FIRST air cannon robots PVC??

[Billfred]

Originally Posted by KenWittlief

I would look into using a pnuematic cylinder, with a mechanical trigger

if you try to use a cylinder to launch stuff the airflow into the cylinder is too restricted by the small connectors and hoses

to fix that, you put a mechanical stop, about half way down the cylinders travel, that holds the pressurized piston back

to fire the launcher you pressurize the cylinder against the trigger/ release pin, then you pull the pin out of the way, allowing the cylinder to extend all the way

in effect you have a large pnuematic spring.

I think you will find this takes much less air, than blowing Tee shirts and stuff out the end of a PVC tube. Also the thing you are launching does not have to fit snugly into the tube, you could put just about anything in there.

The plus for safety is you can build this will all standard (approved) pnuematic parts and valves, you wont have to worry about pressurized PVC failing in a room with thousand of people around.

It can still be made to look like a cannon, in fact you could release CO2 or smoke to make it look more omminous. Only you would know whats really happening inside.

I get the basic concept, but what would work for the pin? It'd have to be something strong enough to handle the force of the cylinder, yet also be able to be yanked out of place (or so I think).

[KenWittlief]

Originally Posted by Billfred

I get the basic concept, but what would work for the pin? It'd have to be something strong enough to handle the force of the cylinder, yet also be able to be yanked out of place (or so I think).

it could be something you pull like a trigger on a gun. It doent have to be pulled out of the way quickly, because the cylinder is already pressurized behind the stop.

Also, I think the largest cylinders you can get from the supply list has a 2 foot piston. That should be big enough to shoot just about anything!

[Gdeaver]

A Sabot casing for the tee shirts would cut down on the volume of air and flow rate needs. Properly designed the sabot casing would only go a couple feet. Sabots need a very smooth bore. Seam to remember a few casing designs for water balloons online. Starching and a tight roll will keep the shirt in a good projectile form.

[Matt H.]

Our team is working on a similar project. We considered the use of PVC, but decided to use a tank instead. Our local Walmart sells a 7 gal 120 psi tank for 20 dollars; the decision to use the air tank is about as expensive as PVC tanks would be for a similar volume and has left everyone on the team feeling safer. If you do use PVC consider netting around the PVC similar to that used around pressurized laboratory glassware.

[amos229]

not that i support nor encourage the use of pvc for pressure tanks i have heard however of wrapping the pvc tanks in many layers of duct tape incase of a blowout it prevents shard from injuring someone. just a thought.

[KenWittlief]

Im having a hard time with some of the ideas being presented of ways to make a PVC tank less dangerous by wrapping it with nets or tape or wire

that seems like a seat of the pants kinda thing to do. You would have to test the end product many times to see whether it contained all the little pieces, or whether it made things worse. You would have to repeatedly blow up PVC tanks in a controlled environment, and find a way to measure the energy of the resulting fragments. (Sound like a job for the MythBusters!)

And dont forget the concussion if it blows in a small area. It doesnt take much of a bang to permanently destroy your hearing.

This reminds me of people who put an X of tape across their windows or sheets of plywood, when a cat 5 hurricane is coming. When they return their house is completely gone!

[Nitroxextreme]

If an type of pressurized system blows in close proximity to your ears....ouch

[dtengineering]

We have had some luck with building a PVC water balloon cannon. I only let the students take it up to about 75 psi until well after they had graduated and were "responsible" for their own safety. We took it up to about 90-100 psi for a couple of shots, then destroyed it before fatigue (or cockiness) could set in. Your mileage may vary, compressed air is inherently dangerous and we took appropriate safety precautions.

The book "Backyard Ballistics" describes a pneumatic petard that was the basis for our design. I do recommend a video on youtube, however, http://www.youtube.com/watch?v=_zxlFoZhOC4 to illustrate both the majesty and power of compressed air.

Jason

[FourPenguins]

Originally Posted by KenWittlief

Im having a hard time with some of the ideas being presented of ways to make a PVC tank less dangerous by wrapping it with nets or tape or wire

that seems like a seat of the pants kinda thing to do. You would have to test the end product many times to see whether it contained all the little pieces, or whether it made things worse. You would have to repeatedly blow up PVC tanks in a controlled environment, and find a way to measure the energy of the resulting fragments. (Sound like a job for the MythBusters!)

And dont forget the concussion if it blows in a small area. It doesnt take much of a bang to permanently destroy your hearing.

This reminds me of people who put an X of tape across their windows or sheets of plywood, when a cat 5 hurricane is coming. When they return their house is completely gone!

I'm definitely not advocating PVC tanks. I'm just saying that if you're going to be an idiot, please be slightly less of an idiot.

[Donut]

A question on the irrigation valve; we've gotten it to open by applying power on the relay forward, do you need to apply reverse power to close it or just turn off the forward part?

[Andrew Blair]

The valve will close when power is removed. The solenoid is only used to open the valve- the return spring closes it.

[Arkorobotics]

Air cannon + PVC + tennis ball = http://www.duke.edu/~jwc13/tennisball.html

I think these guys went up to 125 PSI with PVC tubes.

If you are really scared of it blowing up, you can always add a pop safety valve.

[mizitchell]

i thaught about the whole pop-saftey valve but the only thing is if its rated for 125 PSI and its filled to 124 PSI when it fails what good does it do you?

[Arkorobotics]

Originally Posted by mizitchell

i thaught about the whole pop-saftey valve but the only thing is if its rated for 125 PSI and its filled to 124 PSI when it fails what good does it do you?

When you say "it fails" do you mean it explodes? Why would it explode if you were to cap off the pressure lower than its maximum? If you were to know the maximum PSI of your tube, you can then buy a pop safety value 40 - 50 PSI less than the maximum. Therefore you can't fail, unless all the stress on the tube from charge and discharge weakens the material.

[ahecht]

The problem is that you can never know what the maximum the tube will withstand is. Hairline fractures, sun damage, dents, improper gluing, manufacturing defects, etc, can all lower the amount of pressure needed for a catastrophic result.