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Pneumatic System Design
By Jon Szpakowski

To many, the hiss of compressed air conjures up childhood memories of theme park rides and haunted houses. The soft yet startling sound can be just as effective as music at setting the mood of a haunt. The use of compressed air pneumatics in commercial Haunted Attractions is well established. These systems are a very powerful, yet inexpensive alternative to electro-mechanical devices such as linear actuators. It is so inexpensive in fact that these systems have found a niche in Home Haunting as well. If used properly compressed air is safe and reliable. However, the use of extreme pressures and undersized or homemade cylinders can create a very dangerous situation, one that can cause injury, death or at least an expensive lawsuit. With an understanding of a few basic concepts and a little algebra (I know, I know, when you subscribed they promised no math, but trust me this is easy) you can custom design and build safe reliable pneumatic cylinders. Basically, a pneumatic system consists of compressed air, forced into a chamber that pushes a piston and therefore a rod, which is then attached to something you want to move. The size of the piston, and the pressure needed to move it has been guesswork until now.

Instructions for building PVC cylinders can be found on various prop oriented web sites (or see issue #14 of Haunted AttractionMagazine), however, these 'stock' designs may not always meet your needs. There is often a need to design your own cylinders, which are just the right size for the job.  This customizing creates less wasted air so that your compressor runs less often.

When designing your own cylinders the first thing you need to consider is the amount of weight you need to lift. A larger diameter cylinder, or bore, will allow you to lift more weight with less pressure. To find out how much weight a cylinder can support use the following balance formula:

F = P x A

'Balance' is achieved when the force (F) in pounds, equals the pressure (P) in pounds per square inch, multiplied by the area of the piston head (A) in square inches.

The piston head is a circle, and to calculate the area of a circle, multiply 3.14 (P) by the radius of the circle, squared. For example a 2 inch diameter piston head has a 1 inch radius and an area of 3.14 (3.14 x 1^2). Using the above calculation tells us that a 2 inch cylinder can support about 157 pounds with 50 PSI of compressed air (157 = 50 x 3.14). This 'Balance Point' is the amount of weight the cylinder can hold up at the specified pressure, however, since you want your props to pop out quickly in front of unsuspecting victim...err...patrons, we need to subtract 75% from this weight. That leaves a prop that must weight 39 pounds or less. If you need to lift more weight you can either increase the pressure (WARNING: exceeding 60 PSI is dangerous) or increase the area of the piston head (bore size). 

If you already have the weight of the prop and want to find out what size cylinder will lift it at what pressure, we can turn this formula around to solve for the of the piston head area.

A = F / P

'Balance' is achieved here when the area of the piston head (A) in square inches equals the force (F) in pounds, divided by the pressure (P) in pounds per square inch.

To lift a 22 pound prop, we increase this weight by 75% (remember we want it to pop out quickly), which gives us about 38.5 pounds. 50 PSI is a good, safe operating pressure so we will hold that as a constant. We can now plug in the numbers and solve the equation.

A = 38.5 / 50

A = .77 square inches

The radius of a circle is equal to the square root of, the area (A), divided by 3.14 ([insert symbol for pie]) and the diameter is twice the radius, (find a calculator to do the square root for you).  For a 22 pound prop pop up, diameter of the bore required is about 1 inch.  (However for a small prop such as this you may consider increasing the bore size in favor of lower (safer) operating pressures.)

The length of the cylinder (or throw), is determined by how far you want the prop to move. In some cases, like the Scare Factory corpselator, levers are used to increase the distance that the prop moves, without increasing the length or throw of the cylinder.

When choosing the size of the air inlet for your cylinder, bigger is better. The greater the airflow into the cylinder, the faster the prop will move. Use an air inlet that is as close as possible to the inside dimension (ID) of the air hose. The standard inlet connection is a brass nipple threaded into the PVC pipe, and care must be taken when installing and sealing this connection. It is usually the weakest link in the system, and I have seen more than a few blow out of the PVC cylinders.

When selecting an air compressor the volume of air it can supply (measured in cubic feet per minute) is the most important factor. Again, Bigger is better, but for most haunts a compressor that can provide 6.4 CFM @ 40 PSI should do the trick. The Home Haunter can do with 3.7 CFM @ 40 PSI unit. If the compressor has a relatively small tank, adding a portable air tank to the system will greatly increase the performance of your props, not to mention decrease the ware and tear on the compressor.

Another piece of the puzzle is how the air pressure is applied to the cylinder. If the prop only has one cylinder movement or you do not mind all your props activating at once, then a simple hand operated valve inline between the compressor and the props will serve the purpose. If on the other hand, the prop has multiple movements or you wish to activate multiple props at separate times, you will need to invest in solenoid valves to control the action. Inexpensive solenoid valves can even be scavenged off of old appliances such as washing machines and dish washers. Most solenoid valves are operated by 120 VAC household current and are connected between the air line and the cylinder. Now you can operate your entire attraction from a bank switches in one location, or the props can also be automated to activate on their own by using electric eyes and/or motion sensors.

In the last few years, pneumatics has drastically changed the direction of the Haunted Attraction industry, and Home Haunting as well. Using compressed air is relatively straight forward, and the concepts presented in this article will allow you to produce a very efficient system. Whether you are running one prop with one motion, or several props at once, these guidelines will provide you with excellent results. Do, however, remember that compressed air can be dangerous if used improperly. Use common sense when designing props, keep the air pressure below 60 PSI and thoroughly test each prop before they are used as a public display.

 

Jon Szpakowski is the Technical Supervisor for "A Haunting On The Ridge" in Lockport, NY. He can be reached at JonS999@aol.com

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