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First Piston......please comment

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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First Piston......please comment

Unread postAuthor: limbeh » Mon Oct 13, 2008 10:18 am

Hi,

This is my first piston. 2 leftover 6mm acrylic disks bolted together with a M10 bolt, with a 3mm thick rubber disk on top of one of them. I haven't epoxied it together though, so the thing can be taken apart again.

It fits pretty tight in the tee. I will probably go add O rings sometime in the future, but I think I'd rather get the piston right first than to dabble with equalization holes and O rings. That said I have not finished making the pilot valve system yet, so no tests yet.

Please give your comments & suggestions on the thing, esp how it can be improved. I'm sure the pros like Ragnarok and JSR would have something to say.

Again, please understand that I live in Singapore, which has the strictest rules, plus my brother is a policeman, so I don't dare try it out for myself at home for fear of accidents.......
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1.JPG
The piston from the side view. From left to right, nutt + washer, rubber disk, 6mm acrylic disk, UPVC JIS AE 3" pipe, 6mm acrylic disk, washer + bolt head.
1.JPG (15.7 KiB) Viewed 603 times
2.JPG
The piston from the rear. I forgot to add another rubber disk to the back. The beige thing is my hand. You can't see the nuts from this angle.
2.JPG (15.32 KiB) Viewed 603 times
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Re: First Piston......please comment

Unread postAuthor: Ragnarok » Mon Oct 13, 2008 10:37 am

limbeh wrote:I'm sure the pros like Ragnarok and JSR would have something to say.

Wow, I'm a professional. :P

That looks like a good start, although the brittle acrylic might not fare well under the forces incurred by the valve's operation, so be careful when you're testing it.

Remember that the forces on the pistol will be transferred by compressive forces along the length of the piston, so if you haven't already, I'd recommend making sure there are nuts on the bolt which can cope with compression loads as well as tension loads.

Of course, if the piston's interior isn't entirely airtight, these problems will be lessened by some balancing pressures inside the piston.

Good start - now test it!
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: starman » Mon Oct 13, 2008 10:39 am

If it fits snug but friction free in the tee, you should be good. With the bolt holding things together, you probably won't need to epoxy everything together. Just make sure it is air tight from the rear to barrel opening and the sealing face is flush with your rubber seal.

Looks like a good design!!

Edit: Yes, Rag is a professional... :wink:
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Unread postAuthor: limbeh » Mon Oct 13, 2008 10:53 am

Thanks all.

Some further data on the piston

Snug = yes
Airtight = no. Quite a good number of places which air can leak out from.
Friction free = No. Seems quite resistant to moving.
Intended usage = < 120psi. Not very high compared to some of you.

I'll post the pictures of the thing in the tee, when my policeman bro isn't around. More nuts will be added once I get them.

Thanks once again.

Limbeh
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Unread postAuthor: Ragnarok » Mon Oct 13, 2008 10:59 am

120 psi can be a lot in a larger cannon. Anything above about 150 is more getting into the preserve of smaller, metal launchers.
I'd be surprised to see a larger 200 psi PVC cannon, put it that way.

starman wrote:Edit: Yes, Rag is a professional...

I'm probably more of an expert or adept, but not so far as a master or authority - professional sounds wrong, as I'm not making a profession out of it.

But enough of my semantics.
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Unread postAuthor: starman » Mon Oct 13, 2008 11:04 am

limbeh wrote:Airtight = no. Quite a good number of places which air can leak out from.
Friction free = No. Seems quite resistant to moving.
Intended usage = < 120psi. Not very high compared to some of you..


Airtight to the barrel opening is a requirement and resistant to moving isn't good. However, it sounds like you are very close to making this work right. Be very careful not to take too much away in freeing the movement. Use some fine sand paper to take ever so small amounts from the edges.

120 psi is a lot for this size cannon.
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 13, 2008 11:23 am

It looks like a sturdy and relatively lightweight way to make a piston and should work well in practice. Shame it isn't airtight though, you could save yourself some dead volume, plus air leaking from the piston into your pilot volume could deacrease the efficiency of your piloting. Also, it's a little long, however I'm thinking in coaxial terms so if it's made for a tee I imagine it's as short as you can make it.
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Unread postAuthor: VH_man » Mon Oct 13, 2008 11:45 am

didnt i just post in this? whered it go?

anyway, Nice piston, im semi-envious.

The only problem i see is the fact that Lexan is not very impact resistant, and may shatter upon its impact with the rear of the piston housing.

Try it anyway tho, and if it breaks, well then... now you know! and if it doesnt, well then WOHOO! Its what spudding is all about.....
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Unread postAuthor: limbeh » Mon Oct 13, 2008 12:17 pm

Actually, 120psi is stretching it quite a bit. I'm using a normal, unmodded bicycle pump here, and I highly doubt that the thing could get me beyond 100psi. Must add the 20% safety margin
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Cannon.JPG
This is a picture of the cannon. Barrel diameter 3&quot; JIS AE, tee size 4&quot; JIS AW, pilot valve is JIS 1/2&quot; ball valve.
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Unread postAuthor: jrrdw » Mon Oct 13, 2008 12:28 pm

pros like Ragnarok and JSR


Pro what? is my question! :P

The disk on each end look bigger in diameter then the piston body. I for-see a problem with the sealing disk catching the cylinder while opening, piston tipping and so forth.

For your sake I hope I'm wrong because the above sernerio will cause catastrophic failure at 100psi.... :shock:
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Unread postAuthor: ALIHISGREAT » Mon Oct 13, 2008 12:30 pm

thats a nice looking piston (i'm not gonna repeat everything everyone has already said)

but you could do with some more chamber volume if you want to have decent power.
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 13, 2008 4:52 pm

ALIHISGREAT wrote:you could do with some more chamber volume if you want to have decent power.


Agreed, especially if you're not using high pressure. Unless of course noise is a factor and you want to keep things quiet, in which case you're on the right track ;)
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Unread postAuthor: limbeh » Tue Oct 14, 2008 8:29 am

jackssmirkingrevenge wrote:
ALIHISGREAT wrote:you could do with some more chamber volume if you want to have decent power.


Agreed, especially if you're not using high pressure. Unless of course noise is a factor and you want to keep things quiet, in which case you're on the right track ;)



Correct guess. Given my circumstances, noise is not a good thing.

jrrdw was correct in that the disk would catch the cylinder while moving back. Operationally, not too bad a problem, it just means that I can't fire with the cylinder pointing downwards.

Ideally, I would go try for the best possible / foolproof cannon, but I can well put up with the inconvenience for the time being until I make a better one.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 14, 2008 9:14 am

limbeh wrote:jrrdw was correct in that the disk would catch the cylinder while moving back. Operationally, not too bad a problem, it just means that I can't fire with the cylinder pointing downwards.


Can't you just trim the disk a little? I don't see the problem, nor the relevance of the launcher's orientation :?
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Unread postAuthor: limbeh » Tue Oct 14, 2008 9:28 am

jackssmirkingrevenge wrote:
Can't you just trim the disk a little? I don't see the problem, nor the relevance of the launcher's orientation :?


No problem trying. I have plenty of spare acrylic sheet and disks, leftovers from a school project.

The problem is that a large part of the piston overhangs out over the cylinder. So the front part of the piston does "drop" into the cylinder - and the part of the disk sticking out hits the rear wall of the cylinder.

But then and again, that can wait - constructing the pilot valve and getting it to work is the headache now on my side.............I lack the experience in that aspect.
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