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Heavy Metal Hybrid 6.3L

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: Moonbogg » Mon Mar 26, 2012 11:26 pm

Keep in mind with the threaded holes into aluminum. If you have 2" worth of thread, don't expect the pullout strength to increase linearly. You will benefit from about 6 threads of engagement, and beyond that they will no longer carry any load. Can the aluminum threads withstand the pullout force created from the pressure and impact of the piston? How did you come to the conclusion that the threads in the aluminum are sufficient to hold that end cap?

If your nut material (the chamber) is good for 30kpsi, then assume that the bolt, no matter how strong it is independantly, is also only good for 30kpsi. Imagine yourself using aluminum bolts instead of steel ones. Would you do that? If not, then why would you use aluminum as a nut material?
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Unread postAuthor: SirStarchCrusher » Tue Mar 27, 2012 11:06 pm

Thank you Moonbogg, I did not think of that. How about this instead:

http://postimage.org/image/epc3wypb5/

Nobody has posted about the barrel-lock mechanism so why not use it at the back as well? I would, of course, extend the chamber length to maintain the chamber volume.

The only downside I can see is extra dead space in the pilot chamber
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Unread postAuthor: Moonbogg » Wed Mar 28, 2012 12:35 am

If I had to choose between your first idea and the second for holding the end cap on, I would choose the second as you have shown the bolts in shear. At least you benefit from the bolt strength in some way. Bolts in shear are generally 60% as strong as when normally used, so keep that in mind. If I had my way with the design (which I don't because its not mine) I would use high strength, alloy steel tie rods and a steel end cap and then have another steel end cap on the front of the chamber. I would not rely on aluminum to hold the caps in any way, but thats just me. It doesn't have the elasticity or strength of many steels and you are using a high mix. Hydro test it at twice the pressure you expect to see during use, and when using it, never stand behind the cannon if fastening with aluminum unless you have truly engineered this cannon and know it to be safe.
I have second guessed my own designs for this reason, and as a result, any new cannons will use steel for anything that has to be fastened. Not just any steel either. Use 4140 or equivalent high strength steel. If you want stainless, consider 17-4 PH and heat treat to H900 condition. Its actually pretty cheap to get this done for small parts as they usually charge by the pound and have a reasonable minumum charge of about $75.00 where I live. Aluminum can break. Be careful.
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Unread postAuthor: SirStarchCrusher » Wed Mar 28, 2012 1:03 pm

I will go for the high-strength steel rods and bolts then. I like your idea of hydro testing first. Keep in mind I won't be anywhere near this thing when firing, remote ignition is mandatory.

assuming 1500psi max pressure...

that would put about 19,000 lbs of on each 4" section and 42,500 lbs total on each 6" end. Will the steel bolts and rods be enough to hold this? Remember that I will also be JB welding the cylinder to the chamber in addition to the bolts. I'm not an engineer, so I'm not too sure how to calculate the strength of this....

The end caps are a good idea but they're very expensive form what I've seen, might not be as good-looking either.

Thanks again for the advice!
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Unread postAuthor: Moonbogg » Thu Mar 29, 2012 1:48 am

Try to eliminate as much guesswork as possible. Google search "tensile stress area calculations" to learn about how to find the strength of bolts or tie rods. Multiply the psi of the bolt material for the yield by the tensile stress area of the bolt (if I remember correctly, research it yourself). Divide total force by number of bolts. Each tie rod or bolt should be able to hold about 2.5X the load that they will experience before bolt tension is reversed, not before they break. Keep in mind when you tighten a bolt to about 80% of its maximum tensile rating, that the load it can theoretically hold is reduced by about 20%. The cannon itself should withstand at least 4X the load before breaking. This ensures the tie rods will break before the cannon does and this is the ideal way to go as it is at least predictable. Anyway, like you said, stay far away from it and make sure nothing is behind or infront of it.
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Unread postAuthor: SirStarchCrusher » Sun Apr 01, 2012 10:17 pm

Okay, I went with a Max 1500psi pressure in mind, with a 2x safety factor of 3000psi working pressure. Very generous, because I doubt that I would go over 8x mix to begin with.

12 5/8" grd 5 steel bolts on each end works out to about 265K lbs of shear strength on each end, that should be plenty.

If I use 4x 3/4 grd 5 steel tie rods, they should be at least able to hold at least 127K lbs comfortably on each end.

The chamber is good for 5500psi (1.5x safety factor) of working pressure

Am I missing anything else? If there are any last-minute suggestions, now is the time. Also, if anybody knows and good cnc places for milled cylinders and pistons, do post. Thanks for the detailed feedback so far gentlemen!
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