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Barrel Sealing Piston Valve Question

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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Barrel Sealing Piston Valve Question

Unread postAuthor: AngryChauncey » Thu Aug 13, 2009 12:18 am

I am in the process of making a standard barrel sealing piston valve in a 1.5" tee, and I am wondering about the pilot valve. The pilot chamber is 1.5" sch.40 and is about 4" long. The 3 options I had in mind for the valve was a 1/4" ball valve, a standard blowgun valve, or a 1" sprinkler valve. I know there is a recent topic that states "a pilot valve can never be too big", which I understand, but I would rather use the blowgun to avoid messing up the design of my pilot chamber (springs bumpers etc.).
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Re: Barrel Sealing Piston Valve Question

Unread postAuthor: jackssmirkingrevenge » Thu Aug 13, 2009 1:04 am

AngryChauncey wrote:I know there is a recent topic that states "a pilot valve can never be too big", which I understand, but I would rather use the blowgun to avoid messing up the design of my pilot chamber (springs bumpers etc.).


A pilot valve can never be too big, but with a carefully designed piston setup, you can pilot your piston with an incredibly small valve.

The main issue to understand is about airflow. If more air can flow around your piston than it can through the blowgun, your launcher will simply (and painfully) empty through the pilot, as whatever pressure is lost through the pilot is replaced from the chamber. That is to say, if your piston is 100% airtight, you can pilot a 4" piston with a schrader valve if you want.

So basically, make sure your piston is as tight as possible and you can easily use a blowgun.

Why is the pilot chamber so long? For say a 1" barrel, travel of more than 0.5" for the piston is a bit overkill, theoretical maximum flow is achieved at just 25% of the barrel diameter - then you allow a little bit for piston bounce.
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Unread postAuthor: MrCrowley » Thu Aug 13, 2009 3:36 am

JSR is right, you can pilot your piston with ridiculously small pilot valves, but that doesn't mean it performs better. Whether your piston can be piloted via blowgun, 1/4" ball valve or 1" sprinkler valve, the 1" sprinkler valve will offer the best results in most cases.

I'd go with a 1/2" ball valve if you're not using o-rings or machining the piston, a 3/4" or 1" sprinkler valve would be better though. I use 1" sprinkler valves for my 2" housed pistons.
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Unread postAuthor: jackssmirkingrevenge » Thu Aug 13, 2009 3:42 am

MrCrowley wrote:I'd go with a 1/2" ball valve if you're not using o-rings or machining the piston, a 3/4" or 1" sprinkler valve would be better though. I use 1" sprinkler valves for my 2" housed pistons.


Probably the blowgun is preferred for the sake of ergonomics. You can enhance performance and keep a blowgun trigger by using it to pilot a sprinkler valve which in turn will be your piston pilot.
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Unread postAuthor: MrCrowley » Thu Aug 13, 2009 4:04 am

jackssmirkingrevenge wrote:
MrCrowley wrote:I'd go with a 1/2" ball valve if you're not using o-rings or machining the piston, a 3/4" or 1" sprinkler valve would be better though. I use 1" sprinkler valves for my 2" housed pistons.


Probably the blowgun is preferred for the sake of ergonomics. You can enhance performance and keep a blowgun trigger by using it to pilot a sprinkler valve which in turn will be your piston pilot.

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Unread postAuthor: jackssmirkingrevenge » Thu Aug 13, 2009 4:27 am

MrCrowley wrote:We're such a good team, you and I. :D


Oh, if I were living in your part of the world we'd have built a pneumatic launcher big enough to propell sheep at unreasonable velocity through a shearing device in no time at all :D
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Unread postAuthor: AngryChauncey » Thu Aug 13, 2009 1:21 pm

I am making the piston airtight, and jacksmirkingrevenge my barrel is 1.5" not 1", (it is just ported to 1" and converted up), which is why I had about 0.5" of travel, a hair more than 25%.
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Unread postAuthor: jackssmirkingrevenge » Thu Aug 13, 2009 1:40 pm

AngryChauncey wrote:my barrel is 1.5" not 1", (it is just ported to 1" and converted up)


So your barrel is 1.5" but where the piston actually seals is 1"? This means that the dimension that counts when getting your 25% value is still the 1" of the seat.

The math is painfully explained here but I've also felt the need to qualify this rule to account for phenomena such as piston bounce.
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Unread postAuthor: AngryChauncey » Thu Aug 13, 2009 10:45 pm

Ok yeah that makes sense. I went ahead today and used the 1" sprinkler valve just to be safe. But now I can't get the piston to seal. :?
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Unread postAuthor: MrCrowley » Thu Aug 13, 2009 11:19 pm

If you're using a bolt to attach the sealing face to the piston, air could be leaking out the bolt, so epoxy that up.

Make sure your 'seat' (where the piston seals against the barrel) is flat and level.

Make sure your sealing face is of correct material.

Make sure you are using an air compressor or high flow track pump to seal the piston. Some pistons will require an air compressor to get them to seal because they don't seal well enough in the piston housing. If your piston is a nice fit, a track pump might be able to seal it.
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Unread postAuthor: FORE!!!! » Fri Aug 14, 2009 3:22 am

AngryChauncey wrote:Ok yeah that makes sense. I went ahead today and used the 1" sprinkler valve just to be safe. But now I can't get the piston to seal. :?


i had the same problem like 3 hours ago untill i epoxied the bolt going through the piston and sand papered the end of the pipe for like half an hour
with 800-1000 grain sandpaper,it made the end of the copper pipe look like a crome finish ,now i have 100% perfect seal..
just thort id let u know cause i was in ur position a couple of hours ago :shock:
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Unread postAuthor: M.J.P. » Fri Aug 14, 2009 7:08 am

I am having the same problem getting my piston to seal.
Does rounding the seat result in a better seal?
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Unread postAuthor: inonickname » Fri Aug 14, 2009 7:10 am

The end of the barrel really needs to be perfectly flat, otherwise you will certainly end up with issues.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 14, 2009 7:22 am

inonickname wrote:The end of the barrel really needs to be perfectly flat


I like to round the edges off slightly, as having it perfectly flat tends to create a "stamp" effect which cuts the piston seal.
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Unread postAuthor: Hubb » Fri Aug 14, 2009 7:51 am

jackssmirkingrevenge wrote:
inonickname wrote:The end of the barrel really needs to be perfectly flat


I like to round the edges off slightly, as having it perfectly flat tends to create a "stamp" effect which cuts the piston seal.


For reference, I think he meant to say even. If it's not cut exactly perpendicular with the rest of the pipe, you will have problems. Also, I round off these edges as well. The main reason is what JSR said. The other reason is air tends to flow around bends better than it does around corners. Take a look inside a sprinkler valve and you will see that there exhaust ends are rounded rather than flat.
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