When selecting a pneumatic cylinder, you must pay attention to:
A "single-acting" cylinder has a single port for compressed air.
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.
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.
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.
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.
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.
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
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.
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:
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:
This is addressed by similar hobby hacks:
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:
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).
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