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A place to ask general spud cannon related questions.
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Unread postAuthor: Desmo » Tue May 22, 2012 2:25 am

Nice work on the design. Seems pretty similar to the one i made just before (have a look in the pneumatic show case section). If you want to discuss it drop me a PM and i'll give you my MSN :)

Was about to ask myself actually so this works well, But I'm not sure if turning a shoulder into the piston behind the barrel seal would be very ideal. The surface area on the barrel side isn't far off that of the pilot side. The surface area of a little less than the m10 is the only difference. Do you think it will actuate fast enough? Seems you would need a serious pilot valve for that to work better than something like a low ratio piston valve.

That and if you make it from engineering plastic, you really don't save that much weight from turning the shoulder. On a 55mm x 57mm piston Only about 40 grams out of 215 total would be saved, that's with delrin which is one of the heaviest plastics.

Image

What i was thinking of doing was turning a large recess between the two o rings on the piston so the area on either side of the recess is equal, cancelling the net force out. That would save much more weight, keep the surface area of the barrel side small for quicker valve actuation and probably be easier to make.

However what I'm not sure of is how the pressurised air between the o rings would effect the piston operation. When the valves fired it would basically stay pressurised minus leakage through the equaliser port (or completely pressurised with a check valve). Would it slow the piston down or what? Or is it just "dead volume" that doesn't affect anything?

With your tolerance question (forgive me if i'm reading this all wrong i kinda glanced over this page lol) I made the piston about .025mm underiszed in diameter which let it drop slowly through the tube under gravity. Even then it had too much leakage to work as a piston valve but works good with an oring, because the plastic piston can still take the load if it has to, meaning you only need one o ring = lighter piston.

The O ring diameter i used was 3.25mm and i think the slot it sits in is roughly 3.2mm deep. I'll have to double check it though! In any case its not extending very far. A firm push with two fingers can move the piston though as an indicator.

As for material and what not, I guess NZ would be pretty similar to Aus. The only o-rings i can find are in those kits jackssmirkingrevenge mentioned. But even then there aren't really that many sizes. the biggest is 32mm radius and the one under that is 31mm, neither of which fit into any metric pvc locally without stretching.

Best thing you can do is buy this stuff from mcmaster

http://www.mcmaster.com/#urethane-round-belts/=hn39qr

which unfortunately you can only buy if you have an account with them (i know someone that does luckily :)! ). Its basically o ring you melt together and sand off the excess. The stuff is so damn strong its not funny. To get a floating o-ring working in our end of the world, that's probably what you will have to use. Bonus is the o-ring isn't the driving dimension in a design so you can be a bit more flexible with things. Let me know if you want some and i'll grab a few extra meters :)

As for plastics there is a place in melbourne that is very cheap. Got a 1m long piece of 80mm diameter delrin for $1 per cm. $1.50 per cm shipped to Sydney. Most "engineering" stores or bearing stores retail it from cut to size plastics or dotmar in Aus. Perhaps NZ has the same buisinesses?

But yeah man if you wan't anything machined or some materials sent over let me know hey :)
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Unread postAuthor: Crna Legija » Tue May 22, 2012 2:45 am

tl'dr but its for a piston hybrid not barrel sealer like yours.
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Unread postAuthor: MrCrowley » Tue May 22, 2012 2:46 am

Nice work on the design. Seems pretty similar to the one i made just before
Yeah I caught that earlier. It's refreshing to see a new member with such a decent build :wink:

With your tolerance question (forgive me if i'm reading this all wrong i kinda glanced over this page lol) I made the piston about .025mm underiszed in diameter which let it drop slowly through the tube under gravity. Even then it had too much leakage to work as a piston valve but works good with an oring, because the plastic piston can still take the load if it has to, meaning you only need one o ring = lighter piston.
I use cheap Chinese digital callipers so I can barely trust them to 0.1mm let alone 0.01mm :shock:

With the hybrid piston in question, I don't need the piston itself to have such good tolerances, I should be able to leave that to the o-rings. Perhaps I'll make the piston's final diameter 51.0mm instead of 50.5, I'm just worried about making it too large in case the cheap callipers are wrong.

I imagine the problem with your piston wasn't the 0.025mm difference, it would be the combination of a large pilot volume with poor flow and the low ratio piston difference. If I'm not mistaken, you were piloting the valve with a blowgun attached to ~6' of air compressor hose. That sounds like a nightmare to pilot. I would use a 1/2" ball valve attached directly to the pilot setup behind the piston. If you want a trigger, use a 1/2" or 3/4" solenoid valve with electric actuation or a hose (but preferably not as long as that long stretchy stuff). Most people here don't get tolerances anywhere near 0.025mm when making their pistons but they make up for it by having a decent sized pilot valve.

The O ring diameter i used was 3.25mm and i think the slot it sits in is roughly 3.2mm deep. I'll have to double check it though! In any case its not extending very far. A firm push with two fingers can move the piston though as an indicator.
Have you tried to see if it provides a 100% seal by blocking the eq. hole? In a hybrid piston design, I need a 100% seal so I might err on the side of caution and use a higher compression ratio. If the diameter of the piston housing is indeed 51.30mm, I shouldn't have too much trouble squeezing in the 51.90mm diameter o-rings (once stretched on the piston) but hopefully it wont require a sledge hammer to seat like my current piston :D
If the o-rings don't fit, I can always find some others and use teflon tape to help them fit if required.

As for plastics there is a place in melbourne that is very cheap. Got a 1m long piece of 80mm diameter delrin for $1 per cm. $1.50 per cm shipped to Sydney. Most "engineering" stores or bearing stores retail it from cut to size plastics or dotmar in Aus. Perhaps NZ has the same buisinesses?

But yeah man if you wan't anything machined or some materials sent over let me know hey

Might stick with the aluminium just because I don't trust a plastic piston of that size in a piston valved hybrid when I probably have an inadequate bumper. Thanks for the offer thugh, hopefully Crna can sort out the machining for me but if not I'll be sure to drop you a PM :)

By the way, do you have a recommendation for an easy to use CAD software? I probably can do something basic on Google SketchUp but it's a bit annoying to use for this application.
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Unread postAuthor: Desmo » Tue May 22, 2012 2:57 am

Ah a hybrid! Fair enough then. Thought it was a barrel sealer for whatever reasaon :S I'll go through all the posts later tonight so i actually know whats goin on XD. And yeah the pilot is shyte. It needs a second QDV/QEV attached to work proper. Thats being worked on :)

For cad, the best free one thats easy to use is probably the rhino3d free evaluation. It's complete but you only get 30 saves or something like that. I dropped you a PM about it :)
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Unread postAuthor: MrCrowley » Tue May 22, 2012 3:03 am

Technically, the valve is still a barrel sealer :wink:
The main differences are the requirement of an isolated pilot volume (100% seal by o-rings) and a self-piloting mechanism to dump the pilot air when the chamber fuel mix has been ignited and the piston starts to move back.
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Unread postAuthor: jackssmirkingrevenge » Tue May 22, 2012 3:06 am

MrCrowley wrote:I use cheap Chinese digital callipers so I can barely trust them to 0.1mm let alone 0.01mm :shock:


I bought some of these recently, surprisingly accurate compared to my Mitutoyo.
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Unread postAuthor: Petitlu » Tue May 22, 2012 3:09 am

cool !

How to Mix?
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Unread postAuthor: Crna Legija » Tue May 22, 2012 6:22 am

I'm trying to work out how much pressure i need to add to the pilot of my piston hybrid, it uses a spool like MrCrowley's. Do i need to take the pilot spool into account in anyway?

How i see it the pressure is pushing both ways but it will move to the side with more surface area right? So does it mean over all surface area the pressure has to act on, is the back of piston's surface area - the spool surface area?
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Unread postAuthor: pneumaticcannons » Tue May 22, 2012 9:54 am

Looks like you have the answer to your own question :) , yes that is correct
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Unread postAuthor: MrCrowley » Tue May 22, 2012 4:51 pm

Yeah if you use a design like the one I did, the spool part will need to be taken in to account. Usually, you just take in to account the surface area of the spool minus the area taken up by the bolt because you have the bolt connected to both the piston and spool so they effectively cancel each other out.

So when calculating the force of X pressure in the pilot, you would find the surface area of the back of the piston (minus the area the bolt takes up), and then deduct from that the surface area of the spool (minus the area the bolt takes up) which will leave you with your final surface area that will have pressure pushing the piston forward.
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Unread postAuthor: Technician1002 » Tue May 22, 2012 6:19 pm

The spool shape has no effect.

How i see it the pressure is pushing both ways but it will move to the side with more surface area right? So does it mean over all surface area the pressure has to act on, is the back of piston's surface area - the spool surface area?


The only two measurements that affect the force on the piston is the cross sectional area on the pilot seal vs the valve seat area. Any fancy long slopes for a spool between these two seals do not change the forces on the piston.

The piston opens when the force holding it closed is exceeded by the force pushing it open.

The force holding it closed is defined by the pressure on the barrel seal times the cross section area of the barrel seal. For example a 1 inch barrel seal would have Pi times R squared or 0.5 inch times Pi. If filled to 100 PSI the piston would have 0.5 times Pi times 100 PSI force holding it closed.

When firing, the force holding it closed drops. At the same time the pilot seal which had no differential force now has a pressure differential between the chamber and pilot trying to fire the piston into he pilot. When the force holding it closed is exceeded by this, it will move.

Due to the larger pilot seal cross sectional area, not surface area due to spool shape is the only dimension used.

Opening force is the pressure based on the difference of the barrel seal subtracted from the pilot seal cross section. This is why low ratio pistons require a lower pilot pressure to pop open and wide ratio pistons open at higher pilot pressure and are subject to closing early as the pilot pressure may re-close it as the chamber pressure drops.

A 1.75 OD piston will require a lower pilot pressure to fire than a 2 inch piston.

In summary, What is the OD of the piston in the pilot area. What is the diameter if the valve seat the face touches. The length shape or diameter of the bolt or piston or spool in between does not matter except for weight.
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Unread postAuthor: MrCrowley » Tue May 22, 2012 6:27 pm

No one mentioned spool shape so I assume that was in response to me saying surface area instead of cross sectional area? Take note of that, Crna. I assume you would have a flat face for a spool and piston anyway but as Tech said, it's the cross sectional area (which would be the same as the surface area on a flat piston, if I'm not mistaken).

The length shape or diameter of the bolt or piston or spool in between does not matter except for weight.
I'm not sure what you mean here but the diameter of the spool in a design like mine does need to be taken in to account. Well, the cross-sectional area subject to pressure in the pilot.

Petitlu wrote:How to Mix?

Sorry, who is this question for? I suspect you're asking about the piston hybrid we've been talking about but I'm not sure :)
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Last edited by MrCrowley on Wed May 23, 2012 12:12 am, edited 1 time in total.
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Unread postAuthor: Crna Legija » Wed May 23, 2012 12:59 am

Ok thanks guys.

MrCrowley wrote:? Take note of that, Crna. I assume you would have a flat face for a spool and piston


Yeah flat washer and a flat nut in front of it, should i work surface area
as if it flat though like the nuts not there, right?
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Unread postAuthor: MrCrowley » Wed May 23, 2012 1:02 am

Yeah, so you just take in to account the diameter of the spool itself. Maybe you should chuck up a drawing that we can check just to be sure.
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Unread postAuthor: Crna Legija » Wed May 23, 2012 1:21 am

back off piston OD ~ 23.18mm
pilot spool OD ~ 14.01mm
rod connecting them OD is 5.72mm

piston sealing face OD ~ 30.03mm
piston seat OD ~ 25.90mm

I think you'll get that but don't post result ill never learn if I don't try my self.
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