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Soon to be built 4inch housing piston, 3 inch porting

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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Soon to be built 4inch housing piston, 3 inch porting

Unread postAuthor: LikimysCrotchus5 » Sun Jan 20, 2008 6:40 pm

Well i am soon going to make a 3 inch porting gun in a couple of weeks but i wont be able to use it because it is winter now.

Now it is going to be a 3 inch cap piston and i might make a configuration to it so that it take up piolt volume. The only concerns i have now are that my fiitings are not rated to a maximum pressure.

I am not sure also about the bumper, which maybe paddle board, like he ones used in a pool, or expanding foam which im not sure if it will make a good bumper.

The following product numbers are corresponding to the parts below it on http://www.mcmaster.com.

4880K48
Std-Wall (Schedule 40) White PVC Pipe Fitting 4" Pipe Size, Tee

48925K98
Std-Wall (Schedule 40) White PVC Unthrd Pipe 4" Pipe Size X 5' Length

4880K28
Std-Wall (Schedule 40) White PVC Pipe Fitting 4" Pipe Size, 90 Deg Elbow

4880K615
Std-Wall (Schedule 40) White PVC Pipe Fitting 4" X3" Pipe, Hex Bushing, Pipe End Male X Socket Fem

4880K87
Std-Wall (Schedule 40) White PVC Pipe Fitting 3" Pipe Size, Fem Adapter, NPT Female X Socket Fem

2389K62
Drain, Waste & Vent Standard Wall PVC Fitting 3" Pipe, Male PVC Adapter, NPT X Socket, Sch 40

4880K58
Std-Wall (Schedule 40) White PVC Pipe Fitting 4" Pipe Size, Cap

4880K57
Std-Wall (Schedule 40) White PVC Pipe Fitting 3" Pipe Size, Cap

Just about all of these parts are not rated for a maximum pressure but they are NSF - 61, and ASTM D2466, ASTM D1784

Im also worried about the 4inch T, which has been discused
here.

Also would it be okay if i took 2 pieces of 3 inch pipe and glue them together to make a ~10 foot barrel. I know it would look ghetto but would it have alot of stress on the coupling but would it be okay to use as a barrel. I just dont want to buy a 10 foot section when i can glue 2 pieces together. Its the cheaper method.

So plz give me your thoughts on it, safety performance, etc.
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Unread postAuthor: MrCrowley » Sun Jan 20, 2008 6:47 pm

Don't use expanding foam as a bumper, but a pilot volume filler. Make sure the piston only has contact with the paddle board.

Yeah that will work, may decrease muzzle velocity a tiny bit, and make sure to have a good barrel support.
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Unread postAuthor: DYI » Sun Jan 20, 2008 7:03 pm

As far as I have gleaned from my somewhat limited experience with plastic plumbing parts, PVC fittings don't have a specific pressure rating. A general rule of thumb is that a PW SCH 40 PVC fitting will be safe to use with at least the pressure that SCH 40 pipe of the same size is rated to.

As long as you bore out the coupling, your extended barrel is viable. As long as the supports extend past the joint, and they hold the barrel straight, there shouldn't be any problems.
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Unread postAuthor: LikimysCrotchus5 » Sun Jan 20, 2008 7:34 pm

ok thx for the responses.

i will most likely attach the paddle board bumper to the piston.

This will be considerable cheaper because i dont have to buy another 10 foot section of 3 inch as a barrel.

I plan to use a big chamber because i can get a good amount of power without reaching a high psi, and since i dont know the pressure rating, it makes it safer, to me atleast.

DYI i do understand what you are saying and it does make good sense.

I have never made a barrel support so what do you think is good? Wood, pvc?
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Unread postAuthor: MrCrowley » Sun Jan 20, 2008 7:39 pm

Wood will do, just stand the 3" pipe up on it, trace half way around the pipe with a pen, figure the distance between your barrel and chamber, then trace half of your chamber (4") that distance away from the 3" barrel semi circle.

Then just cut it out with a saw or jig saw.

You may want two like DYI said, one before and after the coupler. 3 would be better. Secure them with rope, bungy cords, hose clamps or what ever.
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Unread postAuthor: DYI » Sun Jan 20, 2008 8:27 pm

For the barrel supports on the Paris Gun style SCTBDC stand, I just used plywood with the right diameter circles cut out in the exact center. It's the least crooked part of the whole stand (which is pretty damned crooked).
I have a 3" x 12' barrel, and it only needs 2 supports. That barrel is ABS, which is even more flexible than PVC pipe (and also safer :D ). 3" pipe doesn't bend too much.

There are far more beautiful ways to accomplish the barrel support, but I've never been concerned with making my launchers look beautiful, or even decent for that matter, so I'm not the one to ask if you want it to be elegant.
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Unread postAuthor: LikimysCrotchus5 » Sun Jan 20, 2008 8:34 pm

Well i dont care much about elegancy, if it looks decieving, then it will wow people more :D .

Isnt the Paris gun from WWI? Im pretty sure its what the Germans used against the French when they were raining shells and bombs on Paris for 23 days i think.

Well now that i think of your cannon DYI, it might be more convenient for me to make it a coaxial, but then would it yield less power than a barrel sealing?
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Unread postAuthor: DYI » Sun Jan 20, 2008 8:41 pm

Yeah, the Paris Gun is one of the longest range (perhaps the longest range) conventional artillery pieces ever. JSR ( who else :roll: ) was the first one to point out the similarity between its bracing system, and that of the SCTBDC.

I don't think I've ever gone to the trouble of making any of my guns look good. I only painted one with some leftover spray bombs to keep it from rusting. Rusting steel pipe sucks, but it's better than aged PVC in that you can at least tell that it's old.

For a given chamber volume, I would be inclined to believe that a coaxial would generally perform better than a barrel sealer, due to the increased piston acceleration times. However, these differences would likely be in the fractions of milliseconds range, so the difference shouldn't be detectable.
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Unread postAuthor: LikimysCrotchus5 » Sun Jan 20, 2008 8:55 pm

I was just thinking of a coaxial only for the reason that it will be more compact and easier for storage, and somewhat cooler in my opinion.

If you think of a barrel sealer that i would be making with a 5 foot chamber then a 10 foot barrel above it, it would be more of a hassle to do than just a coaxial.

Even though i have never made a coaxial before, the same principle applies in when making a barrel sealer.

I thought a coaxial was not as powerful as a barrel sealing because of my stupid reasoning with the famous diagram made by clide that shows how a piston works in coaxial, barrel sealing and chamber sealing. I never thought that the diagram was not to scale, so to speak. If you look at the coaxial diagram again, you can see that it yields less power than the barrel sealing diagram of his.

Also, i dont ever recall ne coaxial cannon on this site that uses a 3 inch barrel, so then i would be the first. Finally be able to put my name in the record books. :)
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Unread postAuthor: LikimysCrotchus5 » Mon Jan 21, 2008 12:46 pm

Well i am posting again to get attention here because i have still not recieved enough information.

After doing some calculations with the windows calcualator and spudtoolonline, it has come to me that if i had 10 foot chamber of 4 inch pvc, made into a coaxial with 3 inch pvc down the whole chamber, my total volume in there would be 352.51 cubic inches, which would not yield good performace comepared to a barrel sealer inside a tee with a 5' chamber of 4 inch pvc that has the volume of 753.23 cubic inches, which would yield much more power, as confirmed by jimmy's results on his post about C:B ratio.

So yes a barrel sealing piston housed in a 4 inch tee would be more powerful only because it would has more volume. In order to get the same volume with the coaxial, i would have to exceed 20 feet of chamber length so there is not way i would do that because i dont have room for that.

So do you concur DYI? what do you think i should go with? Coaxial or Barrel Sealing housed in Tee?
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Unread postAuthor: jrrdw » Mon Jan 21, 2008 1:11 pm

If you configour the tee set up rite, it really wont take up much more room then the coaxle. With the coaxle you need longer room, with the tee you need wider room. How many inchs wider compaired to how many inchs longer????
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Unread postAuthor: LikimysCrotchus5 » Mon Jan 21, 2008 1:38 pm

I dont understand what you are saying, but there is a significant decrease in chamber volume because the barrel takes up volume in the chamber in a coaxial design compared to a barrel sealing piston housed in a tee.

According to jimmy's experiment results, it shows that i would have much less performance with a fixed barrel and different chamber volume, which is essential what is happening here.
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Unread postAuthor: psycix » Mon Jan 21, 2008 1:43 pm

jrrdw wrote:If you configour the tee set up rite, it really wont take up much more room then the coaxle. With the coaxle you need longer room, with the tee you need wider room. How many inchs wider compaired to how many inchs longer????

With the tee you indeed need wider room BUT.
I have to disagree with you that a coaxial is longer then a tee-overunder.

Wether it is a barrel with a T behind or a coaxial with the same length barrel, they are about just as long.
"barrel length + tee length + pilot valve length = barrel length + piston area length + pilot valve length"

Barrels are the same, pilot valves are the same and the space the piston needs is the same, but on the coaxial it is a shortenable length! And in the tee its not.

By this I conclude that a coaxial is the same length OR shorter then a one with a tee.
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Unread postAuthor: jrrdw » Mon Jan 21, 2008 1:51 pm

Sorry for not being more clear, I was refering to the storage room you have to work with in compairasin to the cannon you want to build. You mentioned that in one of your post. If I was in your situation, I would build the tee piston valve. I think more power is all ways more fun!
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Unread postAuthor: LikimysCrotchus5 » Mon Jan 21, 2008 1:53 pm

Yes i agree a coaxial is shorter but i am just saying that the chamber volume is much less because you have to minus the volume of the barrel to the volume of the chamber, therefor you have less chamber volume and less performance.

I am talking about performance in this case. Performance would be less in a coaxial due to less chamber volume because the barrel takes up volume.

EDIT: I understand what you mean jrrw. I would like to get more performace and i agree that a piston housed tee does get more performance than a coaxial, but its just gonna cost more for me. So ill make a decision after i get more opinions.
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