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Piston Construction help : bumper, piston &, overall design

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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Piston Construction help : bumper, piston &, overall design

Unread postAuthor: lazercatz286 » Mon Nov 25, 2013 9:15 pm

Hi guys I'm a new user and I would like some assistance with the construction on my piston and some help with the bumper

1) I'm thinking about using the piston design shown in the diagram below it uses a bolt, rubber, washers, and hex nuts for the piston. do you guys think it's okay?

2) Can you use hot glue put inside of the piston housing to act as the bumper?

Please provide feedback to help improve design, thank you.
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: Ragnarok » Tue Nov 26, 2013 8:50 am

lazercatz286 wrote:1) I'm thinking about using the piston design shown in the diagram below it uses a bolt, rubber, washers, and hex nuts for the piston. do you guys think it's okay?

I use pretty similar things for pistol valves of this size. You may need to glue some parts together, as the design can leak through the thread at the centre, and the steel can make the pistons a bit heavy, but they're certainly a proven concept.

2) Can you use hot glue put inside of the piston housing to act as the bumper?

I've never been fond of hot glue as a construction material, to be honest.

However, it's somewhat elastic, moderately impact resistant and, while not exactly spectacularly adhesive, thus could be practical if well supported from behind. That might call for a redesign of the rear of your valve - reducing to 3/8" or 1/4" would be preferable to give the bumper a greater area to be supported by (as well as reduce the rather large pilot volume that will result from using 1/2" pipe).
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Last edited by Ragnarok on Tue Nov 26, 2013 11:36 am, edited 1 time in total.
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: jackssmirkingrevenge » Tue Nov 26, 2013 9:57 am

You're either quite young or your school taught handwriting has survived the test of time :)

Your piston looks like it would be quite heavy. A heavy piston is a killer, both of performance and your bumper. I would use a small diameter bolt and steel washers as the main contact points, and instead of nuts use a section of say ABS pipe as a spacer.
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: Ragnarok » Tue Nov 26, 2013 2:17 pm

jackssmirkingrevenge wrote:A heavy piston is a killer, both of performance and your bumper.

Well, that's not as true as it sounds.

It's fairly simple to calculate the effects of piston weight on performance.

The opening force on a piston valve is easy - the point at which a piston valve starts to open has the pressure in the chamber times the outer seat area equal the pilot pressure times the overall piston diameter (i.e. the force on the piston is zero).
It's the air gaining access to the inner seat area that suddenly increases the force and "slams" the piston open. And assuming given an equal seat diameter to barrel diameter, as well as taking pressure on the projectile and piston face to be the same (it more or less will be, unless the dead volume is stupidly large), the force opening the piston is often the same as the force moving the projectile.

At that point, it's just a ratio of masses. Multiply the distance the piston needs to move to be fully open by its mass, then divide by the projectile's mass. This will give the distance the projectile moves while the piston is opening - and hence, the distance the projectile is under only partial acceleration.

At that point, HEAL's 68.7 gram piston (as an example of this type) doesn't seem quite so heavy. Sure, it's a lot heavier than many of the projectiles, but it never moves more than 20mm - and may well achieve full flow quite some time before that. (I'm not sure the D/4 rule works brilliantly for that kind of valve design) - so a 15 gram projectile might take as much as 10 centimetres before the valve reaches full flow... but at these low velocities, you don't NEED particularly high valve flow to maintain a high pressure. So it'll have effectively been at a very high acceleration for a lot of that, making it the equivalent of only a couple of centimetres difference in barrel length as compared to an instant valve.

Given I cut down the barrel by a fifth years ago (the first 80% of its length counted for ~95% of the velocity of most projectiles), meh.

As far as how hard the piston hits... well, the above note on piston force applies again. Energy is force times distance. The force and distance will be much the same regardless of piston mass, so a light piston is similarly energetic in opening as a heavy one.
A heavy one's momentum will be higher, but it'll also strike slower, so the impact will happen over a longer time. There is a difference, but it's not as great as might be imagined.

Actually, these days, HEAL's piston doesn't even have much of a conventional bumper. Most of it is the responsibility of the rubber washers at the back...
CIMG0230-600.jpg

... which partly work because of the high Poisson's ratio and coefficient of friction of rubber to work. Aside from just absorbing energy in elastic deformation, they compress outwards as the rear of the piston reaches a cursory rubber ring on the rear fitting and create friction against the valve walls. This reduces the shock of the impact on the actual fitting dramatically - and friction damped pistons will never bounce.

That said, I will be re-engineering the piston quite a bit if I ever finish the rebuild, replacing some parts with aluminium or plastic, changing the layout to improve the effectiveness of the friction damping, and probably changing the shape of the piston face slightly.
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: jackssmirkingrevenge » Tue Nov 26, 2013 2:42 pm

been through this already here: spud-cannon-related/topic24254.html
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: Ragnarok » Tue Nov 26, 2013 4:18 pm

With all due respect, I'm not sure that I agree with past you more than I do present you...

It doesn't really matter if we evaluate it by momentum or energy. To actually look at the maths:
A 20 gram piston at 10 m/s = 10 Nm (or Joules), 0.2 Ns
An 80 gram piston at 5 m/s = 10 Nm, 0.4 Ns

Working out the decelerating force can be done either by dividing energy by stopping distance or by stopping time. However, an deceleration over the same distance at half the starting velocity will take twice the time - which cancels out the doubled momentum.

As the impact time is changed by the impact velocity, energy is by far the more relevant measure.

Now, a heavier and slower piston could have two effects:

1) it may give the pilot more time to vent, increase the pressure differential, accelerating force and overall result in a more energetic piston, but that's not a dramatic difference (especially if we're talking about the kind of optimised pistons you were on about); not to mention that if the piston bumper really is that marginal in its ability to do its job, it's probably not up to the task.

2) heavier pistons are probably also harder and less flexible. In the absence of a compressing bumper to be the primary definition of impact distance/time, a stronger piston would reduce both.


Lightness in a piston is a good thing - but a heavy piston is not a black mark (if anything, extreme mechanical strength is a quite the virtue when most launchers have no redundancy preventing an accidental discharge in the case of piston failure).
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: velocity3x » Tue Nov 26, 2013 5:13 pm

Ragnarok wrote:
jackssmirkingrevenge wrote:A heavy piston is a killer, both of performance and your bumper.

Well, that's not as true as it sounds.


Rag,
Is that your piston? No offence intended but......Holy S**t.....I don't believe I've EVER seen ANYTHING quite like it :shock: :shock: :shock:
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Re: Piston Construction help : bumper, piston &, overall des

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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: Ragnarok » Tue Nov 26, 2013 5:41 pm

Yes, that's HEAL's piston - it's not pretty, but it's got an integral check-valve and is friction damped. Function before form.
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Re: Piston Construction help : bumper, piston &, overall des

Unread postAuthor: mark.f » Tue Nov 26, 2013 8:24 pm

Mine's pretty heavy as well, although I was mostly concerned with using jellybean parts without any special tools rather than having a lightweight piston.
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