17-03-2009 12:21

s_forbes

Re: pic: T-Shirt Launcher, still needs a chasis

Nice revolver! That's something we wanted to add to our t-shirt cannon but never got around to doing. We were worried about air leaks where the revolver seals on the barrel... do you notice any issues like that with your set up? I like the use of 3 compressors too, how long is the shot to shot recharge time?

Very cool project, hope you get it mounted on a base soon and start shooting some engineering-enthusiasm into crowds.


EDIT: oh yeah, pvc and exploding and all that... someone's bound to go into more detail later in this thread

17-03-2009 12:36

IKE

Re: pic: T-Shirt Launcher, still needs a chasis

17-03-2009 13:07

gorrilla

Re: pic: T-Shirt Launcher, still needs a chasis

sweet!! how did you guys get revolvers' chamber to rotate? or is it manual?

17-03-2009 14:45

amariealbrecht

Re: pic: T-Shirt Launcher, still needs a chasis

Thats awesome...good work!
Hopet that you can have some fun with that later in the season, good choice of an offseason project!

17-03-2009 14:48

tseres

Re: pic: T-Shirt Launcher, still needs a chasis

forget a chassis!!!!! i want that thing shoulder-mounted!!!

17-03-2009 14:54

Zach O

Re: pic: T-Shirt Launcher, still needs a chasis

Man that's cool! Nice revolver design also. How does loading work?

17-03-2009 19:35

surferacf

Re: pic: T-Shirt Launcher, still needs a chasis

I see so much WIN in this picture... I want to cry.

17-03-2009 20:37

Creator Mat

Re: pic: T-Shirt Launcher, still needs a chasis

got to do something with all that old equipment laying around.

An yes that is amazing.

17-03-2009 20:46

reversed_rocker

Re: pic: T-Shirt Launcher, still needs a chasis

alright i'll try to answer the questions in the order they arrived:

1.The revolver hasnt caused any leaking, the chambers are slightly smaller than the barrel itself and are mounted to a single piece of acrylic. Also, the parts were all CNCed so they fit together extremely well. Theres a switch at the bottom of the revolver that will stops it each time a new chamber is lined up. Since the chambers are cylindrical, the switch will only trigger at the absolute lowest point on the chamber (which also happens to be the center).

2. We used all pressure rated PVC and never let it get above 60 psi, no matter how much we want to . I know where you're coming from tho, using PVC gave me the jitters at first

3. I havent really timed how long it takes to fill with air... im guessing around 20 seconds? i'll see if i can get you a more proper estimate.

4. There are 3 motors on the cannon, one to rotate the base for aiming, one on a worm gear to change the launch angle, and finally one to rotate the revolver.

5.You can load tshirts in any chamber thats not in the barrel. You just shove the tshirt in any of the rotating chambers that you can get to. Normally the revolver spins only when the cannon is shot, but we also put a loading button on the joystick so we can cycle through the chambers to load them all. When we're firing, i mentioned the switch below the revolver earlier. After each shot, the revolver spins until the bottom of the chamber triggers the switch. If anyone's interested i can post the code

at the moment we have two chasis for consideration. We are either going to build a new chasis with tank treads or possible 6 wheel tank drive, or steal the chasis and crab drive from our lunacy bot (with much bigger off road tires of course) I for one am voting for the tank treads

17-03-2009 21:00

artdutra04

Re: pic: T-Shirt Launcher, still needs a chasis

Rule #1 when making air cannons is do not use PVC.

Rule #2 is DO NOT USE PVC.

There are huge safety concerns with PVC, because PVC IS NOT DESIGNED TO HOLD ANY KIND OF COMPRESSED GAS, AND HAS BEEN BANNED BY OSHA FOR USE WITH PRESSURIZED GAS IN INDUSTRY. PVC has the awful tendency to spontaneously explode when pressurized with any kind of gas, even if that pressure is below the maximum pressure stamped on the pipe. (The max pressure rating for PVC is ONLY for liquids, such as water).

On a personal note, one of my friends had a PVC air cannon explode while holding it a few weeks ago. He wasn't even moving, and the air cannon didn't hit anything. He was just standing there, holding the pressurized air cannon, when it spontaneously exploded between his arm and his body. He was very lucky in that the way the shrapnel flew out, nearly all of it missed him. His jacket on that side of his body was not so lucky, and literally was torn to shreds.

Since this air cannon is being shown as for "marketing", it seems like it would most likely be used at public appearances. In order to protect any innocent bystanders from possible exploding PVC shrapnel, I would seriously urge your team to avoid using PVC on the air cannon at all costs.

I don't want to sound negative here, but at the same time I don't want to see people put themselves in dangerous situations.

17-03-2009 21:15

ZInventor

Re: pic: T-Shirt Launcher, still needs a chasis

I agree with the concerns about the PVC, but, is there an alternative that would be anywhere light enough?(not metal)

I've seen 400PSI rated ABS blow a hole in the ground at 60PSI when a cannon of mine fired, not a pretty site...

overall, the only solution i have found is to wrap the whole thing in Molecular Bonding Tape (adheres to itself on a molecular level giving a shock strength of 1500PSI) then again in duct tape for appearance (and cachage of schrapnel that gets through the molecular tape)

after that, i've gone to metal for most of my cannons... since it'll bend, not shatter if it has to...

Do you by any chance have Inventor drawings of this???

I've been working on a similar project,but haven't gotten the revolving part down quite yet...

video? Please?

thanks,

-Z

17-03-2009 22:25

IceStorm

Re: pic: T-Shirt Launcher, still needs a chasis

so are there any drawings of this thing out there that you would be willing to share with other teams. I would love to look closer at it.

i totally agree about upgrading to metal though. the last tater cannon we built was air powered and we actually had a remote firing system on it so that we could be in the barn watching it incase it blew up.

17-03-2009 22:48

Fe_Will

Re: pic: T-Shirt Launcher, still needs a chasis

Since you have access to a CNC I recommend Using a Geneva Drive to index the chambers. We have some experience with one and it works well if manufactured to close tolerances and well lubed.

17-03-2009 23:56

reversed_rocker

Re: pic: T-Shirt Launcher, still needs a chasis

i'll talk to one of the mechanical guys and see if we can get some drawings out on the internet (they lock me in a dark closet and make me write code all day). As for the exploding part, the only part of the cannon that holds pressure is the air tank on the bottom of the barrel, i'll see if we can get that replaced with a safer material, all other valves, pipes, ect. are metal.

Video: http://www.youtube.com/watch?v=0cHf3iY4h9k

sorry the video isnt that great, we finished the cannon right before build season. the website isnt quite up to date either.

18-03-2009 11:51

ahecht

Re: pic: T-Shirt Launcher, still needs a chasis

If you're concerned about weight of using metal, don't forget that only your pressure reservoir needs to be metal. If you store your air at 120psi (or higher) you can get away with quite a small reservior.

18-03-2009 12:51

Bruceb

Re: pic: T-Shirt Launcher, still needs a chasis

I knew we would get some warnings about the PVC. We have 2 GE engineers on our team and they don't see how this PVC can be a problem. We did not buy the cheap unrated PVC from Manards, this is all pressure tested and pressure rated PVC. The 3 in dia reservoir is rated at something like 320 psi. Don't remember the exact spec but it was WAY higher than the 60psi we intend to carry at max. I was thinking about wraping it in fiberglass cloth and epoxy just for safety but I realy don't think it is an issue. I can't help thinking that the accidents have been caused by using the unrated pipe or overpressurizing it. Any way. The chambers are gasketed at both ends with a spring loaded deal that seals both ends of the chamber. We do get a little leakage here but not much. I think at max elevation and 60 psi we could launch a rolled up t-shirt about 200 ft. Thats a guess as we have not tried it outdoors yet but it hits real far up the wall indoors.
Bruce

18-03-2009 14:27

IceStorm

Re: pic: T-Shirt Launcher, still needs a chasis

Before I really got involved with FRC too much we had actually built two different potatoes cannons that used air pressure. We were doing a very similar design only difference was that our barrel actually went down through the air tank but we were easily pushing 100 + psi into the 4 inch air tank with no long term issues. Again though we were always careful to make sure that we were using at a very minimum schedule 40 PVC and in most cases used SCH60 pvc so that we would have at least a 3x safety factor on our intended PSI and the max we could do.

So what also has me extremely curious is how do you fold/roll the Tshirts so that they come out of the barrel and stay somewhat together on the way to there intended target and not just unfold and act like a big sail/parachute and come right down to the ground?

18-03-2009 15:11

Bruceb

Re: pic: T-Shirt Launcher, still needs a chasis

Fold them in thirds the long way so they are about 10 inches wide then roll them up. Secret weapon.....rubber bands.
Bruce

18-03-2009 15:16

Bruceb

Re: pic: T-Shirt Launcher, still needs a chasis

Sorry, I misspoke a bit before. The cannon barrel is 3" pvc with a pressure rating of 260psi and the tank is 4" pvc with a pressure rating of 220 psi.
Bruce

18-03-2009 15:30

billbo911

Re: pic: T-Shirt Launcher, still needs a chasis

This is exactly the folding method we used when I mentored 1147. One variation, we didn't use rubber bands, we used electrical tape. Rubber bands can come off in the chamber or barrel yielding unsatisfactory and unpredictable flight characteristics.

03-04-2009 13:00

Steigerwald

Re: pic: T-Shirt Launcher, still needs a chasis

I would like to point out that the components on that gun are not assembled correctly and they are not all pressure rated as you say. The components on the barrel are not assembled correctly. Those are also the components that are the most sensitive to blowing out because they receive a large pressure shock when the gun is fired.



You use a tapered bell reducer to go from the valve size to the barrel size. These components are not available in pressure ratings. While they are Schedule 40 components, that does not mean that they can be run at the same pressure as other schedule 40 parts. They take a large amount of force pushing them apart and because of that, they do not rate them for pressure.



Additionally, the components are not even pushed in all the way, this is very bad as the strength of the joint is based on the amount of material which is bonded together.

I would recommend you reconsider your design. I believe you are mistaken that this is acceptable for pressure. It is a great design and I would love to see it work, however, it is not safe and I would hate to see someone get hurt because of a explosion.

03-04-2009 13:13

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

Interesting....I wonder what the actual maximum pressure is on the T shirt side of the valve? Mainly I wonder this because we have a similar situation, using PVC (although not with a reducer like that) on our T shirt cannon on the same side of a very similar valve.

04-04-2009 00:47

artdutra04

Re: pic: T-Shirt Launcher, still needs a chasis

Let's say that an air cannon has a 120 PSI storage accumulator, and when the valve is opened, for some reason the tee shirt jams in the barrel. Since the 120 PSI just expanded to a slightly larger volume (between the valve and the tee shirt), the overall pressure will drop slightly, but will still remain very high. Regardless of whether it jams or not, all fittings in the entire shooter have to be able to withstand the maximum pressure in the accumulator, because all of the fittings near the barrel will see similar pressures in very short bursts.

04-04-2009 01:01

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

Let's assume that I'm trying to figure out what the actual maximum pressure is, and that T shirts don't "jam" in the barrel...at most it takes about five pounds of force to move the shirt thru the barrel. Since the barrel bore has about 5 or so square inches of cross sectional area, the shirt will begin to move when the pressure in the barrel exceeds about one psi. The actual pressure probably depends on the mass of the shirt, and how fast it accelerates, how fast the valve opens, and all kinds of other interesting dynamic things that would be pretty hard to calculate.

04-04-2009 10:49

IceStorm

Re: pic: T-Shirt Launcher, still needs a chasis

How could a T-Shirt ever Jam inside? And even if it did wouldn't the air still be able to pass through the T-Shirt.

Before I got involved with anything FRC my buddies and myself used to build air powered pottato cannons that used very similar parts. The only difference is that we had our air storage chamber wrap around our barrel and their was a bell reducer at the end of that with the barrel sticking out of it. The only issue that we ever encountered was once the glue let loose and the barrel slid out. This happened well before we ever hit our highest PSI level.

And speaking of PSI level the highest we ever put through the PVC was nearly 200 psi. Now in saying that we were using solenoids so that the cannon would be well away from us as we both pressurized and fired it so that if something did blow up we would be inside of the polebarn looking out a door or window.

04-04-2009 12:31

Daniel_LaFleur

Re: pic: T-Shirt Launcher, still needs a chasis

I can tell you, first hand, that those warnings are for good reason. I have had a 1 liter, 400+ PSI pressure rated PVC volume explode less than 2 feet away from me while holding less than 100PSI air. I can tell you that I am truely lucky that I came away with no injury (to speak of ...).

This accident was not the result of unrated PVC or overpressurizing it, or even shock loading it (Like you are doing with the cannon). This was the nature of PVC and of how it fails. PVC gives little to no warning before it fails and when it fails it fails catastrophically.

While I Highly recommend that you replace the PVC, if you are dead set on using PVC then I would suggest wrapping all of the PVC parts (My suggestion would be Twaron over your idea of Fiberglass).

While building devices like this is cool and a great learning experiance, please, please, please don't ignore the safety concerns just because you don't think there is an issue. You need to Know all the safety issues, especially if it's going to be shown to the public.

04-04-2009 13:14

Steigerwald

Re: pic: T-Shirt Launcher, still needs a chasis

It really does not matter whether there is a clog in the barrel. Assume there is one by some random occurrence. That would mean that the bell reducer which is not pressure rated would be experiencing the full pressure. Not only that, but when you open the valve, there is a sudden change in the pressure which causes a shock to the assembly making it more likely to blow out.

Every component on a project like this one has to be rated to the absolute maximum pressure that is to be put in. If air flows through a component, there is a chance that it will be at high pressure. Possibly not under normal conditions, but you should always plan for the worst.

04-04-2009 14:18

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

Such as?.....

04-04-2009 15:26

Steigerwald

Re: pic: T-Shirt Launcher, still needs a chasis

If that revolving mechanism gets stopped halfway between the barrels when there is a shirt in both slots. While the air may eventually leak out. It will be a full pressure for a brief time.

04-04-2009 15:31

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

I don't see how it could develop full pressure, one or both shirts will move as soon as pressure builds up to a few psi.

04-04-2009 15:40

Fe_Will

Re: pic: T-Shirt Launcher, still needs a chasis

For all of the posters concerned about safety:

Why don't you help this team find properly rated parts so this design can be safe, find a suitable alternative that serves the same function or, post a new design altogether...

Over half this thread is, " OMG PVC EXPLODES!!!!" It reminds me of the propeller threads...

Finding a solution > Whining about the problem

P.S. My team is working on a launcher of our own and we are excited about the PR opportunity it brings.

04-04-2009 15:48

Steigerwald

Re: pic: T-Shirt Launcher, still needs a chasis

I'm not very good with paint, but here is what I mean. Say the barrel is 1 and where the tshirts are is number 2. Assume that both tshirt tubes are filled and have something in them. The shirts would not be able to move. Assuming they are relatively packed in the barrel, they will let air through, but it will not be instant and it will expose the assembly to the full pressure.

04-04-2009 16:00

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

Since neither tube is sealed at the back end where air enters, the air will all leak out immediately. It won't have to go thru the shirts.

04-04-2009 16:36

artdutra04

Re: pic: T-Shirt Launcher, still needs a chasis

You should always design things for the worst case scenario.

99.997% of the time there aren't any 100-year-storm hurricanes in New York City. Does that mean that the structural engineers there should ignore designing their buildings to withstand 150+ mph winds? Absolutely not!

"will move" is not the same as "should move", the latter of which is more correct in assuming the worst case scenario. The engineers behind the Tacoma Narrows Bridge thought that wind would never pose a problem to bridge design.

The levees around New Orleans weren't designed well enough to protect against a direct hit of a Category Five hurricane. Engineers knew about the problem, but political leaders thought the risks of a direct strike weren't high enough to divert billions to rectify the problem as quickly as possible.

How about if someone told you that all car accident rollovers will always happen at only high speeds, and that it is impossible for someone to flip over a car going only 10 mph? And yet, it's been done.

The I-35 Bridge in Minnesota was known to be structurally deficient as far back as 1990 (along with design flaws), but again the powers-that-be deemed the risk to be low enough to carry on as is without spending money to fix the problem immediately.

PVC also has a design flaw in that it is very brittle, and several people here have posted in this thread with real life examples of what happens with PVC fails. The odds of an accident happening are low, just as the odds were low with New Orleans and the I-35 Bridge.

But ask yourself this: do you want to be the one responsible for making the decision of "don't worry, the risk of PVC exploding is low, so it's safe" if something does happen?


If you search through this thread, and the dozens of similar threads here on ChiefDelphi over the past few years about air cannons, you'll find people suggesting plenty of alternative pipe materials, such as ABS, aluminum, steel, copper, brass, or polycarbonate. Of these, steel and copper fittings are extremely easy to get at your local Home Depot or Lowe's, and brass fittings can be found at most truck part stores in the Hydraulics section. All of these can be ordered from McMaster or similar online retailers and be at your school within a few days.

The reason people keep saying " OMG PVC EXPLODES!!!!" is because there are a lot of people don't believe the warnings, and keep insisting that there isn't any problems with PVC. Eventually I'll stop caring to convince these people (I haven't reached that point... yet), and if I see any thread that deals with PVC air cannons, I'll just avoid it like the plague. If these people are seriously convinced there is no danger with PVC, then nothing we write here will convince them otherwise until an accident happens, and all we can hope for is that no one gets injured in the process.

04-04-2009 17:11

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

Thanks for your concern, Art, keep it up! I agree that pressurized PVC is a bad thing. I'm not so sure that designs that can't build pressure on the PVC part should be dismissed out of hand, on the vague supposition that "something" could go wrong. If there is a specific concern or scenario that could cause problems, please let us know. Bringing up unrelated cases of bad judgement by engineers or politicians doesn't really help us evaluate this case.

04-04-2009 18:17

Steigerwald

Re: pic: T-Shirt Launcher, still needs a chasis

I respectfully disagree, I believe that is exactly what is needed here. I think that people need to see that sometimes things that aren't expected happen.

I have seen that same argument that things don't have to be designed for the worst case scenario on our robot. Why do the construction properly the first time when you can just fix it in competition. Why over design something the first time when you can just make it work temporarily. It's things like that that make a huge difference.

04-04-2009 18:40

EricH

Re: pic: T-Shirt Launcher, still needs a chasis

Especially in elims, when that part that wasn't designed/built right fails suddenly because it took a load that it couldn't take.

Do it right the first time, and it'll keep going. Right, Mr. Lavery?

As for "How could a T-shirt jam?", most launched T-shirts are wrapped somehow for distance reasons. Usually, it's a rubber band or three. Guess what? Said rubber band or other wrapping has a much higher coefficient of friction than a T-shirt, and may stick in the barrel. The T-shirt then sticks too, and you suddenly have a jam. Now you have pressure that can't get the shirt out and can't go elsewhere. Your best bet in this case is to drain pressure immediately and use a stick to remove the shirt, but sometimes, it takes care of that for you, in spectacular fashion.

04-04-2009 18:46

Mr. Freeman

Re: pic: T-Shirt Launcher, still needs a chasis

Yes, but there is a point at which it reaches absurdity. Bringing up cases of bridges collapsing doesn't help anyone determine failure modes of a potato cannon.
I do agree though that this cannon should be able to withstand a T-shirt getting stuck in the barrel. I do see this as actually being a possibility due to T-shirts being pressed into the barrel too tightly, rubber bands around them increasing friction with the barrel, etc. That said, air will move through the T-shirts. I don't know how fast, but it might not be as bad as being completely clogged.

Anyway, on the subject of PVC. Why do I keep hearing stories about pressure rated PVC exploding? Air/water shouldn't make any difference regarding what pressure it fails at, only what happens AFTER the PVC fails.

Seems to me that if PVC rated at 400+ PSI fails at anything less than 400 PSI then it was either:
A) Not actually rated
B) Not properly used (exposed to conditions it wasn't rated for, etc.)

04-04-2009 19:12

gorrilla

Re: pic: T-Shirt Launcher, still needs a chasis

actually,

since PVC is pressure rated for water only....

Air compresses differently than water(dont remember the exact science)....

which is why, PVC can(not will)-fail at much lower psi's than its rated for....

04-04-2009 19:37

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

A compressed gas has far more energy stored in it than a liquid compressed to the same pressure. It took a lot more work to compress the gas, and it's volume changed considerably. Liquids are "incompressible", meaning the volume will change very little while the pressure increases greatly. It's this energy that causes problems with pressurized air in PVC, the plastic can shatter and sharp pieces of it will fly at high speed all over the place.

If you're making something that will hold pressurized air, make it out of something besides PVC.

04-04-2009 21:12

ubermeister

Re: pic: T-Shirt Launcher, still needs a chasis

This means that your cloth T-shirt is able to hold more pressure than your welded 3/16" thick pressure-rated plastic. Just sayin...

I built an airgun (2, actually) and have had the projectile jam in the barrel. The air just leaked out of the poor seal between the movable barrel and its seat. And that seal is (I think) much better than this one. And that was at 120 PSI.

I'm just trying to say that you should be more worried about the chamber, and that your very awesome airgun does not need a complete redesign.

04-04-2009 21:22

XXShadowXX

Re: pic: T-Shirt Launcher, still needs a chasis

PVC is rated to 350 psi of air by UL;
I got this from a team mentor who builds PVC potato cannons, and has never had one explode at pressures as high as 150 psi (fired like a bazooka)

04-04-2009 23:22

Steigerwald

Re: pic: T-Shirt Launcher, still needs a chasis

Well put!

The fact is that on this cannon, the barrel assembly is not made properly. The fittings are not inserted and some are not pressure rated. The need to be replaced to be safe. Period.

You asked for a way to solve the problem instead of just complaining? Fine, here you go!
Plumbing Supply
McMaster

I'll put up some pictures of what I have done which would work perfectly for your application.

05-04-2009 03:14

Mr. Freeman

Re: pic: T-Shirt Launcher, still needs a chasis

Do you have a reference for this by any chance. I agree that compressed gas has more stored energy than a liquid under pressure, but it doesn't make sense that this would cause the PVC to fail, as long as the force exerted on the PVC remains the same in both cases.

05-04-2009 10:19

MrForbes

Re: pic: T-Shirt Launcher, still needs a chasis

Read the warnings provided by PVC pipe manufacturers....

"WARNING: Do not use PVC pipe for pressurized air systems. Injury or death may result due to the catastrophic nature of pipe failure should failure occur. Rapid expansion of compressed air could propel shards of plastic throughout the area."

05-04-2009 12:56

Daniel_LaFleur

Re: pic: T-Shirt Launcher, still needs a chasis

Apparently you did not read my previous post.

PVC is not rated for compressed gasses. It is only rated for liquids. The difference is not in the pressure itself (as 400 PSI is 400 PSI). The difference is in the failure effect. With liquid the PVC will split and release it's pressure quickly (as liquid is only compressed as far as the gas within the liquid allows) while the gas will rapidly expand and sunder the PVC causing it to expel the shards in all directions.

Whats worse is that PVC gives no (or very little) warning that it is about to fail and that the PVC item in question may have been operating "properly" for years (as was the case with the 1 liter volume I discribed earlier).

Oh, and another thing ... The forces on the barrel and breach are almost instantanious (sp?), whereas the t-shirt takes time to be expelled from the barrel ... thus the entire force of the air is applied to every surface (universal gas laws) within the cannon. So when you fire the cannon it puts almost 60 PSI (assuming thats the pressure you are using) on every portion of that cannon being used until the t-shirt leaves the barrel. Not only that but those forces are applied as a shock load which is about as bad as you can apply to PVC due to its brittleness properties.


as to another question in this thread:
And as far as making PVC safe for use with compressed air ... the only way you can do this is to ensure that the blast from a catastrophic failure (when it happends) is contained.

And lastly ... if someone doesn't want to listen and makes a pneumatic device out of PVC, well I personally don't care too much. If they want to hurt themselves thats fine. But when they state that they are going to use it for 'marketing' then I have to stand up and be heard, because those near the device may not fully understand the dangers that the device presents.

05-04-2009 14:35

ZInventor

Re: pic: T-Shirt Launcher, still needs a chasis

i just say if you're going to put it on a chassis anyways, give the drive strong motors, and make the whole think out of steel.

that's what i'm working on right now, is backpack version (with exoskeleton like legs to carry the incredible weight (over 150 lbs)

when it's done, it'll be a dual-wield (one for each arm) 10 or 12 shot revolver (total of 20-24 shots)...we'll see if i can make it work...

-Z

05-04-2009 19:33

Mr. Freeman

Re: pic: T-Shirt Launcher, still needs a chasis

I did read your previous post. My question was not why the failure modes are different (I understand the difference in compressed gasses vs. liquids), but why it fails in the first place.

05-04-2009 19:51

ZInventor

Re: pic: T-Shirt Launcher, still needs a chasis

the reason it fails is that PVC was never rated to go from 0-60 psi and back to 0 in less than a second.

the instantaneous stresses are just too much.

think of it this way, your tires give you traction at 60MPH, but if you try to go from 0-60 too fast, you'll spin out...

i know, it's totally different forces at work, but the result is essentially the same.

it's MUCH harder to create instantaneous pressure with liquids, as they do not compress, and once given more volume, return to 0 without much trouble.

i've seen first-hand the problems with PVC AND ABS!!! IMHO, don't use plastics of any kind for air pressure, unless they are flexible (ie. Pneumatics tubing)

just use metal.

Just my $0.02...

-Z

20-04-2009 17:43

Beener

Re: pic: T-Shirt Launcher, still needs a chasis

I like the revolver design. How much does it weigh?
This is what we came up with: Vex Air Cannon

There is no PVC used anywhere on this robot.

05-05-2009 15:55

Bruceb

Re: pic: T-Shirt Launcher, still needs a chasis

Well it took me a while but I read this intire thread. What a great discussion. First let me say that I think I will build a sheet aluminum clamp to completely encase the pressure tank. Think of it like a hose clamp that is as wide as the tank is long. Make it from .050 aluminum and roll it to the exact diameter of the pressure tank then bolt the ends together every 2 inches. That should eliminate the potential failure of the pressure tank. The chambers are sealed on both ends by a spring loaded piece of 3 inch pvc. These seals open somewhat every time the gun is fired and I am sure they will release all the pressure very rapidly in the event the t-shirt refuses to go down the barrel. As was said before, if the gun is fired when the chambers are not properly indexed then the seals do not ....seal.. :>) So that is not a problem. Squirrel, this looks like your valve because I am far better at copying an idea than I am at coming up with my own. It is your valve as best as I could tell from the pictures. At least I think it was you. It is only a 3/4 in gate valve. I believe the small size of the valve actually limits the pressure buld up in the barrel quit a bit. As far as the big reducer at the back of the barrel not being pressure rated goes, that is not a problem either as it is not carrying ANY pressure. The one inch pvc that is connected to the union goes all the way through the reducer to the back of the chamber and is sealed at that end as well so no pressure can get into the reducer at all.

Why not build it all from something stronger(read that more expensive)? Budget is all I can say. If anybody wants to donate some 4 inch steel pipe and the necessary fittings I would be happy to use them. No, I'm not ignoring the safety concernes due to budget constraints. We built the thing with the aid of 2 GE engineers who don't see how the PVC can be a problem. That was before this thread got started and all the concerne got started so now I am trying to minimize the risk and concern using materials we have available.
Thanks for all you input. Keep it coming.

Bruce

05-05-2009 15:58

Tyler Hicks

Re: pic: T-Shirt Launcher, still needs a chasis

That is one of the coolest things I have ever seen......this gives me an idea!!!

05-05-2009 19:33

cbale2000

Re: pic: T-Shirt Launcher, still needs a chasis

One suggestion that I've heard before but I'm not sure has been mentioned here, if someone does plan on using PVC for a cannon, perhaps just build shielding around it. I seem to recall another launcher design that incorporated a larger diameter PVC tube around the main cannon that would block any flying parts in case of a catastrophic failure.

In the case of this design, maybe even just a polycarbonate box around the launcher so that onlookers can still see all the mechanisms but are protected?

16-11-2010 12:15

Tahti101

Re: pic: T-Shirt Launcher, still needs a chasis

cool....im new to robotics and my team has to build a t-shirt launcher..