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What Makes An Ideal Piston Bumper

A place to ask general spud cannon related questions.
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Unread postAuthor: ramses » Sun May 27, 2012 10:34 am

Hey, you remember me! I have not done a lot of testing with my hybrid, but I have not had any issues with my sealing face or a lack of power. I am at an advantage in that I use a solid aluminum piston (<1" diameter) and a 1/4" thick sealing face. After a shot, there is always a weak vacuum in my chamber, so I am, at worst, loosing about one "mix number" worth of power.

In my mind, the ideal system would be a lot like an air strut used in cars.

The piston push pre-pressurized hydraulic fluid through a check valve (negligible restriction). This would be designed with minimal dead volume so that the system could never "bottom out". This way, it moves back mostly un-arrested, until the end. In the opposite flow direction, the oil flow would only take place through a small orifice. The piston would return to its closed position fairly slowly so as not to honk the valve or mangle the piston face. You would get the artillery like recoil movement (back quickly, forward slowly) with your piston

The energy of the piston would be absorbed in the movement back, but the dissipated in the movement forward. Most physical bumpers do much of the dissipating on the way back.

I suppose you could use the air as the working fluid, but you would have serious issues with the dead volume induced by the check valve.

I think that for a design to be successful, it needs to be made in such a way that it is completely contained in a solid bumper sized object that can be placed behind the piston in a given cannon and "forgotten about"
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Unread postAuthor: POLAND_SPUD » Sun May 27, 2012 12:41 pm

well who said you've got to have any problems with piston bounce ??
It's just a matter of choosing the right dimensions of the piston itself, the part of it that provides air cushion, its stroke etc.

I am totally with JSR (and Kelly Johnson :-) ) on this -> K.I.S.S.
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Unread postAuthor: velocity3x » Sun May 27, 2012 1:17 pm

jackssmirkingrevenge wrote:air.

I completely agree. I've been using the same setup jsr described for some time now and it works flawlessly.
http://www.spudfiles.com/forums/piston-seal-bumper-info-t23060.html
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Unread postAuthor: MrCrowley » Sun May 27, 2012 4:03 pm

POLAND_SPUD wrote:Well for a start they are used in pneumatic valves :D and air cylinders

I already mentioned those in my first post. Does no one read anything anymore? :D

As ramses pointed out, an air bumper similar to a hydraulic one in the sense that it uses the movement of a fluid to damper the piston, there are still some problems that can make it more difficult, and expensive, to implement than a simple piece of material. You might need to buy a commercial one instead of fiddling around dynamic rotary o-ring seals (I think that's the kind i'm thinking of).

jackssmirkingrevenge wrote:When we talk about collisions, it's momentum rather than energy that is discussed, so we're talking of mass x velocity.

A 50 gram piston at 20 metres per second has the same momentum as a 100 gram piston at 10 metres per second.

In practice though, if you half the weight there will not be a doubling of velocity, so you will be reducing the momentum.

As to diameter, there's an easy weight saving there - a 0.9 inch diameter piston will weigh just 81% of a 1 inch diameter piston of the same length and material.

Well I did mention impulse :wink:

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.

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

ramses wrote:Hey, you remember me! I have not done a lot of testing with my hybrid, but I have not had any issues with my sealing face or a lack of power. I am at an advantage in that I use a solid aluminum piston (<1" diameter) and a 1/4" thick sealing face. After a shot, there is always a weak vacuum in my chamber, so I am, at worst, loosing about one "mix number" worth of power.
Yeah I did go and read your topic and saw that you mentioned you used a special kind of rubber that was a b|tch to cut. I didn't think an air bumper would hamper performance either as by the time the piston has moved back and is being pushed forward again, the projectile is probably on its way. I wonder then if an air bumper would affect the performance of a cannon shooting relatively heavy projectiles.

What was the diameter of your piston again? Surely smaller pistons would fair better than larger pistons in terms of piston bounce? I guess one could always double up on sealing face material and use a solid piston.

The hydraulic idea you mentioned is similar to the one I had in my head where I mentioned in the first post that it could be mounted externally. It could be done with some fuffing around and it would actually look pretty cool.

I think that for a design to be successful, it needs to be made in such a way that it is completely contained in a solid bumper sized object that can be placed behind the piston in a given cannon and "forgotten about"
Exactly.With material bumpers I'm worriyng about whether they've been destroyed or got caught in the piston and with an air bumper I'd be worrying about damages from piston bounce.


Okay, now a rant (not directed at you, ramses!):

I think maybe some people are confused as to the purpose of this thread. I'm not specifically looking for a bumper. Considering all the crazy synthetic crap being made today, surely there must be a material that would be suitable as a bumper?

Like I mentioned earlier (why do I have to keep saying this?), I'm well aware of air bumpers as well as pneumatic and hydraulic shock absorbers, but can't we think outside the box for a little bit? Surely there is some material that would work perfectly even if it is $20 for a small piece of it.

So far no one has mentioned anything about what I originally wrote this thread for: material bumpers. Let's drop the air and hydraulic shock absorbers for now and explore something that hasn't been explored, and what seems like a taboo around here: bumper materials. If you're confused and think I'm talking about air bumpers again, you'd be wrong. I'm talking about a material that is placed behind the piston to absorb (not hydraulically!) the impact of the piston. :D

It may not be practical, it may be expensive, it may not perform as well as a bloody air bumper and it may require maintenance, but I don't care. At the moment our hobby consists of two bumpers: a broken tennis ball and an air bumper. Can't we expand on this? Can't we find a material replacement for a tennis ball that was developed at MIT 10 years ago and can now be bought for relatively cheap?

Maybe everyone should actually read my first post this time, it pretty much contains all you guys said on page 1 :wink:

velocity3x wrote:
jackssmirkingrevenge wrote:air.

I completely agree. I've been using the same setup jsr described for some time now and it works flawlessly.
http://www.spudfiles.com/forums/piston-seal-bumper-info-t23060.html
I didn't know your valve used an air bumper, interesting! How feasible would the idea be if your piston isn't machined? On a small scale, epoxy can be used well but I'm not so sure about epoxy on a large scale (2" valves).
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Unread postAuthor: POLAND_SPUD » Sun May 27, 2012 4:30 pm

I didn't know your valve used an air bumper, interesting! How feasible would the idea be if your piston isn't machined? On a small scale, epoxy can be used well but I'm not so sure about epoxy on a large scale (2" valves).

I don't think you really understand this pic (or the one I posted)
Image
you could build one with an all threaded rod, nuts and washers
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Unread postAuthor: MrCrowley » Sun May 27, 2012 4:51 pm

What's not to understand?

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?

Even if they are safe to use on PVC cannons, this is not what this thread is about. All it takes is for someone to say that there are no commercially available, viable and cheap materials suitable to absorb high forces in spudguns; but I doubt that is the case.
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Unread postAuthor: POLAND_SPUD » Sun May 27, 2012 6:26 pm

well I suspect that this>>
http://en.wikipedia.org/wiki/High-density_polyethylene
fares quite well

But still it's a better idea to use air cushioning effect rather than look for stuff that can directly handle impact


I don't really understand what's up. Lol the way I see it you started a thread and wanted to have a loooooong discussion on high strenght materials or something but the first response ended it :D
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Unread postAuthor: Technician1002 » Sun May 27, 2012 7:18 pm

My take on air bumpers.. AKA Dash-pot.

http://en.wikipedia.org/wiki/Dashpot

To be effective, the dash-pot must keep acceleration forces within safe boundaries of the material involved.

F=MA. Learn it. Dash-pots work well in toasters and other items needing to slow moving objects. They don't work well in High Speed items when using air. Air compresses. Consider an air cannon with 7 Bar chamber pressure. The piston has 7 Bar on one side providing Acceleration. On the other side the dash-pot. This Dash-pot provides an increasing deceleration force as the air compresses. It is balanced when the pilot and chamber are at the same pressure, at about 7 BAR. Note the volume in the pilot.. About one 7th of the total travel remains. With any significant void for the air, such as in the pilot valve, this piston will encounter not a soft landing in the dash-pot, but an impact of very high force in the pilot housing. In PVC the result is often separation of the pilot area from the air chamber.

Air dash-pots are LOW pressure devices because of this. You will want some other energy absorbing material in there to increase the deceleration zone and reduce the deceleration force spike of a collision.

Liquid dash-pots limit opening speed. This is used in normal sprinkler valves with water in them so they soft open and close.

Image

I have found high density open cell foam rubber to be an effective bumper material for air cannons. Foam rubber has worked well.
Image
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Last edited by Technician1002 on Sun May 27, 2012 7:26 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Sun May 27, 2012 7:25 pm

Yuss finally materials! :P

Now HDPE is interesting, I wonder how it would fair if it was crafted like a hemisphere and implemented similar to a tennis ball bumper. Are there things that are like solid, rigid, materails like a plastic that can absorb a crap load of energy through some weird physics? When I say solid, I mean solid like a hockey puck; one would assume it would transfer most of the energy but perhaps there's a plastic or other material that can actually absorb it.

I don't really understand what's up. Lol the way I see it you started a thread and wanted to have a loooooong discussion on high strenght materials or something but the first response ended it
:lol:

I just wanted to see if there were any cool materials that had ridiculous properties, think along the lines of flubber :D

Before I posted this thread, I was pretty tempted to give a hydraulic shock absorber a go just because it would be pretty friggin' cool.

Does anyone know anything about gel absorbers? If they were under pressure, I assume they wouldn't work as well but is this the case?

Liquid dash-pots limit opening speed. This is used in normal sprinkler valves with water in them so they soft open and close.

Good point.
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Unread postAuthor: Technician1002 » Sun May 27, 2012 7:30 pm

How much does it compress? F=MA. Short distances provide high Acceleration and high Force.
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Unread postAuthor: shardbearer » Sun May 27, 2012 7:34 pm

I too would like to find an easy material that we can cut to shape and throw it behind a piston, rather than having a complex mechanism and have to adapt it for different valves.

I was thinking some kind of cushion, I have seen pieces of foam sleeping pads used. They are only about 1/2" thick but can be layered together, but it seems too hard. Something I think would work even better is Memory or Tempurpedic foam. We want something that will slow down the piston at a constant rate (not slowing it suddenly at the beginning or end of the travel) and not bounce back.

Also, I like springs. What were the problems with them when people used them in the past, other than bounce? If you allowed space for the piston to sit without being in the way of the valve or compressing the spring, and have fairly light piston and high friction, it would work well. Another problem its that compression springs can get very damaged when overcompressed, warping to the sides and bending permanently.
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Unread postAuthor: MrCrowley » Sun May 27, 2012 7:38 pm

What were the problems with them when people used them in the past, other than bounce?
Exactly what you said after this quote; they were over-compressed and warped.

So how do we prevent this? You would need the piston travel to be less than the length of the spring at full comression but the material would also have to be strong enough to not be warped while not slowing down opening speed.
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Unread postAuthor: Lockednloaded » Sun May 27, 2012 8:21 pm

With the extreme forces in your cannon, what about a golf ball? They can compress a whole lot under simply the force of a golf club being swung. Plus they're meant to be struck repeatedly without failing
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Unread postAuthor: DYI » Sun May 27, 2012 8:45 pm

[rant] 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. Surely you read my thread about thick-walled pressure vessels. Moonbogg was near-frantically imploring me to stop caring about the topic, because I didn't need to know (I did, but that's not really the point), and most other responses were either complete non-sequiturs or "practical" responses which cleverly skirted around the fact that the poster had no idea what he was talking about, but really wanted to say something anyway.
[/rant]


Now, on to the bumpers. Much as I know you're more of an artsy type, if you want to find a better bumper material you really do have to look at this from an engineering design perspective. Find what criteria define a useful bumper material.

1. When it comes right down to it, you're looking to minimize the force applied to the structure behind the bumper and piston (with the constraint that you must avoid excessive pilot volume, of course, but this doesn't translate directly to the achievable acceleration distance). You can rattle on about "impact" and such, but applied force is what this boils down to. The shorter the distance the piston has to accelerate over for a given initial speed, the higher the force applied to the valve casing (I won't talk about reducing initial impact speed here, as it is irrelevant to the topic). This implies that you want a material capable of high elongation at break.

2. Any deviation from constant acceleration of (and thus applied force on) the piston is undesirable, much like deviation from constant projectile base pressure in a gun is undesirable. It increases the maximum force for a given velocity change over the available distance, and thus should be avoided (this is difficult to achieve in practice though, and not possible with Hookean materials, which I'm sure you already know).

3. Any deformation of the bumper must be elastic. Considering the relatively large velocity change to be achieved and the resultant desired distance, this suggests a material with low elastic modulus (barring geometric "tricks" like springs which allow large deflection per unit length without large strain, which I sense is not what you're looking for).

4. Dampening properties are desired - in the limit of a perfectly elastic material, the piston would rebound at nearly the same speed it started off with owing to the relatively high mass of the launcher. What you want, then, is a viscoelastic material, which is a rare class indeed.

5. You require that the material be in the solid phase at your ambient conditions.

Although you've doubtless already seen it in your research, Sorbothane appears to fit the bill. There are other viscoelastic materials out there which you may also want to look into. I'm not sure whether you have any particular chemical or temperature resistance requirements which may discount those materials.
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Last edited by DYI on Sun May 27, 2012 8:47 pm, edited 1 time in total.
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Unread postAuthor: velocity3x » Sun May 27, 2012 8:46 pm

MrCrowley wrote:I didn't know your valve used an air bumper, interesting! How feasible would the idea be if your piston isn't machined?


IMO....Not at all feasible if the parts are not machined to close tolerances.


Technician1002 wrote:
My take on air bumpers..
Air dash-pots are LOW pressure devices.

tech,
Do you realize that my "air bumper" is still working after 2 1/2 years of shooting at 69 bar?

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