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coaxial so far need some 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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coaxial so far need some help.

Unread postAuthor: hawktalk » Sun Dec 22, 2013 10:33 am

need help with sealing surface, piston design.
because its so close to Christmas its going to be impossible to get any more materials, so this is what i have: hot glue, duck tape, little bit of epoxy, foam tape, old inner tube rubber, bit of mdf, hardwood, softwood.
i haven't got a vice any more as I stripped the thread on my old one. no lathe (wood/metal) :( ,
i do have a pillar drill, angle grinder, files and that's about it really.
any suggestions please comment.
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Attachments
barrel and chamber(0)[1].jpg
8mm OD aluminium tube on the right and 22mm pipe on the left about 19mm ID
barrel and chamber(0)[1].jpg (13.5 KiB) Viewed 1595 times
old bbgun piston about 14mm wide[1].jpg
an old piston from a bbgun i found its smaller than the 22mm pipe could i wrap tape around it?
old bbgun piston about 14mm wide[1].jpg (12.57 KiB) Viewed 1595 times
assemble 1[1].jpg
fill port is angled away from stock for easy refill
assemble 1[1].jpg (12.72 KiB) Viewed 1595 times
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Re: coaxial so far need some help.

Unread postAuthor: jackssmirkingrevenge » Sun Dec 22, 2013 12:39 pm

Hard to see anything in those tiny photos.

I would suggest casting a hot glue piston directly in your chamber.

Also, you might need to beef puf the tip of the barrel to stop the piston digging into it if you intend to use high pressures.
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Re: coaxial so far need some help.

Unread postAuthor: hawktalk » Sun Dec 22, 2013 2:58 pm

like this

https://drive.google.com/file/d/0B7Wd5t ... sp=sharing

this is what i had in my last attempt
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Re: coaxial so far need some help.

Unread postAuthor: jackssmirkingrevenge » Mon Dec 23, 2013 6:59 am

Yep, something like that. Not too big a diameter as it will obstruct flow, and its important to make sure it is parallel to the piston face.
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Re: coaxial so far need some help.

Unread postAuthor: hawktalk » Mon Dec 23, 2013 7:56 am

i have one more plastic thing any suggestions on diameter, i found a plastic wine cork should i use this? (il need to trim the diameter down a bit)
how much play should there be ?

edit: for pictures il be using links to my google drive for now on so you guys will be able to see them in full definition and screen merry Christmas to all!!.
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Re: coaxial so far need some help.

Unread postAuthor: jackssmirkingrevenge » Mon Dec 23, 2013 8:50 am

For the barrel support, you have a 8mm bore in a 19mm ID pipe.

The maximum support diameter where the gap between support and chamber is equal to the barrel area is 17mm, so you don't need more than a 1mm gap.
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Re: coaxial so far need some help.

Unread postAuthor: hawktalk » Mon Dec 23, 2013 11:08 am

1mm gap around the barrel support the same for the piston?
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Re: coaxial so far need some help.

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Re: coaxial so far need some help.

Unread postAuthor: Ragnarok » Mon Dec 23, 2013 12:32 pm

The ID of a 22mm pipe tends to be closer to 20mm.

I guess it depends a bit on manufacturer, but the ID tends to be very slightly elliptical - however, my personal reckoning* is that the official 20.2mm (22mm, minus two 0.9mm walls) is a good average.
*I filled a 10cm section of pipe with water, weighed it to a hundredth of a gramme and then found the average cross sectional area from the known volume and length.

If a pipe cutter was used, that might be the source of a deceptive measurement - they crush the ends of the pipe in a bit.
I almost always use a hacksaw over a pipe cutter these days, as it means I don't have to ream the end of the pipe out again. Far better anywhere flow or being able to insert full diameter projectiles/pistons is needed.

As far as your piston having a 1mm gap? Hell no. As tight a fit as possible where it'll still easily move.

What concerns me is that a barrel support like that design will probably rip off on the first shot, when the pressure in the valve falls.
Because the flow is dependent on pressure differential, the flow will only be equal between chamber to valve as from valve to barrel if the valve is at a considerably lower pressure than the chamber. Hence, there'll be no small force on that support. It needs to have a far higher flow area so the flow between chamber -> valve flow will match valve -> barrel flow at a far lower pressure differential.

The equal area rule is really quite a fallacy. Thing is, flow is defined by pressure differential.
But when flow is not negligible, any restriction causes a pressure drop - hence reducing the available pressure differential at the next restriction and thus the flow as compared to if there wasn't a flow restriction upstream of it.
(And, inversely, having a flow restriction downstream bounces back and reduces the pressure differential at the previous restriction).

The electrical analogy would be like adding resistors in series. Every resistance adds up.
The small resistances (like the natural friction of straight pipe) or even medium resistances (elbow fittings and the like) aren't all that significant when there's a big one (the valve), but other big resistances will have a major effect on the current.

I'd suggest it'd be better to have a separate barrel support to a design like boogieman/advancedspuds did here, with lots more flow area - and instead increase the sealing face edge separately with a considerably less drastic increase in diameter.
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Does that thing kinda look like a big cat to you?
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Re: coaxial so far need some help.

Unread postAuthor: hawktalk » Mon Dec 23, 2013 1:44 pm

so the piston should be tight fit but easily moved.
the plastic bit (isn't a support) should be smaller than the piston.
the barrel supports are made of small plastic washers glued onto the barrel.
i will take pics keep u guys updated :)
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Re: coaxial so far need some help.

Unread postAuthor: hawktalk » Wed Feb 26, 2014 6:09 am

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