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Pneumatic Pistons?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Pneumatic Pistons?

Unread postAuthor: jacoblesann » Tue May 07, 2019 11:55 pm

I’ve researched a lot of compressed air spearguns and have noticed that they’re way different than I thought. Instead of a simple air tank and barrel these guns actually have a sealed piston which is used to launch the spear. The rear end of the shaft is pushed against the face of the piston, and the spear is pushed down into the barrel by hand until it catches onto the trigger. There is no pumping to be had except for some pressure maintenance every once in a while. It’s effectively a pneumatic spring gun.

Here’s a video describing all this:
[youtube]https://m.youtube.com/watch?v=n9PNanZjVFA[/youtube]

It seems like all the piston needs to be is a plug with two o-ring seals. I’m not too sure of how the trigger is sealed but it seems to be not too complicated. I’m wondering how fast this design is. Does anybody know the velocity of these spearguns? I’m very interested in this design.
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Re: Pneumatic Pistons?

Unread postAuthor: jackssmirkingrevenge » Wed May 08, 2019 8:58 am

Some previous discussion here: pneumatic-cannons/topic24525.html

It’s effectively a pneumatic spring gun.


... meaning you have to generate the power in a single stroke, and mechanically retain the force until the moment of firing.

This negates the advantages of pneumatics as mentioned in your other thread.

I went for a similar concept in my STBD:



The difference (apart from pumping up the pressure chamber separately) is that I balanced the force on the piston while it was in the retained position, meaning there was relatively little force to hold back mechanically.

It's basically this idea, but rather than opening the chamber to the barrel, the piston becomes the pusher that travels down the "barrel" and fires it.

Image
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Re: Pneumatic Pistons?

Unread postAuthor: jacoblesann » Wed May 08, 2019 5:34 pm

What a coincidence, I’ve been studying that Aquatech site all day. Very interesting.

You’re right about the disadvantages. But I’ve been considering it for use as a small pistol size dart gun. I have a whole bunch of little crossbow bolts I bought for dirt cheap on Amazon one time and never got around to using up. Since actual pistol crossbow are illegal here, I’ve spent a few months now designing some alternative designs to meet the laws. Anyhow, I’ve figured out a compression spring design that is cocked with no interruptions to the barrel or piston in any way. I don’t know how to post pictures but it’s basically just a lever attached to the back end of the spring. Pull back til trigger catches, push forward to compress spring. It’s surprisingly simple and could be adapted for this.

Oh! So that is your YouTube account. I’m subscribed and I’ve watched many of your vids but I didn’t know it was you. Good work man, I really enjoy your vids.

Ive also thought of that design. Sort of like a Quick Dump Valve. Im wanting a design which didn’t release any air, but if I decide to suck it up and deal with how loud it will be, I’ll probably end up using your design. What pressure are you running it at?

OH WAIT I have just realized that you’re making what I’m designing. I thought you made it like that diagram shown. I watched the video and became super curious at the lack of muzzle report then realized you’ve made the piston gun I’m describing.

Do you have any images to describe the internals a bit better? Is the piston sealed with just an O-ring? I ask lord of questions but it’s a little cloudy to me and I don’t want to reinvent the wheel lmao.
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Last edited by jrrdw on Thu May 09, 2019 5:51 am, edited 1 time in total.
Reason: Double post.

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Re: Pneumatic Pistons?

Unread postAuthor: jackssmirkingrevenge » Thu May 09, 2019 9:33 am

jacoblesann wrote:Oh! So that is your YouTube account. I’m subscribed and I’ve watched many of your vids but I didn’t know it was you. Good work man, I really enjoy your vids.


Glad to have been of some inspiration :)

Do you have any images to describe the internals a bit better? Is the piston sealed with just an O-ring? I ask lord of questions but it’s a little cloudy to me and I don’t want to reinvent the wheel lmao.


This is basically what the STBD looks like internally:

Image

The crucial part is the balanced piston that reduces the load on the mechanical lock.

You can make it in such a way that you push down on the piston to pressurize the air inside the chamber, but this would necessitate a bleed screw in order to have the benefit of the balanced piston:

Image

It must be said that having the chamber pressurized separated greatly increases the power potential, the damage you see in the video was done with 850 psi of CO2.
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Re: Pneumatic Pistons?

Unread postAuthor: jacoblesann » Thu May 09, 2019 10:51 am

I suppose that pressurizing it separately would be best. I thought the air remains inside upon firing, though. Does it leak out any air upon firing?

You’re right though, I’m sure that to get to this to fire powerfully while relying on the pressure of compressing the piston, it’s gotta be very high pressure. I believe spearguns of this sort operate at 3000 PSI and above, so that’s a pretty dangerous idea.

Have you considered ways to hold the piston back with a trigger sear? That’s what I’m looking at right now. Probably finicky to seal the trigger; I bet I’ll end up following your design
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Re: Pneumatic Pistons?

Unread postAuthor: jackssmirkingrevenge » Fri May 10, 2019 6:03 am

jacoblesann wrote:I suppose that pressurizing it separately would be best. I thought the air remains inside upon firing, though. Does it leak out any air upon firing?


Yes, you have to drain it through the filling valve before reloading.

Have you considered ways to hold the piston back with a trigger sear? That’s what I’m looking at right now. Probably finicky to seal the trigger; I bet I’ll end up following your design


It's feasible with a bit of imagination:

Image

You could also have a balanced piston that doesn't need to be mechanically held back, that is then pneumatically triggered. In this example you would fire by pulling the red component to the rear:

Image
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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Re: Pneumatic Pistons?

Unread postAuthor: jacoblesann » Sat May 11, 2019 12:41 pm

That’s perfect, thank you so much. I was trying to design my own and was close to that first image but not quiet. This helps tremendously.

I think I pretty much know what I need to know now, thanks for all the help.
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Re: Pneumatic Pistons?

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