Mounting and Using Pnumatic Cylinders

So, you bought a pneumatic cylinder, or improvised one from a screen door closer or a bicycle pump. How do you actually put it to use?

The short answer is "mount the door-closer cylinder with the door-closer hardware." This is true enough, but leaves off the essential question of geometry: where in the prop do you mount the cylinder? This question is germane to all cylinders, improvised or commercial.

Safety First
The Problems
Some Applications
- Straight Up
- Sit Up

Safety First

The first thing that you must do is insure that there is no way that the pneumatic cylinder (or the device that it activates) can hurt somebody. This must take into account both the cylinder operating properly (e.g. "if the cylinder extends as far as possible, can it hit anybody?") but also possible failures (e.g. "if too much pressure gets in and the end blows off the cylinder, can it hit anybody?").

The Problems

Using a pneumatic cylinder isn't exactly rocket science, but there are some things you need to think about.

We can't provide solutions for everything that you might run into, but at least we can present you with some problems to think about before you start building.

Desired Motion

First, you have to think about what kind of motion you want:

What do you want to move?
How should it move (turn to the left; jump up; sit up)?
Where do you want it to go?

I think that it is important to use one cylinder per motion - especially when designing your first few props. For example, you might want to make a coffin that slams open, and a skeleton sits up - and be tempted to use a single cylinder to do them both. You might even have a drawing of pivots and bars that take the push of one cylinder and accomplish both motions.

Solutions like this are simple in theory, but may be very difficult to make work in practice. Getting the lid to open will take a lot of force. But it must be applied 90-degrees apart from the force needed to lift the skellie. I see problems with the asymmetrical force.

I suggest two pneumatic cylinders. One for the skellie, one for the coffin lid. Now you have two independent mechanisms, and if you don't perfect the lid (or it breaks halfway through the night), you just prop the lid open and the rest at least continues to work. [Note that you don't lose much scare by leaving the coffin open. You might even get more, because the victims think "oh, cute - a coffin with a dummy in it" and then it pops!]

Two pneumatic cylinders doesn't mean that you need two valves. You can take the output of the valve and split it in two. Since you want the lid to open first, I would put an adjustable flow restrictor in the line going to the skeleton. With a little adjustment you can slow down the skellie enough to guarantee that the lid opens first.

My point is that, by giving each motion a separate cylinder: the mechanism is simpler to design and build, the design is more likely to work properly, and if one part fails the other will still work.

Geometry

This problem involves figuring out where to put the cylinder in order to get the motion that you desire when it is activated.

Your best bet is to measure the cylinder at rest, and extended. Then think about two triangles, with triangle #1 having two sides exactly like those of triangle #1. They differ only in the length of the third side (the cylinder).

Cylinder Body

Now, where do you hide the body of the cylinder?

It's a fact of life that a normal pneumatic cylinder will less than double when activated. If the cylinder is 2' long at rest, when activated it will be a tad less than 4' long. [There are exceptions, multistage cylinders that telescope like radio antennas. We can't afford those.]
It works the other way, too. If you want something to jump up 2', you start with a 2' cylinder that extends to a total of 4'.
Where do you hide the extra 2' of cylinder while the prop is at rest?

In our Pumpkin Pop-Up, we put the cylinder inside the body of the prop.

Air Feed Position

In order to activate a pneumatic prop, you need a way to feed it a supply of compressed air. Commercial air cylinders usually have air connections nicely placed on the cylinder body pointing out, perpendicular to the axis of the cylinder.

Some Applications

Here are some examples of cylinder use and mounting.

Straight Up

The simplest use of a pneumatic cylinder is to make something pop straight up.

Please see our Pumpkin Pop-Up.

Sit Up

Here's how we mounted a screen door closer in our Mad Monk prop:
[photo]

Here's how we mounted a screen door closer in our pop-up skeleton prop:
[photo]

Here's how we mounted a screen door closer in our crate beast prop:
[photo] There are close-ups on the crate beast pneumatics page.

Thank you for visiting. Your comments are welcome.
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