What Makes An Ideal Piston Bumper

[jackssmirkingrevenge]

The problem with threads like this, Crowley, is that you're pitching it to the wrong crowd. Spudfiles members don't like hypothetical discussion. They, for the most part, lack the background and the desire.

Ne'er a truer word was spoken.

But just because something has the same momentum, doesn't mean it has the same effect on something, right? Like if I fired a tennis ball fast enough that it would have the same momentum as a cricket ball at 160kph, if you were hit by the tennis ball it wouldn't feel the same. A cricket ball would be a lot worse.

Just wondering if that would have anything to do with how pistons would react with different bumpers.

It's all about reducing the impulse, and means spreading out the deceleration. In this regard even piston material is important, something like delrin which can deform to some extent will cause less damage that an aluminium piston. Essentially you're shooting at something wanting to achieve the opposite of what one would normally see as desireable traits. In this regard, the piston needs to be low weight and density, with a large contact area to spread the impact. Focusing on these parameters will make choice of bumper material practically irrelevant.

As for the diameter, why would someone want to sacrifice 0.1" to save weight. PERFORMANCE! PERFORMANCE! PERFORMANCE!
The 0.1" could be what they need for the piston to have work properly.

If you're optimising, a small diameter piston is ideal! As Tech often takes great pains to point out, the closer the diameter of the piston is to your barrel, the greater pressure drop in the pilot area needed to make it work, so there is less air in the pilot area to oppose its motion.

Aside from the weight benefits, the 0.1" different I mentioned in my example also leads to a 19% reduction in pilot volume, meaning better performance for the same pilot valve.

I do see your point about straying from your intended subject, but I think the point about piston weight really has to be hammered home. Would you rather be hit by a bicycle while strolling along, or wear a suit of armour and be hit by a car? In looking for a solution in bumper material, that's asking the wrong question in my opinion.

On a small scale, epoxy can be used well but I'm not so sure about epoxy on a large scale (2" valves).

Putting some thought into casting to make a compound piston instead of just a cylindrical plug gives you a lot of opportunities to save weight, and make a bumper at the same time. Using the tape-wrapping method, you can keep things centred enough for them to work. Also, the bumper doesn't need to be part of the piston either.

How much pressure do air bumpers 'generate' anyway? Safe enough to use on a PVC cannon? Probably not, I assume. So why are we still talking about them?

Here's an obviously failed attempt to stop a very fast and (pre-lathe) not too light "piston" with an air bumper, that's a lot of pressure to bulge the metal like that, and the space between alloy and PVC was filled with epoxy! The solution to that problem was the one I am suggesting here, a smaller diameter and lighter weight piston which made bumper material almost unnecessary.

[MrCrowley]

What about a heavy spring with an external adjustment screw to set the spring depth in the pilot area?

IIRC, Fnord did something similar in his first piston hybrid. Correct me if I'm wrong, Fnord (or anyone else) but I assume that is when he ran in to spring problems. To carry on from my earlier spring discussion a few posts above, a spring could be used in conjuction with a 'stopper' that prevents the spring from being over compressed and hopefully allows the piston to be slowed down sufficiently before hitting the stopper.

Surely you read my thread about thick-walled pressure vessels

I'm pretty sure I did, but I probably didn't pay much attention to all the equations and such. That's not to say they're not useful for a member like me, who lacks a decent engineering knowledge, but just that I didn't require them at that point in time. If I ever need to calculate the yield of certain materials when used as pressure vessels, the thread would be informative.

Now, on to the bumpers

DYI that's just what I was after, cheers. Looking at the problem from an engineering perspective is what I had in mind. Obviously, I lack the knowledge to contribute meaningfully to such a discussion but that's partially why I started this thread. There are a handful of members who, like you, can look at anything from an engineering perspective and also have the relevant know-how, but unless these members document their thought process it leaves most members out of the loop. The five points you stated help many of us understand the ideal physical properties of a bumper.

I'll have to do some more research tonight with those points in mind.
I did come across Sorbothane when looking at shoe insert material but only now that you have mentioned it have I realised its true potential, I even found some shotgun recoil pads made from the stuff.

Much as I know you're more of an artsy type

I'm a science buff (I follow science blogs with everything from geology to particle physics and I enjoy reading 'popularised' books by people like Krauss and Hawking) but I major in Biological Anthropology and Statistics so on a forum like this I probably would come across as artsy (to quote a famous Kiwi "All science is either physics or stamp collecting").

Aside from the weight benefits, the 0.1" different I mentioned in my example also leads to a 19% reduction in pilot volume, meaning better performance for the same pilot valve.

And if your piston is already low-ratio optimised...

I do see your point about straying from your intended subject, but I think the point about piston weight really has to be hammered home. Would you rather be hit by a bicycle while strolling along, or wear a suit of armour and be hit by a car? In looking for a solution in bumper material, that's asking the wrong question in my opinion.

In that case, you'll be happy to know my aluminium piston has been reduced in length from ~10cm to ~9cm as Crna reckons he has a suitable piece already cut to 9cm (saves me money)

Here's an obviously failed attempt

Damn!

[Crna Legija]

what about a air bumper like this the red part can moving the yellow part it should be a good fit, and a spring or string tied to the piston could be used to reset it.

i know you don't like air bumpers lol but others might like it.

edit, the hole in the middle is only if you have a spool type vent like your piston hybrid.

[jackssmirkingrevenge]

I major in Biological Anthropology and Statistics so on a forum like this I probably would come across as artsy (to quote a famous Kiwi "All science is either physics or stamp collecting").

And if your piston is already low-ratio optimised...

In that case, as a fairly decent chap once said:

Simplify, then add lightness.

In that case, you'll be happy to know my aluminium piston has been reduced in length from ~10cm to ~9cm as Crna reckons he has a suitable piece already cut to 9cm (saves me money)

I still think there's a lot you can shave off in terms of weight, and that combining the threaded rod piston with some custom "washers" might have been a better solution, but what do I know

Damn!

Pretty much my same reaction, and it happened with a number of prototypes - but enter lightweight synthetic pistonset voila

[Crna Legija]

In that case, you'll be happy to know my aluminium piston has been reduced in length from ~10cm to ~9cm as Crna reckons he has a suitable piece already cut to 9cm (saves me money) :

[jackssmirkingrevenge]

That's what, around half a kilo? Terrible

[Labtecpower]

you will definitely have to hollow it out a lot, otherwise your pilot area will be smashed to pieces

I would have gone for a piston made out of UHMWPE, nylon or delrin. metal pistons are just too dense

[Zeus]

Edit: Seems I should have read the thread.

[Crna Legija]

should be 420ish grams when it done, how much is your one now MrCrowley?

[Labtecpower]

i'm already bending steel with a 100 gram one in my QDV

[jackssmirkingrevenge]

I would have gone for a piston made out of UHMWPE, nylon or delrin. metal pistons are just too dense

I tried to tell him

[MrCrowley]

That's weird, I wasn't notified of all these replies even though I'm "watching" this topic. Perhaps I would've heeded your advice, JSR, had I saw the other replies

Anyway, I was thinking of how I can keep all the pilot material from being scrunched up like it usually is after a dozen shots in the hybrid. I thought I could fill the 2"f x 1"f reducer at the end of the pilot area with silicon sealant to hold in bumper (very similar to SB15's one) and help reduce the impact.
If I get time today, I can do a few impact tests with some silicon sealant to see how it holds up.

If the weight is still too much, I'll just pop over to that mag wheel garage that welded my S/S barrel for me and get them to take a chunk out.

[jackssmirkingrevenge]

That's weird, I wasn't notified of all these replies even though I'm "watching" this topic. Perhaps I would've heeded your advice, JSR, had I saw the other replies

[youtube][/youtube]

If the weight is still too much, I'll just pop over to that mag wheel garage that welded my S/S barrel for me and get them to take a chunk out.

I think you should do that regardless.

[Fnord]

Yeah, missed this topic...

IIRC, Fnord did something similar in his first piston hybrid. Correct me if I'm wrong, Fnord (or anyone else) but I assume that is when he ran in to spring problems.

The first version didn't have spring problems so much as sucky power problems/leaky orings. In the original, I used a very stiff spring which could be compressed completely without yielding. It fit snugly inside the pipe, so it could not bow off to the side when being compressed. The pancaked spring I posted a picture of was from the 2.0 version and was a little more loose in the housing. I'm not really sure how I got it that flat honestly.

The partially-finished 3.0 version just used a solid rubber piston, so there was no need for a bumper. I had fired it repeatedly at 5x but never got to do documentation. I really wish I had finished it now... it was a nice little gun.


If you want to look for an ideal bumper material, just think about this:

You have a piston traveling at a velocity, and it carries some amount of energy.
You don't want that energy to deform your gun/piston, so it has to be dissipated... somewhere.

An air piston will absorb that energy almost perfectly, but will just spit it back out again by shooting the piston forward, probably chomping your sealing face in the process. The only energy not returned will be lost into the chamber walls because of friction and adiabatic heating.

It would be more desirable to have all the energy dissipated by frictional heating. You'll want friction to be applied in a way that doesn't cause mechanical wear, so the best way is to use a bumper material with a high level of internal friction when it is deformed. So the less it bounces the better, usually.
Ideally you'll also want the bumper to deform quite a bit before the piston bottoms out, just to lengthen the deceleration.

For more exotic alternatives you could look at inductive braking... basically the same thing that happens when you drop a strong magnet down a copper pipe.
This might have an advantage if done correctly, since the braking effect will increase with piston velocity. And it's basically 100% efficient at turning the energy absorbed into heat; there is no 'spring-back'
The disadvantage is you'll need a strong magnet and a lump of copper in there somewhere, both of which add considerable weight.


A well-designed hydraulic brake would also have considerable advantages, but would probably get messy at some point

[MrCrowley]

Interesting, definitely some food for thought! For some reason, I have a feeling that clear silicone sealant would be good at absorbing energy. I remember we had a tube of it that got exposed to air and cured over a great deal of time. It was quite spongy and was difficult to pierce deeply with a sharp object like a screwdriver.

I was going to go and test whether it could stop a peanut from being crushed by a hammer but I can't find it, it must have been chucked out. I reckon it would do a pretty good job at protecting it; although possibly due to the fact that it would mould around the peanut and most of the force would be transferred to the surface its sitting on.

How it would perform under pressure is probably a different story.

@JSR
I f*cking love Randy Marsh, pretty much the only reason I ever watch the show