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A few piston hybrid questions...

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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A few piston hybrid questions...

Unread postAuthor: PeteS » Fri Dec 20, 2013 12:46 pm

I decided to build a piston hybrid. I started collecting parts and honed the 2" pipe that the piston will be fitted to. The honed pipe measures 2.076" ID. How tight should I fit the piston? (it will have o-rings)

I was planning to order a piece of stock to turn the piston from, probably Delrin unless that is a bad idea. Can I expect to get a decent piston for that 2.076" bore from Delrin rod that is nominally 2" diameter? I am guessing not, but it would be nice.

Any hints on o-ring selection and o-ring groove dimensions and shape?

Would it be a bad idea to use a pop off valve to evacuate the area behind the piston rather than have a little piston connected to the main piston?

I have one of those 7x10 lathes from harbor freight and am fairly new to it, but getting the hang of it fairly well.
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My wife upon seeing my latest hybrid and hearing an explanation of it:
"That really isn't a potato cannon anymore, is it?"
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Re: A few piston hybrid questions...

Unread postAuthor: jackssmirkingrevenge » Fri Dec 20, 2013 1:03 pm

PeteS wrote:I decided to build a piston hybrid. I started collecting parts and honed the 2" pipe that the piston will be fitted to. The honed pipe measures 2.076" ID. How tight should I fit the piston? (it will have o-rings)


As tight as possible :) as tight as you can make it while it while you can still push it easily with one finger.

I was planning to order a piece of stock to turn the piston from, probably Delrin unless that is a bad idea. Can I expect to get a decent piston for that 2.076" bore from Delrin rod that is nominally 2" diameter? I am guessing not, but it would be nice.


Delrin is a suitable material. Typically material rods are slightly bigger than the nominal diameter, you should be fine.

Any hints on o-ring selection and o-ring groove dimensions and shape?


Select an o-ring size lightly under your bore diameter. Square grooves are fine, groove width should equal o-ring thickness and groove depth should be (in my experience) calculated as follows:

(o-ring thickness) - (bore radius - piston radius) - 0.004"

So say your bore diameter is 2.076", your piston is 2.05" and the o-ring is 0.1" thick.

Groove depth should be (0.1) - (1.038 - 1.025) - 0.004 = 0.083"

Again this is based on my experience, the 0.004" tolerance is something I found works based on trial and error.

If you want some more scientific musings, have a look at the "dynamic seal" section of this handbook: http://www.parker.com/literature/ORD%20 ... ndbook.pdf

Would it be a bad idea to use a pop off valve to evacuate the area behind the piston rather than have a little piston connected to the main piston?


It will work, but you need to find one with reasonably high flow.
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Re: A few piston hybrid questions...

Unread postAuthor: PeteS » Fri Dec 20, 2013 2:00 pm

Thanks. That covers my questions nicely.

UPDATE:
I ordered some Delrin, o-rings, rubber sheet, and a high flow pop off valve. I also have done some of the fabrication and assembly. I will post a build thread with more details.
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Re: A few piston hybrid questions...

Unread postAuthor: MrCrowley » Sat Dec 21, 2013 9:07 pm

A 2" piston valve will need a bigger pop-off than those 1/4" ones. It does depend a lot on your pilot volume and your bumper as well, but a pilot valve will poor flow may cause piston bounce. With a delrin piston, that may be fatal after awhile.
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Re: A few piston hybrid questions...

Unread postAuthor: PeteS » Fri Dec 27, 2013 5:13 pm

jackssmirkingrevenge wrote:
PeteS wrote:I was planning to order a piece of stock to turn the piston from, probably Delrin unless that is a bad idea. Can I expect to get a decent piston for that 2.076" bore from Delrin rod that is nominally 2" diameter? I am guessing not, but it would be nice.


Delrin is a suitable material. Typically material rods are slightly bigger than the nominal diameter, you should be fine.


I got my order and the nominally 2" Delrin is a way loose fit. It was barely over 2". I am kind of bummed. I forget the actual measurement, but it is something like 0.050 under the size of the bore.

I have some fat 2" o-rings (actual outside dia. 2.020", actual inside dia.1.600", actual width 0.210"). Will this work with a piston that under size given the thickness of the o-rings, or would I be wasting the Delrin and my time to try?
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Re: A few piston hybrid questions...

Unread postAuthor: MrCrowley » Fri Dec 27, 2013 5:34 pm

I think fat o-rings would do the job. I'm sure I've had o-rings extend more than 1mm outside of the groove to increase the diameter of the piston by 2mm total. Those o-rings you have do sound pretty big though, perhaps something with a larger inside dia. but similar outside dia. would be better.
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Re: A few piston hybrid questions...

Unread postAuthor: jackssmirkingrevenge » Sat Dec 28, 2013 3:25 am

MrCrowley wrote:I think fat o-rings would do the job.


I agree.
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Re: A few piston hybrid questions...

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Re: A few piston hybrid questions...

Unread postAuthor: PeteS » Mon Dec 30, 2013 5:23 pm

Ok, so I went ahead and made a piston to use the o rings I had. The piston was even more undersized than I thought so I stuck with the really fat o rings. Initially I made it too tight, but then cut the o ring grooves deeper and it seems fine now if a bit tight. It works fine as a pneumatic. More work to do to finish it up as a hybrid though.
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Re: A few piston hybrid questions...

Unread postAuthor: jackssmirkingrevenge » Tue Dec 31, 2013 7:23 am

PeteS wrote:It works fine as a pneumatic. More work to do to finish it up as a hybrid though.


That's the first step, good luck ;)
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Re: A few piston hybrid questions...

Unread postAuthor: PeteS » Tue Dec 31, 2013 10:02 am

I wish I had taken some pictures of the piston, but I don't want to take the gun apart again now.

That said here is the external view. It is 2" parts with a 1-1/2" EMT barrel. A $7 ViperTek stun gun and NGK 4629 spark plug provide ignition and there is a manifold for charging using the manometric method.

The pop off valve is a high flow 1/4" one and is adjustable. It has the choice of 4 different springs and the tension on those springs is also adjustable so it has a wide range. Not sure if it is high enough flow, but it seems fast and the pilot chamber is quite small. The pop off goes off with a bang when set at 100 psi if I overcharge past 100 psi. I may be reworking the tee, quick connect, and pop off arrangement depending on how it works out.

Right now it has a stubby little barrel (~20"), but I plan to use a longer one. I have not decided what length to use yet.

Image

Update... At first with the old two piece manifold I was having problems with getting the mix right. The unsuccessful drill was:
1. Plug pump into quick connect for pilot chamber and pressurize to desired pressure.
2. Plug propane manifold into quick connect on chamber and pressurize chamber to desired pressure of propane.
3. Unplug propane manifold.
4. Plug pump into quick connect for chamber and pressurize to desired air pressure.
5. Unplug manifold.

I think the problem was that the quick connect let propane escape when plugging the pump into the quick connect.

The current setup has the pump and propane both connected to the manifold at the same time. A ball valve protects the propane gauge when pressurizing it with air. The whole manifold pops on and off as one piece.

The new and successful drill is:
1. Plug pump into quick connect for pilot chamber and pressurize to desired pressure.
2. Plug combined propane and air manifold into quick connect on chamber.
3. Open the ball valve and pressurize chamber to desired pressure of propane.
4. Close the ball valve.
5. Pressurize chamber to desired air pressure.
6. Unplug manifold.

It seems to work fine now. Oh and it was crazy loud with the short barrel and no projectile.

Now that it is set up and working, I will have to figure out what barrel I really want on this gun. It definitely deserves something longer than the little stubby barrel that is on it now. It was just a leftover piece of EMT. Something in the 5-6' range is probably where I will wind up. Longer would probably be better for optimum performance, but where I usually shoot keeping a lower profile is desired. I figure a moderate length barrel attracts less attention.

Another Update:
Ok, so the rubber face on the piston was quickly shredded. I decided to smooth out and round the breech face that the rubber bears against, but after doing so decided that the tube was thin enough that it was likely to still be a problem. At that point I made a bushing to spread out the load at the cost of restricting flow a bit.

Image

I installed a 42" barrel that I had laying around, but it would definitely benefit from a longer yet barrel. I had been wondering what the gun weighed as I lugged it around so I weighed it and it turns out to be a stout 19 pounds!
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Re: A few piston hybrid questions...

Unread postAuthor: jackssmirkingrevenge » Wed Jan 01, 2014 6:45 pm

Looking good!

...but where I usually shoot keeping a lower profile is desired. I figure a moderate length barrel attracts less attention.


I don't think it's possible to keep a low profile with a hybrid this size...
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Re: A few piston hybrid questions...

Unread postAuthor: PeteS » Fri Jan 03, 2014 6:49 am

jackssmirkingrevenge wrote:
...but where I usually shoot keeping a lower profile is desired. I figure a moderate length barrel attracts less attention.


I don't think it's possible to keep a low profile with a hybrid this size...


True enough! That said, I can fit the chamber assembly in a backpack and put a oversize crutch tip on the barrel. With that I can pass myself off as an eccentric old hiker. That is easier with smaller bore barrels though. I also try to go when I have the whole area to myself.

I have a spot only a couple hundred yard from where I can park that is sheltered from view by the lay of the land. It is pretty far from any homes or businesses and the lay of the land also tends to help tone down the sound and direct it toward an unpopulated area. The published rules for the area do not forbid spud guns, but I suspect they would change them if spud guns were on their radar. My police officer brother in law thinks I'd be OK if the law enforcement folks there saw me, but I have my doubts so I keep as low of a profile as I can with such a beast of a gun.
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Re: A few piston hybrid questions...

Unread postAuthor: jackssmirkingrevenge » Fri Jan 03, 2014 7:23 am

Is it acceptable to fire shotguns or other firearms in the area?

You should consider an integral suppressor of some sort to muffle the noise, though that doesn't help with the sonic crack from a supersonic projectile.

For this one, a bit of drain pipe made a world of difference.



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Re: A few piston hybrid questions...

Unread postAuthor: PeteS » Fri Jan 03, 2014 9:12 am

jackssmirkingrevenge wrote:Is it acceptable to fire shotguns or other firearms in the area?

No.
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