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
Hydraulic Elevators 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
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
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
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
The roped elevator design gets around both of these
problems. In the next section, we'll see how this system
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
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.
Safety Systems 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
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
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
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
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
In the next section, we'll look at one of the coolest
components in an elevator: the automatic doors.
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
For more information on elevators, including the elevator
technologies of the future, check out the links on the next