flex and jam its ends into the guide rails if tension on the rope or cable was released. What he had invented was a type of speed governor that translated an elevator's downward motion into a sideways, braking action. On March 23, 1857, Otis installed the first commercial passenger elevator in the Haughwout Department Store in New York, and the age of the skyscraper was begun. Until then, large city buildings were limited to five or six stories which was the maximum number of stairs people were willing to climb. When the iron-frame building was developed by architects in the 1880s, the elevator was ready to service them. By then, electric power had replaced the old steam-driven elevator, and the first commercial passenger elevator to be powered by electricity was installed in 1889 in the Desmarest Building in New York. In 1904, a "gearless" feature was added to the electric motor , making elevator speed virtually limitless. By 1915, automatic leveling had been introduced and cars would now stop precisely where they should.


Modern elevators

Today's passenger elevators are not fundamentally different from the Otis original. Practically all are electrically propelled and are lifted between two guide rails by steel cables that loop over a pulley device called a sheave at the top of the elevator shaft. They still employ the counterweight principle. The safety mechanism, called the overspeed governor, is an improved version of the Otis original. It uses centrifugal force that causes a system of weights to swing outward toward the rails should the car's speed exceed a certain limit. Although the travel system has changed little, its control system has been revolutionized. Speed and automation now characterize elevators, with micro-processors gradually replacing older electromechanical control systems. Speeds ranging up to 1,800 ft (550 m) per minute can be attained. Separate outer and inner doors are another essential safety feature, and most now have electrical sensors that pull the doors open if they sense something between them. Most also have telephones, alarm buttons, and emergency lighting. Escape hatches in their roofs serve both for maintenance and for emergency use.

Modern elevators can also be programmed to provide the fastest possible service with a minimum number of cars. They can further be set to sense the weight of a car and to bypass all landing calls when fully loaded. In addition to regular passenger or freight elevators, today's specialized lifts are used in ships, dams , and even on rocket launch pads. Today's elevators are safe, efficient, and an essential part of our daily lives.

See also Building design/architecture.

Resources

books

The First One Hundred Years. New York: The Otis Elevator Company, 1953.

Strakosch, George R. Vertical Transportation: Elevators andEscalators. New York: John Wiley & Sons, 1983.

periodicals

Jackson, Donald Dale. "Elevating Thoughts from Elisha Otis and Fellow Uplifters." Smithsonian (November 1989): 211+.


Leonard C. Bruno

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Centrifugal force

—The inertial reaction which causes a body to move away from a center about which it revolves.

Counterweight

—The principle in which the weight of one object is used to balance the weight of another object; for an elevator, it is a weight which counterbalances the weight of the elevator car plus approximately 40% of its capacity load.

Hydraulic elevator

—A power elevator where the energy is applied, by means of a liquid under pressure, in a cylinder equipped with a plunger or a piston; a direct-plunge elevator had the elevator car attached directly to the plunger or piston which went up and down a sunken shaft.

Microprocessor

—The central processing unit of a microcomputer that contains the silicon chip which decodes instructions and controls operations.

Sheave

—A wheel mounted in bearings and having one or more grooves over which a rope or ropes may pass.

Speed governor

—A device that mechanically regulates the speed of a machine, preventing it from going any faster than a preset velocity.

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"Elevator." The Gale Encyclopedia of Science. . Encyclopedia.com. 13 Sep. 2018 <http://www.encyclopedia.com>.

"Elevator." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 13, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/elevator-0

"Elevator." The Gale Encyclopedia of Science. . Retrieved September 13, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/elevator-0

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