Need input: Trouble shooting prototype pb marker

[Thorpydo]

Hey,
Finished up the first prototype today. Its got a few problems, nothing we can't fix. The reason I'm posting: We've got a O-rings on the bolt. When the marker cycles, the O-ring on the bolt that passes over the feed tube is forced up and is pinched. The O-rings hold approx 250psi. The bore in the gun is .700, the bolt .692. Its a real nice fit, same with the o-rings. The O-ring is required to be there, we can't remove it.

Anyone have any advice to stop the o-ring being pinched?

[Thorpydo]

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[ShinyGuy]

Sweeping an o-ring across the feed port with that much pressure behind it just isn't going to work. Without know what that o-ring is doing exactly I can't offer an exact solution. I had a similar problem on one of my designs and I solved it by setting the oring at an angle such that the top of the o-ring never crossed the feed port while the bottom was well forward of the port. The o-ring was perfectly happy working at an angle but the slot geometry was a real pain (the groove had to be narrower and shallower towards the top and bottom when the o-ring was streatched tighter). Note that you'll also need a turning center or screw machine that can cut the tilted groove or you'll need to do it on a 4-axis mill. Hope this helps. I've since elliminated the need for that o-ring to come past the feed tube in my design and I much prefer that, but the tilted o-ring worked fine.

[ShinyGuy]

And hey, where are your safety glasses at that lathe?

[Thorpydo]

Yeah, That's what I figure is the problem... We have a solution. Its fairly simple, it just lengthens the bolt an inch or so. Right now the bolt measures only 1.7 inches =)

Saftey glasses... I've got some, they are just so dirty you cant see through them. Where can I get some? local hardwhere store? How much?

Also, would you mind explaining your other solution? If you can't I'm perfectly cool with that, I'm the same way about my stuff.

[vantrepes]

I think this is what he is talking about:
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The grey area is the bolt, the black line is the o-ring, and the purple makes the feed tube. This would seal the back area of the bolt from the barrel, but it wouldn't stop air from going up the feed port, so it may not be what you need.
You can try a slightly deeper slot, with a higher durometer(sp? beats me) o-ring. That would help with the distortion, and help keep the seal where you wanted it. It doesn't take much exposed o-ring to get a seal. You could also try a thicker o-ring with a smaller ID so it is stretched tighter on the bolt, that might help keep it in place also.

[Thordic]

Try using a harder oring. If you are using something like a neoprene o-ring thats really soft, it'll be blown around by the pressure more.

[athomas]

Does the o-ring have to be at that exact location? Can it be moved further back on the bolt so that it never crosses the feed tube. Obviously if it crosses the feed tube, it isn't used for holding air. If it is used for sealing the barrel against blow back, then make your groove a little deeper so there is less pressure on the o-ring as it passes through the feed opening.

If it is used to prevent blowback then I don't think it is needed. As a ball is fired, most of the air is used to push the ball down the barrel. Very little escapes around the bolt and up the feed tube as long as the bolt is forward. Most blowback in guns is caused by timing issues where the bolt returns before the ball exits the barrel.

[toymyster]

Like Athomas said, try moving the O-ring back a bit, which might mean relocating your gas ports, and/or elongating your bolt!!!

[Thorpydo]

Thats the solution Toy

The design that were going to use now is what we came up with in the first place. But I insisted on going this other route... And look where we end up. Oh well, learned a bit.

Also I'm gonna try putting a O-ring like... Half the size of this one so it really stretches. Anyone got think it will work?

Thanks for all the help and suggestions!

[SLICEnDICE]

Question

So, how does the thing work? Fill us all in on the creation.