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My First Pressure gun ever. Over/Under Design, need help.

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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My First Pressure gun ever. Over/Under Design, need help.

Unread postAuthor: Yukondano2 » Fri Oct 29, 2010 7:27 pm

Hey, I've dry fit a somewhat done PVC Gun.... I'm fairly worried on a few design flaws already and I'm running short on cash.
I was reading another newbies thread, and i saw that Flat end caps aren't pressure rated. :shock:
Does anyone know where i can get my hands on a 2" dome shaped end cap?
Only ones at the true value I'm going to are flat....
Also, in the image you can see a sag. Where can i find/make a brace to connect them in order to hold them together.

Oh and, is the gun dangerous at all? I don't know the pressure rating on my connectors, other than the ball valve. Barrel and chamber are 2", and the ball valve section is 1/2". I'm worried when the air passes from the chamber through the 1/2" its going to blow.
Any other thoughts?


http://filesmelt.com/dl/DSCN2267.JPG
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Unread postAuthor: EvilFish » Fri Oct 29, 2010 7:49 pm

The sag is caused because all of the weight of the large barrel is held up by the thinner connector pieces (the elbows and thin pipe there). you could fix this by sticking a chunk of wood between the chamber and the end of the barrel. also, I wouldn't worry too much about the flat end caps, the work fine so long as you don't take it past 50-60 psi, and you can always cut it off later and replace it. I would worry a bit more about the tiny ball valve as your firing valve, as it wont give enough airflow for powerful shooting.

BTW, what country do you live in? Might help people tell you where to find pressure rated encaps. :wink:
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Unread postAuthor: Technician1002 » Fri Oct 29, 2010 7:51 pm

One of the biggest things you will need to watch is the leverage the barrel and chamber place on the small pipe in between. A little wiggle and it will break. Some support between the barrel and chamber is highly recommended. It needs to provide protection from recoil forces so the barrel recoiling will not bend the pipe.

The launcher has a decent chamber to barrel ratio, but the valve and pipe in between will severely limit the peak flow resulting in limited performance.

In smaller sizes sometimes schedule 40 caps are flat. Check the fitting. It should be clearly marked if it is schedule 40 pressure rated.

I used to use schedule 40 1 inch pipe caps (flat) as pistons in my Mouse Musket. Unfortunately PVC does not like repeated flexing or hard impacts and tends to shatter. For a low pressure cannon, ABS is a viable alternative with safer failure modes. See my ABS cannon in my sig for more information on using ABS instead of PVC. Do not use the thin test caps or plugs. They are meant for system leak testing under low pressure only.

Edit; Linked to the photo.
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Last edited by Technician1002 on Sat Oct 30, 2010 5:13 pm, edited 1 time in total.
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Unread postAuthor: Yukondano2 » Fri Oct 29, 2010 7:56 pm

EvilFish wrote:The sag is caused because all of the weight of the large barrel is held up by the thinner connector pieces (the elbows and thin pipe there). you could fix this by sticking a chunk of wood between the chamber and the end of the barrel. also, I wouldn't worry too much about the flat end caps, the work fine so long as you don't take it past 50-60 psi, and you can always cut it off later and replace it. I would worry a bit more about the tiny ball valve as your firing valve, as it wont give enough airflow for powerful shooting.

BTW, what country do you live in? Might help people tell you where to find pressure rated encaps. :wink:


I wanted to get past 60 PSI... actually. I was hoping around 80-100 PSI. Is that excessive? Especially for my first shot? I've.. Never actually handled anything with a recoil. Ever. So uh.. that'll be an interesting experience.


EDIT:
Crap, Sorry guys i should have put that into one post. I'll remember that for next time.
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Unread postAuthor: saefroch » Fri Oct 29, 2010 8:11 pm

80-100psi is usually safe for PVC, but if you can find something with a pressure rating stamped on it, that's best, since you can almost always push at least to the rating given by the manufacturer.

I know Home Depot has a LOT of pressure-rated PVC fittings and pipe.
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Unread postAuthor: Technician1002 » Fri Oct 29, 2010 8:43 pm

Home Depot has many of the caps on their website. Most of the caps 1.5 inch and smaller are flat. The page does not list a 2 inch cap so I don't know if it is flat.

http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=973157&Ntt=973157&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&N=0&Ntk=P_PartNumber
1.5 cap sched 40

http://www.homedepot.ca/catalog/caps/173045
Home Depot's page of caps.
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Unread postAuthor: Yukondano2 » Fri Oct 29, 2010 9:52 pm

Well that's... Frustrating.
So.. my other worry was that as the air passes from the compression chamber through the 1/2" pipes it will cause it to burst due to the reduced space. Is that an issue?

-EDIT-
Oh and also.. what do you suggest to use as a brace between the barrels? Like.. a piece of wood to jam it and duct tape to hold it? Or is there an actual BRACE i can buy? Or something else? Really anything that holds it would be fine.
-EDIT2-
So.. if i can't get my hands on a dome shaped End cap.. what else can i use to seal the compression chamber? I'm thinking of killing the idea of using all these bushings.. and with all the leftover parts i COULD down Size it at the end and put on a 1/2" Dome cap i have lying around.
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Unread postAuthor: Technician1002 » Fri Oct 29, 2010 10:17 pm

A flat cap is OK if it is pressure rated. In large sizes, they are domed for the pressure. In small sizes flat is OK as long as it is rated for the pressure. Check where you bought the cap to see if it is pressure rated.

Your cap that you are using might be OK.

The problem with small pipe is it flexes easily and not much air flow can get through it to fill the large barrel very quickly to push the projectile fast.

The small diameter pipe won't cause breakage from the flow. The breakage is from the barrel waving around from carrying it and firing it. Bending the pipe is bad. The brace is to prevent it from being bent.
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Unread postAuthor: Yukondano2 » Fri Oct 29, 2010 10:19 pm

Technician1002 wrote:A flat cap is OK if it is pressure rated. In large sizes, they are domed for the pressure. In small sizes flat is OK as long as it is rated for the pressure. Check where you bought the cap to see if it is pressure rated.

Your cap that you are using might be OK.

The problem with small pipe is it flexes easily and not much air flow can get through it to fill the large barrel very quickly to push the projectile fast.

The small diameter pipe won't cause breakage from the flow. The breakage is from the barrel waving around from carrying it and firing it. Bending the pipe is bad. The brace is to prevent it from being bent.


The flat cap is Pressure rated.. But would it help at ALL to downsize it and cap it with a smaller cap? Because i don't want to waste those Bushings, I'm short on cash as it is.
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Unread postAuthor: saefroch » Fri Oct 29, 2010 10:51 pm

No, right now you won't get much benefit at all from replacing the cap with something of a smaller size.

Three things about this build bother me (before even discussing valves):
1) You are choking the flow before it enters your barrel.
2) Your barrel and chamber have almost the same volume.
3) This looks terribly fragile with that small 180 elbow design (see my signature, if you're stressing that elbow construction, you might as well ignore the pressure rating).

I am fully aware that you're short on cash and therefore cannot address most of our concerns, but all of us here are short on cash to some extent, and that's why we design, share, then build. Just a hint for the future, I pray this isn't the last pneumatic you build. :wink:
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Unread postAuthor: Yukondano2 » Fri Oct 29, 2010 11:14 pm

saefroch wrote:No, right now you won't get much benefit at all from replacing the cap with something of a smaller size.

Three things about this build bother me (before even discussing valves):
1) You are choking the flow before it enters your barrel.
2) Your barrel and chamber have almost the same volume.
3) This looks terribly fragile with that small 180 elbow design (see my signature, if you're stressing that elbow construction, you might as well ignore the pressure rating).

I am fully aware that you're short on cash and therefore cannot address most of our concerns, but all of us here are short on cash to some extent, and that's why we design, share, then build. Just a hint for the future, I pray this isn't the last pneumatic you build. :wink:


Thanks for the help. I want to just scrap it.. only issue is that those massive ball valves are a bit :l. Thanks for the tip though. What do you mean that my barrel and chamber have almost the same volume.. Why is that an issue?
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Unread postAuthor: Technician1002 » Fri Oct 29, 2010 11:18 pm

Keep the ball valve if possible and make a chamber and barrel to match the flow of the valve. It could still make a nice over\under for smaller ammo.
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Unread postAuthor: saefroch » Sat Oct 30, 2010 9:24 am

As a general rule for pneumatics, your chamber should have 2-3 times the volume of your barrel, this is based on a generalization of efficency. If your barrel is the same volume as your chamber, by the time the projectile exits the muzzle, you're down to half the pressure you started with. Now the problem isn't at the muzzle, but what I'm saying is that you'll see a significant decrease in acceleration (or jerk, not totally sure), which pretty much just robs you of efficiency.

Don't scrap it, that's silly. Keep the chamber and valve at least. Get a smaller diameter barrel, ideally the same size as your valve or smaller, not bigger, make some sort of support for the barrel, and you'll have a pretty good ball valve cannon.
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Unread postAuthor: Technician1002 » Sat Oct 30, 2010 11:27 am

The maximum efficiency and the maximum power are not the same ratio. For maximum efficiency, the gas expansion would be just enough to expand to 0 PSI on projectile exit. For a chamber operating at 7 bar and a very high efficiency valve, this would be a chamber about 1/4 the volume of the barrel as the expansion is reduced due to cooling during expansion.

This volume will vary depending on operating pressure. Generally I have found a 1:1 volume ratio is an excellent compromise between efficiency and power. Both my Mouse Musket and ABS test cannon are very near 1:1 ratio and pack a punch.

As a general rule for pneumatics, your chamber should have 2-3 times the volume of your barrel, this is based on a generalization of efficiency.


My 2 inch QDV with most barrels has this larger chamber so it simply takes a lot more air to pump it up for a marginal increase in power.

My 1 inch QDV is often used with a barrel volume much larger than the chamber with good results at normal operating pressure. It makes an excellent t shirt cannon with very efficient use of the compressed air supply when used with the 3 foot tennis ball barrel. A typical range is 200 feet with t shirts with this small chamber.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 30, 2010 11:32 am

saefroch wrote:As a general rule for pneumatics, your chamber should have 2-3 times the volume of your barrel, this is based on a generalization of efficency.


Too much of a generalisation, have a look through threads like this one.
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