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?
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:
- One or more cars (metal boxes) that rise up and down.
- Counterweights that balance the cars.
- An electric motor that hoists the cars up and down, including a
- A system of strong metal cables and pulleys running between the cars and the motors.
- Various safety systems to protect the passengers if a cable breaks.
- In large buildings, an electronic control system that directs the cars to the correct floors using a
so-called "elevator algorithm" (a sophisticated kind of mathematical
logic) to ensure large numbers of people are moved up and down in
the quickest, most efficient way (particularly important in huge,
busy skyscrapers at rush hour). Intelligent systems are programmed
to carry many more people upward than downward at the beginning of
the day and the reverse at the end of the day.
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?
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.
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
- 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.
- Since less force is involved, there's less strain on the cables—which makes the elevator
a little bit safer.
- 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.
- 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
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:
Greatly simplified, here's how it works:
- 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!
- 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!
- 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.
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
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.
Find out more
On this website
Other useful sites
- The Vertical Transportation Handbook by George R. Strakosch and Robert S. Caporale. John Wiley, 2010. An up-to-date reference about modern elevator systems produced with Elevator World magazine.
- Elevator Traffic Handbook: Theory and Practice by Gina Carol Barney. Spon, 2003. Explores the theory of designing elevators (and other transportation systems) for the most efficient movement of large numbers of people.
- A History of Engineering in Classical and Medieval Times by Donald R. Hill. Routledge, 1984. Although this book doesn't cover elevators (so far as I remember), it does go into excellent detail about ancient water-raising machines used for irrigation, which were among the earliest mechanical lifting devices.
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