Most Powerful FIRST Team created T-Shirt Cannon [Archive]

NickE

22-05-2010, 02:13

Nice shots but I don't think I'd want to be near it when pressurized.

PVC really shouldn't be used to store compressed air. In short, pressure rated PVC is not rated to hold compressible gas and can fail violently when holding compressed air. See these threads for more information:
http://www.chiefdelphi.com/forums/showthread.php?t=75929
http://www.chiefdelphi.com/forums/showthread.php?t=77871

JohnFogarty

22-05-2010, 02:50

JohnFogarty

22-05-2010, 02:55

http://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html
Please do your reasearch we did this is "AIR" tests

Chris is me

22-05-2010, 03:36

I think we're ok...3inch 350psi PVC and we never go above 100psi that shot was at 80psi. Plus the mentor who built it with us he made the gun, is plastic engineer/machine specialist he would know a bit more about this that most people. Plus we have members of our team who create 1000psi potato guns with PVC.

PVC is not rated for air pressure, only liquid pressure. PVC can catastrophically fail at 80PSI, causing serious injury.

http://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html
Please do your reasearch we did this is "AIR" tests

Those aren't air tests. Those are liquid tests. "Please do your research".

Jack Jones

22-05-2010, 07:21

More research: (http://www.osha.gov/dts/hib/hib_data/hib19880520.html)

JohnFogarty

22-05-2010, 10:25

Correct me now if i'm wrong cause I did the right homework on ABS pipe. Safe or no?

MrForbes

22-05-2010, 10:34

PVC is NOT safe with compressed air....please don't use your cannon until you replace the pipe with something more suitable for use with compressed air.

ABS appears to be not as dangerous.

JohnFogarty

22-05-2010, 11:51

I'm not seeing this happen?? I have a special program protocol I put into my inventor. I have tried 3" Shcedule 40 and 80, with an air pressure simulation. using FIRST regulators and compressors. and the couplings we primed and had machined. The castostophic failure doesnt occur because the REG is INPUT 150psi OUTPUT only 50psi. we must have done almost 300-400 shots last night. Doing the reashearch on how the system we use flows air, the equalibrium is maintained everywhere. The back of the cannon has a dump tank which in a sense also has a quick disconnect. the cannon barrel is the same design used in most launchers never seen to fail..the barrel. Now i CAN SEE NOW THAT THE ACCUMULATORS WE MADE are possibly dangerous 2 1.5 foot long 3 inch pressure rated 370psi capped and primed with..intresingly a air PVC cap. ABS seems to no safer. only one type Air-Line Xtra is...and it's not sold anywhere. I forwarded this to our mentor and he said maybe aluminum, but we have homework to do because dual layered 3in shedule 40 layer with 4in sechudule 40 doesnt rupture unless you are at an air pressure of 1500psi...two of our members made a nailgun with these specifications.

Ian Curtis

22-05-2010, 11:51

PVC is not rated for air pressure, only liquid pressure. PVC can catastrophically fail at 80PSI, causing serious injury.

Those aren't air tests. Those are liquid tests. "Please do your research".

What is the difference between gas pressure and liquid pressure? If they are both Force/Area, how do they make the material fail in one case, and not in the other? Does it have something to do with the rate of expansion of a gas vs. a liquid?

Does it specify that they are liquid tests, or does everyone in the thermoplastics business just know that gas pressure is not liquid pressure?

I don't want air cannons blowing up in my face. :)

NickE

22-05-2010, 12:31

What is the difference between gas pressure and liquid pressure? If they are both Force/Area, how do they make the material fail in one case, and not in the other? Does it have something to do with the rate of expansion of a gas vs. a liquid?From another thread:
Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block ... if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it's original shape but rockets off of the surface it was on.

Hawiian Cadder

22-05-2010, 12:37

what i usually do is put the PVC tube inside a cardboard packaging tube, it adds strength and then even if the PVC ruptures, the cardboard prevents it from creating shrapnel.

Chris is me

22-05-2010, 13:52

I'm not seeing this happen?? I have a special program protocol I put into my inventor. I have tried 3" Shcedule 40 and 80, with an air pressure simulation. using FIRST regulators and compressors. and the couplings we primed and had machined. The castostophic failure doesnt occur because the REG is INPUT 150psi OUTPUT only 50psi.

The PSI doesn't matter. PVC can catastrophically fail without warning.

We must have done almost 300-400 shots last night. Doing the reashearch on how the system we use flows air, the equalibrium is maintained everywhere.

Unfortunately, PVC fails without warning. It could be the first shot, or the 1,000th shot, or never.

I forwarded this to our mentor and he said maybe aluminum, but we have homework to do because dual layered 3in shedule 40 layer with 4in sechudule 40 doesnt rupture unless you are at an air pressure of 1500psi...two of our members made a nailgun with these specifications.

Again, not air pressure, water pressure. Compressed air is different.

If you use aluminum and it ruptures, air will leak out, but it won't explode into tiny little pieces of shrapnel. PVC will.

JohnFogarty

22-05-2010, 13:58

Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block ... if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it's original shape but rockets off of the surface it was on.????

Umm Yeah the principle of compressing water has to happen other wise when you put your finger over the end of a hose nothing would happen. Water is compressable. 1000psi water jets are used to cut steel.....so again logic isn't following here. It's compressable to a very very small degree.

sdcantrell56

22-05-2010, 14:06

PAR_WIG1350

22-05-2010, 14:14

Would lamination (with carbon fiber or fiber glass) prevent fragmentation? Metal Mesh?

JohnFogarty

22-05-2010, 14:26

I may end up making my half inch thick lexan boxes after all, We probably won't be able to replace the PVC but i'm going to install bulletproof box around them for saftey. The dump tank on the gun however i'm not sure yet.
http://maikenmagic.com/?ref=gallery&dir=2010%20T-Shirt%20Launcher

Ian Curtis

22-05-2010, 14:43

Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block ... if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it's original shape but rockets off of the surface it was on.

I still don't see how that contributes to making the material fail. I can see how it would make the failure explosive, but it seems to me that if the gauge reads 100 psig, there is a net force on the tank walls of 100 lb/in^2, no? If not, how does the compression add a force that the gauge doesn't see? Don't pressure gauges just measure the deflection of a spring?

,4lex S.

22-05-2010, 15:45

I think the real problem is the 'glassy' behaviour of PVC at room temperature. This is a characteristic of the polymer, and means it will fail in a brittle manner (fatigue and brittle fracture, neither of which would be non-catastrophic in a pressure vessel) rather than the ductile failure mode of a tank made of most metals or a different plastic.

I don't get why people choose to build a PVC air cannon after all the warnings that have come out over the years. There has to be a safer way of doing this.

EDIT: fatigue is based on the pretty much uncontrollable microcracks in your material, it could fail after 1 cycle or a million, and it is pretty much impossible to tell when it will fail.

Dale

22-05-2010, 16:38

Eye protection too, please. If this thing fails in the way people have been describing it may be the last thing your operators see!

gblake

22-05-2010, 18:30

Water is most definitely incompressible.

Umm Yeah the principle of compressing water has to happen other wise when you put your finger over the end of a hose nothing would happen. Water is compressable. 1000psi water jets are used to cut steel.....so again logic isn't following here. It's compressable to a very very small degree.You are both mistaken.

SDCantrell - Water will compress when you press upon it, just like every ordinary real material will. However, as John said, it compresses very little, and for that reason pretending it is incompressible is a useful approximation.

John - Water's compression has nothing (or at least very little) to do with whether a 1000psi waterjet can cut something. That pressurized water will fly out of a nozzle and cut things is a separate topic from how much or how little it's volume changes when it is put under pressure.

Blake

Akash Rastogi

22-05-2010, 19:26

To be very blunt about this, can people please stop being unintelligent and just purchase something that is certified for air pressure instead of just assuming it will work for an air cannon?

Your robot for demo's doesn't need to come under any weight constraints so feel free to use a high capacity, industrial, CERTIFIED, tank.

ie: http://www.toolsoutlet.ca/osc/images/5-gallon%20air%20tank%20copy.png

Here's your testing: http://www.youtube.com/watch?v=Cal71EN5Gt8

Or you could be idiots like these guys and laugh about it: http://www.youtube.com/watch?v=AQo81nqx-W8&feature=related

Have fun with the shrapnel. ::ouch::

JohnFogarty

22-05-2010, 19:41

efoote868

22-05-2010, 23:19

Umm Yeah the principle of compressing water has to happen other wise when you put your finger over the end of a hose nothing would happen. Water is compressable. 1000psi water jets are used to cut steel.....so again logic isn't following here. It's compressable to a very very small degree.

The point everyone here is trying to make is better safe than sorry. Your PVC may be rated for pressure on the inside, but what happens when you've used your gun time and time again? You know, storage, moving it from place to place. What if you drop it? Chances are you'll make defects in the exterior, and it'll weaken as time goes on.

Your cannon has a chance of failure. What happens after failure is shrapnel, bits of plastic flying in every direction at ridiculous speeds.

By definition, liquid water isn't compressible (liquids have a definite volume). Since water doesn't violently expand or change shape when its depressurized, there is no chance that it'll take the plastic with it.

Jack Jones

23-05-2010, 00:27

http://blog.makezine.com/128388565_52a6211bd4.jpg

A good way to win the Darwin Award?

.

CENTURION

23-05-2010, 01:13

By definition, liquid water isn't compressible (liquids have a definite volume). Since water doesn't violently expand or change shape when its depressurized, there is no chance that it'll take the plastic with it.

Last I checked, liquid water IS compressible, just very little. Heck, a solid steel block is compressible, you just need to apply enough pressure :P

Call me a bit of a n00b, but I'm not sure I see the difference between water pressure and liquid pressure, PSI is PSI, no matter what is CREATING the pressure on the walls of the PVC, the pressure is still there, and (assuming it is regulated) the same amount in both cases.
I understand that the PVC wouldn't be safe in this situation, I just don't really understand the difference between the ratings (liquid vs. air)...

EricH

23-05-2010, 01:28

It's how it fails that's the issue. Water, you get an incompressible (or close enough as to be easily approximated as such) fluid that, if it's under some pressure (as it might be in a sprinkler system), and it gets an outlet, it will leave in a reasonably orderly fashion, spraying out until somebody turns the water off.

Air, on the other hand, will try to all leave at once, causing further mayhem as it breaks whatever it's in further. If you have something that's rated for air, presumably it would have failure modes to safely release the air even if something breaks.

What we're concerned about is the "mayhem" as the PVC breaks. I've seen PVC that's fallen from a height of about 3.5' break (fortunately, no pressurized air inside).

Bryan Herbst

24-05-2010, 17:02

Umm Yeah the principle of compressing water has to happen other wise when you put your finger over the end of a hose nothing would happen. Water is compressable. 1000psi water jets are used to cut steel.....so again logic isn't following here. It's compressable to a very very small degree.

Just to clear things up- The water in your hose wants to leave the hose at a constant rate. For example, lets say 5 gallons a minute. Now, if you have a one inch diameter opening at one end of your hose, the water will come out relatively gently. If you shrink that opening (such as with your thumb), the water still wants to exit the hose at 5g/min, but because the opening is now smaller, it must exit faster, and thus with more force.

On a side note, our team operates our tshirt cannon off a 5-gal tank designed for air pressure (craftsman tank, I believe). It works fantastically well. PVC can not only fail in the piping itself, but it is also highly susceptible to end caps and joints failing.

gorrilla

24-05-2010, 17:28

You commentary is unnecessary..we are having an intelligent discussion about why the dangers occur. I'm going to be getting our 1600psi firefighter air tank to use. soo thank you.

You should listen to Akash, he does know what he's talking about.

I'm glad to know that your getting a metal airtank It's alot easier to spend some money and be safe, than to pay a hospital to fix you.

By the way, a SCUBA tank would work fine, it holds more air at 3000psi, and is way more safe than any PVC storage tank you could practically come up with. Plus they have standard size fittings!

JohnFogarty

25-05-2010, 12:51

http://maikenmagic.com/photos/2010%20T-Shirt%20Launcher/0508101531.jpg
That's what we are going to use until we get a new tank, this one is rated 2560psi

Peter Matteson

25-05-2010, 15:18

From a safety standpoint I would not want to use a material that has unknown fatigue characteristics and can fail brittley creating shrapnel. You should only use a well characterized material that has a well researched fatigue characteristics for which you can get a Goodman diagram. Cyclical loading application like this are far harsher an environment than you realize. Proper design would keep the loading either below the endurance limit of the material or take it out of service before the failure occurs. This is why all aircraft parts have service life limits to prevent a fatigue failure like Aloha Flight 243 from happening.
http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243

Fatigue causes parts to fail after continually being cycled and they don't even have to be cycled close to their yeild or ultimate strength for this to be an issue. Please keep this in mind when designing something like this.

Azores

25-05-2010, 16:50

Peter Matteson

26-05-2010, 08:15

Off-topic sorry, but it's odd that Peter used Aloha 243 as an example... the episode of MayDay(?) that Discovery was playing this morning was about Aloha 243. I would not want my PVC t-shirt cannon doing that right next to me. Or anywhere within direct line of sight of me really.

When I was in undergrad that was the go to example of fatigue failure for all Stress Analysis/Strengths of Materials professors. I think the Minnesota bridge failure from a couple years ago might be replacing it though.

JohnFogarty

26-05-2010, 13:21

The Barrel and Dump tank are going to be left the same due to the fact our engineers have made it indestuctable. But on the underside our 2 1.5 footlong 3inch airtanks are getting replaced with 2 1/2 gallon airtanks. Our mentors are still confused and i'm wondering if anyone has proof of this happening to pressure rated PVC not crap PVC PRESSURE RATED ONLY! They had never heard of it happeneing only low grade and the internet does not provide any either.

billbo911

26-05-2010, 14:08

Off-topic sorry, but it's odd that Peter used Aloha 243 as an example... the episode of MayDay(?) that Discovery was playing this morning was about Aloha 243. I would not want my PVC t-shirt cannon doing that right next to me. Or anywhere within direct line of sight of me really.When I was in undergrad that was the go to example of fatigue failure for all Stress Analysis/Strengths of Materials professors. I think the Minnesota bridge failure from a couple years ago might be replacing it though.

What?? Pop Top Airlines being replaced by the Minnesota Disappearing Bridge? No way! Soda can airplanes are much cooler. Besides, that sucker landed safely. :ahh:

dtengineering

26-05-2010, 16:29

The argument here is "What is 'safe'?"

And the thing is... it varies. What I consider 'safe' to do on my own encompasses much, much more than what I consider 'safe' to do with my students.

Personally, I think that PVC cannons can be made safe enough for me to use, by myself... or with a few adult friends... away from students and the general public. But then again, I ride a motorbike, and bicycle to work in city traffic. Safe? Maybe not, but those are risks that as an individual adult that I'm willing to take.

But the risk of catastrophic failure is such that I no longer use PVC in compressed air applications with students. As slim as the risk may be, if a student were seriously injured and I were to be asked, "Were you aware of the potential for serious injury from this use of PVC?" and "Were you aware that PVC was not rated for compressed air applications?" and "Were you aware of safer options that could have achieved the same educational outcome?".... What would I say?

So in this context, rather than trying to argue over an absolute "safe" or "not safe" when using PVC for compressed air, I think it is sufficient to say that the potential for hazard does exist and that we should each evaluate that risk, and the steps required to mitigate that risk.

Be safe... whatever that means to you.

Jason

P.S. Thank you to the people who have provided references, etc. that have helped educate me as to the potential risk.

JamesCH95

26-05-2010, 16:45

Would lamination (with carbon fiber or fiber glass) prevent fragmentation? Metal Mesh?

Carbon fiber (and I believe fiberglass too) fail catastrophically, that is to say that once they yield, they fail entirely. I know some high-pressure air tanks are wrapped in CF or fiberglass, but they have a METAL tank on the inside that will yield and (I am guessing here) bleed off the compressed gases more slowly.

Eye protection too, please. If this thing fails in the way people have been describing it may be the last thing your operators see!

Or hear... wear very good hearing protection. I can't hear so good out of one ear since a soda bottle rocket blew up near me. You know, the little water-and-compressed-air-propelled dealies you can make with a 2L soda bottle and some cardboard.

Tanner

26-05-2010, 18:58

Not to put myself in this PVC argument, but I found this video interesting.

http://www.youtube.com/watch?v=9tg0CagbU9I

Though PVC is cheap, readily available, and easy to use I think there is a point to which it is better to use something else. Though that point depends on, as others have said, what is considered "safe".

-Tanner

JohnFogarty

26-05-2010, 20:24

Once again I keep seeing people posting videos of PVC being shot at with arrows..and .22 caliber handguns no proof that this happens on it's own.

Tanner

26-05-2010, 21:06

Once again I keep seeing people posting videos of PVC being shot at with arrows..and .22 caliber handguns no proof that this happens on it's own.

I'm not saying the cause is the same, but the results are what matters. One crack/ding/hole/some sort of stress causes disintegration.

-Tanner

Chris is me

27-05-2010, 00:50

Once again I keep seeing people posting videos of PVC being shot at with arrows..and .22 caliber handguns no proof that this happens on it's own.

Other than the multiple linked to videos, posts of people's first hand accounts, and OSHA safety expert warnings...

Brandon Holley

27-05-2010, 01:30

When I was in undergrad that was the go to example of fatigue failure for all Stress Analysis/Strengths of Materials professors. I think the Minnesota bridge failure from a couple years ago might be replacing it though.

We learned about the Aloha plane in my undergrad ME design class, so its still around.

Other than the multiple linked to videos, posts of people's first hand accounts, and OSHA safety expert warnings...

I've seen the aftermath first hand of a PVC cannon exploding. A member of our team while I was in high school constructed a PVC cannon on his own. We were in the parking lot at our school and he was launching some things off with it. He charged the tanks and was walking away from the group to fire a shot when he slipped and dropped the charged gun on the ground. The "storage tank" made from PVC EXPLODED, sending shrapnel everywhere, exactly as everyone is describing. The result was, him being lucky to be alive. He had a piece of PVC embedded in his skull and other parts of his body, and to this day (~4 years later) has a scar on his head that looks awfully similar to harry potters.

I'm not going to take a stance here as to "PVC is okay to use" or "PVC is not okay to use", but this first hand account may add some insight to people's decisions in the future. Sure, he dropped the cannon and it was his own fault, but anything can happen. Using the cannon is a lot more fun when no one is going to get hurt, regardless of how it happens. Theres a lot of energy stored inside a tank for a t-shirt cannon (look at it! you launched a tshirt across a football field!!) its important to be smart about it.

-Brando

efoote868

27-05-2010, 11:30

RMiller

27-05-2010, 12:33

I did a little internet searching on PVC material strength.

One thing that you need to know is that its rated for that PSI at a certain temperature - probably 20C. If the temperature is lower, the PVC becomes very brittle and loses its strength rapidly.

http://www.madisongroup.com/services/failure/casepipe/pipeanalysis.html

Also, there is a high end too. You hook that up to a compressor, which gets hot and bad things begin to happen as well. Hot water is enough to cause issues which is why even in liquid service, only cold water is allowed.

EricH

27-05-2010, 14:09

Food for thought: Has anybody else noticed that any time the Mythbusters are doing something with compressed gas of any sort, they use metal?

I wonder if there could be any connection there... Nah, can't be that PVC in compressed gas situations is dangerous, so insurance won't let them do it. That's hard to believe...[/:rolleyes:]

JamesCH95

27-05-2010, 15:43

I also seem to recall that in a particular experiment on Myth Busters (shooting a bird out of an air cannon?) they they were getting essentially the same results if they used around 100psi or around 20psi. I can't recall the exact details... Maybe a t-shirt cannon can operate at a much lower pressure, reducing the potential danger.

One could make the barrel wider to extract the same PV work from the lower-pressure air, but heaven forbid we actually do some engineering math :eek:

As an aside: I'd be amazed if any of the people on the show actually have a "pressure vessel" welding certification from ASME or AWS.

EricH

27-05-2010, 16:30

As an aside: I'd be amazed if any of the people on the show actually have a "pressure vessel" welding certification from ASME or AWS.
I would suspect that they mainly use stock tanks, or get someone else who does have it to build the custom ones.

Or, maybe someone does have one. I'd actually not be that surprised, with the range of experience they have available...

davidthefat

03-06-2010, 20:39

Now, that gives me an idea... Make a tshirt launcher on the existing robot, are 2 servos strong enough to support the launcher (obviously not the WHOLE launcher) I can just use the existing compressor and stuff just mount a barrel to the servos

JamesCH95

03-06-2010, 21:56

If you're talking about the hobby-type servos that come in the KOP then do the math:

Typical holding torque of about 2.8lb-inches

So it will hold 2.8lbs 1 inch from its center of rotation, 1lb 2.8 inches from its center of rotation, etc. You won't have much of a cannon...

apalrd

03-06-2010, 21:59

Of course, you could always make a servo with a big motor and a pot. A window motor would probably work, unless it was really really heavy, then you could go to a FP or gear it down a ton.

davidthefat

03-06-2010, 22:04

http://www.truetex.com/servomod.htm

that? I might ask one of the electrical people to do it, my mentor will NEVER trust me to mess around with motors, only as a programmer

apalrd

03-06-2010, 22:22

Why not just have feedback through whatever processor is already on the robot?

Mount the pot and motor (or a sprocket chained to the motor) on the shaft of the cannon. If there is no place for the pot it can be on a sprocket on the motor chain as well. If the shaft is hollow, you can put in a plastic (HDPE, Delrin, etc.) plug with a set-screw for the pot, and mount the pot with a piece of folded sheet metal.

Then, wire the pot to an analog input, and the motor, through a speed controller, to a PWM output.

You can then have fun writing and tuning PID code. If it dosen't have a whole lot of mass to it then P would work fine, if it can't get to the setpoint without overshooting you can add a little bit of I to help it along, and if it has a giant amount of mass (e.g. a big steel pipe) you can add D to help with momentum.

JamesCH95

04-06-2010, 08:43

billbo911

04-06-2010, 10:30

I don't think that any of the KOP motors are rated for continuous stall (the exact term is escaping me right now) the way a servo motor is generally used. To use a FP or CIM you would really want to gear it down through a worm gear, like the window motor, to prevent back-drive. Alternatively you could use a Dewalt or other drill transmission assuming that it uses an anti-back-drive mechanism.

Continuously running any motor in a stalled state is never a good idea. Even a hobby servo doesn't like it. Position feedback keeps the servo in a set position. Once the desired position is achieved, motor current is removed.

Your best bet is to keep the mechanism you are positioning in balance. That way, the motor is primarily used to move the mechanism, not hold it.

JohnFogarty

04-06-2010, 10:56

how about what we did, Lazy Susan+Window Motor

JamesCH95

04-06-2010, 11:16

Continuously running any motor in a stalled state is never a good idea. Even a hobby servo doesn't like it. Position feedback keeps the servo in a set position. Once the desired position is achieved, motor current is removed.

Your best bet is to keep the mechanism you are positioning in balance. That way, the motor is primarily used to move the mechanism, not hold it.

After doing a little more research I found that some motors have a published "Maximum Continuous Stall Torque" rating, usually a small fraction of their maximum rated torque. It would be safe to operate the motor under these conditions, though a non-KOP motor designed for this purpose would be capable of a higher sustained torque.

On GOOGLE Books one can find Handbook of Electric Motors By Hamid A. Toliyat, G. B. Kliman which has many of its pages readable and searchable for more information. It has block diagrams and equations one could use to start making a PID loop to control a DC motor.

I heartily agree with your second point, mounting the servo so it is close to the CG of what it's manipulating will undoubtedly reduce the load on whatever motor or servo is used.

David Brinza

27-06-2010, 08:19

Prior post reported.

martin417

28-06-2010, 08:52

Just a note about safety. In industry, there are several factors that are considered when characterizing the "safety" of a device or design. When considering a dangerous event that might occur, the two most important are:

Likelihood of the event occurring
Cost of the event occurring.

If either one of these is high, that device or design is considered unsafe, even if the other is low. The reason PVC is not rated for pressurized gas is the cost of failure, not the likelihood of failure. When gas is pressurized in a reservoir, energy is stored (not so much the case with non-compressible fluids like water or oil). If the reservoir fails, that energy is released. If the material is brittle plastic, that energy release is almost instantaneous, and is accompanied by flying shrapnel.

Metal reservoirs can also fail catastrophically. If you look at pressure vessels like SCUBA tanks, you will see dates stamped on the side. Each SCUBA tank must have a visual internal inspection every year (to look for corrosion or visible flaws), and a hydrostatic test every three years (the tank is filled with water and pressurized to measure the growth of the tank. Water is used instead of air so that if the tank fails, little or no energy will be released). With higher pressures and higher volumes come more energy and more danger. Here is the result of a SCUBA tank failure.

http://www.autospies.com/images/nos-explosion.jpg

So, PVC pipe is not more likely to fail when pressurized with gas instead of water, but the cost of that failure, in terms of safety, is much higher. Please consider the cost of failure of any design you wish to build, as well as the likelihood of that failure.

will_1359

06-08-2010, 12:02

lol i'm sure the boys doing the t-shirt cannon for team 1359 will take this as a challenge.

right now our most powerful launch we've had was using a 3 foot x 4in(i don't feel like getting the cubic inches) air reservoir(yes it was PvC, but it was the highest rated pressurized PvP we could find, what we're worried about blowing are the seals holding everything together), pumped to 125psi. It may have actually traveled further then a football field lol, might have to try it again.

only downside is that we havn't set up the robot to fire it yet, it's all hand operated.

JamesCH95

06-08-2010, 13:26

Doing the *quick* numbers shows that it holds on the order of 24,000joules of potential energy. That would be roughly equivalent to a 9lb weight going 170mph, think of a sledge hammer swinging many times faster than you could possibly hope to swing it, that's the sort of energy you're playing with here. Do a little Energy=Mass*(speed)^2 math and figure out what would happen if one of your end-caps blew off. Bad news.

will_1359

06-08-2010, 13:54

so, i'll have to finish my wireless robot-human are system to safely work on this.... oh well, i'll be wearing my safty goggles and cup, and not standing behind it,

JamesCH95

06-08-2010, 15:11

Worry a lot about hearing damage, sound is waves of air pressure, and you've got a lot of air pressure. An engineer I knew at a local research lab developed PTSD after an experimental fixture failed while storing a couple cubic feet of ~100psi air.

will_1359

06-08-2010, 17:30

when we fire ours it's most of the time at 80psi, the 125psi was just a test, and when we fire shirts at crowds it will be down to about 40-60 psi.

P.S. sorry for hijacking the thread lol.

you might be able to see this video
http://www.facebook.com/#!/video/video.php?v=125764667445518

JesseK

10-08-2010, 13:59

T-shirt cannons are a fun topic in general; looks like we may have some cannon fodder for the Mythbusters to test!

JamesCH95

10-08-2010, 14:58

Yes they are. I had several in high-school, including a propane powered one. I feel the fuel-powered cannons are safer because the cannon is not sitting at full pressure with any small cracks continuously growing waiting for failure. Instead the pressure is applied rapidly, where the high strain rate tends to result in higher material strengths, and the pressure is prevented from building to critical levels because the projectile is already flying out and there are inevitably leaks around the ignition source and the threaded-on end cap.

I also found that one only needs a small combustion chamber, my best one was 1ft of 4" pipe, it could utilize the full length of a 10ftx2in barrel.

The only failure I've ever experienced with a pvc cannon was the pneumatic one.