The automatic doors at grocery stores and office buildings are mainly there for convenience and as an aid for handicapped people. The automatic doors in an elevator, on the other hand, are absolutely essential. They are there to keep people from falling down an open shaft.
Elevators use two different sets of doors: doors on the cars and doors opening into the elevator shaft. The doors on the cars are operated by an electric motor, which is hooked up to the elevator computer. You can see how a typical door-opener system works in the diagram below.
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The electric motor turns a wheel, which is attached to a long metal arm. The metal arm is linked to another arm, which is attached to the door. The door can slide back and forth on a metal rail.
When the motor turns the wheel, it rotates the first metal arm, which pulls the second metal arm and the attached door to the left. The door is made of two panels that close in on each other when the door opens and extend out when the door closes. The computer turns the motor to open the doors when the car arrives at a floor and close the doors before the car starts moving again. Many elevators have a motion sensor system that keeps the doors from closing if somebody is between them.
The car doors have a clutch mechanism that unlocks the outer doors at each floor and pulls them open. In this way, the outer doors will only open if there is a car at that floor (or if they are forced open). This keeps the outer doors from opening up into an empty elevator shaft.
In a relatively short period of time, elevators have become an essential machine. As people continue to erect monumental skyscrapers and more small buildings are made handicap-accessible, elevators will become an even more pervasive element in society. It is truly one of the most important machines in the modern era, as well as one of the coolest.
For more information on elevators, including the elevator technologies of the future, check out the links below.