[cockerpunk]

a continuation of the discussion we were having earlier this month.

my question ...

i get barrel beaks with a pump gun - why?

i know they are not chops, because i can feel that, but i do get barrel breaks. does anyone have a theory as to why?

[Nicholai]

Edited for stupid content, wasn't aware of who cockerpunk was at the time.....

This post has been edited by Nicholai: 18 November 2008 - 06:12 AM

[Leftystrikesback]

The easiest answer is that your paint isn't perfect, it has some surface flaws that create stress concentrations when the force of the air blast hits it.

Categorizing the surface flaws is more of a chore, I would say the worst kind is what you see in "bad paint," such as dimples, oblong shape, or inconsistent wall thickness.

Obviously no paintball is a perfect sphere with isotropic surface properties (constant everywhere). For one thing each has a stiff seam, which likely increases stress concentrations to either side of the seam.

I start it off with that.

[cockerpunk]

in all my pumps, with all the different barrels (and bores) i have had barrel breaks.

[ChrisB]

My guess is that its the paint. Even if you got the best paint there's gonna be surface defects or stresses. Maybe not noticeable to the eye but it's there.

But that's just my 2 cents.

[Leftystrikesback]

Speaking of paintballs and stresses, does anyone have or know where I can get some material properties of the gelatin that paintball shells are made of?
Even if you knew a more specific name for the gelatin mixture that they are made out of that would help my searches for the material properties, "gelatin" doesn't turn up anything useful on google or in a library search.

I'm going to see if I can set up a FEA model to show how paintballs might deform in a barrel, and if stresses occur around a seam.

[brycelarson]

Leftystrikesback, on Oct 30 2008, 08:49 PM, said:

Speaking of paintballs and stresses, does anyone have or know where I can get some material properties of the gelatin that paintball shells are made of?
Even if you knew a more specific name for the gelatin mixture that they are made out of that would help my searches for the material properties, "gelatin" doesn't turn up anything useful on google or in a library search.

I'm going to see if I can set up a FEA model to show how paintballs might deform in a barrel, and if stresses occur around a seam.

first I'm going to say - it's all the paint.

I think that any hope of alleviating barrel breaks through bore choice is a false hope.

and in direct answer to your question - my guess would be is that each manufacturer has their own formula for shell and fill. I'm pretty sure that they are secretive - but, as CP and I have found - paintball manufacturers can be cool. I suggest you spend a couple hours on the phone and call some companies. These are small businesses - and it's likely that they might help you out. Might not give exact formulas - but maybe they would be helpful with information that would make your math a good addition to the work we're trying to do in PunkWorks.

[cockerpunk]

already on the phone with joey again for the compression test

[Troy]

Leftystrikesback, on Oct 30 2008, 08:49 PM, said:

Speaking of paintballs and stresses, does anyone have or know where I can get some material properties of the gelatin that paintball shells are made of?
Even if you knew a more specific name for the gelatin mixture that they are made out of that would help my searches for the material properties, "gelatin" doesn't turn up anything useful on google or in a library search.

I'm going to see if I can set up a FEA model to show how paintballs might deform in a barrel, and if stresses occur around a seam.

I would be very interested in this. You may have better luck trying to figure out the exact ingedients by looking at the back of a NyQuil box, or some other kind of gelatin capsule.

What kind of property info do you need? If we have the answer to that, we may be able to organize some tests to get this figured out.

I imagine that we'll be able to get some kind of model that is close enough to test with that can shed some light on what causes breaks without knowing THE EXACT formula each manufacturer uses.

If it absolutely comes down to it, I can get ahold of a flame emission spectrograph and figure out the exact molecular composition of that damn shell... but I would rather not if its at all possible.

[cockerpunk]

Troy, on Oct 30 2008, 10:55 PM, said:

I would be very interested in this. You may have better luck trying to figure out the exact ingedients by looking at the back of a NyQuil box, or some other kind of gelatin capsule.

What kind of property info do you need? If we have the answer to that, we may be able to organize some tests to get this figured out.

I imagine that we'll be able to get some kind of model that is close enough to test with that can shed some light on what causes breaks without knowing THE EXACT formula each manufacturer uses.

If it absolutely comes down to it, I can get ahold of a flame emission spectrograph and figure out the exact molecular composition of that damn shell... but I would rather not if its at all possible.

damn punkworks is awesome!

i think the compression test will tell us alot about how paintballs resist loading. me and bryce will take pictures if any kind of failure trend emerges (like we should have done on the barrel break test!)

FEA ... damn, thats a sweet idea!

[Leftystrikesback]

The material properties I would need would be for the shell only and would be young's modulus, shear modulus, poisson's ratio, ultimate tensile strength. If we get some results from the compression test that was mentioned then I may be able to derive some of those values. I don't think the recipe or flame emission spectrograph will help, thanks for the offer though!

I did some work on the FEA model and figured out a plan:

What I want to do is show where the stress concentrations are on a stiff spherical shell filled with liquid when it's hit with a pressure along its rear half and constrained along its circumference (paintball held in a barrel hit with air).
If I get a base model for that then I'll start to look at what changes the stresses to a point where failure might occur:
I'll put a seam on it to see if that adds anything interesting. I'll make a bit of the shell thinner and see what happens to that part. Maybe I'll add a dimple, or make it more oblong... things like that in different orientations could lead to some insight into what defects in the ball contribute to barrel breaks. This model would be a static approximation of a dynamic action so it's limited by that and the assumptions made to constrain the model.

I can show stress concentrations on a spherical shell under similar loading circumstances, that part is not too difficult. I ran some models earlier today and got some results that are consistent with observation... however they aren't really valid since I haven't "filled" the shell yet.

For that I need to figure some stuff out with multi-body analysis I think, possibly wait a new software version my lab is getting soon.

If I only want to look at where the stress concentration's occur and ignore the actual forces at work, I can probably use any linearly elastic material so that's where I'm going to start.

[Troy]

Leftystrikesback, on Oct 31 2008, 02:15 AM, said:

I don't think the recipe or flame emission spectrograph will help, thanks for the offer though!

That's kind of what I was thinking. It would only really help if we had to recreate the shell to test it. Unfortunately, knowing what its made of doesn't tell us it properties.

Leftystrikesback, on Oct 31 2008, 02:15 AM, said:

What I want to do is show where the stress concentrations are on a stiff spherical shell filled with liquid when it's hit with a pressure along its rear half and constrained along its circumference (paintball held in a barrel hit with air).
If I get a base model for that then I'll start to look at what changes the stresses to a point where failure might occur:
I'll put a seam on it to see if that adds anything interesting. I'll make a bit of the shell thinner and see what happens to that part. Maybe I'll add a dimple, or make it more oblong... things like that in different orientations could lead to some insight into what defects in the ball contribute to barrel breaks. This model would be a static approximation of a dynamic action so it's limited by that and the assumptions made to constrain the model.

I can show stress concentrations on a spherical shell under similar loading circumstances, that part is not too difficult. I ran some models earlier today and got some results that are consistent with observation... however they aren't really valid since I haven't "filled" the shell yet.

For that I need to figure some stuff out with multi-body analysis I think, possibly wait a new software version my lab is getting soon.

If I only want to look at where the stress concentration's occur and ignore the actual forces at work, I can probably use any linearly elastic material so that's where I'm going to start.

I think you're on to something. Like I hinted at before, the properties of gelatin may not be necessary to make broad assumptions along those lines. You may be able to use some other flexible material, like a plastic, or latex, that you have info for in substitution... that might shed some light on this.

[Leftystrikesback]

I've been using polypropylene in my models thus far. I'll post some if I have the time but it takes a while to describe the assumptions and what conclusions you can make.

[moyster14]

Here's an off the wall idea to bounce off everyone. It goes beyond the forces applied to the paintball and its defects. The rapid expansion of gas propels the paintball, which can be considered an adiabatic process. This rapid expansion will absorb thermal energy and the paintball is right there next to the expanding gas. So could there be some localized cooling taking place that could effect the properties of the paintball? There already been quite a bit of talk regarding the refrigeration of paintballs and how they change with temperature, so there may possible be some interesting thermodynamic effects taking place as well.

Let me know what you all think.

[Snipez4664]

As a chemical engineer, I doubt it. Gas is a terrible medium for heat transfer, and gelatin isn't much better. That and the contact time isn't very long at all. Go shoot a paintball into a tarp, and pick it up - I doubt it will be much less than room temp.

[moyster14]

Well I didn't mean the paintball is going to have frost on it. I'm sure the paintball will be back up to temp by the time you touch it.

I'm talking about cooling on maybe only half the paintball, perhaps only the surface facing the bolt and very, very quickly. Maybe just enough to create a weak spot or enhance a surface defect. I'm just wondering if inducing a temperature gradient through the thickness of the shell plays a role. Especially with other forces acting on it. Now as an electrical engineer, I'm not too experienced with heat transfer and thermodynamics, I'm just throwing the idea out there. Plus there there may also be some heat produced through friction. Will any of the this contribute, maybe not, just thinking outside of the box.

[oerllikon]

Is there a such thing as an "Air Hammer" (Like a water hammer) could it be the air hitting the ball suddenly?
Or is it a weak wall on paint?

[Eskimo]

Could it just be that murphy's law just doesnt want you to go without breaks?

Im thinking it might have to do with the position of the paintball and the acceleration of it.
(for viewpoint sake, looking out from the breech though the barrel and towards the target, the paintballs point of view)
If the paintball is ever so slightly to the left or right, ( so one side) and when it accelerates, ( which it does, greatly) the paintball hits the side of the barrel. when this happens the paintball begins to turn so if it hit the left side it rotates left. ( i think) and when that happens the paintball can do one of three things.
1. it makes contact to the barrel wall, creating the friction between ball and wall ( lets say an aluminum barrel) and the ball simply tears apart from the friction and the rubbing against the wall.

2. it bounces off, still roatating it hits the opposite wall creating an rotation on the other direction, this sudden stop and start causes the paintball to tear open and burst apart. ( so one side of the paintball is being mushed against the stopped portion ( against the wall) and the portion of the ball past the contact point still rotants to the left, this is the tear point, becuase of the streached material, especially if it is the seam)

3. it bounces off and leaves your barrel.

now of course this is assuming your paintball has a small amount of space down the barrel ( not extreme overbore.) but so you can slighltly blow it though. ( when a paintball is shot it changes shape, a little more oblonged.) <- as far as I know. when we do a blow test the ball is still a ball, when its fired extra space is created on the sides becuase it becomes more egg shaped due to acceleration.

thats just my thoughts on barrel breaks.
( But I still believe its murphys law.)

[pb101]

Perhaps we should be asking the people who make paintballs. Maybe they might have some insight into this subject.

[brycelarson]

Eskimo, on Nov 10 2008, 05:38 PM, said:

thats just my thoughts on barrel breaks.
( But I still believe its murphys law.)

All barrel breaks in our test were between 3/4 and 1.5 inches into the barrel - this indicates to me that the breaks were not from shear force or barrel interaction - more just the ball exploding on the contact of the burst of air.

This is held up by Simon's youtube video of the glass barrel and the ball break - it doesn't move, just "pops" in place.

glass barrel break