Pressure induced ball breaks

[Poe]

:)

While I'm at it...

Tom,

When testing spin, what speeds did you test?

[athomas]

Just throwing this out there to see if my math is correct...

Assuming an average pressure behind the ball of 150psi and a barrel length of 0.7", the maximum velocity would be 175fps? A 2" barrel would be 300fps?

Accuracy vs barrel length is interesting. Maybe I'll bring that up on the Punkwork forum. Speaking of which, if you haven't yet visited it's worth a trip.

I just quickly did the math on this and can verify your numbers to be correct, at least for the 2" barrel, so I assume the 0.7" barrel to be correct as well.

[Poe]

I just quickly did the math on this and can verify your numbers to be correct, at least for the 2" barrel, so I assume the 0.7" barrel to be correct as well.

Thank you.

Does anyone know how to contact Tom other than this forum?

[nerobro]

7) Is the GOBLIN 'SOLO' allowed to be used on Paintball and Airsoft fields?
This is strictly up to the folks who run the fields! As the velocity does not exceed 300 FPS (typically 200-250 FPS), chances are good that you'll be allowed to use it on your field.

This.... makes me think that the realistic velocity range for this is really in the 200-250fps range, instead of 300fps. So, lets do our math off of 225fps instead. That's a whole lot easier to reach than 300. (heck, most fields are 285 nowdays anyhow...)

150psi isn't a hard and fast rule. That's a guideline. There's no good reason that the goblin launchers can't really be doing 180psi, and expect say, 1 in 30 or 50 balls to blow up in the barrel. That's not an acceptable number in a gun doing 10bps for 10 minutes at a time, but for a two shot marker? That ain't so shabby.

You'll also note that they sell very small barrels for these things. I think they are banking on the pressure build up behind the ball before the ball gets moving.

Anyone know what the valving system looks like in these things?

[athomas]

You'll also note that they sell very small barrels for these things. I think they are banking on the pressure build up behind the ball before the ball gets moving.

Makes sense. Have a tight barrel to hold the ball in. It keeps the ball from rolling out while carrying it around and also acts like pressure against a cork when you fire it. The ball doesn't start to move right away and the lag allows the pressure to build to a higher value before the ball "pops" out of place and fires out the end of the barrel.

[brycelarson]

Haven't posted on AO for a while - but I might be able to shed some light on a few of the factors involved here:

http://www.punkworkspaintball.com/index.php?p=7&id=32

http://www.punkworkspaintball.com/index.php?p=7&id=16

http://www.punkworkspaintball.com/index.php?p=7&id=40

http://www.punkworkspaintball.com/index.php?p=7&id=42

to summarize - bore size isn't a factor on accuracy in any test we've done. Barrel length certainly effects efficiency - longer control bores do help efficiency.

We were never able to chrono an emag up to 280 shooting just CCM sizers (1.5"). That may be due to the low pressure and gentle curve of the mag's power pulse. It's possible that some guns can do it - but the efficiency will be terrible.

Bore size didn't effect the number of barrel breaks we got until we pushed it WAY past reasonable use. at .01 and greater underbore the instance of breaks went up by a lot. We don't have testing to show the breech pressure in situations like this - but it would make sense that when the underbore got that extreme the breech pressure should go up.

If we get the opportunity I would love to test breech pressure on a bunch of modern guns. I think that would really open some people's eyes.

I've never seen any evidence of significant ball deformation in the breech, barrel or just outside the barrel. I don't know if the balls really deform. In our insanity underbore test we had a number of balls come apart at the seams - simply splitting in half. That seems to be the most likely failure type for the ball under a high speed impact situation. We tested ball toughness at one point and under a steady load pretty much every ball deformed a lot before breaking (.125" or so) even the brittle, chilled ones.

I would like to do some additional testing and filming in a system that we can adjust the breech pressure. I think that would be really useful information.

[Poe]

This.... makes me think that the realistic velocity range for this is really in the 200-250fps range, instead of 300fps.

That's due to the trigger. Like I said earlier, how fast the valve opens depends on how fast you pull the trigger. This effects velocity to the point that I can shoot as low as 100fps just by pulling the trigger really slow. Pulling the trigger like most normal adults typically yields 200-250fps. When you hear about persons not breaking 200fps it's normally a good sign of their age/strength.

I have modified two of my goblins with a spring hammer assembly to reliably shoot near 300fps every time. Since I have never broken a ball in the past one hundred plus shots, this is a good indication that pressure alone was not causing Tom's breaks.

So, lets do our math off of 225fps instead. That's a whole lot easier to reach than 300. (heck, most fields are 285 nowdays anyhow...)

That would be an average pressure in excess of 250psi.

150psi isn't a hard and fast rule. That's a guideline. There's no good reason that the goblin launchers can't really be doing 180psi, and expect say, 1 in 30 or 50 balls to blow up in the barrel. ...

The reason is the velocity and average number of ball breaks. 180psi = 180fps. Even at 300fps I rarely break a ball.

Bryce, I don't see how those tests relate to the topic at hand (Tom experiencing increase ball breaks when breech pressures exceeded 150psi). Unless Tom was using really large paint or a really small ID barrel that is.

[athomas]

Bryce, I don't see how those tests relate to the topic at hand (Tom experiencing increase ball breaks when breech pressures exceeded 150psi). Unless Tom was using really large paint or a really small ID barrel that is.

The breaks could have been started by the bolt. In an open bolt design like the mag, the increased pressure behind the ball is usually related to increased pressure in the chamber. The chamber pressure is directly linked to the bolt speed/force. Therefore, it is possible that the bolt pushing on the ball had as much to do with the breaks as the pressure itself. Even other markers with regulated air pushing the bolts didn't have soft bolt movement back when the experiment was conducted, so they would also be subject to bolt impact. In the Goblin, there is no bolt movement so this part of the equation is different.

[leloup]

My experience with the goblin: I did not have a chrono, but I got it to go 40-50+ feet (then into woods, so I don't know how far). I had better luck with a freak barrel. I modded the goblin to accept freak inserts as barrels and get a tighter fit, plus more accuracy. Also, you can load up to 5 paintballs in the barrel, 3 for the best, and get almost the same velocity as one, but a nice 10in spread at 35ft.

[Poe]

The breaks could have been started by the bolt. In an open bolt design like the mag, the increased pressure behind the ball is usually related to increased pressure in the chamber. The chamber pressure is directly linked to the bolt speed/force. Therefore, it is possible that the bolt pushing on the ball had as much to do with the breaks as the pressure itself. Even other markers with regulated air pushing the bolts didn't have soft bolt movement back when the experiment was conducted, so they would also be subject to bolt impact. In the Goblin, there is no bolt movement so this part of the equation is different.

Do you think the bolt caused the ball to break and the air just finished it off? Judging from some of Jack Wood's videos this feels like the most likely scenario to me. Especially since the mag is a blow forward with a normal operating pressure of sub 70psi. The easiest way to get 150psi in the breech is to just increase the pressure in the chamber thus increasing the bolt/ball acceleration/impact.

It would be interesting to see how much pressure a ball can experience before breaking though. Since a paintball only experiences pressure on it's back half this would be a difficult task. Something like a centrifuge test rig forcing a ball down a barrel while pressure pushes back. A light behind the ball would show any changes in the ball shape due to pressure as well.

Anyone have any free time on their hands?

[DevilMan]

Sorry I can't offer any "scientific" data on this, but I'm leaning toward the bolt design/speed as the cause of the issue. I have not watched all of the videos and only saw a few of them long ago.

If you look at a Level 7 bolt you'll see that the face is protrusion with air ports around it and a dished face. I don't think that face would be wide enough to make the best contact patch on the ball, and would/could weaken the shell to cause failure. As stated, the pressure determines the bolt speed. You can hit a nail with a hammer over and over and it won't go anywhere until that hammer reaches X FPS. You double and triple that FPS and the nail goes farther faster. You keep increasing from there and you tear the hell out of whatever it was you were nailing together.

I would bet that the factor that caused the breaks would have been bolt design + bolt speed.

The best way to test any of this would be to chamber up a "gun" that uses air only with no bolt interference. A tube with a valve where you could control the input pressure from 50 to 500 PSI... see what that yields.

Good luck just the same.

DM

[nerobro]

The problem with the L7 bolt and breaking paint wasn't related to pressure, but speed. the breach is larger than a paintball, and the edge of the bolt would nick the next ball inthe stack, causing that ball to break when fired.

The L7 bolt also came as a foamy. And those would break paint just as bad.

As for psi breaking the ball, impulses were known for breaking paint. They had breach peak pressures of 120psi. they also had slow(ish) bolt speed. That was all peak pressures.

[athomas]

For multiple balls in a stack, yes, the speed of the bolt nicking the next ball is a problem. The setup I was referring to, and I think others as well, is a single ball in the breach by itself.

[cockerpunk]

The problem with the L7 bolt and breaking paint wasn't related to pressure, but speed. the breach is larger than a paintball, and the edge of the bolt would nick the next ball inthe stack, causing that ball to break when fired.

The L7 bolt also came as a foamy. And those would break paint just as bad.

As for psi breaking the ball, impulses were known for breaking paint. They had breach peak pressures of 120psi. they also had slow(ish) bolt speed. That was all peak pressures.

rollback si probably the culprit on the Impulse. rollback bascially means every single paintball gets bolt nicked, that is going to cuse some failures no matter what.

SP has yet to fix rollback issues, even in the new Luxe/Imp stuff, they still have rollback problems.

[nerobro]

rollback si probably the culprit on the Impulse. rollback bascially means every single paintball gets bolt nicked, that is going to cuse some failures no matter what.

SP has yet to fix rollback issues, even in the new Luxe/Imp stuff, they still have rollback problems.

I love dual detents. :-)