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Elevator
In agriculture and manufacturing, an elevator is any type of conveyor device used to lift materials in a continuous stream into bins or silos. Several types exist, such as the chain and bucket elevator, grain auger screw conveyor using the principle of Archimedes' screw, or the chain and paddles or forks of hay elevators. Languages other than English may have loanwords based on either elevator or lift. Because of wheelchair access laws, elevators are often a legal requirement in new multistory buildings, especially where wheelchair ramps would be impractical.
There are also some elevators which can go sideways in addition to the usual up-and-down motion.
Machine room-less (MRL) elevators
Machine room-less elevators are designed so that most of the components fit within the shaft containing the elevator car; and a small cabinet houses the elevator controller. Other than the machinery being in the hoistway, the equipment is similar to a normal traction or hole-less hydraulic elevator. The world's first machine room-less elevator, the Kone MonoSpace was introduced in 1996, by Kone. The benefits are:
- creates more usable space
- use less energy (70–80% less than standard hydraulic elevators)
- uses no oil (assuming it is a traction elevator)
- all components are above ground similar to roped hydraulic type elevators (this takes away the environmental concern that was created by the hydraulic cylinder on direct hydraulic type elevators being stored underground)
- slightly lower cost than other elevators; significantly so for the hydraulic MRL elevator
- can operate at faster speeds than hydraulics but not normal traction units.
Detriments
- Equipment can be harder, and significantly more dangerous to service and maintain.
Facts
- Noise level is at 50–55 dBA (A-weighted decibels), which can be lower than some but not all types of elevators.
- Usually used for low-rise to mid-rise buildings
- The motor mechanism is placed in the hoistway itself
- The US was slow to accept the commercial MRL Elevator because of codes
- National and local building codes did not address elevators without machine rooms. Residential MRL elevators are still not allowed by the ASME A17 code in the US. MRL elevators have been recognised in the 2005 supplement to the 2004 A17.1 Elevator Code.
- Today, some machine room-less hydraulic elevators by Otis and ThyssenKrupp exist; they do not involve the use of a piston located underground or a machine room, mitigating environmental concerns; however, code is not yet accepting of them in all parts of the United States.
Double-decker elevators
Double-decker elevators are traction elevators with cars that have an upper and lower deck. Both decks can be serving a floor at the same time, and both decks are usually driven by the same motor. This system increases efficiency in high rise buildings and saves space so that additional shafts and cars do not need to be added.
In 2003, ThyssenKrupp invented a system called TWIN, in which two elevator cars are independently running in one shaft.
Traffic calculations
Round-trip time calculations
The majority of elevator designs are developed from Up Peak Round Trip Time calculations as described in the following publications:-
- Chartered Institution of Building Services Engineers (2015). CIBSE Guide D: Transportation Systems in Buildings. cibseliftsgroup.org (5 ed.). CIBSE. ISBN 9781523106899. Retrieved 26 April 2017.
- Barney, Gina; Al-Sharif, Lutfi (2015). Elevator Traffic Handbook, Theory and Practice (2 ed.). Routledge. ISBN 9781317528463.
- Strakosch, George (30 September 2010). The Vertical Transportation Handbook. Elevatorbooks.com (4 ed.). Archived from the original on 10 September 2017. Retrieved 26 April 2017.
Traditionally, these calculations have formed the basis of establishing the Handling Capacity of an elevator system.
Modern installations with more complex elevator arrangements have led to the development of more specific formula such as the General Analysis calculation.
Subsequently, this has been extended for double-deck elevators.
Otis Elevator Company operates more than 1.9 million elevators worldwide, giving rise to its claim that the equivalent of the world population is transported by its products every five days.
Simulation
Elevator traffic simulation software can be used to model complex traffic patterns and elevator arrangements that cannot necessarily be analysed by RTT calculations.
Traffic patterns
There are four main types of elevator traffic patterns that can be observed in most modern office installations. They are up peak traffic, down peak traffic, lunch time (two way) traffic and interfloor traffic.
Types of hoist mechanisms
Elevators can be rope dependent or rope-free. There are at least four means of moving an elevator:
Traction elevators
- Geared and gearless 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 3 m/s (500 ft/min).
Historically, AC motors were used for single or double-speed elevator machines on the grounds of cost and lower usage applications where car speed and passenger comfort were less of an issue, but for higher speed, larger capacity elevators, the need for infinitely variable speed control over the traction machine becomes an issue. Therefore, DC machines powered by an AC/DC motor generator were the preferred solution. The MG set also typically powered the relay controller of the elevator, which has the added advantage of electrically isolating the elevators from the rest of a building's electrical system, thus eliminating the transient power spikes in the building's electrical supply caused by the motors starting and stopping (causing lighting to dim every time the elevators are used for example), as well as interference to other electrical equipment caused by the arcing of the relay contactors in the control system.
The widespread availability of variable frequency AC drives has allowed AC motors to be used universally, bringing with it the advantages of the older motor-generator, DC-based systems, without the penalties in terms of efficiency and complexity. The older MG-based installations are gradually being replaced in older buildings due to their poor energy efficiency.
Gearless traction machines are low-speed (low-RPM), high-torque electric motors powered either 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 20 m/s (4,000 ft/min), A brake is mounted between the motor and gearbox or between the motor and drive sheave or at the end of the drive sheave 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). But it can also be some form of disc type like 1 or more calipers over a disc in one end of the motor shaft or drive sheave which is used in high speed, high rise and large capacity elevators with machine rooms(an exception is the Kone MonoSpace's EcoDisc which is not high speed, high rise and large capacity and is machine room less but it uses the same design as is a thinner version of a conventional gearless traction machine) for braking power, compactness and redundancy (assuming there's at least 2 calipers on the disc), or 1 or more disc brakes with a single caliper at one end of the motor shaft or drive sheave which is used in machine room less elevators for compactness, braking power, and redundancy (assuming there's 2 brakes or more).
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 is carried along a separate railway 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 computerised 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. Sheave and rope wear may be significantly reduced by ensuring that all ropes have equal tension, thus sharing the load evenly. Rope tension equalisation may be achieved using a rope tension gauge, and is a simple way to extend the lifetime of the sheaves and ropes.
Elevators with more than 30 m (98 ft) 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 tracks.
Regenerative drives
Another energy-saving improvement is the regenerative drive, which works analogously to regenerative braking in vehicles, using the elevator's electric motor as a generator to capture some of the gravitational potential energy of descent of a full cab (heavier than its counterweight) or ascent of an empty cab (lighter than its counterweight) and return it to the building's electrical system.
Hydraulic elevators
- Conventional hydraulic elevators. They use an underground hydraulic cylinder, are quite common for low level buildings with two to five floors (sometimes but seldom up to six to eight floors), and have speeds of up to 1 m/s (200 ft/min). For higher rise applications, a telescopic hydraulic cylinder can be used.
- Holeless hydraulic elevators were developed in the 1970s, and use a pair of above ground cylinders, which makes it practical for environmentally or cost sensitive buildings with two, three, or four floors.
- Roped hydraulic elevators use both above ground cylinders and a rope system, allowing the elevator to travel further than the piston has to move.
The low mechanical complexity of hydraulic elevators in comparison to traction elevators makes them ideal for low rise, low traffic installations. They are less energy efficient as the pump works against gravity to push the car and its passengers upwards; this energy is lost when the car descends on its own weight. The high current draw of the pump when starting up also places higher demands on a building's electrical system. There are also environmental concerns should the lifting cylinder leak fluid into the ground.
The modern generation of low-cost, machine room-less traction elevators made possible by advances in miniaturisation of the traction motor and control systems challenges the supremacy of the hydraulic elevator in their traditional market niche.
Electromagnetic propulsion
Cable-free elevators using electromagnetic propulsion, capable of moving both vertically and horizontally, have been developed by German engineering firm Thyssen Krupp for use in high rise, high density buildings.
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. The Glasgow Tower — an observation tower in Glasgow, Scotland — also makes use of two climbing elevators. Temporary climbing elevators are commonly used in the construction of new high-rise buildings to move materials and personnel before the building's permanent elevator system is installed, at which point the climbing elevators are dismantled.
Pneumatic elevator
An elevator of this kind uses a vacuum on top of the cab and a valve on the top of the "shaft" to move the cab upwards and closes the valve in order to keep the cab at the same level. A diaphragm or a piston is used as a "brake", if there's a sudden increase in pressure above the cab. To go down, it opens the valve so that the air can pressurise the top of the "shaft", allowing the cab to go down by its own weight. This also means that in case of a power failure, the cab will automatically go down. The "shaft" is made of acrylic, and is always round due to the shape of the vacuum pump turbine. To keep the air inside of the cab, rubber seals are used. Due to technical limitations, these elevators have a low capacity, they usually allow 1–3 passengers and up to 525 lbs.
Controlling elevators
Manual controls
In the first half of the twentieth century, almost all elevators had no automatic positioning of the floor on which the cab would stop. Some of the older freight elevators were controlled by switches operated by pulling on adjacent ropes. In general, most elevators before WWII were manually controlled by elevator operators using a rheostat connected to the motor. This rheostat (see picture) was enclosed within a cylindrical container about the size and shape of a cake. This was mounted upright or sideways on the cab wall and operated via a projecting handle, which was able to slide around the top half of the cylinder.The elevator motor was located at the top of the shaft or beside the bottom of the shaft. Pushing the handle forward would cause the cab to rise; backwards would make it sink. The harder the pressure, the faster the elevator would move. The handle also served as a dead man switch: if the operator let go of the handle, it would return to its upright position, causing the elevator cab to stop. In time, safety interlocks would ensure that the inner and outer doors were closed before the elevator was allowed to move.
This lever would allow some control over the energy supplied to the motor and so enabled the elevator to be accurately positioned — if the operator was sufficiently skilled. More typically, the operator would have to "jog" the control, moving the cab in small increments until the elevator was reasonably close to the landing point. Then the operator would direct the outgoing and incoming passengers to "watch the step".
Automatic elevators began to appear as early as the 1920s, their development being hastened by striking elevator operators which brought large cities dependent on skyscrapers (and therefore their elevators) such as New York and Chicago to their knees. Self service elevators were not allowed in New York City until 1922. Prior to this, non-luxury buildings that could not afford an attendant were built as five-story walk ups. These electromechanical systems used relay logic circuits of increasing complexity to control the speed, position and door operation of an elevator or bank of elevators.
The Otis Autotronic system of the early 1950s brought the earliest predictive systems which could anticipate traffic patterns within a building to deploy elevator movement in the most efficient manner. Relay-controlled elevator systems remained common until the 1980s and their gradual replacement with solid-state, microprocessor-based controls are now the industry standard. Most older, manually-operated elevators have been retrofitted with automatic or semi-automatic controls.
General controls
A typical modern passenger elevator will have:
- Outside the elevator, buttons to go up or down (the bottom floor only has the up button, the top floor only has the down button, and every floor in between has both)
- Space to stand in, guardrails, seating cushion (luxury)
- Overload sensor – prevents the elevator from moving until excess load has been removed. It may trigger a voice prompt or buzzer alarm. This may also trigger a "full car" indicator, indicating the car's inability to accept more passengers until some are unloaded.
- Electric fans or air conditioning units to enhance circulation and comfort.
- A control panel with various buttons. In many countries, button text and icons are raised to allow blind users to operate the elevator; many have Braille text besides. Buttons include:
- Call buttons to choose a floor. Some of these may be key switches (to control access). In some elevators, certain floors are inaccessible unless one swipes a security card or enters a passcode (or both).
- Door open and door close buttons.
The operation of the door open button is transparent, immediately opening and holding the door, typically until a timeout occurs and the door closes. The operation of the door close button is less transparent, and it often appears to do nothing, leading to frequent but incorrect reports that the door close button is a placebo button: either not wired up at all, or inactive in normal service. Working door open and door close buttons are required by code in many jurisdictions, including the United States, specifically for emergency operation: in independent mode, the door open and door close buttons are used to manually open or close the door. Beyond this, programming varies significantly, with some door close buttons immediately closing the door, but in other cases being delayed by an overall timeout, so the door cannot be closed until a few seconds after opening. In this case (hastening normal closure), the door close button has no effect. However, the door close button will cause a hall call to be ignored (so the door won't reopen), and once the timeout has expired, the door close will immediately close the door, for example to cancel a door open push. The minimum timeout for automatic door closing in the US is 5 seconds, which is a noticeable delay if not over-ridden.
- An alarm button or switch, which passengers can use to warn the premises manager that they have been trapped in the elevator.
- A set of doors kept locked on each floor to prevent unintentional access into the elevator shaft by the unsuspecting individual. The door is unlocked and opened by a machine sitting on the roof of the car, which also drives the doors that travel with the car. Door controls are provided to close immediately or reopen the doors, although the button to close them immediately is often disabled during normal operations, especially on more recent elevators. Objects in the path of the moving doors will either be detected by sensors or physically activate a switch that reopens the doors. Otherwise, the doors will close after a preset time. Some elevators are configured to remain open at the floor until they are required to move again. Regulations often require doors to close after use to prevent smoke from entering the elevator shaft in event of fire.
- Elevators in high traffic buildings often have a "nudge" function (the Otis Autotronic system first introduced this feature) which will close the doors at a reduced speed, and sound a buzzer if the "door open" button is being deliberately held down, or if the door sensors have been blocked for too long a time.
- A stop switch (not allowed under British regulations) to halt the elevator while in motion and often used to hold an elevator open while freight is loaded. Keeping an elevator stopped for too long may set off an alarm. Unless local codes require otherwise, this will most likely be a key switch.
Some elevators may have one or more of the following:
- An elevator telephone, which can be used (in addition to the alarm) by a trapped passenger to call for help. This may consist of a transceiver, or simply a button. This feature is often required by local regulations.
- Hold button: This button delays the door closing timer, useful for loading freight and hospital beds.
- Call cancellation: A destination floor may be deselected by double clicking.
- Access restriction by key switches, RFID reader, code keypad, hotel room card, etc.
- One or more additional sets of doors. This is primarily used to serve different floor plans: on each floor only one set of doors opens. For example, in an elevated crosswalk setup, the front doors may open on the street level, and the rear doors open on the crosswalk level. This is also common in garages, rail stations, and airports. Alternatively, both doors may open on a given floor. This is sometimes timed so that one side opens first for getting off, and then the other side opens for getting on, to improve boarding/exiting speed. This is particularly useful when passengers have luggage or carts, as at an airport, due to reduced manoeuvrability.
- In case of dual doors, there may be two sets of door open and door close buttons, with one pair controlling the front doors, from the perspective of the console, typically denoted <> and ><, with the other pair controlling the rear doors, typically denoted with a line in the middle, <|> and >|<, or double lines, |<>| and >||<. This second set is required in the US if both doors can be opened at the same landing, so that the doors can both be controlled in independent service.
- Security camera
- Plain walls or mirrored walls.
- Glass windowpane providing a view of the building interior or onto the streets.
An audible signal button, labelled "S": in the US, for elevators installed between 1991 and 2012 (initial passage of ADA and coming into force of 2010 revision), a button which if pushed, sounds an audible signal as each floor is passed, to assist visually impaired passengers. No longer used on new elevators, where the sound is obligatory.
Other controls, which are generally inaccessible to the public (either because they are key switches, or because they are kept behind a locked panel), include:
- Fireman's service, phase II key switch
- Switch to enable or disable the elevator.
- An inspector's switch, which places the elevator in inspection mode (this may be situated on top of the elevator)
- Manual up/down controls for elevator technicians, to be used in inspection mode, for example.
- An independent service/exclusive mode (also known as "Car Preference"), which will prevent the car from answering to hall calls and only arrive at floors selected via the panel. The door should stay open while parked on a floor. This mode may be used for temporarily transporting goods.
- Attendant service mode
- Large buildings with multiple elevators of this type also had an elevator dispatcher stationed in the lobby to direct passengers and to signal the operator to leave with the use of a mechanical "cricket" noisemaker.
External controls
Elevators are typically controlled from the outside by a call box, which has up and down buttons, at each stop. When pressed at a certain floor, the button (also known as a "hall call" button) calls the elevator to pick up more passengers. If the particular elevator is currently serving traffic in a certain direction, it will only answer calls in the same direction unless there are no more calls beyond that floor.
In a group of two or more elevators, the call buttons may be linked to a central dispatch computer, such that they illuminate and cancel together. This is done to ensure that only one car is called at one time.
Key switches may be installed on the ground floor so that the elevator can be remotely switched on or off from the outside.
In destination control systems, one selects the intended destination floor (in lieu of pressing "up" or "down") and is then notified which elevator will serve their request.
Floor numbering
Elevator algorithm
The elevator algorithm, a simple algorithm by which a single elevator can decide where to stop, is summarised as follows:
- Continue travelling in the same direction while there are remaining requests in that same direction.
- If there are no further requests in that direction, then stop and become idle, or change direction if there are requests in the opposite direction.
The elevator algorithm has found an application in computer operating systems as an algorithm for scheduling hard disk requests. Modern elevators use more complex heuristic algorithms to decide which request to service next. An introduction to these algorithms can be found in the "Elevator traffic handbook: theory and practice" given in the references below.
Destination control system
Some skyscraper buildings and other types of installation feature a destination operating panel where a passenger registers their floor calls before entering the car. The system lets them know which car to wait for, instead of everyone boarding the next car. In this way, travel time is reduced as the elevator makes fewer stops for individual passengers, and the computer distributes adjacent stops to different cars in the bank. Although travel time is reduced, passenger waiting times may be longer as they will not necessarily be allocated the next car to depart. During the down peak period the benefit of destination control will be limited as passengers have a common destination.
It can also improve accessibility, as a mobility-impaired passenger can move to his or her designated car in advance.
Inside the elevator there is no call button to push, or the buttons are there but they cannot be pushed — except door opening and alarm button — they only indicate stopping floors.
The idea of destination control was originally conceived by Leo Port from Sydney in 1961, but at that time elevator controllers were implemented in relays and were unable to optimise the performance of destination control allocations.
The system was first pioneered by Schindler Elevator in 1992 as the Miconic 10. Manufacturers of such systems claim that average travelling time can be reduced by up to 30%.
However, performance enhancements cannot be generalised as the benefits and limitations of the system are dependent on many factors. One problem is that the system is subject to gaming. Sometimes, one person enters the destination for a large group of people going to the same floor. The dispatching algorithm is usually unable to completely cater for the variation, and latecomers may find the elevator they are assigned to is already full. Also, occasionally, one person may press the floor multiple times. This is common with up/down buttons when people believe this to be an effective way to hurry elevators. However, this will make the computer think multiple people are waiting and will allocate empty cars to serve this one person.
To prevent this problem, in one implementation of destination control, every user is given an RFID card, for identification and tracking, so that the system knows every user call and can cancel the first call if the passenger decides to travel to another destination, preventing empty calls. The newest invention knows even where people are located and how many on which floor because of their identification, either for the purposes of evacuating the building or for security reasons. Another way to prevent this issue is to treat everyone travelling from one floor to another as one group and to allocate only one car for that group.
The same destination scheduling concept can also be applied to public transit such as in group rapid transit.
Special operating modes
Anti-crime protection
The anti-crime protection (ACP) feature will force each car to stop at a pre-defined landing and open its doors. This allows a security guard or a receptionist at the landing to visually inspect the passengers. The car stops at this landing as it passes to serve further demand.
Up peak
During up-peak mode (also called moderate incoming traffic), elevator cars in a group are recalled to the lobby to provide expeditious service to passengers arriving at the building, most typically in the morning as people arrive for work or at the conclusion of a lunch-time period when people are going back to work. Elevators are dispatched one-by-one when they reach a pre-determined passenger load, or when they have had their doors opened for a certain period of time. The next elevator to be dispatched usually has its hall lantern or a "this car leaving next" sign illuminated to encourage passengers to make maximum use of the available elevator system capacity. Some elevator banks are programmed so that at least one car will always return to the lobby floor and park whenever it becomes free.
The commencement of up-peak may be triggered by a time clock, by the departure of a certain number of fully loaded cars leaving the lobby within a given time period, or by a switch manually operated by a building attendant.
Down peak
During down-peak mode, elevator cars in a group are sent away from the lobby towards the highest floor served, after which they commence running down the floors in response to hall calls placed by passengers wishing to leave the building. This allows the elevator system to provide maximum passenger handling capacity for people leaving the building.
The commencement of down-peak may be triggered by a time clock, by the arrival of a certain number of fully loaded cars at the lobby within a given time period, or by a switch manually operated by a building attendant.