Escalators

Escalator, common name for a mechanical contrivance in the form of a moving stairway, the steps of which ascend or descend, carrying passengers from one floor to another of a building. The steps are separate units but are precision-built to fit together closely. Each has an axle linked with the axles of the other steps by a heavy chain; a large sprocket wheel, connected by worm and spur gearing to an electric motor, drives the chain. In most modern installations, two chains, one on either side, are used. Each step in an escalator has four wheels, running in pairs of tracks so placed in relation to each other that the treads of the steps remain level while ascending or descending the incline. At the head and foot of the stairway the treads form a moving platform level with the floor. On the incline the stairway runs between solid balustrades, usually equipped with a moving belt used as a handrail, traveling at the same speed as the steps.

At a landing, the moving platform may extend forward for about 3 m (about 10 ft), and passengers may be guided in stepping off the escalator by a diagonal barrier. An alternative safety device is a smooth metal comb set in the floor so that it extends over the last visible tread in the landing. Cleats on the treads slide between the teeth of the comb to form an even surface, and by the motion of the stairway the passenger slides onto a stationary metal plate. In all escalators the direction of movement is easily reversed, so that one stairway can be made to travel upward at certain times and downward at others.

Escalators are one of the largest, most expensive machines people use on a regular basis, but they're also one of the simplest.

At its most basic level, an escalator is just a simple variation on the conveyer belt. A pair of rotating chain loops pull a series of stairs in a constant cycle, moving a lot of people a short distance at a good speed.

In this article, we'll look inside an escalator to find out exactly how these elements fit together. While it is exceedingly simple, the system that keeps all the steps moving in perfect synchrony is really quite brilliant.

Each step in the escalator has two sets of wheels, which roll along two separate tracks. The upper set (the wheels near the top of the step) are connected to the rotating chains, and so are pulled by the drive gear at the top of the escalator. The other set of wheels simply glides along its track, following behind the first set.

The tracks are spaced apart in such a way that each step will always remain level. At the top and bottom of the escalator, the tracks level off to a horizontal position, flattening the stairway. Each step has a series of grooves in it, so it will fit together with the steps behind it and in front of it during this flattening.

In addition to rotating the main chain loops, the electric motor in an escalator also moves the handrails. A handrail is simply a rubber conveyer belt that is looped around a series of wheels. This belt is precisely configured so that it moves at exactly the same speed as the steps, to give riders some stability.

The escalator system isn't nearly as good as an elevator at lifting people dozens of stories, but it is much better at moving people a short distance. This is because of the escalator's high loading rate. Once an elevator is filled up, you have to wait for it to reach its floor and return before anybody else can get on. On an escalator, as soon as you load one person on, there's space for another.

Step Speed
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Escalator speeds vary from about 90 feet per minute to 180 feet per minute (27 to 55 meters per minute). An escalator moving 145 feet (44 m) per minute can carry more than 10,000 people an hour -- many more people than a standard elevator.

Briefing:
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An escalator is essentially a moving staircase. The individual steps are supported by metal tracks that run on either side of the escalator. These steps follow one another in a complete loop, driven by an electric motor. When you step onto an escalator at the ground floor, it soon begins to carry you upward and forward at a constant velocity toward the second floor.

Quiz
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a. While you're moving toward the second floor at a constant velocity, what is the net force exerted on you by all outside forces (specify the amount and the direction of the net force)?

b. You know that gravity gives you a weight in the downward direction. What force does the escalator exert on you as you move toward the second floor at a constant velocity (specify the amount and the direction of the force)?

c. Is the escalator doing work on you as you move toward the second floor?

d. As you first step onto the escalator, you begin to accelerate toward the second floor. Is the net force exerted on you by all outside forces the same as in a?

e. If the rapidly moving escalator suddenly stopped moving, you would be thrown forward and might even fall over. What causes you to be thrown forward?

f. You have more energy when you reach the second floor than you had on the first floor. Why aren't you moving faster as a result?

a)
Answer: Zero.
Why: If you are at constant velocity, and thus not accelerating, the net force on you must be zero.

b)
Answer: A force equal in amount (magnitude) to your weight, but in the upward direction.
Why: Since the net force on you is zero and the only two forces you're experiencing are (1) your weight downward and (2) a support force from the escalator upward, those two forces must exactly cancel. Thus the support force must have exactly the same magnitude as your weight, but they must point in opposite directions.

c)
Answer: Yes.
Why: The escalator is pushing upward on you and you are moving (at least partially) upward. Whenever something exerts a force on you and you move at least partially in the direction of that force, that something is doing work on you.

d)
Answer: No.
Why: Since you are accelerating, the net force on you must not be zero.

e)
Answer: Inertia (or your momentum).
Why: If the escalator were to stop suddenly, it would be accelerating backward at an enormous rate. Without some external force to stop you, you'll continue forward at constant velocity. As a result, you will appear to be "thrown" forward, when in fact you are simply obeying the laws of motion and the concept of inertia. No force "throws" you forward--just inertia.

f)
Answer: Your new energy takes the form of gravitational potential energy.
Why: The escalator does work on you as you rise upward and this energy becomes stored in the forces between you and the earth as gravitational potential energy. This energy isn't visible, but it can be released by descending back down to the first floor.