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 electromechanical
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
