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Is a fitting stronger then a pipe?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Is a fitting stronger then a pipe?

Unread postAuthor: BowerR64 » Thu Mar 13, 2014 1:57 am

Say a 4" tee versus a 4" pipe in the same class sch40

Say i make a cannon using a tee or one using just pipe and a cap.

What are the weaker parts to sch40 pvc?

Wich of these 2 would be the stronger?

left,(breech to muzzle) 4" cap, 4" pipe to 4X3 reducer to 3" pipe, 3" coupler (for looks)
Right, (breech to muzzle) 4" threaded plug, 4" thread adaptor, 4" tee with cleanout, bottom threaded cap, 4" pipe to 4X3 reducer, 3" pipe, 3" coupler for looks.
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Re: Is a fitting stronger then a pipe?

Unread postAuthor: mobile chernobyl » Fri Mar 14, 2014 11:07 am

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Re: Is a fitting stronger then a pipe?

Unread postAuthor: noname » Fri Mar 14, 2014 12:58 pm

Crack formation and propagation is inversely proportional to the radius of curvature of the end of the crack. Very small curvatures act as stress concentrators as a result.
This is why cracks often form at right joints before anywhere else; cylinders are much better at holding pressure than rectangular prisms!
In practice, I would expect a T joint to fail along the perpendicular leg's connection.

However, whether the T, pipe, or any other fitting would fail first is mostly dependent on defects in the material's structure, which you probably won't be able to see anyway.
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Re: Is a fitting stronger then a pipe?

Unread postAuthor: jimmy101 » Sat Mar 15, 2014 2:32 pm

What are you trying to do and what is your fuel?

In general a pressure rated fitting is as strong as pressure rated pipe.

Propane in air at 1x has a theoretical maximum pressure of about 135 PSI (120 PSIG). That's for a closed chamber. In an operating gun the movement of the ammo precedes maximum pressure so the max pressure is less than theoretical max pressure. In addition, in a real gun there is a fair amount of heat loss to the chamber walls which further reduces the maximum pressure.

In general threaded clean outs are DWV parts and are not pressure rated. So it doesn't really matter what the rest of the gun is made of the clean out is the weak point. This is actually probably good since blowing out the endcap is probably a bit safer than rupturing the chamber.
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Re: Is a fitting stronger then a pipe?

Unread postAuthor: BowerR64 » Sun Mar 16, 2014 5:23 pm

That is cool to know, do the thread usually go or does the cap just crack or split?

Ive been making cannons simmilar to the 1976 article of popular mechanics called Hal Kellys carbide cannon http://www.toycannons.ray-vin.com/bigguns/kelly.htm They use 6" tee down to 4" barrel with a rear 4" cleanout. Mine is 4" tee to 3" reducer not near as much carbide or air but still SCH40

Ive built 4 the most shots of about 15 shooting a 3" tennis ball (i know that increases pressure) i used a 4" y tee front and rear 4" the cleanout Y was 3" with a cap. Rear had a 4" round cap (not the flat caps they seem weak or thin walled) the trigger was a red button bbq grill igniter in the 3" cleanout section.

1 cup of water or so poured into the 3"y section down into the back where the round cap is. Then 1/2 teaspoon of acarbide, quickly cap it 5 second BOOM! waitng 10 seconds for the carbide to react sent the tennis ball the furthest distance so far.

The grill igniter is hot glued into the Y but for the first few shots it wasnt i just slid it in and used finger pressure to hold it in. It never really felt like it was going to pop out its like a 1" hole or something those igniters are pretty big.

I wanted to make a more relistic looking cannon where everything is smooth using the couplers insted of the pipe for the breech. Say a 4" pip with a cap, the cap puts a 1/4 lip where the cap slips over the pipe i wanted to get rid of those lips. So from the cap all the way to the reducer everything would be smooth.

Here is what i recently built using the couplers. Image

Plastic cup wrapped with a wash cloth pushes out the spent gas and pulls in fresh air between charges.
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Re: Is a fitting stronger then a pipe?

Unread postAuthor: jimmy101 » Sun Mar 16, 2014 6:38 pm

I believe the cleanout cap just bows oputward, which disengages the threads and the cap pops out.

If you are just making carbide fueled noise makers then the material you make the cannon out of doesn't much matter. Once you start putting ammo in the barrel then the pressure ratings come into play. If you launch a Styrofoam cup the pressure rise will probably be only a couple PSI (if that). If you launch a tennis ball (or an Iron cannon ball :P) the pressure will be higher.

Usually carbide cannons are not used for firing projectiles. The classic Bangsite cannons were quite clear in their directions not to try to launch anything from the cannon. (Which of course everyone strictly followed.)


EDIT: BTW, the cannon looks great. Do you have a pirate flag?
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Re: Is a fitting stronger then a pipe?

Unread postAuthor: BowerR64 » Mon Mar 17, 2014 4:28 am

No flag but that sounds cool, maybe a skull and cross bones on the side of it. The tennis balls seem light enough and im thinking about painting them flat black so they look more like a cannon ball. lol
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Re: Is a fitting stronger then a pipe?

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