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Piston shape

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Piston shape

Unread postAuthor: daccel » Mon Jun 08, 2009 9:07 am

Going to machine a piston out of 1/2" aluminum rod soon, as pictured, and I'm debating how to shape the head of the piston.

I think if it's flat it will provide greater flow sooner, but if dome or cone shaped might make for less turbulence which would possibly increase performance even with slightly less flow during opening. Once it is fully open, obviously wouldn't make a difference, just during opening.

Thoughts?
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Unread postAuthor: hi » Mon Jun 08, 2009 9:25 am

I usually make mine round. the square ones Don't work very well.

in all seriousness i don't think its going to make much of a difference, i would say make it flat.
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Unread postAuthor: Carlman » Mon Jun 08, 2009 9:44 am

Wont make any noticeable difference mate
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Unread postAuthor: mark.f » Mon Jun 08, 2009 10:36 am

Instead of thinking of performance, why not think of reliability?

If the piston head is flat, and it has to go into another tube, then you may have problems with jams. If the piston head is rounded or conical, it will self-center and go into the tube without problems.
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Re: Piston shape

Unread postAuthor: Technician1002 » Mon Jun 08, 2009 11:47 am

daccel wrote:Going to machine a piston out of 1/2" aluminum rod soon, as pictured, and I'm debating how to shape the head of the piston.

I think if it's flat it will provide greater flow sooner, but if dome or cone shaped might make for less turbulence which would possibly increase performance even with slightly less flow during opening. Once it is fully open, obviously wouldn't make a difference, just during opening.

Thoughts?


If you extend the smaller barrel outlet into the chamber and cut big ports, the piston will never leave the guide of the pipe. One of my early designs was just like what you drew except it was ported like my QDV. It was an early design and that one had small round ports, so the piston was overly long and the ports were too small so the performance wasn't anything to write home about.

The QDV style piston and ports will work fine with a stepped piston for a QEV style barrel sealer piston. The advantage is the chance of blowing the o ring out the barrel is greatly reduced as the o ring is never fully uncovered so it can be blown off.

It would look like the ports shown below, but the ports would simply be cut off instead of having the pipe on both ends so the piston can be short and still reach in to seal off the outlet. If you guide the nose of the piston, one of the o rings on the other end can be eliminated. The piston can be made shorter and therefore lighter and faster.
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Unread postAuthor: psycix » Mon Jun 08, 2009 4:32 pm

On high-performance launchers, it is advisable to use a cone on the front. Or even better: A cone that is curved, with a very sharp point.
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Unread postAuthor: Ragnarok » Mon Jun 08, 2009 4:35 pm

One question, what's stopping the piston just shooting off upwards in this diagram? As it is, the forces on it are unequal.
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Unread postAuthor: daccel » Mon Jun 08, 2009 11:19 pm

Thanks, good points - I hadn't thought about the actual function as much as I should have.

I screwed up the scale on the diagram though, the barrel and chamber are actually the same size, and they are composed of two pipes sleeved together for strength. Which could make it problematic to extend the 'deadspace tube' for lack of a better phrase down through the chamber. The chamber I wanted to go through all the way for strength, but maybe I can do something with the two layers of pipe to make both work. Sorry I'm not explaining very clearly, too tired right now, but I will think about it and try to post a better diagram and picture later.

When you say pointed curved cone, you mean concave or convex, and because I can't remember which is which, curved in or out :P?

I will probably just put some screws through the piston tube at the step to stop it shooting up. It will be extra thick at this point through the block that is holding all these parts together, so not worried about weakening it.
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Unread postAuthor: psycix » Tue Jun 09, 2009 5:31 am

Hard to explain...
The optimal shape would be a bit like this:

It guides the air smoothly into the barrel, instead of letting it bash into something (or itself).
The performance difference between this and a normal cone is almost unnoticeable though. But if you are really wanting to get out every single joule there is, then go with this.
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Unread postAuthor: USGF » Tue Jun 09, 2009 7:28 am

I made a 3" port, 4" piston for Grock the other day. One of many possibilities.

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http://www.spudfiles.com/forums/4-inch- ... 18112.html

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Unread postAuthor: psycix » Tue Jun 09, 2009 9:27 am

Now thats what I'm talking about!


To make it even more perfect, sharpen or round the tip. (I know, it is a bolt to keep it together, but there are ways.)
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Unread postAuthor: daccel » Wed Jun 10, 2009 12:00 am

I think I will go with that last shape suggested. Yeah, it probably won't make too much difference, but I have to shape it like something so why not.

Roughed in the piston this evening, still have to finalize the dimensions before I cut oring grooves and shorten it up. I need to fill air from the right, but don't necessarily have to run the chamber right through to do so, I realized, so maybe it will just get threaded in there. Forgot in diagram, but barrel will run through as chamber did for bolt and breech.
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Unread postAuthor: daccel » Sat Jun 13, 2009 12:34 am

I know, I know. In my defense, there are shiny new pics :D.

Btw, sweet piston there, USGF. My cannon may have piston envy. It's compensating with the polish bling :lol:.

Finished it up tonight: .57" x .37" porting, 5.8 g. Left the double rear rings because with the full port thought it might need the guidance.

Should I stop the piston with the cone still in the air path (see pic) or have it move back out of the way? This will be a hybrid with an air spring, if it matters. Thinking of using a rubber tube as a bumper to be able to set the piston travel, rather than let the air bumper it.
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Unread postAuthor: Technician1002 » Sat Jun 13, 2009 9:38 am

daccel wrote:I know, I know. In my defense, there are shiny new pics :D.

Btw, sweet piston there, USGF. My cannon may have piston envy. It's compensating with the polish bling :lol:.

Finished it up tonight: .57" x .37" porting, 5.8 g. Left the double rear rings because with the full port thought it might need the guidance.

Should I stop the piston with the cone still in the air path (see pic) or have it move back out of the way? This will be a hybrid with an air spring, if it matters. Thinking of using a rubber tube as a bumper to be able to set the piston travel, rather than let the air bumper it.


Sweet piston. Have it move back just far enough to move the o ring portion out of the path and leave the point in the port area so it directs the flow path for low turbulence.

Well done. It should be a real performer. Are the rings floating for low moving friction? A high friction piston will really slow the valve or cause it to stick.

Make the O ring grooves as deep as the cross section of the o rings. They should not squish flat in the bottom of the groove.

Most here have seen this. I have a video on using o rings in this type of valve. Your grooves look shallow. The side of the o ring needs to seal against the side of the o ring groove, not the bottom. They look above it which will cause binding and the rings to get cut in the ports.

The o rings should sit deep in the grooves with only the edge extending out just enough to seal against the cylinder wall. Air pressure will take care of the sealing force under pressure.
If there is large clearance between the piston and cylinder wall, there is a risk of blowing off the rings. You want less than 10 thousandths of an inch clearance. EDIT; Specified inch. [youtube]http://www.youtube.com/watch?v=BbDiZiRVrnk[/youtube]
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Last edited by Technician1002 on Sat Jun 13, 2009 3:07 pm, edited 1 time in total.
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Unread postAuthor: psycix » Sat Jun 13, 2009 1:19 pm

Sweet piston!

Though I am concerned about the o-ring grooves, they are really shallow. I'd make the diameter of the material next to the groove larger, so that you've got a deeper groove.
This way, the o-ring will handle pressure the best way, with no chance of getting pushed out of its groove OR being extruded/ripper apart under high pressure.
For moving parts, floating o-rings would be best.

Oh, and you could try to drill a hole through the piston, so it becomes lighter.

EDIT: As tech said, optimally the clearance between the material and the wall has to be like a few thousandth of a mm.
Tolerance depends on your working pressure though.
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