Elevator  
  Definition: Elevator



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Type: Glossary & Dictionary

Category: Elevator

Dictionary Definition

An elevator or lift is a transport device used to move goods or people vertically. Languages other than English may have loanwords based on either elevator (e.g. Japanese) or lift (e.g. most European languages, Cantonese). Because of wheelchair access laws, elevators are often a legal requirement in new buildings with multiple floors.

Design
Elevators began as simple rope or chain hoists. An elevator is essentially a platform that is either pulled or pushed up by a mechanical means. A modern day elevator consists of a cab (also called a "cage" or "car") mounted on a platform within an enclosed space called a shaft, or in Commonwealth countries called a "hoistway". In the past, elevator drive mechanisms were powered by steam and water hydraulic pistons. In a "traction" elevator, cars are pulled up by means of rolling steel ropes over a deeply grooved pulley, commonly called a sheave in the industry. The weight of the car is balanced with a counterweight. Sometimes two elevators always move synchronously in opposite directions, and they are each other's counterweight.

The friction between the ropes and the pulley furnishes the traction which gives this type of elevator its name.

Hydraulic elevators use the principles of hydraulics to pressurize an above ground or in-ground piston to raise and lower the car. Roped Hydraulics use a combination of both ropes and hydraulic power to raise and lower cars. Recent innovations include permanent earth magnet motors, machine room-less rail mounted gearless machines, and microprocessor controls.

Which technology is used in new installations depends on a variety of factors. Hydraulic elevators are cheaper, but installing cylinders greater than a certain length becomes impractical for very high lift hoistways. For buildings of much over seven stories, traction elevators must be employed instead. Hydraulic elevators are usually slower than traction elevators.

Elevators are a candidate for mass customization. There are economies to be made from mass production of the components, but each building comes with its own requirements like different number of floors, dimensions of the well and usage patterns.

History
The first reference to an elevator is in the works of the Roman architect Vitruvius, who reported that Archimedes built his first elevator, probably, in 236 B.C. In some literary sources of later historical periods, elevators were mentioned as cabs on a hemp rope and powered by hand or by animals. It is supposed that elevators of this type were installed in the Sinai monastery of Egypt. In the 17th century the prototypes of elevators were located in the palace buildings of England and France.

In 1793 Ivan Kulibin created an elevator with the screw lifting mechanism for the Winter Palace of Saint Petersburg. In 1816 an elevator was established in the main building of sub Moscow village called Arkhangelskoye. In 1823, an "ascending room" made its debut in London.

Henry Waterman of New York is credited with inventing the "standing rope control" for an elevator in 1850.

In 1853, Elisha Otis introduced the safety elevator, which prevented the fall of the cab if the cable broke. The design of the Otis safety elevator is somewhat similar to one type still used today. A governor device engages knurled roller(s), locking the elevator to its guides should the elevator descend at excessive speed. He demonstrated it at the New York exposition in the Crystal Palace in 1854.

On March 23, 1857 the first Otis passenger elevator was installed at 488 Broadway in New York City. The first elevator shaft preceded the first elevator by four years. Construction for Peter Cooper's Cooper Union building in New York began in 1853. An elevator shaft was included in the design for Cooper Union, because Cooper was confident that a safe passenger elevator would soon be invented. The shaft was cylindrical because Cooper felt it was the most efficient design. Later Otis designed a special elevator for the school. Today the Otis Elevator Company, now a subsidiary of United Technologies Corporation, is the world's largest manufacturer of vertical transport systems.

The first electric elevator was built by Werner von Siemens in 1880. The safety and speed of electric elevators were significantly enhanced by Frank Sprague.

The development of elevators was led by the need for movement of raw materials including coal and lumber from hillsides. The technology developed by these industries and the introduction of steel beam construction worked together to provide the passenger and freight elevators in use today.

In 1874, J.W. Meaker patented a method which permitted elevator doors to open and close safely. U.S. Patent 147,853

In 1929, Clarence Conrad Crispen, with Inclinator Company of America, created the first residential elevator. Crispen also invented the first inclined stairlift.

Elevator Safety
Elevators are characterized as being extremely safe. Their safety record of moving millions of passengers every day, with extremely low rate of incident, is unsurpassed by any other vehicle system. Even so, fatalities due to malfunction have been known to occur on occasion. A certain number of passengers do die every year in elevator-related incidents. In 1998, in the United States, it was reported that of the estimated 120 billion rides per year in the approximately 600,000 elevators in the U.S., 10,000 people wound up in the emergency room because of elevator-related accidents.

Past problems with hydraulic elevators meant those built prior to a code change in 1972 were subject to possible catastrophic failure. The code had previously required only single-bottom hydraulic cylinders. In the event of a cylinder breach, an uncontrolled fall of the elevator might result. Because it is impossible to verify the system completely without a pressurized casing (as described below), it is necessary to remove the piston to inspect it. The cost of removing the piston is such that it makes no economic sense to re-install the old cylinder; therefore it is necessary to replace the cylinder and install a new piston. Another solution to protect against a cylinder blowout is to install a "life jacket." This is a device which, in the event of an excessive downward speed, clamps onto the cylinder and stops the car. This device is also known as a Rupture Valve in some parts of the world.

In addition to the safety concerns for older hydraulic elevators, there is risk of leaking hydraulic oil into the aquifer and causing potential environmental contamination. This has led to the introduction of PVC liners (casings) around hydraulic cylinders which can be monitored for integrity.

In the past decade, recent innovations in inverted hydraulic jacks have eliminated the costly process of drilling the ground to install a borehole jack. This also eliminates the threat of corrosion to the system and increases safety.

On traction lifts there is a device called a "Safety Gear" that is fitted to the bottom of the lift car frame. This device connects to another device commonly known as a "Overspeed Governor." There is a separate rope from the main lifting ropes that connects the safety gear to the overspeed governor. The Overspeed Governor usually has a pulley which the safety rope runs on. The overspeed governor usually has an arm type latch. If the device spins too quickly, the arm is forced out from the middle of the unit by centrifugal force. This locks the pulley, which stops the rope. Once the rope stops and the car is still moving down, the rope pulls up on the safety gear causing a wedge type friction roller or a solid plate to clamp very tightly on the lift running guides. This causes the lift to stop suddenly ("instantaneous" safety gear) or in a progressive slowing motion ("progressive" safety gear). There are many different versions of these but they all work in the same way.

Types of elevator hoist mechanisms
In general, there are three means of moving an elevator:

Traction elevators
Geared Traction machines are driven by AC or DC electric motors. Geared machines use worm gears to control mechanical movement of elevator cars by "rolling" steel hoist ropes over a drive sheave which is attached to a gearbox driven by a high speed motor. These machines are generally the best option for basement or overhead traction use for speeds up to 500 ft/min (2.5 m/s). Gearless Traction machines are low speed (low RPM), high torque electric motors powered mainly by AC or DC. In this case, the drive sheave is directly attached to the end of the motor. Gearless traction elevators can reach speeds of up to 2,000 ft/min, or even higher. A brake is mounted between the motor and drive sheave (or gearbox) to hold the elevator stationary at a floor. This brake is usually an external drum type and is actuated by spring force and held open electrically; a power failure will cause the brake to engage and prevent the elevator from falling (see inherent safety and safety engineering). In each case, cables are attached to a hitch plate on top of the cab or may be "underslung" below a cab, and then looped over the drive sheave to a counterweight attached to the opposite end of the cables which reduces the amount of power needed to move the cab. The counterweight is located in the hoist-way and rides a separate rail system; as the car goes up, the counterweight goes down, and vice versa. This action is powered by the traction machine which is directed by the controller, typically a relay logic or computerized device that directs starting, acceleration, deceleration and stopping of the elevator cab. The weight of the counterweight is typically equal to the weight of the elevator cab plus 40-50% of the capacity of the elevator. The grooves in the drive sheave are specially designed to prevent the cables from slipping. "Traction" is provided to the ropes by the grip of the grooves in the sheave, thereby the name. As the ropes age and the traction grooves wear, some traction is lost and the ropes must be replaced and the sheave repaired or replaced. Elevators with more than 100' of travel have a system called compensation. This is a separate set of cables or a chain attached to the bottom of the counterweight and the bottom of the elevator cab. This makes it easier to control the elevator, as it compensates for the differing weight of cable between the hoist and the cab. If the elevator cab is at the top of the hoist-way, there is a short length of hoist cable above the car and a long length of compensating cable below the car and vice versa for the counterweight. If the compensation system uses cables, there will be an additional sheave in the pit below the elevator, to guide the cables. If the compensation system uses chains, the chain is guided by a bar mounted between the counterweight rails.

Hydraulic elevators
Conventional Hydraulic elevators were first developed by Dover Elevator (now ThyssenKrupp Elevator). They are quite common for low and medium rise buildings (2-8 floors), attain speeds of up to 200 feet/minute (1.0 m/s), and use a hydraulically powered plunger to push the elevator upwards. On some, the hydraulic piston (plunger) consists of telescoping concentric tubes, allowing a shallow tube to contain the mechanism below the lowest floor. On others, the piston requires a deeper hole below the bottom landing, usually with a PVC casing (also known as a caisson) for protection.
Roped hydraulic elevators use a combination of ropes and hydraulics.
Twin post hydraulic provides higher travel with no underground hole.
Holeless hydraulic elevators do not require holes to be dug for the hydraulic cylinder. In most designs, the cab is lifted by a pair of hydraulic jacks, one on each side of the elevator.

Climbing elevator
A climbing elevator is a self-ascending elevator with its own propulsion. The propulsion can be done by an electric or a combustion engine. Climbing elevators are used in guyed masts or towers, in order to make easy access to parts of these constructions, such as flight safety lamps for maintenance. An example would be the Moonlight towers in Austin, Texas, where the elevator holds only one person and equipment for maintenance.


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