Advice for pneumatic speargun

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Fri Mar 17, 2023 2:20 pm

Hello everybody!

I am looking for advice regarding a project I would like to build: A pneumatic speargun.

There are already some of those on the market, but they have a lot of shortcomings (maintenance, vendor parts, custom inflation valves, poor quality etc).
The inner workings of these is similar across brands, and can be understood from this video:

I came up with two different ways of building an alternative, one resembling what is usually being built with QEVs.
The gun looks like what is in this picture:
The gun is called a Pelletier Co2 gun and is illegal in most regions now because it is not recharged by human force.
I'd like to build something similar, but rechargeable.
After reading a bit on the topic, I still have some questions and am looking for general advice on how to best build this.

Some specs:
• Operating pressure is usually around 20-30 bars (for it to be legal I have to be able to charge it manually)
• A piston enclosed within a barrel (Al or Stainless Steel) will push a 7mm shaft over its length
• Device will be recharged by pushing the piston back and should not lose air like a conventional pneumatic canon

Some desires:
• Something more powerful than what is on the market (pressure has to be released as quickly as possible).
• I like the idea of having a barrel that's not enclosed in a pressure chamber. Idea would be to make maintenance easier.
• Normal pneumatic spearguns are impossible to discarge without firing, I'd like to be able to release air back in the barrel slowly.
• I would absolutely love being able to have an electonic trigger (normal triggers are hard, and require quite some debatement to release the shaft). Can sort of give up on that one, but maybe it allows for slow discharge.

1. I am not sure I fully understand the difference between a solenoid valve and a QEV (I do get that all solenoids are not QEVs). Am I right to understand that QEV = type of valve and solenoid = actuator (the same way some QEVs are actuated by pressure) ?
2. Which kind of valve(s) should I favour for such a project, especially considering that the device has to be recharged by pushing back the piston?
3. What are the most common physics formulas that would help me calculate what the speed or energy of my piston will be?
4. Why do people mod QEVs with a spring?
5. What if I want that thing to be electronic, should I simply use a solenoid QEV?

I am aware water ingress is something altogether. Consider water not being the problem but rather the fact that the whole system is a 'closed loop'.
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Sat Mar 18, 2023 9:02 am

Welcome to the forum!

Interesting project, I suppose the first question before dispensing advice would be what is your budget and what are your technical abilities and available facilities?

Does it have to be made from off-the-shelf components or are you able to envisage some machined parts?

I'm not sure valves are relevant to such a project if you are compressing it manually, shouldn't you be looking at ways to retain and release the piston?
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Wed Mar 22, 2023 1:49 pm

Hey @jackssmirkingrevenge, I only just saw your reply. I was supposed to receive an email confirming publication which I never got.
The budget is not an issue.
Re. abilities well it is mixed: I am used to building things, albeit not with compressed air.
I disasembled my own pneumatic speargun many times though (for maintenance) and am used to how it works. I am also a software engineer, with some experience in electronics.
As for the mechanical side of things, a good friend of mine is specialised in machining.
The bottom line is that I want something that looks good enough to be open-sourced.

> I'm not sure valves are relevant to such a project if you are compressing it manually, shouldn't you be looking at ways to retain and release the piston?
That's how a normal pneumatic speargun works. There is a lot of value, however, in being able to release air at slower rate (less power). With a band gun you usually just move the bands closer the the arrow tip which reduces the power. With a pneumatic, you always have to shoot once, switch the reducer and charge again. Not great. Same things goes for discharging. With a pneumatic you must discharge it by shooting and that's just less safe. That's why I was thinking about valves.
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Sat Apr 01, 2023 12:45 pm

Fun project!

To empty the charge you can have a bleed valve attached anywhere the launcher is pressurized. Also you could have a separate bleed valve attached to a fill nipple.

It sounds like you want something like a conventional airgun to launch the spear? Maybe a spear with a tail that seals the barrel? It would have to be full bore so there is minimal water in the barrel to move. In the video I could not see how water is prevented from entering the barrel with the spear shaft, meaning the piston has to push out both the spear and the water?

The great thing about the mechanically locked piston in a conventional pneumatic speargun is it opens very very fast, so to come close to that you will need an extremely fast valve... but given how slow spears come out maybe opening speed not so much of an issue... just rambling here.

I have been moving away from QEVs the last few years (still have some as part of ongoing projects) and returning to my roots with hammer valves, but now I'm very curious about piloting a valve under water.

Another option is to build using the conventional mechanically locked piston, just with a valve to slowly empty air, and bigger piston and reservoir for more force.
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