SpudFiles

Some time ago I was reading JSR's post about a preloaded pneumatic cartridge. I'm not sure if he ever built one, but I liked the idea. Just awhile ago I was taking apart a syringe and I noticed that the plunger looked exactly like something I could use to build just such a cartridge. So I just drew up a quick sketch and I was wondering what you guys thought of it.

Soon to come: The actual cartridge! I just need the epoxy.

HunterT wrote:I'm not sure if he ever built one, but I liked the idea.

The thread is full of prototypes, which essentially worked but were not developed further on the basis that they didn't provide enough power to eject the cartridge and cycle a repeating action.

Just awhile ago I was taking apart a syringe and I noticed that the plunger looked exactly like something I could use to build just such a cartridge. So I just drew up a quick sketch and I was wondering what you guys thought of it.

Looks like it would work, provided the end of the plunger is the right diameter for a tight fit in the tube that will allow schrader piloting.

The syringe tube doesn't have to be that long, however the back part of the piston should have a longer full diameter bit to keep the piston from wobbling about and not sealing properly.

You'd want it to be at least 1times the full diameter in length, 1.5 times would be good to be safe.


Or you could have some sort of support at the front of the piston to keep it centred. I think JSR has an illustration of this on his mini air pistol with integrated pump drawing.

al-xg wrote:The syringe tube doesn't have to be that long, however the back part of the piston should have a longer full diameter bit to keep the piston from wobbling about and not sealing properly.

The cruciform body of a typical syringe plunger should keep it centred.



I think this would be a better way of making it:

One problem of pneumatic cartridge is they are too weak, even with high pressure, because unlike hybrid cartridges, the valve (piston) occupies a lot of space, unless you what something like 20mm boltaction rifle that shots bbs

D'oh, I obviously can't read annotations

But now the syringe tube needs better support

I guess this would make a good prototype, but there is room for optimising the volume and flow.

I would be concerned that the plunger would not hold up to the compression forces, I have had them buckle despite the cross construction. I guess the barrel port would have to be very close to the plunger OD.

al-xg wrote: I would be concerned that the plunger would not hold up to the compression forces, I have had them buckle despite the cross construction. I guess the barrel port would have to be very close to the plunger OD.

My thought is that the only part with significant force on it would be the end that is against the barrel. I guess we'll find out soon(like when the epoxy cures).

wyz2285 wrote:One problem of pneumatic cartridge is they are too weak, even with high pressure, because unlike hybrid cartridges, the valve (piston) occupies a lot of space, unless you what something like 20mm boltaction rifle that shots bbs

Well if this doesn't work as a cartridge I could always convert it into a .177 cal airpistol.

jackssmirkingrevenge wrote:I think this would be a better way of making it:

I think I might try that.

wyz2285 wrote:One problem of pneumatic cartridge is they are too weak, even with high pressure, because unlike hybrid cartridges, the valve (piston) occupies a lot of space, unless you what something like 20mm boltaction rifle that shoots bbs

Indeed, this is one of the reasons I moved towards hybrid cartridges. A "vavleless" cartridge likethisis a "best case scenario" where you have maximum use of volume and zero valve opening time with instant maximum flow - and it still doesn't compare with even a relatively low mix hybrid.

You can certainly get it to eject itself if there's a significant disparity between bore diameter and cartridge body diameter, as it becomes a "stepped piston" of sorts, but certainly the size will be disproportionate to the calibre.

I would be concerned that the plunger would not hold up to the compression forces, I have had them buckle despite the cross construction. I guess the barrel port would have to be very close to the plunger OD.

At high pressures this would certainly be a concern, especially as in most syringes a large portion of the plunger is not actually in contact with the tube.

Well I just finished applying the epoxy. Just need to let it set up and then I can start testing it.

EDIT: I managed to fill the pilot area with epoxy. Sigh.... So I guess I'll get a new syringe tomorrow and try it again.

The epoxy is cured and I tried testing it today. Filled it with about 80psi and it does not work. It seals and the pilot valve works fine, but the plunger just sorta falls open. I've held my hand in front of the barrel(with no projectile) and I cannot feel any air coming out. I'm guessing that my piston maybe isn't tight enough. What do you guys think?

jackssmirkingrevenge wrote:Indeed, this is one of the reasons I moved towards hybrid cartridges. A "vavleless" cartridge like this is a "best case scenario" where you have maximum use of volume and zero valve opening time with instant maximum flow

I think you're 100% right jack. The most likely candidate for reasonable power would be a hybrid cartridge.

Is the piston diameter larger than the seat diameter?

Yes the piston diameter is larger than the seat diameter.

Is the pilot dumping quick enough?

I'd have to think so. There's a loud hiss of air when I trigger the pilot valve and I can feel the puff of air on my hand.

I managed to fill the pilot area with epoxy.

Since I started using epoxy I must have done this at least 5 times, it happens.

There's a loud hiss of air when I trigger the pilot valve and I can feel the puff of air on my hand.

Are you using the syringe plunger as the piston seal? It sounds like it's dumping through the pilot. Did you make an equilisation hole? Is it bigger than a pin prick?