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Minimal piston length

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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Minimal piston length

Unread postAuthor: psycix » Mon Dec 03, 2007 4:48 pm

I am on the way of building my first pneumatic, it will be a coaxial piston design, 50mm chamber, 32mm barrel.

I am making a piston out a 40mm endcap wich fits EXACTLY into the 50mm chamber.
The piston I have in mind is kinda short, not even half the diameter (about 22mm long)

Now I have one question:

Does the piston need to have a minimal length in comparison to its diameter?
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 03, 2007 4:54 pm

It depends on how tight the fit is, I would usually recommend having the piston length at least 75% of the diameter, otherwise the tendency will be for the piston to skew and jam in the chamber.
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Unread postAuthor: Ragnarok » Mon Dec 03, 2007 5:14 pm

Less than half is pushing it. I wouldn't normally go under a 1:1 length to diameter ratio on a piston, and I normally go for 2:1 - it makes them a little heavy, but it means I can fit a check valve and spring bumper onto them, probably more important.
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Unread postAuthor: psycix » Mon Dec 03, 2007 5:16 pm

The fit is well-sliding and nearly airtight.
So could a shorter piston jam a bit?

Else ill go get some more 40mm endcaps and make the piston out of 2 endcaps and inner bike tire.

I think ill fill the piston with some sort of foam (or make it airtight), but im uncertain about the way I will attach the sealing face. I wish to avoid a heavy bolt to keep it lightweight.

edit: hmm ok ill make it around the 1:1 to avoid problems
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Unread postAuthor: LikimysCrotchus5 » Mon Dec 03, 2007 5:37 pm

Well i suggest bolting it because some sealing faces have a tendancy to sometimes shoot out the barrel. A bolt wont make it too heavy at all. It will still be pretty light weight and efficiency should be good as well.
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Unread postAuthor: Hawkeye » Mon Dec 03, 2007 6:56 pm

You can use a rubber ball for a piston.
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Unread postAuthor: jrrdw » Mon Dec 03, 2007 6:59 pm

LikimysCrotchus5 wrote:Well i suggest bolting it because some sealing faces have a tendancy to sometimes shoot out the barrel. A bolt wont make it too heavy at all. It will still be pretty light weight and efficiency should be good as well.


I'de like to sagest a big flat washer under the bolt head. One big enough to allmost be the barrel bore diameter. Keeps it from going out the barrel and i beleave helps seal the piston better/quicker, leaves little room for the seal to flap around.
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Unread postAuthor: MrCrowley » Mon Dec 03, 2007 7:09 pm

psycix wrote:The fit is well-sliding and nearly airtight.
So could a shorter piston jam a bit?

Else ill go get some more 40mm endcaps and make the piston out of 2 endcaps and inner bike tire.

I think ill fill the piston with some sort of foam (or make it airtight), but im uncertain about the way I will attach the sealing face. I wish to avoid a heavy bolt to keep it lightweight.

edit: hmm ok ill make it around the 1:1 to avoid problems


If you fill it with foam just use a small bolt with a washer, if you don't you will most likely loose the sealing face on the first shot. With a piston this big, a weight of a bolt wont matter a whole lot, a bolt will be fine.

Also make sure to seal the bolt and washer, otherwise the air could leak through the foam, up the bolt and out where the bolt meets the washer, I had similar problems.
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Unread postAuthor: iknowmy3tables » Mon Dec 03, 2007 8:36 pm

just so it wont topple if you do it right you can even use a sheet of rubber

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Unread postAuthor: psycix » Tue Dec 04, 2007 5:32 am

Thanks for all the replys so far. You guys helped me alot improving the plans and thus the final result.
These are the plans so far, but I am not sure about a few things:
Image

Please tell me what you think and what you would recommend me to do with the remaining things described.
-Filling foam? What kind or could a empty piston work aswell?
-Connecting the two endcaps
-40mm pipe inside?
-Equalisation hole(s)?

Please note I wish to keep the piston as light as possible.

EDIT:
@iknowmytables:
Nah I dont want to use diaphragm. Does that outperform a piston valve then?

@Someguys
I didnt know sealing faces could be shot out when not connected properly.
Quite funny actually, I was firstly planning to hot-glue the sealing face on :P
Oh well, another lesson learned:)

@JSR:
Quote:"With a piston this big, a weight of a bolt wont matter a whole lot, a bolt will be fine. "
Erhm big? This is my mini-design to get started with pneumatics.
Im planning to step it up to double (and later on maybe even triple) size after this.
That will be a 2meter long, 50mm barrel, 75mm chamber (and thus 75mm piston) canon. Where the piston valve is piloted by a... piston valve.
:D
Not even speaking about my "dream": a 110mm chamber, 75mm barrel, and about 3 or 4 meters long.
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Last edited by psycix on Tue Dec 04, 2007 5:41 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 04, 2007 5:40 am

A long bolt won't be *that* heavy vis-a-vis the piston size, but a piece of 40mm pipe is a better idea, better stress distribution and more likely to survive the repeated impacts a piston has to endure. I would leave it empty in this case.

No need for an equalisation hole in this case.
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Unread postAuthor: psycix » Tue Dec 04, 2007 5:45 am

Hmm okay, then it will get a piece of 40mm pipe between, no foam filling and no eq-holes.

Thanks for the quick response:)
Whoa I wasnt even done with editing yet.....


But okay, then that will be the plans and let the building begin!
I have already bought 95% of all parts needed.
You will see this cannon within a week...... stay tuned.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 04, 2007 5:53 am

Erhm big? This is my mini-design to get started with pneumatics.


It's all relative, believe me, a 50mm chamber is HUGE - >>this<< is a mini-design :P :D
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Unread postAuthor: psycix » Tue Dec 04, 2007 12:03 pm

jackssmirkingrevenge wrote:
Erhm big? This is my mini-design to get started with pneumatics.


It's all relative, believe me, a 50mm chamber is HUGE - >>this<< is a mini-design :P :D

No thats NANO.
Mine is mini!
I consider all cannons under one meter length and under 40mm barrels as "small"
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 05, 2007 5:10 am

Fair enough, the biggest pneumatic I ever made was 3/4" calibre so I guess I live in a world with smaller limits :)

I like smaller launchers, you use less material (and waste less if you have to throw it away and start over, a not unheard of eventuality with epoxy) so it works out cheaper, they're easier to store and if you up the pressure, the power is still impressive. If I lived in a rural area though, it would be large calibre beasts all the way :D
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