In the 1800s, new iron and steel production processes revolutionized the world of construction. With sturdy metal beams as their building blocks, architects and engineers could erect monumental skyscrapers hundreds of feet in the air.
But these towers would have been basically unusable if it weren't for another technological innovation that came along around the same time. Modern elevators are the crucial element that make it practical to live and work dozens of stories above ground. High-rise cities like New York absolutely depend on elevators. Even in smaller multi-story buildings, elevators are essential for making offices and apartments accessible to handicapped people.
In this edition of HowStuffWorks, we'll find out how these ubiquitous machines move you from floor to floor. We'll also look at the control systems that decide where the elevator goes and the safety systems that prevent catastrophes.
The concept of an elevator is incredibly simple -- it's just a compartment attached to a lifting system. Tie a piece of rope to a box, and you've got a basic elevator.
Of course, modern passenger and freight elevators are a lot more elaborate than this. They need advanced mechanical systems to handle the substantial weight of the elevator car and its cargo. Additionally, they need control mechanisms so passengers can operate the elevator, and they need safety devices to keep everything running smoothly.
There are two major elevator designs in common use today: hydraulic elevators and roped elevators.
Hydraulic elevator systems lift a car using a hydraulic ram, a fluid-driven piston mounted inside a cylinder. You can see how this system works in the diagram below.
The cylinder is connected to a fluid-pumping system (typically, hydraulic systems like this use oil, but other incompressible fluids would also work). The hydraulic system has three parts:
The pump forces fluid from the tank into a pipe leading to the cylinder. When the valve is opened, the pressurized fluid will take the path of least resistance and return to the fluid reservoir. But when the valve is closed, the pressurized fluid has nowhere to go except into the cylinder. As the fluid collects in the cylinder, it pushes the piston up, lifting the elevator car.
When the car approaches the correct floor, the control system sends a signal to the electric motor to gradually shut off the pump. With the pump off, there is no more fluid flowing into the cylinder, but the fluid that is already in the cylinder cannot escape (it can't flow backward through the pump, and the valve is still closed). The piston rests on the fluid, and the car stays where it is.
To lower the car, the elevator control system sends a signal to the valve. The valve is operated electrically by a basic solenoid switch (check out How Electromagnets Work for information on solenoids). When the solenoid opens the valve, the fluid that has collected in the cylinder can flow out into the fluid reservoir. The weight of the car and the cargo pushes down on the piston, which drives the fluid into the reservoir. The car gradually descends. To stop the car at a lower floor, the control system closes the valve again.
This system is incredibly simple and highly effective, but it does have some drawbacks. In the next section, we'll look at the main disadvantages of using hydraulics.
Pros and Cons of Hydraulics
The main advantage of hydraulic systems is they can easily multiply the relatively weak force of the pump to generate the stronger force needed to lift the elevator car (see How Hydraulic Machines Work to find out how).
But these systems suffer from two major disadvantages. The main problem is the size of the equipment. In order for the elevator car to be able to reach higher floors, you have to make the piston longer. The cylinder has to be a little bit longer than the piston, of course, since the piston needs to be able to collapse all the way when the car is at the bottom floor. In short, more stories means a longer cylinder.
The problem is that the entire cylinder structure must be buried below the bottom elevator stop. This means you have to dig deeper as you build higher. This is an expensive project with buildings over a few stories tall. To install a hydraulic elevator in a 10-story building, for example, you would need to dig at least nine stories deep! (Some hydraulic elevators don't require quite as much digging. Check out this site to learn about these systems.)
The other disadvantage of hydraulic elevators is that they're fairly inefficient. It takes a lot of energy to raise an elevator car several stories, and in a standard hydraulic elevator, there is no way to store this energy. The energy of position (potential energy) only works to push the fluid back into the reservoir. To raise the elevator car again, the hydraulic system has to generate the energy all over again.
The roped elevator design gets around both of these problems. In the next section, we'll see how this system works.
The Cable System
The most popular elevator design is the roped elevator. In roped elevators, the car is raised and lowered by traction steel ropes rather than pushed from below.
The ropes are attached to the elevator car, and looped around a sheave (3). A sheave is just a pulley with a grooves around the circumfrence. The sheave grips the hoist ropes, so when you rotate the sheave, the ropes move too.
The sheave is connected to an electric motor (2). When the motor turns one way, the sheave raises the elevator; when the motor turns the other way, the sheave lowers the elevator. In gearless elevators, the motor rotates the sheaves directly. In geared elevators, the motor turns a gear train that rotates the sheave. Typically, the sheave, the motor and the control system (1) are all housed in a machine room above the elevator shaft.
The ropes that lift the car are also connected to a counterweight (4), which hangs on the other side of the sheave. The counterweight weighs about the same as the car filled to 40-percent capacity. In other words, when the car is 40 percent full (an average amount), the counterweight and the car are perfectly balanced.
The purpose of this balance is to conserve energy. With equal loads on each side of the sheave, it only takes a little bit of force to tip the balance one way or the other. Basically, the motor only has to overcome friction -- the weight on the other side does most of the work. To put it another way, the balance maintains a near constant potential energy level in the system as a whole. Using up the potential energy in the elevator car (letting it descend to the ground) builds up the potential energy in the weight (the weight rises to the top of the shaft). The same thing happens in reverse when the elevator goes up. The system is just like a see-saw that has an equally heavy kid on each end.
Both the elevator car and the counterweight ride on guide rails (5) along the sides of the elevator shaft. The rails keep the car and counterweight from swaying back and forth, and they also work with the safety system to stop the car in an emergency.
Roped elevators are much more versatile than hydraulic elevators, as well as more efficient. Typically, they also have more safety systems. In the next section, we'll see how these elements work to keep you from plummeting to the ground if something goes wrong.
In the world of Hollywood action movies, hoist ropes are never far from snapping in two, sending the car and its passengers hurdling down the shaft. In actuality, there is very little chance of this happening. Elevators are built with several redundant safety systems that keep them in position.
The first line of defense is the rope system itself. Each elevator rope is made from several lengths of steel material wound around one another. With this sturdy structure, one rope can support the weight of the elevator car and the counterweight on its own. But elevators are built with multiple ropes (between four and eight, typically). In the unlikely event that one of the ropes snaps, the rest will hold the elevator up.
Even if all of the ropes were to break, or the sheave system were to release them, it is unlikely that an elevator car would fall to the bottom of the shaft. Roped elevator cars have built-in braking systems, or safeties, that grab onto the rail when the car moves too fast.
Safeties are activated by a governor when the elevator moves too quickly. Most governor systems are built around a sheave positioned at the top of the elevator shaft. The governor rope is looped around the governor sheave and another weighted sheave at the bottom of the shaft. The rope is also connected to the elevator car, so it moves when the car goes up or down. As the car speeds up, so does the governor. The diagram below shows one representative governor design.
In this governor, the sheave is outfitted with two hooked flyweights (weighted metal arms) that pivot on pins. The flyweights are attached in such a way that they can swing freely back and forth on the governor. But most of the time, they are kept in position by a high-tension spring.
As the rotary movement of the governor builds up, centrifugal force moves the flyweights outward, pushing against the spring. If the elevator car falls fast enough, the centrifugal force will be strong enough to push the ends of the flyweights all the way to the outer edges of the governor. Spinning in this position, the hooked ends of the flyweights catch hold of ratchets mounted to a stationary cylinder surrounding the sheave. This works to stop the governor.
The governor ropes are connected to the elevator car via a movable actuator arm attached to a lever linkage. When the governor ropes can move freely, the arm stays in the same position relative to the elevator car (it is held in place by tension springs). But when the governor sheave locks itself, the governor ropes jerk the actuator arm up. This moves the lever linkage, which operates the brakes, or safeties.
In this design, the linkage pulls up on a wedge-shaped safety, which sits in a stationary wedge guide. As the wedge moves up, it is pushed into the guide rails by the slanted surface of the guide. This gradually brings the elevator car to a stop.
Elevators also have electromagnetic brakes that engage when the car comes to a stop. The electromagnets actually keep the brakes in the open position, instead of closing them. With this design, the brakes will automatically clamp shut if the elevator loses power.
Elevators also have automatic braking systems near the top and the bottom of the elevator shaft. If the elevator car moves to far in either direction, the brake brings it to a stop.
If all else fails, and the elevator does fall down the shaft, there is one final safety measure that will probably save the passengers. The bottom of the shaft has a heavy-duty shock absorber system -- typically a piston mounted in an oil-filled cylinder. The shock absorber works like a giant cushion to soften the elevator car's landing.
In addition to these elaborate emergency systems, elevators need a lot of machinery just to make their stops. In the next section, we'll find out how an elevator operates under normal conditions.
Making the Rounds
Many modern elevators are controlled by a computer. The computer's job is to process all of the relevant information about the elevator and turn the motor the correct amount to put the elevator car where it needs to be. In order to do this, the computer needs to know at least three things.
Where people want to go
Where each floor is
Where the elevator car is
Finding out where people want to go is very easy. The buttons in the elevator car and the buttons on each floor are all wired to the computer. When you press one of these buttons, the computer logs this request.
There are lots of ways to figure out where the elevator car is. In one common system, a light sensor or magnetic sensor on the side of the car reads a series of holes on a long vertical tape in the shaft. By counting the holes speeding by, the computer knows exactly where the car is in the shaft. The computer varies the motor speed so that the car slows down gradually as it reaches each floor. This keeps the ride smooth for the passengers.
In a building with many floors, the computer has to have some sort of strategy to keep the cars running as efficiently as possible. In older systems, the strategy is to avoid reversing the elevator's direction. That is, an elevator car will keep moving up as long as there are people on the floors above that want to go up. The car will only answer "down calls" after it has taken care of all the "up calls." But once it starts down, it won't pick up anybody who wants to go up until there are no more down calls on lower floors. This program does a pretty good job of getting everybody to their floor as fast as possible, but it is highly inflexible.
More advanced programs take passenger traffic patterns into account. They know which floors have the highest demand, at what time of day, and direct the elevator cars accordingly. In a multiple car system, the elevator will direct individual cars based on the location of other cars.
In one cutting-edge system, the elevator lobby works like a train station. Instead of simply pressing up or down, people waiting for an elevator can enter a request for a specific floor. Based on the location and course of all the cars, the computer tells the passengers which car will get them to their destinations the fastest.
Most systems also have a load sensor in the car floor. The load sensor tells the computer how full the car is. If the car is near capacity, the computer won't make any more pick-up stops until some people have gotten off. Load sensors are also a good safety feature. If the car is overloaded, the computer will not close the doors until some of the weight is removed.
In the next section, we'll look at one of the coolest components in an elevator: the automatic doors.
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.
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. There are 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 on the next page.