"In the world of spuds today"

mark.f · Fri Apr 08, 2011 9:05 pm

ITWOST, I rolled up a 3' piece of PVC and stuffed it in my pocket.

PVC doesn't make good barrels for semi-auto combustions.

saefroch · Sat Apr 09, 2011 9:26 pm

ITWOST, Parts for a hybrid QDV are lined up, and ready for ordering. Will be shipping Monday, arrival probably on Wednesday. It's great to live so close to a McMaster distribution center.

I almost splurged for the titanium internal piston, but alas convinced myself otherwise.

Sun Apr 10, 2011 6:39 am

I remember when building my hybrid there were a few others doing the same, were you one of them saefroch? It was funny because no one else ended up finishing theirs.

ITWOST, blew the seals on my 1.5" steel ball valve on my piston hybrid.

saefroch · Sun Apr 10, 2011 10:59 am

Nope, I wasn't one of them. I only really decided to build a piston hybrid recently, after realizing it was possible to do with Fnord's design.

I'm planning for specs much like the Trident, and it's quite interesting what happens because of the incredibly short burn time. My "burst disc" can fail when combustion is only halfway complete and I'll lose almost no performance.

I'm desperately praying I don't blow out any seals, because that may mean destroying an $84 gauge. The pressure rating on the 1/4" ball valve I'll be using is 600 psi. I've decided to skip that darned meter, and to not bother with mixes below 5X.

SpudBlaster15 · Sun Apr 10, 2011 11:06 am

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Fnord · Sun Apr 10, 2011 1:11 pm

Nope, I wasn't one of them. I only really decided to build a piston hybrid recently, after realizing it was possible to do with Fnord's design.

Hey saefroch,

Are you referring to the hybrid QDV sketch I posted a while back?

If you're building the former, hang on; I want to give you a modified verson that may be easier(tomorrow). You'll be able to use the parts you already ordered, minus the spring. You can probably get one from a hardware store or make it with some music wire.

What's your piston diameter?

You may need to buy teflon-coated orings, regardless. I can't see neoprene lasting too long if you decide to up the mix.

saefroch · Sun Apr 10, 2011 1:18 pm

Okay, so what type of o-ring do you suggest for higher temperatures? I still have time to modify my order. I was counting on using the spring I'm ordering, because it has a very high spring constant and is short.

Piston diameter: .5"
Internal piston diameter: .25"

EDIT: McMaster has some silicone o-rings that say they're good up to 450F. High enough?

Lockednloaded · Sun Apr 10, 2011 1:30 pm

ITWOST I guess it really doesn't take much to get a PB moving at field legal speed, I just threw together a pretty nice PB pistol with a 1/4" QEV, 1/8" slide valve, 1/2"x4" chamber, and a nice laser

JDP12 · Sun Apr 10, 2011 1:33 pm

lockednloaded... thats the beautiful thing about paintball/airsoft/ nerf guns. You don't need much to get them moving. I realized how easily I can design a nerf gun with an incredibly tiny chamber, use a high pressure to get hundreds of shots out of a small CO2 tank. plus, since you don't need them going more than 250 ft/s, any inefficiencies really don't matter, since you are propelling such a lightweight projectile at relatively low speeds.

Fnord · Sun Apr 10, 2011 1:36 pm

Piston diameter: .5"
Internal piston diameter: .25"

Oi, You're not giving yourself much room for error on this one

Ok, basically what I did was move the spring from inside the main piston to below it. The forces acting on the piston will remain the same, but you'll have more room for the spring and will be able to adjust the opening pressure on-the-fly by (un)screwing the cap or plug that's holding it all in.

The only thing you need to worry about is adding a spacer so the piston can't bottom out and smash the spring. If you're using a built-in reset rod you could modify it to do this.

You can pick any spring that will fit in the dead space below your piston, just make sure your compression force is correct.

As to the orings- I don't know- you'll have to cross that bridge when you come to it. Go with normal orings at first, since they will probably not fail immediately. If they prove ineffective, buy either hard red silicone or teflon coated o-rings. They are fairly cheap on ebay though I haven't checked mcmaster.

EDIT: McMaster has some silicone o-rings that say they're good up to 450F. High enough?

Hell no

...but you're not going to get much better than that. They should hold up better than standard ones at least.

Lockednloaded · Sun Apr 10, 2011 1:37 pm

Yea, the gun was built to be an airsoft semi, but it got too complicated for my tastes so I thought I might as well throw on a PB barrel... and hey now it shoots great with the target balls I use

JDP12 · Sun Apr 10, 2011 1:54 pm

yep. what kind of magazine setup do you have on there?

saefroch · Sun Apr 10, 2011 2:16 pm

My piston is pre-made

Not entirely sure what you mean by moving the spring to below it, but I ran the numbers on the compression forces, but the spring won't move appreciably fueling for a 20X mix, and will compress fully on combustion of a 5X mix, which are my parameters.

I'm going with the McMaster silicone o-rings, 100 for $5.69 isn't shabby at all. I'm thinking I'll just skip the o-rings on the inside of the piston, and instead just machine the UHMWPE rod so that it forms a seal on its own.

What's this about adding a spacer so the piston can't bottom out? I'm drastically modifying your design in any case

. The piston body is a solid steel spacer, with the spring epoxied into the front, and the internal piston passing through the spring, and sealed towards the pilot side to the inside of the piston. The barrel side of the internal piston will be modified to be of a greater diameter, so that it catches on the barrel assembly, which is of course a flow restriction, but with any luck it'll behave a little like a DeLaval nozzle. The entire piston is housed inside a cross.

Fnord · Sun Apr 10, 2011 2:34 pm

Not entirely sure what you mean by moving the spring to below it

I don't know how much more clearly I can state it without a picture. The spring fits in the pipe below the main piston, and pushes the main piston against the internal piston. There is no internal spring.

***
Is your internal piston "stepped", like a pop-off valve or JSR's "rattlesnake" valve?
In that case you shouldn't need orings on the internal piston. My design just had a piece of neoprene sealing it.

***

What's this about adding a spacer so the piston can't bottom out?

If you modify the design like I've mentioned above, the spring will be below or behind the piston; when the mix ignites the piston will slam into the spring.
Add a spacer for it to hit before the spring is completely compressed.

Labtecpower · Sun Apr 10, 2011 2:35 pm

ITWOST I didn't launch any taters.

I did demolish a bucket with the muzzle blast of my QDV however, fixed some problems on my copper gun, and shot my mini cannon.

It had some pretty impressive damage. (went through a sheet of 1.5 mm plate steel, and a can of whipped cream)empty and dented a brick. All was done with a 1 gram piece of brass rod

"In the world of spuds today"

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