Pneumatics
by Chris Woodford.
Last updated: July 31, 2021.
Remember a time when you blew up a balloon, held it a moment, then let it zoom madly round the room? What seemed like a great big, goofy
old joke back then was really a basic lesson in pneumatics—putting
pressurized air to practical use. From factory machines and road
drills to paint-spraying robots and power tools, all kinds of
everyday things rely on pneumatics. It sounds like a huge
contradiction: how can something as useless and empty as air do
such wonderfully useful things? How can you lift a helicopter, drill a hole,
or power a train with just a big old bag of nothing?
It's all down to the simple science of pressure
and the way gases store energy when you squash them up.
Let's take a closer look!
Photo: Pneumatics is used in some ingenious machines. Here's a pneumatic cylinder with air hoses attached, forming part of a NASA
electric arc shock tube, which is used to carry out tests on space-vehicle reentry. Photo courtesy of NASA on the Commons.
What is pneumatics?
Pneumatics is the science and technology of pressurized
air—using piped, compressed air (or a similar gas, such as nitrogen) to
transmit force and energy.
So a pneumatic drill (or jackhammer) can blast the pavement apart with a metal chisel
pumped up and down by compressed air feeding in from a hose, while a
robot paint machine uses compressed air to make an even spray across something like a car body. As we'll see shortly, pneumatics is similar to hydraulics, where we use
water (or another
liquid) to transmit force and
energy in something like a bulldozer or
a crane; both are examples of fluid power, but where hydraulics uses
liquid, pneumatics uses gas.
Photo: Pneumatic machines, like this hammer drill, are powered by hoses that deliver energy and force in the form of pressurized (compressed) air. Photo by US Bureau of Reclamation courtesy of US Library of Congress.
Think back to that balloon you blew up and let go. You used your
lungs to blow air inside it. Each breath you forced inside inflated
the balloon a little bit more, pushing outward with some force
against the air pressure and the stretchiness (elasticity) of the
rubber pushing in from outside. So, in physics terms, you used a force
(your breath) over a distance (the amount you inflated the balloon):
you did what scientists call work on the balloon. You
compressed the air and forced it into the balloon under pressure. Now
compressing a gas stores energy in it. It takes energy to do the work
that squeezes a gas, and the energy you use isn't wasted—it's
stored in the compressed gas as potential energy you can use later.
In other words, inflating a balloon is a bit like filling up a small
energy tank (or reservoir) made of rubber. When you let go of the
balloon, the trapped gas is released: it expands, so its stored
potential energy rapidly turns back to kinetic energy, making a jet
of air that powers the balloon round the room—a very simple kind of
jet engine.
Photo: Pneumatic? An Aeropress coffee maker is like a big plastic syringe that uses air pressure to push hot water through coffee. It has some of the components of a pneumatic machine, but not all of them.
Some people prefer to define pneumatics more widely, so it covers
pretty much any technology that uses moving air. Such a broad
definition can take in everything from vacuum cleaners (which use suction) to
jet engines—even Aeropress coffee machines—but I find it more vague
and less useful. With a jet engine, for example, just like with a
balloon, the rush of gas shooting back through the exhaust nozzle
powers the plane forward, but another part of how a jet engine works
is combustion (getting energy out of burning fuel) and the
"air" flowing through it doesn't really do the same job, in quite
the same way. A truly pneumatic plane would have something like a
giant tank full of compressed air in the wing (a balloon-like
reservoir) feeding through a hose to a turbine or a
propeller, and
driving the plane forward that way; there wouldn't be any combustion
going on.
So, just to be clear before we start, this article is based on a
tighter definition of pneumatics—with the focus on using compressed
air to transmit forces and store or carry energy—and the examples I
use will tend to reflect that. It won't cover jet engines,
vacuum cleaners, hovercraft,
and other air-moving
technologies that rely less directly on piped, compressed
air (you can find my articles about those things by following the
links).
How do pneumatic machines work?
Pneumatic machines need five basic components to make, store, control, move, and use compressed air:
- A compressor—makes air.
- A reservoir (or receiver)—stores air.
- One or more valves—control air.
- A circuit—moves air between the other components.
- An actuator or motor—uses air to do something.
Pneumatic devices get all their power from the energy in the
compressed air they use, so you can probably see straight away that
they need at least two key components: something to compress the air
(the compressor) and something that uses compressed air to
lift, move, or hold an object (the actuator). We also need a
pipe or network of pipes (the circuit) to get air from the
compressor to the actuator. Something to switch the air on or off (a
valve) and maybe reverse its direction would also be handy (so
we can make our machine lower things as well as lift them).
There's one more thing we need in a pneumatic system. Since air is
a very compressible gas, a basic system linking a compressor to an
actuator through a circuit and a valve would work very slowly. When
you switched it on, it would take time for the compressor to push air
through the circuit and build up enough pressure to make the actuator
move (just like it takes some time for a bicycle tire or a balloon to
really start inflating as you wait for the pressure to build). So a
pneumatic machine also has a reservoir (a balloon, in effect)
where quite a bit of compressed air is stored under pressure, ready
to deliver near-instant force as soon as the operating valve is
opened.
Animation: How pneumatics works (in theory). Here you can see the five key components of a pneumatic machine: 1) Compressor (red); 2) Reservoir (blue); 3) Valve (orange); 4) Circuit of pipes (gray); 5) Actuator (green). The yellow line shows the flow of compressed air. In this case, the actuator is a simple pneumatic cylinder and a piston that goes up when the valve flips up, allowing air into the bottom of the cylinder; it goes down when the valve flips down, so air flows into the top of the cylinder. For the sake of simplicity, I've missed out some of the details, including the cylinder's exhaust (outlet) valves and the engine that powers the compressor.
Photo: How pneumatics works (in practice). Here an airman is lifting an overturned helicopter with the help of a giant pneumatic airbag (light gray). The setup here is much like it is in the animation, though you can see only three of the key pneumatic components (the circuit of pipes, the valves controlling them, and the actuator bag that lifts the helicopter); you can't see the compressor or reservoir.
Photo by Liliana Moreno courtesy of US Air Force.
Compressors
The compressor is the machine that turns ordinary air into
compressed air, squeezing it to around 7–10 times atmospheric
pressure (in scientific units, 7–10 atmospheres, 700–1000 kPa, or
100–150 psi); to give you a rough idea, that's about 25–30 percent more pressure than in a
champagne bottle or 2–3 times the pressure in a car tire.
That's quite a bit of pressure but nothing too extraordinary, and it tells us that
we're going to need quite a lot of compressed air to do anything really useful.
The compressor is the starting point for any pneumatic circuit:
it's the bit that puts energy into the system by squeezing
air into a much smaller space. Now it's important to note that air isn't like the fuel you load
into a gasoline engine: it doesn't contain much useful energy to
begin with. Squeezing energy into ordinary air is the job that the
compressor does, although it doesn't actually "make" this energy
out of nothing. Typically, it's powered by a gasoline or diesel engine, so
it's simply converting energy from one form to another—burning
gas or diesel from its tank, in an engine, and so converting the
energy stored in that fuel into energy stored in pressurized air. I
have a separate article about compressors and pumps and you can read
more about them there.
Actuators and motors
"Actuator" is a bit of engineering jargon that just means "mover";
it's the business end of a pneumatic tool; the bit that shifts about
and does some useful job for us. It might be a pneumatic drill bit
thumped up and down by a piston, a factory ramp that lifts things and
lowers them, a mechanical arm that swings things around, or something
like that. Actuators typically move back and forth in a straight line
(the technical word for that is reciprocate) and, as in the animation above,
they're often powered by pistons that slide back and forth in cylinders as compressed air flows in and out
of them. They turn the potential energy stored in compressed air back
into kinetic energy and movement.
What if we want a tool that spins around (rotating) instead of
just moving back and forth (reciprocating)? Then we can use an
air-powered motor, in which moving gas makes a shaft rotate. It works
in a similar way to a turbine, which is a machine powered by a kind
of internal windmill. As gas flows through an air motor, it pushes
against vanes and makes an axle spin around, turning a drill bit or something
like that. Pneumatic tools like grinders, polishers, and dentist drills work this way.
Photo: Pneumatic machines like this grinder use air motors to produce high-speed, powerful rotary force. How do we know it's not electric? The giveaway, as always, is the thick gray air hose leading to the machine (bottom left). Photo by Lawrence Davis courtesy of US Navy.
Circuits
Although some pneumatic machines might have a single compressor,
actuator, valve, and reservoir, most are more complex than this.
There are many kinds of actuators and valves, and a factory might
have all kinds of machines driven from a complex circuit by a single,
large compressor. You can make complex electronic circuits from
different arrangements of the same basic components, and you can make
complex pneumatic circuits in exactly the same way. There are dozens
of little pneumatic symbols to help you draw circuits out clearly on
engineering plans, just as there are symbols for electronic and hydraulic
components.
Taking things a step further, it's possible to make very advanced
logic circuits entirely from fluid-powered components; an entire
field of engineering called fluidics (fluid logic) is devoted
to this. Just as electronic components like resistors, capacitors,
and transistors can control complex devices by making electricity
flow in different ways round circuits, so fluidic components can do
equivalent things by changing the flow of air around analogous
components. There are fluid equivalents of AND/OR logic gates, timer
circuits, latching units, switches, and amplifiers, for example.
Photo: Maintaining steady air pressure is the key to pneumatics; most pneumatic tools and machines
have pressure gauges on them somewhere, which help to identify leaks. Photo by Marc I. Lane courtesy of US Air Force.
What can we use pneumatics for?
Put these basic components together in different ways and you can
make many different machines to do many different jobs.
Things you might do with an electric motor or a hydraulic
machine can be done just as well (or better) with a pneumatic machine
(we'll come to why you might use one of these technologies rather
than another in a moment).
Air-powered tools are probably the most familiar, everyday example
of pneumatic technology (whenever I hear the word "pneumatic,"
the next word that leaps into my head is invariably "drill.")
Using either a piston-cylinder actuator or an air-powered motor,
virtually any kind of construction tool can be powered by compressed
air, from screwdrivers and hammers to wrenches, polishers, and
piledrivers.
Photo: Pneumatic power tools, like this sander, are lightweight and easy to use. Photo by Ryan D. McLearnon courtesy of
US Navy.
In the world of industry, factory machines are a much more
widespread and perhaps interesting application of pneumatics. When we
think of robots, we usually think of electronic circuits
making arms move using things like stepper motors (electric motors
that turn through precise amounts, one step at a time), but they're
just as likely to use hydraulics and pneumatics—as well, or
instead. Computer-controlled robots might be holding pneumatic
paint-sprayers, for example, or hydraulic cutting tools. Pneumatic
factory machines are widely used to pick up and place objects using
suction to hold things or squeeze them very gently, then release them
some time later. Robotic cow-milking machines that automatically
attach and release suction cups to an animal's udders are a clever
variation on this pneumatic theme, and they illustrate an important
advantage of pneumatics: it can apply force very gently.
Photo: A Porter-Cable pneumatic nail gun, powered by the blue compressed air tube you can see immediately underneath. Photo by Steve Chan courtesy of US Navy
and Wikimedia Commons.
Pneumatic devices and machines are also widely used in
transportation—and in quite a range of different ways. Since the 19th
century, pneumatic transport tube technology, has been widely used by banks, hospitals, and even
hamburger restaurants to move things swiftly and securely down
networks of air tubes from one part of a building to another.
The 21st-century's answer to Thomas Edison, electric car
pioneer and prominent inventor Elon Musk, has garnered lots of
publicity for an idea he calls Hyperloop, a high-speed railroad running inside
a giant, closed tube. Although it sounds similar to an ordinary pneumatic tube
system, it's radically different: the passenger cars zip through the low-pressure
tubes propelled by linear motors rather
than compressed air.
In pneumatic tube transportation, compressed air makes things move, but
in other systems, it can also bring things rapidly to a halt: for
example, it powers the air brakes in large vehicles such as trucks
and railroad engines and exploding automobile airbags
(though they're not really pneumatic, according to my strict definition).
Pneumatics might seem humdrum and dull, but it has entertaining
applications too. How about player pianos and pipe organs; exercise
machines where the resistance comes from pneumatic pistons; and even
bouncy castles? Nothing dull about any of those things!
Pneumatics or hydraulics?
As we've already seen, pneumatics and hydraulics use fluids to
move force and energy through simple (and not so simple) machines.
Both are well suited to making machines move back and forth
(reciprocal motion) and can operate at considerable distances from
their compressor or pump without any need for things like conveyor
belts or gears. Both can achieve considerable power with relatively
lightweight machines (that's one reason why dentists sometimes use
pneumatic tooth drills, which are extremely powerful but very light).
If both types of fluid power do similar jobs, why would you use a
pneumatic machine instead of a hydraulic one, or use either of these
in place of something electric? There are all sorts of different
considerations to weigh up.
Power
For medium-power, high-speed, applications where accuracy isn't
critical, and soft action or cushioning (force absorbing) are
important, pneumatic systems are often preferred to hydraulic ones.
Hydraulics tends to win for high power, high accuracy, high-force
transmitting applications, and any application where variations in
air temperature or pressure might cause problems for a pneumatic
system. But hydraulic machines do tend to move slowly.
Photo: Light and maneuverable, this pneumatic wedge catapult is perfect for launching drones
from onboard navy ships. Notice the pneumatic cylinder (the black thing just above the wheels). Photo by Patrick W. Mullen III courtesy of
US Navy.
Accuracy
Pneumatic machines are generally (but not always) less responsive
and less accurate than hydraulic ones, largely due to the
compressibility and relative unpredictability of air. They're less
efficient and more expensive to run, because it takes a relatively
large amount of electrical energy to run a compressor and store some
of that energy in compressed air—and a fair bit of that energy is wasted when
the spent gas is released as exhaust.
Control
Electric motors are relatively heavy, spinning things, so they
have a high moment of inertia—a kind of flywheel effect—that
makes them relatively slow to start and stop. That means they take
time to supply maximum force and time to stop or reverse.
Fluid-controlled machines, on the other hand, have relatively little
momentum, so they can (generally) be started, stopped, and reversed
more quickly.
Convenience
Pneumatic tools use lower pressures and smaller forces, so they
tend to be lighter and more compact, which can be extremely important
in the case of things like hand tools. Pneumatic tools might be made
of relatively light plastic, where hydraulic ones have to be made
from metals to cope with higher forces and pressures.
Safety
Unlike electric motors, pneumatic and hydraulic machines don't use
any electricity at the point where work is being done, so there's
less risk of a spark or gas explosion (a very important consideration
in places like underground mines). Also, unlike electric motors, both
are self-cooling (their fluids disperse heat), and there is no chance
of overheating or electrical burn out. Fluid leakage is a potential
problem in both hydraulic and pneumatic systems. While pneumatic
tools and machines invariably exhaust their working gas to the air
once it's expanded and done its job, hydraulic ones are sealed units
designed to keep the same fluid recirculating. Since hydraulic fluid
is flammable, pneumatic systems are inherently much safer than
hydraulic ones in dangerously explosive environments.
Practical operation
Since hydraulic machines use oil, they are self-lubricating and
often much quieter than pneumatic tools (which don't self lubricate). The compressors and exhausts of pneumatic tools can be especially noisy, though silencers can be fitted.
Pneumatics in a nutshell
So that's a basic overview of pneumatics—a technology that handles everything from balloon-powered
toys to air-driven drills. Next time you see someone using a power tool or machine, take a closer
look and see if you can figure out whether it's electric, hydraulic, or pneumatic. Can you figure it
out from the size of the force the machine is likely to need? (A bigger machine, like a crane or bulldozer, is much more likely to be
hydraulic.) Is the speed of the machine something of a giveaway? (Pneumatic machines tend to be faster than
hydraulic ones.) Is it the kind of thing that needs the gentle touch of pneumatics (like a cow-milking, machine)?
Is it noisy—is that the bang of a pneumatic exhaust you can hear and the drone of an air compressor in the background? Do safety considerations (for example, working underground or with explosive natural gas) make it more likely for the machine to be pneumatic than electric? Is the power line too thin for a pneumatic air hose and too flexible to be hydraulic (making it more likely to be electric?)? If you're an engineering geek, you can have a little bit of fun playing pneumatic eye-spy!
A brief history of pneumatics
Artwork: A milestone in pneumatic history. George Westinghouse's railroad brake from his 1869 patent.
The compressor (orange, right) powered by steam from the locomotive, fills a reservoir (blue, left) with compressed air.
When the brakes need to go on, this feeds pressurized air through the yellow hose to the brake cylinder (red), shifting
a piston back and forth, applying the brake shoes (purple) to the wheels (green) or releasing them.
From US Patent 88,929: Steam power brake by
George Westinghouse, April 13, 1869, courtesy of US Patent and Trademark Office.
- ~2000BCE: The pot bellows is invented to help people make fire
more efficiently. It's a pot with a leather skin stretched over it like
a flexible lid. You pump up and down on the skin to direct a powerful draft of air (and
oxygen) into a fire.
- ~350BCE?: Aristotle (384–382BCE) coins the physics principle
"nature abhors a vacuum," which is a (very indirect) way of
saying that air pressure is powerful and useful.
The Ancient Greeks also give us the word pneumatic (their word
"pneumatikos"), meaning air-powered.
- ~50CE?: Hero of Alexandria (10–70CE) develops a number of basic
machines powered by moving steam or air, including a simple spinning
steam engine (the aeolipile) and a wind-powered organ.
- Middle Ages: Leather bellows are invented; originally for
blacksmiths, they're later used as a source of pneumatic power in
things like organs.
- 1654: German physicist Otto von Guericke (1602–1686) develops the
vacuum pump. Famously, he demonstrates its power by pumping the air
from in between two huge hemispheres. Atmospheric air pressure clamps
the spheres together so firmly that teams of horses cannot pull them
apart.
- 1700: Frenchman Denis Papin (1647–1713) uses a waterwheel to
compress air and pipe it to a machine. (His work on pressurized gases
helps others to develop the steam engine and the diving bell.)
- ~1799: Scottish engineer William Murdoch (1754–1839) first
proposes the idea of pneumatic tube transport.
- 1799: British engineer George Medhurst (1759–1827) proposes an
early pneumatic ("atmospheric") railway powered by air pressure
instead of steam, including the idea of using compressed air to power
a motor (a device he calls an aeolian engine). In 1847,
Mechanics Magazine
summarizes his idea as "a new method of driving carriages
of all kinds without the use of horses, by means of an improved
aeolian engine, and which engine may also be applied to various other
useful purposes (the power applied to the machinery being compressed
air, and the power to compress the air being generally obtained by
wind, etc."
- 1865: British inventor George Law (about whom little seems to be known) develops the pneumatic rock
drill, in which compressed air pumping a piston bangs a hammer into
the ground. The following year, a four-cylinder drill is used to dig
the Hoosac tunnel in Massachusetts, USA.
- 1869: US industrial engineer George Westinghouse (1846–1914) is granted a patent for a
pneumatic steam railroad brake.
- 1879: While developing better methods for carrying cash around
buildings, US inventor William Stickney Lamson revives William
Murdoch's pneumatic tube transport idea, now widely known as the
Lamson tube.
- 1960s: Fluidic (fluid-logic) machinery first starts to become
popular.
- 2013: US inventor Elon Musk proposes Hyperloop, a mass-transit
people carrier based on giant pneumatic tubes, not unlike Lamson
tubes.
