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Space elevator illustration by artist Pat Rawling.

Elevators

by Chris Woodford. Last updated: June 25, 2013.

Hit the top button on the elevator and prepare yourself for a long ride: in just a few days you'll be waving back from space! Elevators that can zoom up beyond Earth have certainly captured people's imagination in the decade or so since space scientists first proposed them—and it's no wonder. But in their time ordinary office elevators probably seemed almost as radical. It wasn't just brilliant building materials such as steel and concrete that allowed modern skyscrapers to soar to the clouds: it was the invention, in 1861, of the safe, reliable elevator by a man named Elisha Graves Otis of Yonkers, New York. Otis literally changed the face of the Earth by developing a machine he humbly called an "improvement in hoisting apparatus," which allowed cities to expand vertically as well as horizontally. That's why his invention can rightly be described as one of the most important machines of all time. Let's take a closer look at elevators and find out how they work!

Photo: How far will the top button take you? All the way to space? NASA is already working on an elevator that could carry materials from the surface of Earth up to geostationary Earth orbit, 35,786km (22,241 miles) up. Illustration by artist Pat Rawling courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

What is an elevator?

A modern steel and glass elevator.

Photo: A typical, modern, electronically controlled elevator. If you wait for the cars to move out of the way, you can often see some of the workings and figure out which bits do what.

The annoying thing about elevators (if you're trying to understand them) is that their working parts are usually covered up! From the viewpoint of someone traveling from the lobby to the 18th floor, an elevator is simply a metal box with doors that close on one floor and then open again on another. For those of us who are more curious, the key parts of an elevator are:

How elevators use energy

Scientifically, elevators are all about energy. To get from the ground to the 18th floor walking up stairs you have to move the weight of your body against the downward-pulling force of gravity. The energy you expend in the process is (mostly) converted into potential energy, so climbing stairs gives an increase in your potential energy (going up) or a decrease in your potential energy (going down). This is an example of the law of conservation of energy in action. You really do have more potential energy at the top of a building than at the bottom, even if it doesn't feel any different.

To a scientist, an elevator is simply a device that increases or decreases a person's potential energy without them needing to supply that energy themselves: the elevator gives you potential energy when you're going up and it takes potential energy from you when you're coming down. In theory, that sounds easy enough: the elevator won't need to use much energy at all because it will always be getting back as much (when it goes down) as it gives out (when it goes up). Unfortunately, it's not quite that simple. If all the elevator had were a simple hoist with a cage passing over a pulley, it would use considerable amounts of energy lifting people up but it would have no way of getting that energy back: the energy would simply be lost to friction in the cables and brakes (disappearing into the air as waste heat) when the people came back down.

How much energy does an elevator use?

African elephant in Tanzania, Africa.

If an elevator has to lift an elephant (weighing let's say 2500 kg) a distance of maybe 20m into the air, it has to supply the elephant with 500,000 joules of extra potential energy. If it does the lift in 10 seconds, it has to work at a rate of 50,000 joules per second or 50,000 watts, which is about 20 times as much power as a typical electric toaster uses.

Suppose the elevator is carrying elephants all day long (10 hours or 10 × 60 = 600 minutes or 10 × 60 × 60 = 36,000 seconds) and lifting for half that time (18,000 seconds). It would need a grand total of 18,000 × 50,000 = 900 million joules (900 megajoules) of energy, which is the same as 250 kilowatt hours in more familiar terms.

In fact, the elevator wouldn't be 100 percent efficient: all the energy it took from the electricity supply wouldn't be completely converted into potential energy in rising elephants. Some would be lost to friction, sound, heat, air resistance (drag), and other losses in the mechanism. So the real energy consumption would be somewhat greater.

That sounds like a huge amount of energy—and it is! But much of it can be saved by using a counterweight.

Photo: An African elephant in Tanzania. Photo by Gary M. Stolz courtesy of US Fish and Wildlife Service.

The counterweight

Counterweight on wheels and tracks inside an elevator shaft.

In practice, elevators work in a slightly different way from simple hoists. The elevator car is balanced by a heavy counterweight that weighs roughly the same amount as the car when it's loaded half-full. When the elevator goes up, the counterweight goes down—and vice-versa, which helps us in four ways:

  1. The counterweight makes it easier for the motor to raise and lower the car—just as sitting on a see-saw makes it much easier to lift someone's weight compared to lifting them in your arms. Thanks to the counterweight, the motor needs to use much less force to move the car either up or down. Assuming the car and its contents weigh more than the counterweight, all the motor has to lift is the difference in weight between the two and supply a bit of extra force to overcome friction in the pulleys and so on.
  2. Since less force is involved, there's less strain on the cables—which makes the elevator a little bit safer.
  3. The counterweight reduces the amount of energy the motor needs to use. This is intuitively obvious to anyone who's ever sat on a see-saw: assuming the see-saw is properly balanced, you can bob up and down any number of times without ever really getting tired—quite different from lifting someone in your arms, which tires you very quickly. This point also follows from the first one: if the motor is using less force to move the car the same distance, it's doing less work against the force of gravity.
  4. The counterweight reduces the amount of braking the elevator needs to use. Imagine if there were no counterweight: a heavily loaded elevator car would be really hard to pull upwards but, on the return journey, would tend to race to the ground all by itself if there weren't some sort of sturdy brake to stop it. The counterweight makes it much easier to control the elevator car.

In a different design, known as a duplex counterweightless elevator, two cars are connected to opposite ends of the same cable and effectively balance each other, doing away with the need for a counterweight.

Photo: The counterweight rides up and down on wheels that follow guide tracks on the side of the elevator shaft. The elevator car is at the top of this shaft (out of sight) so the counterweight is at the bottom. When the car moves down the shaft, the counterweight moves up—and vice versa. Each car has its own counterweight so the cars can operate independently of one another. On this picture, you can also see the doors on each floor that open and close only when the elevator car is aligned with them.

The safety brake

Everyone who's ever traveled in an escalator has had the same thought: what if the cable holding this thing suddenly snaps? Rest assured, there's nothing to worry about. If the cable snaps, a variety of safety systems prevent an elevator car from crashing to the floor. This was the great innovation that Elisha Graves Otis made back in the 1860s. His elevators weren't simply supported by ropes: they also had a ratchet system as a backup. Each car ran between two vertical guide rails with sturdy metal teeth embedded all the way up them. At the top of each car, there was a spring-loaded mechanism with hooks attached. If the cable broke, the hooks sprung outward and jammed into the metal teeth in the guide rails, locking the car safely in position.

How the original Otis elevator worked

Thanks to the wonders of the Internet, it's really easy to look at original patent documents and find out exactly what inventors were thinking. Here's one of the drawings Elisha Graves Otis submitted with his "Hoisting Apparatus" patent dated January 15, 1861. I've colored it in a little bit so it's easier to understand:

Original patent diagram showing how the safety brake of an elevator works drawn by Elisha Graves Otis in 1861

Greatly simplified, here's how it works:

  1. The elevator compartment (1, green) is raised and lowered by a hoist and pulley system (2) and a moving counterweight (not visible in this picture). You can see how the elevator is moving smoothly between vertical guide bars: it doesn't just dangle stupidly from the rope!
  2. The cable that does all the lifting (3, red) wraps around several pulleys and the main winding drum. Don't forget this elevator was invented before anyone was really using electricity: it was raised and lowered by hand!
  3. At the top of the elevator car, there's a simple mechanism made up of spring-loaded arms and pivots (4). If the main cable (3) breaks, the springs push out two sturdy bars called "pawls" (5) so they lock into vertical racks of upward-pointing teeth (6) on either side. This ratchet-like device clamps the elevator safely in place.

Wheels guiding an elevator car.

According to Otis, the key part of the invention was: "having the pawls and the teeth of the racks hook formed, essentially as shown, so that the weight of the platform will, in case of the breaking of the rope, cause the pawls and teeth to lock together and prevent the contingency of a separation of the same."

If you'd like a more detailed explanation, take a look at the original Otis patent, US Patent #31,128: Improvement in Hoisting Apparatus. It explains more fully how the winch and pulleys work with the counterweight.

Photo: A modern elevator has much in common with the original Otis design. Here you can see the little wheels at the edges of an elevator car that help it move smoothly up and down its guide bars.

Did Otis invent the elevator?

No! He invented the safety elevator: he noted how ordinary elevators could fail and came up with a better design that made them safer. The Otis elevator dates from the middle of the 19th century, but ordinary elevators date back much further—as far as Greek and Roman times. We can trace them back to more general kinds of lifting equipment such as cranes, windlasses, and capstans; ancient water-raising devices such as the shaduf (sometimes spelled shadoof), based on a kind of swinging see-saw design, may well have inspired the use of counterweights in early elevators and hoists.

Other safety systems

Elevator shaft showing a car supported by multiple cables.

Modern elevators have multiple safety systems. Like the cables on a suspension bridge, the cable in an elevator is made from many metal cables twisted together so a small failure of one part of the cable isn't, initially at least, going to cause any problems. Some elevators also have multiple, separate cables so the complete failure of one cable leaves others functioning in its place. Elevators also have a safety braking system similar to the one Otis originally designed with spring-loaded arms locking the car into (or onto) vertical guide rails. Even if all the cables break, this system will still hold the car in place or at least reduce its descent to a safe and slow speed. Finally, if you've ever looked at a transparent glass elevator, you'll have noticed a giant hydraulic or gas spring buffer at the bottom to cushion against an impact if the safety brake should somehow fail. Thanks to Elisha Graves Otis, and the many talented engineers who've followed in his footsteps, you're much safer inside an elevator than you are in a car!

Photo: Elevators don't just hang from a single cable: there are several strong cables supporting the car in case one breaks. If the worst does happen, you'll find there's often an emergency intercom telephone you can use inside an elevator car to call for assistance.

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Text copyright © Chris Woodford 2009, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009) Elevators. Retrieved from http://www.explainthatstuff.com/how-elevators-work.html. [Accessed (Insert date here)]

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