How do pneumatic machines use compressed air?

1. This machine pushes box after box onto a moving belt. Each push ends with a hiss. There is no motor on the pusher. So what is doing the pushing?
2. A pump called a compressor squeezes more and more air into a strong tank. The crowded air pushes hard on the tank's walls. That push is called pressure, and the dial shows how big it is.
3. Turn the first valve and squeezed air rushes down the hose into the left end of the cylinder. It pushes the piston along, and the rod pushes the box. The other valve lets the old air on the right escape.
4. To pull the piston back, swap the valves. Now air flows into the right end, and the left end is opened to the room. The used air escapes with a hiss, and the piston slides home.
5. Air in on one side pushes. Swap the valves and it pulls back. Bus doors, factory robots and the dentist's tools all work with squeezed air like this.

Tap each part and follow the air from the pump to the piston and out again.

• Compressor. The compressor is a pump driven by a motor. It sucks in ordinary air and squeezes it into the tank.
• Air tank. The strong steel tank stores the squeezed air until it is needed. Look through the window: the air inside is very crowded.
• Pressure gauge. The gauge shows the pressure: how hard the air in the tank pushes on each bit of the wall.
• Hoses. Thin, bendy hoses carry the air from the tank to the valves and on to the cylinder. They can snake around corners easily.
• Valves. Each valve has a turning channel inside. One way, it joins the tank to the cylinder. The other way, it lets the cylinder's air out. The valves decide which way the piston moves.
• Cylinder. The cylinder is a smooth tube, closed at both ends. Air can be let into either end, so the piston can be pushed both ways.
• Piston and rod. The piston fits snugly inside the tube. Air pushes on its flat face. The rod carries that push outside to do a job, like moving a box.
• Silencer. Used air leaves through the silencer. It breaks the rushing air into many tiny streams, which turns a loud bang into a soft hiss.

Tap each valve to send air in or let it out. Which settings push the piston out? Which bring it back? What happens when both sides get air?

Experiment: What happens if you send air into both ends of the cylinder at once?

1. Set the left valve to air in and the right valve to let out. Watch the piston.
2. Now swap them both. Listen for the hiss.
3. Then set both valves to air in. Does the piston move?

What we found: A piston only moves when the air pushes harder on one side than the other. Air in on the left and out on the right pushes it out. Swap them and it comes back. With air on both sides the pushes cancel, so it stays where it is.

Turn on JavaScript to try it yourself.

Push all 5 boxes onto the belt before the tank runs out of air. Gray boxes are heavy and need more pressure. More pressure uses more air.

Air is made of tiny bits called molecules that zoom around and bump into everything. Each bump is a tiny push. When a compressor squeezes lots of air into a tank, there are many more molecules in the same space, so there are many more bumps. All those bumps add up to a strong, steady push called pressure.

A pneumatic machine uses that push. A valve opens a path from the tank to one end of a cylinder. The crowded air rushes in and pushes on the piston. At the same time the other end is opened to the room, so the air there can get out of the way. The piston moves toward the side with less push.

To go back, the valves swap. The used air escapes through a silencer with a hiss. That hiss is the sound of pneumatics: you can hear it when a bus opens its doors or a truck sets its brakes.

Air can be squashed, a bit like a spring. That makes pneumatic machines quick and gentle, and a leak only lets out air. But it also means they are not good at holding a heavy thing perfectly still. For that job, machines use liquid instead. Our straw comparison is not perfect either: your breath is weak, while tank air pushes many times harder.

Key words

Pneumatics

Using squeezed air to make machines move.

Compressed air

Air that has been squeezed into a smaller space, so it pushes harder than the air around us.

Pressure

How hard the air pushes on each little patch of a surface.

Valve

A part that opens, closes or redirects the path for air.

Cylinder

A tube with a sliding piston that air can push along.

Check your thinking

1. What pushes the piston along the cylinder?

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   Squeezed air pushing on one side of it. Air from the tank pushes on the piston's face, and nothing much pushes back on the other side.

2. Where does the hissing sound come from?

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   Used air escaping to the room through the silencer. Before the piston can come back, the squeezed air behind it has to get out. It rushes out and hisses.

3. You want the piston to come back. What should the valves do?

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   Let the air out behind the piston and send air into the rod end. The piston moves away from the side with more push. So the push has to be on the rod side, with the other side open.

4. A heavy box will not move. What could you change so the same cylinder can push it?

   Show the answer

   Raise the air pressure. More pressure means a harder push on the same piston.

5. Why are pneumatic machines not good at holding a heavy load perfectly still?

   Show the answer

   Air can be squashed, so it gives a little like a spring. Push on trapped air and it squashes, so the piston moves a bit. Liquid does not squash, so liquid holds firm.