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Double Piston Conceptual Design

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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Double Piston Conceptual Design

Unread postAuthor: rad14701 » Mon Mar 12, 2007 8:38 pm

After browsing these forums over the past several weeks as a non-member I decided to join so I could share an idea that I haven't found in any of the forum threads... I apologize if this concept has already been presented, as well as for my lack of skills in graphic representation...

The theory behind this design is that a second piston is used to seal the exhaust port... If the stem of the piston is either hollow or slotted then once depressed, using a hammer like a standard rifle, then the pilot pressure would rapidly exhaust... The relatively small size of the exhaust piston in comparison to the chamber diameter should easily be overcome by a spring loaded hammer... The functionality should exceed both blow guns and ball valves according to my initial research...

Although I haven't included the location of the schrader valve it would obviously be placed where it would rapidly seat both pistons... I have also considered placing a weak return spring on the piloting piston...

I'm hoping to be able to start work on a prototype in the near future... I also have ideas for the trigger and hammer mechanism but will need to brush up on my graphics skills before I can share them...

Comments welcomed...
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Last edited by rad14701 on Tue Mar 13, 2007 4:29 pm, edited 1 time in total.

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Unread postAuthor: willarddaniels » Tue Mar 13, 2007 12:46 am

Sounds good... but then again it is really late for me.
A compression spring would be a good addition to go between the two pistons. You could mortice a recess for the spring and the exhaust piston into the main piston so that you can get full compression and be able to fully open your main piston.
This could be incorporated into a mortar-type pneumatic as well, You would just have a guide that you slid the whole cannon down to a plate that pressed your exhaust piston in, actuating the sequence. With the exhaust piston/valve small enough, there wouldn't hardly be any pressure/resistance to overcome. One drawback would be hitting the exhaust/pilot piston at an angle and ruining the seal, if not the whole mechanics.
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Unread postAuthor: Hotwired » Tue Mar 13, 2007 5:34 am

So its basically a hammer valve to pilot the main piston.

It should work fine but theres no need for the hammer valve to touch the main piston.
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Unread postAuthor: deusXmachina » Tue Mar 13, 2007 7:47 am

It looks interesting but I think you're overcomplicating it a bit. You can make up for lost air (I assume that is what the second seal is for) by compensating with a larger chamber.
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Unread postAuthor: rad14701 » Tue Mar 13, 2007 8:27 am

I should probably add that this is intended to be a shoulder fired rifle more than a cannon... And the two pistons are not intended to touch one another and would be separated either by a spring or some other form of snubber... It would probably take me longer to draw up accurate technical details than it would to build it... After thirty-some years of seat of the pants inventing I've found this to be the case... Let me loose in the workshop and I'll show you how quicker than I can explain how...

@deusXmachina

The second piston is the pilot valve (wastegate) which is used in place of a standard air valve or sprinkler valve... People have been concerned about the internal pressures not allowing this to work but we used this method years ago when designing farm equipment so I know it works...

I offer up some napkin scratch logic... 100psi is only 50psi for a half a square inch... And a .625 piston only has .3 square inches or 33psi to overcome... Using a 3:1 leverage you would only need 11psi to cause a release... This is the same approximate amount of force required to fire a handgun or a rifle... So if you used a spring loaded hammer you could easily overcome the pressure in the pilot to cause the unit to fire... You only need just over 11 pounds of spring pressure on your firing hammer...
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Unread postAuthor: willarddaniels » Tue Mar 13, 2007 9:34 am

"If you build it, he will come."
I'd love to see it in action. Show some pics when you are done.
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Unread postAuthor: joannaardway » Tue Mar 13, 2007 10:12 am

rad14701 wrote:I offer up some napkin scratch logic... 100psi is only 50psi for a half a square inch... And a .625 piston only has .3 square inches or 33psi to overcome... Using a 3:1 leverage you would only need 11psi to cause a release... This is the same approximate amount of force required to fire a handgun or a rifle... So if you used a spring loaded hammer you could easily overcome the pressure in the pilot to cause the unit to fire... You only need just over 11 pounds of spring pressure on your firing hammer...


I have to correct this, because you seem to have a total misunderstanding between pressure and force.

The simple pressure*area = force equation has been used to correct the maths. You seemed to be using pressure/area = pressure which makes little sense. (I don't know where you got it from, but it won't help you if I just ignore it and let you keep using damaged science. Please forgive me.)

I offer up some napkin scratch logic... 100psi puts 50 pounds of force on a half a square inch... And a .625 piston only has .3 square inches so needs 30 pounds of force to overcome... Using a 3:1 leverage you would only need 10 pounds of force to cause a release... This is the same approximate amount of force required to fire a handgun or a rifle... So if you used a spring loaded hammer you could easily overcome the pressure in the pilot to cause the unit to fire... You only need just over 10 pounds of spring force on your firing hammer and an equivalent momentum that produces a peak of force in excess of that on impact
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Unread postAuthor: rad14701 » Tue Mar 13, 2007 12:07 pm

@joannaardway
You are correct in that I used improper verbage... However, I happen to run a very busy forum myself and if I was to go around making corrections all day long I would have little time to do much else... It must be nice to have nothing better to do with ones day... This was, after all, napkin logic - not exact science... Or should I say physics...
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Unread postAuthor: joannaardway » Tue Mar 13, 2007 3:49 pm

I'm a full-time student, with massive piles of work most of the time. I always have something else to do, but if I spent 24/7 doing school work, I'd go mad.

I hope I didn't cause any offence. I often rush in too fast without much thought for politeness.
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Unread postAuthor: rad14701 » Tue Mar 13, 2007 3:55 pm

joannaardway wrote:I'm a full-time student, with massive piles of work most of the time. I always have something else to do, but if I spent 24/7 doing school work, I'd go mad.

I hope I didn't cause any offence. I often rush in too fast without much thought for politeness.

No problem... If it weren't for the fact that we are presently a single income household I'd probably be posting pictures of the completed and tested product by now... Priorities currently dictate spending available funds on more important items than grownup toys... Ho Hum... Such is life...
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