Elevators

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!

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 braking system.
  
• 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.

The counterweight

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.

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

Artwork: The Otis elevator. 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, courtesy of the US Patent and Trademark Office, is 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:

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.
   

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.

Speed governors

Most elevators have an entirely separate speed-regulating system called a governor, which is a heavy flywheel with massive mechanical arms built inside it. Normally the arms are held inside the flywheel by hefty springs, but if the lift moves too fast, they fly outward, pushing a lever mechanism that trips one or more braking systems. First, they might cut power to the lift motor. If that fails and the lift continues to accelerate, the arms will fly out even further and trip a second mechanism, applying the brakes. Some governors are entirely mechanical; others are electromagnetic; still others use a mixture of mechanical and electronic components.

Artwork: How a governor works. The lift motor (1) drives gears (2) that turn the sheave (3)—a grooved wheel that guides the main cable. The cable supports both the counterweight (4) and lift car (5). A separate cable (6) is attached to the lift car and the governor mechanism on the right. The governor consists of a flywheel with centrifugal arms inside it (7). If the lift moves too quickly, the arms fly outward, tripping a safety mechanism that applies brakes to the governor cable (8), bringing the car safely to a halt.

Artwork: An example of a fully mechanical governor mechanism designed by Otis engineers in the 1960s. You can see the flywheel (gray) with its centrifugal arms inside (light blue) and the springs that hold them in (yellow). When the wheel turns too fast, the arms fly outward, tripping a braking device that applies a pair of spring-loaded arms (darker blue) to the rope (brown). From US Patent 3,327,811: Governor by Joseph Mastroberte, Otis Elevator Company, patented June 27, 1967. Artwork courtesy of US Patent and Trademark Office (with colors added to make it easier to understand).

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 strands of wire rope twisted together so a small failure of one part of the cable isn’t, initially at least, going to cause any problems. Most elevators also have multiple, separate cables supporting each car, so the complete failure of one cable leaves others functioning in its place. Even if all the cables break, this system will still hold the car in place.

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!