Question RE: Pvc Pipe Pipe Part's Names

[watto]

I'm building my first piston gun after the huge success of my sprinkler cannon (largely due to this site) The trouble I had last time is that I don't know any of the names of different parts, and because I only use class 18 pipe I have to go to a specialist plumbing store where you tell the bloke all the bits you need and he goes out the back to get them for you.

The main thing I need is a class 18 thread to go OVER the outside of 50mm class 18 pipe, but when I asked for that last time they gave me a fitting that went over the outside, but then reduced down for the thread part, If you know what I mean. The cap is obviously for access to the piston so I need the piston to be able to slide out of the 50mm pipe when the cap is off. Does anyone know what I could ask for to get what I'm after?

The place I go to has they're catalog listed online, so If someone could point out the name of what I want that would be great. http://www.galvins.com.au/plumbing/cata ... lect&cat=6

Much appreciated

[thespeedycicada]

hmmm what kind of piston are you using and a diagram would be alot of help to allow me to understand what you are saying.But if its co axial a pressure rated threaded end plug works great!

[MrCrowley]

Hmm, i'm guessing you're using it to have a serviceable piston?
Are you planning on sleeving the inside of the tee with 50mm pipe for the piston to travel in, and want that threaded adapter to go on the outside of that pipe so you can have a threaded end cap?

All I can suggest is get rid of the 50mm pipe that sleeves the tee, and just use a 50mm socket x 50mm male bsp threaded adapter, and have that go inside the tee and let your piston travel inside of that.

I'm sure there is a way, it's just slipping my mind right now.

Btw my post only applies to you if you are doing what I think you are, which I explained in the first paragraph

[watto]

I went and got all the stuff today, surprisingly bunnings actually had the things I needed, except some 25mm pipe above PN9 for the 25x15 reducer.

The thing I was looking for is called a "F.Socket" which I assume means "Faucet Socket" (Faucet is when americans call a tap I think)

I got some neoprene and an expensive tyre valve ($6 what a scam) Now I just gotta find something the right size to reinforce my piston, I'm thinking skinny deoderent can...

A few more questions, has anyone seen those black plugs that screw into the faucet sockets, do you reckon those are pressure rated? They look real thick and they seem to be the only plugs around, the markings on it say "philmac, BSP, iPLEX, MLE"

Do you reckon its alright to use possibly un-pressure rated 25mm pipe just in the reducer between 25mm and 15mm? It would be completely covered by the class 18 fittings. I found it on the side of the road and it would save me another trip to buy some pressure rated 25mm.

Thanks

[MrCrowley]

Black plugs? Hmm are you talking about the black hansen fittings?
They are fine for low pressures, like around 120psi i'm sure.

Well for starters I hope your 'reducer' is infact pressure rated or a bushing, and usually non-PR fittings don't fit in PR fittings because of the different O.D's. And I wouldn't trust it, trust me its really alot better feeling safe when you are charging up your gun to know you have all PR fittings.

Though I don't trust PVC with pressure anymore after 260psi rated PVC blew on me at 110psi.

[Mr.Russ]

i would assume he has a rpessure rated reducer, at bunnings they have them in some sizes. walls are about 3mm thick and it says pn18 on them

[watto]

Well for starters I hope your 'reducer' is infact pressure rated or a bushing, and usually non-PR fittings don't fit in PR fittings because of the different O.D's. And I wouldn't trust it, trust me its really alot better feeling safe when you are charging up your gun to know you have all PR fittings.

Though I don't trust PVC with pressure anymore after 260psi rated PVC blew on me at 110psi.

Yeh the reducers are class 18, I guess I'll just get some proper pressure rated 25mm, I think I'll get 2m then I'll use the left over bit for a 25mm barrel for my sprinkler gun.

What happened when the pvc blew up? are you talking about class 18?? Was it a really cold day? What part of the gun blew up? Was it an old gun? Thats really scary, I thought I was safe because I don't go above 100PSI.

[MrCrowley]

Not much, sh|t hit the fan, 3 of us were standing within 1m of it, one of us 3 was holding it where it blew. It didn't injure us in anyway, shape or form, only our ear drums. Part of the chamber on my SGA golfball gun, the front 'handle' part with the 45* elbow. Nope, brand new gun.
It was about 14-16* Celcius.

We think it was because we had been shooting it for about 40minutes, filling it up every 4 or 5 minutes, so we guess it was just stress from rapid compression and decompression. It was a 2" piston gun btw.

[watto]

It didn't injure us in anyway, shape or form, only our ear drums.

Was there shards of jagged pvc like everyone says or just a kinda clean break at a fitting or something? Was it class 18?

[watto]

how big do I want my piston, how much space does it need behind it and should it be a very small hole that goes through to the chamber to keep pressure behind the piston?

[MrCrowley]

Yeah it was PN18, not class 18. Class system uses the alphabet, A-F I think it is. PN system, the new one, uses rating numbers in BAR.

1BAR = 14.5PSI
PN18 = 18 x 14.5 = 261PSI
PN18 rating = 261PSI

10BAR = 145PSI = 1mPa = 1000kPa

And yeah it sent shards everywhere like everyone says it does.

A piston in a co-ax you want big enough without getting too heavy to take up as much pilot volume as possible and have the recommened space to actuate back. In a co-ax this is usually 1/4 of the barrels diameter.
So allow it to move back 1/4 of the barrels diameter, maybe a bit more to be on the safe side.

As long as the piston is lubed and can be pushed without too much effort, equalization holes aren't necassary.

So just try and keep pilot volume to a minimum, let the piston be able to be actuated back(when the gun fires) at least 1/4 the barrels diameter, not too much more or less. If you figure that out you will know how long to make the piston, weight usually isn't too much of an issue, just minimize it if necassary or where possible.

[watto]

As long as the piston is lubed and can be pushed without too much effort, equalization holes aren't necassary.

How does the air get to the main chamber, around the outside of the piston?

[MrCrowley]

Sure does. This is usually enough, and helps if you only have a bike pump. If it's too slow you can always add a hole after.

[jackssmirkingrevenge]

how big do I want my piston, how much space does it need behind it and should it be a very small hole that goes through to the chamber to keep pressure behind the piston?

The piston should be long enough to be stable in the chamber, making it as long as the chamber is wide should suffice, you can have it shorter as long as it doesn't wobble - it depends how tight it is. A piston only needs to move back 1/4th of the barrel diameter to be fully efficient, anything more is just a waste. In the case of a 15mm barrel, it only has to move back half a centimetre.

Unless you're using o-rings, the piston won't have a 100% airtight seal in the chamber so there won't be need for an equalisation hole. Also, if you're filling from behind the piston, you don't need the spring in your design - a rubber pad is enough to serve as a bumper.

[watto]

The piston should be long enough to be stable in the chamber, making it as long as the chamber is wide should suffice, you can have it shorter as long as it doesn't wobble - it depends how tight it is. A piston only needs to move back 1/4th of the barrel diameter to be fully efficient, anything more is just a waste. In the case of a 15mm barrel, it only has to move back half a centimetre.

Unless you're using o-rings, the piston won't have a 100% airtight seal in the chamber so there won't be need for an equalisation hole. Also, if you're filling from behind the piston, you don't need the spring in your design - a rubber pad is enough to serve as a bumper.

Thanks, I can always count on you and mrcrowley for some quality answers.