Pneumatic Cylinders

Compressed air is a form of energy. In order to put it to work, you need to turn the energy into something like motion. This is frequently done with a pneumatic cylinder.

Introduction
Action Types
Commercial Pneumatic Cylinders
- Sources
Improvised Pneumatic Cylinders
Warning: Compressed Air Is Special
Rotation Problems
Bore Size and Force Multiplication

Introduction

A pneumatic cylinder simply converts air pressure into linear motion.

When selecting a pneumatic cylinder, you must pay attention to:

how far the piston extends when activated, known as "stroke"

surface area of the piston face, known as "bore size"

action type
pressure rating, such as "50 PSI"
type of connection to each port, such as "1/4" NPT"
must be rated for compressed air use
mounting method

Action Types

By "action type", we mean how the cylinder acts when pressure is applied and removed.

A "single-acting" cylinder has a single port for compressed air.

[photo] When air pressure is removed, the cylinder does nothing to retract the piston. Whatever the piston pushed out must push the piston back in.

A cylinder that is "single-acting with spring return" has hookups like the "single-acting" cylinder.

[photo] When air pressure is removed, the spring pushes the piston back into the resting position.

In some cases the cylinder is sealed and the trapped air performs as an "air spring".

A single-acting cylinder with spring return requires more air pressure to activate because you are pushing against both the load and the spring.

A "double-acting" cylinder has two ports for compressed air.

[photo]

Note that a double-acting cylinder requires a more complex solenoid valve. When you want to return the piston to its resting position, you must not only apply pressure to the second port, but also open up the first port so that the gas in it can be expelled.

A double-acting cylinder consumes more air than the equivalent single-acting cylinder, because you need air to push the cylinder in each direction.

Commercial Pneumatic Cylinders

Pneumatic cylinders are available commercially in a wide range of sizes and designs.

Using a commercial pneumatic cylinder is not a guarantee of safety - you can still do bad things with good parts - but it does provide additional safety margins that improvised pneumatic cylinders simply can not provide.

Commercial pneumatic cylinders are surprisingly affordable and easy to find.

Sources

The web sites of pneumatic cylinder manufacturers can provide product information, as well as dealer referals:

Here are some vendors of surplus equipment who often have commercial pneumatic cylinders in stock:

You should also check on eBay. Pneumatic cylinders are almost always available there.

Improvised Pneumatic Cylinders

When anything is expensive or hard to obtain (or perceived as such), people turn to alternatives. For many years, builders of dark attractions and animated Halloween props have been tempted by various alternatives to commercial pneumatic cylinders.

Honestly, your best course of action is to find a commercial pneumatic cylinder. They are in fact relatively inexpensive, available in many different configurations, and easier to find than you think.

For those determined to improvise a pneumatic cylinder, the choices range from

Screen Door Closers [most common]
Bicycle Pumps
PVC Pipe [cheapest, most dangerous]

Please be aware that PVC pipe is the worst possible material to use for compressed air. It's tricky, because it seems to work really well, and is often used for water under pressure. But compressed air is special. And PVC is brittle, so when it does break under pressure, it explodes into many sharp fragments propelled at high speeds. You can die this way.

Warning: Compressed Air Is Special

Those who are fond of improvised pneumatic cylinders often argue that they are safe because they are adapted from materials that work fine for water under pressure. This is an incorrect, misleading, and dangerous argument.

Liquids and gasses react very differently when under pressure.

When you squeeze a quantity of liquid, it doesn't get significantly smaller. This is one of the basic characteristics of a liquid: it can flow to fill the shape of a container, but the volume is constant.

But even a child can push down on a bicycle pump and squeeze a quantity of air into a smaller space. When you squeeze something into a smaller space, you are putting energy into it. And when you stop squeezing, the energy wants to come back out.

Think of it this way:

A compressed liquid or solid can conduct energy
A compressed gas stores and conducts energy

So when you rupture a compressed air system, all of the energy stored inside bursts out. This is a lot different from a water leak. And a lot more dangerous.

Rotation Problems

As the name implies, pneumatic cylinders tend to be round. Since the piston is a round shape fitted into a round cavity, it might spin around. I sometimes call this "torsion effects".

In many cases, this is not a problem. But if you were doing something like putting a mask on the end of a piston to pop up and startle visitors to your haunt, you need to worry about this. Each time the piston pops up and down, it might twist a little, and after a few minutes the mask is pointing away from the audience.

This is addressed commercially by:

use of a special pneumatic cylinder that is designed not to rotate - look for "non-rotating" models
addition of an external guide rod parallel to the cylinder
use of a special pneumatic cylinder assemblies that incorporate a guide rod - look for "guided cylinders"

This is addressed by similar hobby hacks:

retrofitting a pneumatic cylinder with a square rod
addition of an external guide rod parallel to the cylinder
weights
bungees or springs to provide tension in the correct direction

Bore Size and Force Multiplication

"Fluid power" uses energy transmitted and controlled through the use of a pressurized fluid. To a scientist, "fluid" can mean either liquid or gas. So "fluid power" applies to both hydraulics (using pressurized liquid, like oil or water) and pneumatics (using compressed air or other gases).

The key to fluid power is Pascal's Law: "Pressure exerted by a confined fluid acts undiminished equally in all directions."

Consider this thought experiment:

You have a strong, sealed box full of oil, with two pistons sticking up from the box.
One piston has a surface area of 1 square inch pressing against the oil.
The other piston has a surface area of 10 square inches pressing against the oil.

Place a one-pound weight on the smaller piston. This will press the piston down with a force of 1 pound, exerting a pressure of 1 pound per square inch (PSI). [One pound weight pressing down on a 1-inch surface.]
Pascal's Law says that the same pressure of 1 PSI will be pressing out on all sides of the box.
The same pressure of 1 PSI will be pressing up against the inside surface of the larger piston.

But the larger piston has a surface area of 10 square inches, with a pressure of 1 pound pressing against each square inch of it.
So the larger piston will push up with a force of 10 pounds.
You don't get anything for free: For every inch you want to lift up the large piston, you must push down the small piston 10 inches! Remember that the volume of the oil inside the system is constant.

Because the pressure is the same everywhere inside the fluid system, the force is proportional to the surface area.

Multiplying the area of the bore (square inches) by the pressure (pounds per square inch) gives the output force of the cylinder (pounds).

Examples:

20 PSI input pressure X 2 square inches = 40 pounds of force
40 PSI input pressure X 3 square inches = 120 pounds of force

This explains how a hydraulic jack works - it multiplies the incoming force. It also explains why pneumatic devices can be dangerous!

Thank you for visiting. Your comments are welcome.
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