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Piston 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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Piston help

Unread postAuthor: Juggernaut12121 » Sat Apr 12, 2014 11:21 am

I've been trying to make a golf ball cannon and have nearly everything put together except the actual piston valve which doesn't seem to want to work. Every time I try to pump it up the piston never seals and all the pressure just escapes. I tried fattening up the back end slightly but it just results in the piston getting stuck in the housing (The again all I've worked with is duct tape and grease). I've thought about making an epoxy piston but I don't know how long to make it. From the back of the piston housing to the pipe the piston seals against is about 5" and the diameter is 2". Any help would be greatly appreciated!
P.s. The pilot valve is a 1" modified sprinkler valve and the pilot to that is a blow gun, sorry the pictures aren't very good, I took them with my ipod
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Attachments
1.jpg
Inside of piston housing
2.jpg
Pilot setup
3.jpg
The piston (I know it sucks)
4.jpg
Sealing face of the piston
5.jpg
Whole picture
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Juggernaut12121
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Re: Piston help

Unread postAuthor: MrCrowley » Sat Apr 12, 2014 4:47 pm

How are you filling the chamber? With a sprinkler valve pilot, a large pilot volume, and a poorly fitting piston, you would have real trouble sealing the valve with anything less than an air compressor (not the small 12V car types).

If you are using an air compressor to fill the chamber, the sealing face may be leaking air through to the barrel. Air can seep around the bolt and large washer, past the inner ring of the rubber sealing face, and out between the small front washer and bolt nut. This is a very common problem but is easily fixed by applying epoxy over the front nut, the front washer, around where the sealing face meets the front washer, and also behind the large washer and around the nut holding it in place. Before you do this though, I'd recommend doubling the large washer for added strength, sanding the surfaces of the nuts and washers for improved epoxy grip, and making sure that the piston is the right length and all.

The duct tape could be the culprit but people have got it to work before. If the above doesn't work, you'll need to make a new piston. If you can afford to buy the right diameter hole saw, you can easily cut one from plywood and layer it together. I've also made pistons from deodorant cans filled with hot glue but I'm not sure if they fit 2" pipe in the US (I'm in NZ). Making one out of epoxy or bondo is a good idea and pretty easy. Don't worry too much about the dimensions, I would just do something like: sealing face >>> 2cm of bolt >>> 6cm of piston body.

You can play around with the dimensions a bit, just leave about 3-4cm at the back for the piston to move back in to when it opens. Depending on your piston bumper, you may need to adjust the size of the piston. The bumper will take up room, if it is 4cm at rest, but can compress to 1cm when the piston slams against it, you need to account for that 1cm in distance that the piston can't open. As I said, I'd leave about 3-4cm so that when the piston is fully back, there is about 3-4cm gap between the sealing face and the barrel.
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Re: Piston help

Unread postAuthor: Juggernaut12121 » Sat Apr 12, 2014 5:14 pm

Thanks! I'll try using a compressor. If that doesn't pan out then I'll make a bondo or epoxy piston :D
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