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Springs, Valves and Automatics - Air is your friend.

A place to ask general spud cannon related questions.
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Springs, Valves and Automatics - Air is your friend.

Unread postAuthor: SPG » Sun Mar 15, 2009 3:50 pm

Almost any kind of automatic ends up needing some form of spring to cycle, Why not use air ? Both JSR and Fnord have used air as a spring material.

Things we'll need to bear in mind:

  • Boyle's law and specifically: p1 x v1 = p2 x v2
  • Constant Force Spring: Force remains the same through spring compression.
  • Rising Force Spring: Force needs to increase through spring compression.

But why use air?

  • It's simple: if you can make a piston gun, you can make an air spring.
  • It's adjustable: changing the pressure changes the force needed to compress the spring.
  • It's calculable: if we know the piston size and the air pressure we can work out the force of the spring.

So what's the stuff about Rising and Constant Forces?

Boyle's Law says that pressure and volume are linked by a constant. So in an air spring, piston travel is also linked, we'll call this swept volume and what is important is the ratio of swept volume to overall volume.

  • If the swept volume is a small proportion of overall volume, we have a near constant force spring
  • If the swept volume is a large proportion of overall volume, we have a rising force spring.

Why's any of this important? Because we use lower pressures, and so we only have a lower forces to play with.

For instance in a blow back we want a large travel but we don't want the spring force to increase over that travel. So we'd want a near constant force spring.

But in a piston hybrid we might want the opposite, and have a rapidly rising force spring so that our piston only travels a short distance. So that's a rising force spring.

Because we know Boyle's law we can make just the right spring for each and every scenario.

EDIT: As btrettel points out below I got my rates and forces the wrong way round, now corrected.
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Last edited by SPG on Mon Mar 16, 2009 12:05 pm, edited 2 times in total.
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Unread postAuthor: btrettel » Sun Mar 15, 2009 4:13 pm

Interesting summary. I'm sure some will find this useful.

I've got two nitpicks though:

- Boyle's law is only accurate with good heat transfer to ensure the temperature is constant. Polytropic process relationships should be suggested in general. Boyle's law is one of the polytropic process relationships. The adiabatic equation is probably most accurate for most situations. For small displacements either isothermal (Boyle's law) or adiabatic assumptions are perfectly acceptable.

- A constant rate means that the force increases or decreases with distance at a constant rate (like a normal helical spring). A constant force means that the force is constant with respect to distance (like a constant force spring). There also are non-constant rates in materials such as rubber.

Edit: Friction's also something to think about, but that applies to springs too.
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Re: Springs, Valves and Automatics - Air is your friend.

Unread postAuthor: jackssmirkingrevenge » Mon Mar 16, 2009 12:57 am

SPG wrote:It's simple: if you can make a piston gun, you can make an air spring.


Not necessarily, most pistons aren't 100% airtight so it is a bit more involved but still no massive chore. Syringes are definitely a way to go here ;)

As to what you were saying about constant force, here's an example I had made for the recoil system of my combustion cartridge prototype:

Image

Note the size of the air reservoir compared to the piston's swept volume, this meant that the piston could recoil fully without ramping up the resistance too much, also it made the bolt return slower which would have give time for the next cartridge to fall into the breech.
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OOOPS

Unread postAuthor: SPG » Mon Mar 16, 2009 12:00 pm

You're right a constant force spring, sorry it comes out of reading all about it in French. (I've changed it in the orginal now).

As for Boyle's Law, yes of course there's heat built up if you rapidly compress air, which will make a difference, but I think the general principle still stands, the bigger the overall volume compared to swept volume the more even the force needed to compress.

I think the remote reservoir (like you'd find in high end shock absorbers) is a great idea, keeps things compact and who knows might even help cushion against excessive heat build up.
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Unread postAuthor: psycix » Mon Mar 16, 2009 1:24 pm

To make a normal spring approach constant force, pre-tension it.
Instead of going from zero to maximum force it will go from something like half to max. Depends on how much you preload it.
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