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Advice on Piston

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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Advice on Piston

Unread postAuthor: limbeh » Thu Mar 12, 2009 10:51 pm

Hi,

I've just built a barrel seal piston in a 4" tee supplying a 3" barrel.

The sealing face of the piston is 3mm thick neoprene of 70% hardness. The input is a schrader, while the output is a small blowgun.

Now what happens is that the piston is already assembled, but is leaking - most of the air goes down the barrel when it is being pumped. The barrel is probably not even, but I'm not sure where is the imperfection to file off.

I'd like to ask the more experienced / experts among you some things:

How would you find where the leak is coming from in an already glued cannon? The cannon is not made of clear PVC and it's huge, so the normal underwater look for bubbles can't really be adapted here.

Would an old mousepad be a good subsitute for the 70% neoprene? I an't seem to find any neoprene softer than this yet. This might be another cause of the piston not sealing.

Is the input and pilot powerful enough? Or should I add more schraders and change the pilot to a ball valve?

Thanks.
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Unread postAuthor: dudeman508 » Thu Mar 12, 2009 10:52 pm

On a 4" piston you need ALOT more than a blowgun (A sprinkler or a ball valve would be better).
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Unread postAuthor: Gippeto » Thu Mar 12, 2009 11:01 pm

I'll second that, you need a bigger pilot. One schrader should be fine for filling though.

Growing up on the farm, when we had to find small leaks in large implement tubes, we use a squirt bottle full of soapy water.

Just spray, and watch for bubbles. Easy.

Mouse pad makes great ... mouse pad. (JMO) Try some pieces of inner tube or similar.

If your barrel is not "square" to the tee, you can replace your sealing face with peel and stick sandpaper, and rotate your piston against the end of the barrel, sanding it true.
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Unread postAuthor: pharmboy » Thu Mar 12, 2009 11:07 pm

As you didn't mention a spring, I'd also take it that you're not getting filling quickly enough to seat the piston. If you're filling with a compressor, I'd strongly suggest a larger input on top of a larger pilot as dudeman508 just suggested.
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Unread postAuthor: limbeh » Thu Mar 12, 2009 11:26 pm

Would a 1/2" ball valve be enough? A 1/2" ball valve like those used in taps here costs only $4.50, while a 3/4" one cost $10 and takes longer to open.

The problem with the barrel not being square is that the tee is not transparent. So it is not easy to see what is going on inside the tee. But I'll try the soapy water out and report the results.

I don't even know if the rate of fill is high enough, because all testing has the air going out of the barrel (I put a plastic bag to block the barrel, and by the first pump the bag flew off)
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Unread postAuthor: mark.f » Fri Mar 13, 2009 12:56 am

If you look at Home Depot they'll usually have 1/16" thick rubber gasketing, if not 1/8" thick gasketing, for sale in small squares (but big enough to make a sealing face out of).

I use 1/8" thickness for small valves (1-1/2" porting, two layers of the stuff), so maybe try three layers for a 4" piston.

Along with changing the pilot valve to a bigger one (a 3/4" Q.E.V. should work nicely) and making sure the piston seals well in the pipe it fits in, that's all I can really think of at the moment.

Take a look at Velocity's AmeriCannon for some inspiration, if need be.
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Unread postAuthor: Gippeto » Fri Mar 13, 2009 12:59 am

Color the end of the barrel with a marker, and then sand a little bit.

Any remaining marker will demonstrate the presence of "low" spots.
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Unread postAuthor: chrissilvermancs » Fri Mar 13, 2009 1:42 am

i myself built a 4" piston but my piston goes to 2" now i have to say o-rings on the piston r your best friend for a 4". u really need a great seel you have to be able to put your mouth on one end and blow the piston to the other if o-rings are too hard try that blue panters tape adding little by little till u get a good seel but its only a subsitute for o-rings

also i use a modifed 1" sprinkler valve to port my piston because the ball valve didnt work and its 1/2 so think about that.

also i like the sandpaper on the piston and marker but make sure you piston face is flat!

good luck
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Unread postAuthor: limbeh » Fri Mar 13, 2009 10:25 am

Rubber gaskets - I know their purpose but I'm not very sure whether they sell them in exact sizes. I haven't exactly started buying it yet.

As for mousepads, the thing is that when I soak them in water bubbles come out. I would suppose that means they aren't airtight. Is it so?

Would foam rubber be good? I can find foam rubber at a lot cheaper, plus I can cut the shape as I want.

Thanks for the sandpaper tip. I didn't think of that. Quite foolproof method. Plus I have lots of sandpaper to spare.

Hmmmm......I'll find a bigger metal ball valve then. Sprinkler valves are generally rare here because there are few lawns that would normally require a sprinkler valve.

Thanks for the advice on Home Depot but sorry, the nearest one to me is one ocean away (I live in Singapore, in Asia).
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Unread postAuthor: jmeyer1022 » Fri Mar 13, 2009 10:41 am

Mousepad's will work OK, only ok low PSI, 20,30 ish, after that they fail.

If your going to use the rubber foam I think your going to use, I used it on a piston seal face, worked amazing until high PSI, I only used 1 layer so maybe 2 will do it? Your best bet is to look around for rubber. If you have a hardware store or a Lowes nearby, they also usually sell these sheets.
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Unread postAuthor: limbeh » Fri Mar 13, 2009 11:26 am

As expected.

My maximum pressure (inclusive of safety margin) is 150 psi, beyond that I'm asking for an explosion already. My bike pump has a maximum capacity of 130 psi, though even that you need to rest your bodyweight on the pump or be darn strong to compress the air into the pump.

I know what you mean by rubber sheet, will check it out asap when I get back to my workshop on Monday.
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Unread postAuthor: chrissilvermancs » Fri Mar 13, 2009 11:42 am

i think i know your main problem did u say that you r using a bike pump if so correct me if im wrong but theirs no way you can get enough starting pressure to slam the piston on. you have to get a compressor

also there not that expensive look up the ball valves made of pvc u can get that kind in big sizes.

also im just mentioning this just in case you didn't know but you do have your fill valve on the same side of your porting valve or other wise the air is not pushing the piston the right direction

http://www.plumbingsupply.com/pvcvalve.html

this sight is your best friend if you cant find a part you can also ask your local hardware store and ask them to special order it
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Unread postAuthor: john bunsenburner » Fri Mar 13, 2009 12:53 pm

I dont have a link at hand but i think hubb once made a post where he discribes a wa of makign our piston seal better what ou do is ou take felt, put theat on ou piston and then put a laer of rubber over. I hope ou dotn mind the little hjack here but:
1.how man laers of inner tube do ou recomend
2.for a 1inch piston will blowgun be good as a pilot?
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Unread postAuthor: Hubb » Fri Mar 13, 2009 1:10 pm

john bunsenburner wrote:I dont have a link at hand but i think hubb once made a post where he discribes a wa of makign our piston seal better what ou do is ou take felt, put theat on ou piston and then put a laer of rubber over. I hope ou dotn mind the little hjack here but:
1.how man laers of inner tube do ou recomend
2.for a 1inch piston will blowgun be good as a pilot?


http://www.spudfiles.com/forums/piston-valve-improvement-t10720.html

If the piston is serviceable, you may just be able to put a coupling on the barrel. It may have to be cut and shaped, but it might be possible.

I always use a coupling on the end of my barrels because (1) they are somewhat concave meaning the airflow is just a little bit better and (2) they are always square.
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Unread postAuthor: jrrdw » Fri Mar 13, 2009 1:26 pm

Post pictures of what your working with. From your discriptions it sounds to me like your not filling fast enough or venting fast enough. How is your piston built? Do you have a center bolt? If so, is the air leaking past the bolt and going out the barrel? It may not be the sealing face.
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