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Compact pistons

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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Compact pistons

Unread postAuthor: goose_man » Fri Apr 20, 2007 9:31 am

When I see pistons that are housed entirely inside a tee, I am always impressed. I am able to do that as far as the barrel-end goes, but how do you have a fitting inside the part that the piston needs to slide in? Is there a lot of custom machining involved? Maybe a really short piston?

Feedback is greatly appreciated because I want to make a 1/2" porting piston housed entirely inside a 1" tee. Thanks in advance!

-goose_man
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Unread postAuthor: chaos » Fri Apr 20, 2007 9:55 am

yer man ive done a 5/8" porting piston inside a 1" tee, it wa sdone out of metal but it worked all the same. i am currently working on a piston that fits inside a 1/2" tee. but it is yet to work.

pics of my piston plan



Image

oh dw bout the duct tape u can put o-rings instead
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Unread postAuthor: VH_man » Fri Apr 20, 2007 10:54 am

Try using 1/2 inch CPVC for the porting of a valve........... that would be TINY! in fact, im working one one of those........
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Unread postAuthor: scatdawg » Fri Apr 20, 2007 11:37 am

your piston can be s ahort lenght as long as it is a close enough fit so it cant twist and parralel and bind itself.
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Unread postAuthor: goose_man » Fri Apr 20, 2007 12:36 pm

Chaos: your design seems like you'd need pipe + fittings extending from the back before the pilot. My goal is to make it all fit inside the tee, like one of sgort's beautiful valves. No pipe or fittings extending past the openings of the tee.
Thanks!
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Unread postAuthor: CS » Fri Apr 20, 2007 1:23 pm

Adding threaded ports to the tee is fairly easy as you said. Usually involves turning down a female adapter, and possibly boring a pipe for it to be inserted into. From there it has to be cut to length, as not to interfere with the pistons travel. This is done on the "slip" connection of the female adapter.

Next there needs to be a place for the piston to travel upon. I like to call this the "piston chute". Two methods I have seen used for this, either using a pipe to house the piston, or using the original - unmodified port of the T as the "piston chute". (I might note that using the original port of the tee presents problems in that its ID (Inner Diameter) is tapered.)

The piston is cylindrical in shape, flat on either end. The sealing face being on one end with neoprene, or something similar. On the opposing end O-ring grooves are made to make a seal along the piston chute. Then depending on the style of valve (barrel or chamber sealing) hole(s) will be drilled to connect the pilot to either the barrel or chamber port. Also sometimes springs are used to set the piston against the "seat" for the sealing face.

There then needs to be a place in which the sealing face of the piston can seal against. Which I like to refer to as the "seat". This can be the female adapter protruding past the initial stop of the T, or a section of pipe inserted in the stop. (Generally either, or both the pipe/ fitting being inserted into the stop, and the stop of the tee have to be bored, and turned.)

Last of all there needs to be a plug, this fitting in the "vent" end of the tee. Several different items can be used to accomplish this task for instance - PVC round stock. This is relatively short as to restrict minimal space, although long enough to accept the vent being attached. Then a tap can be used to create a female NPT thread for the vent to be inserted into.

I know my writing was a bit sparse, and not as general as I wanted it to be, but I think it will be of some help.
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Unread postAuthor: pyro6314 » Fri Apr 20, 2007 2:09 pm

I had the exact same question for a while.

What I did was take a piece of 3" Sch40 pipe (3" Tee as well) and cut down the OD to go past the stop on the socket. I left a small piece about a 1/2" long the same OD as the normal pipe to stop it at the desired depth in the end of the socket. I bored out the inside as well (not necessary and just caused problems from the machining marks). As for the end plug I took a 3" to 1" threaded bushing and cut some off the back of it to make more room in the socket for piston travel and to accommodate the "sleeve". This was O-ringed and screwed in radially so it could be removed. The piston was just a 3" plug turned down, Oringed and parted off to the desired length.

If anyone cares for pictures I could whip up a diagram (cant see much on real pictures now that its all welded together.
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Unread postAuthor: goose_man » Fri Apr 20, 2007 4:43 pm

Thank you very much, I'm pretty sure I can do it now. I'll post the pics when it's done.
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