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New Coax Design and idea, 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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New Coax Design and idea, HELP !

Unread postAuthor: Shrike88 » Tue Apr 19, 2011 11:22 pm

Ok so I am new to spud files and have been working on a typical COAX pneumatic design with a piston Valve. I am totally dead in the water and need some help

Using 4'' pipe for chamber
Using 2.5'' Pipe for barrel (tennis Balls for ammo)
3'' Coupler for Piston, with modified extension (kind of a coolie valve)
4mm Neoprene as piston sealant material.
Using a Tire Valve Schrader stem to fill
Modified 3.4'' Sprinkler valve / blowgun
Valve weights about 9oz

first off I tried with softer material and was able to get the cannon to fire a couple of times, however wasn't getting nearly the power I was hoping for, using the GGDT (~550f/sec) @ 100psi.

I am having trouble getting the piston to move and seal. I am guessing either there isnt enough flow of air behind the piston to move it, I am guessing its the Small Schrader valve ? I need to upgrade to something larger ?

This is my idea, I was thinking about making a spring, so that it is guided by the threaded rod throughout the piston and pushes forwards on the piston to help it seat up against the barrel. However I dont know how much this would negatively impact the firing of the gun, i.e. a performance decrease in the duration of the piston valve being open.

I will send more pictures to and explain much more in detail to anyone that wants to help me get this thing firing. I have tried many different options and trials, just starting to give up hope. I want to get this working !


PS. I have tons of images however they are all ~1mb each and dont feel like converting them all to fit for this forum. If you want ill email any specific part of the cannon you think might be an issue.


Thanks

Shrike
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Attachments
Piston Assembly.jpg
This is the "almost" completed Piston Assembly, Version 5. The possible spring would go aft of the piston using the threaded Rod as a Guide. The Bumper material is a silicone filled drain plug cut to size. The red material at front is the Neoprene from Lowes.
Rear assembly 1.jpg
This is the Completed Rear Assembly

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Unread postAuthor: hi » Tue Apr 19, 2011 11:50 pm

I recommend you do this-

Get rid of the sprinkler valve, tee, and pressure gauge entirely. Put the pressure gauge on the cannon it self. For the pilot, use a ball valve with a quick connect fitting for an air compressor. This will allow you to fill it very quickly and will guarantee a fast actuation. Simpler is usually better.

If you do this you connect it to a compressor, open the ball valve and pressurize the cannon, then close the ball valve and disconnect the compressor. Then open the ball valve to fire.

As for the piston- get some grease and give it a good coat. Don't over do it, but a little bit helps. As for the sealing face, I always used inner tubes for the sealing face. It works well as long as you don't have too big of a barrel.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: Shrike88 » Wed Apr 20, 2011 12:01 am

Oh Not Pictured but the sprinkley valve has been since modified with a blowgun attachment.

How do I use the innertubes for the sealing face ? glue them down in a circle around the diameter of the piston ?

I understand simpler is better. I will consider again, have in the past the ball valve. However seems that the reaction would be faster in a sprinkler valve. i.e. as soon as you start turning that ball valve, small amounts of air start to leak. So a faster dump would be a modified sprinkler valve, no ?

Also this still doesnt fix my origional problem of the piston failing to slide forwards and seat against the barrel and seal.

I am using the right neoprene material that has proven to work right ?
piston slides forwards and backwards easily with my finger, as tested so I shouldnt need grease, and the air should move it no ? (what kind of grease do you use ? wont grease break down the rubber ?)

Guessing its still a flow problem with the schrader valve....also What do you think about the buffer spring idea ?
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Unread postAuthor: hi » Wed Apr 20, 2011 12:32 am

Are you using a compressor for filling? for most coaxials, you need to slam that piston against the sealing face and build up pressure as quick as possible. most need at least 20 psi or so to seal. That is why i am recommending you use a quick connect fitting rather than a schrader valve.

If it seals, it seals. If it does't seal, try using an inner tube. Just cut it so that it fits on the piston.

As for grease, just go to any hardware store or autostore and ask for grease. Lithium grease, chassis gease, or wheel bearing grease will work fine. you just need to a tiny little bit.

And finally, ball valve vs. sprinkler valve, I doubt it would make any real noticeable difference. If you have it modded, just leave it.


If it is a Coaxial, why do you have the space between the piston??? there is not point in that. Just make a single one piece piston. You may actually be having problem because the air inbetween the piston is not releasing. The idea is that when you actuate it, all the air behind the piston it dumped out and the piston slams back. You may be prohibiting the piston from doing this because of your design. People usually put that type of piston into a tee, not a coaxial.


I think your main problem is not getting enough air into the chamber fast enough.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: Technician1002 » Wed Apr 20, 2011 1:26 am

A long skinny crecent of leakage around the edge of the piston can be a much LARGER opening than the small fill valve will provide. To use air to close the piston, the fit has to be glove tight and still move freely. If the piston has any sideways wobble room, there is too large a gap. An alternative is to add a layer of electrical tape or a layer of felt to restrict the air blowing by the side of the piston. The felt when greased restricts flow well.


Take a look at the Wiki in the piston valve page. It will give some of the factors that affect how fast a piston will pop open and if it will try to close too soon.

For sealing the piston on the barrel, it is essential that the end of the barrel is prepared to mate the face of the piston with no gaps. Placing sand paper on the face of the piston and polishing the end of the barrel and then reversing it to polish the face of the piston will help greatly in getting it to seal without needing a big spring. In piston cannons, springs tend to get smashed.

Here is an example of a very small piston gap.
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Unread postAuthor: Shrike88 » Wed Apr 20, 2011 1:40 am

The spring will not get smashed as it will be recessed in a little pocket created in the 2'' to 3/4'' reducer fitting. I measured it it should fit perfectly. As far as the Flow on the fill is concerned I am using a 20 gal compressor at 100psi, so I am guessing it must be the valve.

For HI, the reason that there is a space between the first larger coupler (PISTON) and the second is that the coupler is the same diameter as the threaded male adapter that I Used at the end of the barrel. So it basically does not move all the way to the back of the cannon when fired. It is restricted and hits the coupler. I alleviated this by making a large sing axle (the threaded ROD) and placing a second piston valve in the rear that fits exactly in the coupler at the end, THUS creating a smaller empty pilot volume.

This in theory should increase performance and only allow the piston to travel 1.5'' ~ as the barrel is only 2.5'' that should be perfect. The threaded Rod allows me to make changes to the amount of travel the Piston moves. Here are some pictures to show you the 3'' coupler (piston) is the same diameter of the threaded male end cap. Thus my need to create a type of coolie piston to:
1.... Allow cushioning and dampening of Piston forces
2.... restrict movement of the piston for proper travel during firing
3.... Reduce the amount of dead space and air behind the piston (pilot area volume) to increase performance of that Sprinkler valve effecting the piston during firing.

Still haven't answered my question about the buffer spring. I am thinking its a good idea, however might reduce performance by prematurely closing the piston as the air during firing gets to be neutral to ambient pressure. Thus not allowing the valve to be open as long.........
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Attachments
Buffer Spring in End Cap.jpg
This is the small pocket that would fit a Buffer Spring and would not allow it to be "crushed" when the piston traveled Backwards when fired. I think its a perfect fit for a spring, and the Threaded Guide Rod would provide a guide for the spring.
Piston Cap.jpg
See how when placed in the end Cap The Piston will not travel into it. It is restricted cause the Threaded male cap and the piston are the same diamter, So there is all that empty pilot volume at the end if i didnt extend the Piston
Piston Front.jpg
I am going to cut the Threaded Rod Shorter on the front, This is the Front of the Barrel Seating Piston Valve

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Unread postAuthor: jhalek90 » Wed Apr 20, 2011 9:01 am

when i made my four inch piston coax, and had the same issue, i was able to fix it by super gluing some felt around the piston.

Just be sure to lube the crap out of it.

Also, lubing you sealing face is always a good idea.
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Unread postAuthor: Shrike88 » Wed Apr 20, 2011 9:29 am

ok what kind of Lube did you use ? that doesn't make a too TIGHT of a fit ? you were still able to move the piston ?

The felt you glued around the outside diameter of the piston ?(around the circumference ?)

seems like there is barely enough room to get that thing in there, I think felt would just make it tighter. Epoxy as the glue ? or super glue ?


thanks

Shrike
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Unread postAuthor: hi » Wed Apr 20, 2011 10:09 pm

Use masking tape (painters tape) to make it fit. It doesnt have very much friction and also is not super thick, like duct take. Just use lithium grease or wheel bearing grease. You could probably even use vasoline.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: Shrike88 » Fri Apr 22, 2011 11:30 pm

OK GUYS THANKS FOR ALL YOUR HELP. I did some trimming down on the pistons rubber gaskets, Lubed her up in some good ol vasolene. And cut the threaded Rod to size and pumped her up to 20 psi to test.

BOOOOOOM ! Put a Tennis ball about 140 Yrds down range. Paced it off.

Tried it again and was able to get about 20 more shots off, the largest of which was never more than 40 psi of Air. The last shot destroyed the back end of my forward piston. Pulled the wooden Buffer right out of the 3'' Coupler, Epoxy and all. Did get the Tennis ball about 200 Yards dowrange. Man this baby kicks !

I am sure when I start making my PVC Sabot Darts they will fly much farther. Ill upload Pictures of some I have made Later. 12'' Long, Flaired End with little slits out of the skirt for stators to make the round spin in flight.

However it was SWEET until it lasted. Hoping to get her up to 110 psi this weekend after some tweaking and Paint.

I am going to replace the 3'' coupler with the wood front and back covers to just 3-4 pieces of plywood cut in circles. I am sure this would hold up and be roughly the same weight.

Thanks again..
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Attachments
Broken Piston.jpg
This is the wooden epoxied part that was pulled out from the pilot suction.

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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 12:39 am

In my cannons, I tend to use a short piston and a filler behind the piston to limit the travel distance. Would eliminating the rod make the piston stronger?
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Unread postAuthor: Shrike88 » Sat Apr 23, 2011 12:54 am

I Doubt it. I had to make a Rod and 2 pistons because of some short sightedness on my part. Already glued the barrel's and stuff so the Piston had too far to travel between the end of the male cap and the front of the barrel. Would be like 7 inches of travel for that 3'' coupler. Plus the Coupler didnt fit into the male threaded 4'' reducer fitting in the end of the barrel. ( as shown pictured above) so i made a piston that fit in that endcap reducer fitting, and one for the 4" chamber.

Basically its design flaws on my part but I made it work, and it worked nicely. Just trial and error.

Cant decide if I want to fashion the new piston from 1 large block of wood (a cylinder cut and sanded from a 2X4 or several pieces of plywood ? Guessing would be about the same strength. Difference would be workload and weight.

The Threaded Rod Tech, I use to make adjustments on the travel of the pistons, I like that, so it allows me to tweak. Have a Chronograph, plan on testing and submitting my data for different piston travel settings. I know its supposed to only be 1/4 of the diameter of the piston seating.
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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 9:45 am

The common 1/4 of the diameter number is where the piston gap area = the barrel orifice area. I've always rejected that value as it is two restrictions in series of the same size. By making one restriction larger, the contribution to flow restriction by one of the restrictions can be effectively eliminated or reduced to an insignificant value. This is why I have designed my cannons to have the first flow restriction to be about twice the area of the second restriction which is the barrel ID.

This should be considered to be a minimum opening, not optimum.

In real numbers the 2 inch valve on my larger cannon has a valve seat cross section of about 3.14 sq inches. The area of the ports when open is 6 square inches.

By eliminating the pressure drop of one of the 2 areas of pressure drop, the effect of them should drop.
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Unread postAuthor: warhead052 » Sat Apr 23, 2011 9:56 am

If it still is not sealing, just flip it so the barrel faces down, and the piston is covering the barrel, then fill, and if that doesnt work, try removing that space... I have never dealt with a coaxial, but a friend has, and it worked perfect for him when he did it...
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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 10:51 am

If the rod between the pistons becomes bent, it will cause miss alignment of the piston face and valve seat. This is one of the reasons I am a fan of solid pistons. There is nothing to go out of align.

Did you check if the rod is bent?
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