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Pressure Equation Confusion

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Pressure Equation Confusion

Unread postAuthor: grumpyoldman » Tue Apr 22, 2014 6:12 am

Hey guys. I'm a bit confused on how much pressure I can safely use with the chamber I've made.

I've seen some members here use the calculator at this link: http://www.engineersedge.com/pipe_bust_calc.htm.
With that calculator I get a working pressure of 1300 PSI with a safety factor of 5. Not that I want to go that high. But I would like to have about 600 PSI.

I also got an equation from D_Hall at this link: http://www.spudfiles.com/materials-ammo/topic24833.html. Using that equation, I get a working pressure of 350 PSI with a safety factor of 5.

Why the big difference? What would be the correct working pressure with a safety factor of 5? I must be missing something but I can't figure out what.

The chamber is constructed with the following:

6061-T6 Seamless Tube - 40,000 PSI Yield Strength
Diameter - 3.50 inch
Wall - .25 inch
Length - 6.00 inch

Ends:
6061-T6 Extruded Rod - 40,000 PSI Yield Strength
Diameter - 3.5 inch
Thickness - .5 inch

The ends are bolted together inside the tube using 10-24 threaded rods, high pressure sealing washers (up to 8,500 PSI) and locknuts. The 10-24 threaded rods are 18-8 stainless steel with a minimum Rockwell hardness of B70 and minimum tensile strength of 70,000 PSI. Photos are below.

Thanks in advance!

image_1.jpg
image_1.jpg (16.45 KiB) Viewed 888 times

image_2.jpg
Don't have the locknuts yet.
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image_3.jpg
image_3.jpg (13.44 KiB) Viewed 888 times
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Re: Pressure Equation Confusion

Unread postAuthor: Anatine Duo » Tue Apr 22, 2014 6:54 am

I'm next to useless on your safety calculations but I have been toying with that method of sealing tubes.

Very elegant design. Do you have to seal where the threaded rod comes through?

edit, I think 2 by #10 threaded rod might be your weak link and the mode of failure, um, hazardous
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Re: Pressure Equation Confusion

Unread postAuthor: GonzoInferno » Tue Apr 22, 2014 7:55 am

I agree regarding the threaded rods. Unlike a bolt, which has a solid body blank between threads and head, giving it a bit more strength, threaded rod can stretch at every thread. Couple that with grade 18-8 (304SS) which is pretty good at corrosion resistance but a bit on the soft side, allowing more potential to stretch and allowing the end caps to start moving until the seal is compromised. I like the aesthetics of the design tho. How about a very easy mod and just add more rods to share the load and / or larger rods.
The pressure calculations are confusing me too. My chamber is also 3.5" OD with 1/4" wall, but 36" long (2 of them). I found that I could go to 600 psi with that. I'm trying to find my source .
Please share more of your build.
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Re: Pressure Equation Confusion

Unread postAuthor: grumpyoldman » Tue Apr 22, 2014 8:40 am

Thanks for the replies.

Anatine Duo wrote:I think 2 by #10 threaded rod might be your weak link and the mode of failure, um, hazardous


Yes, I was thinking the same thing. Do you think two 1/4" high-strength 1144 carbon steel rods would do? The yield strength 125,000 PSI. I could just thread the ends.

Anatine Duo wrote:Do you have to seal where the threaded rod comes through?


Yes. But it's very easy with the high pressure sealing washers. They will seal up to 8,500 PSI. I found them at McMasters. Link: http://www.mcmaster.com/#standard-washers/ The 1/4" washers are item number 94154A400.

GonzoInferno wrote:How about a very easy mod and just add more rods to share the load and / or larger rods.


I'll go with slightly larger rods with more yield strength. I don't really want more than two rods cluttering up the inside because there will be a toolie type piston inside. I'm going to use the two rods as guiding rods for the front piston. I'm still doing some experimenting with the design. I'll post everything when it's done.
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Re: Pressure Equation Confusion

Unread postAuthor: Fnord » Tue Apr 22, 2014 9:01 am

Do *not* use those threaded rods. What is the inside thread diameter of 10-24? Something like 0.15?

If your ultimate strength is 70kpsi then you're only looking at 2400 pounds before they(both) snap, and at 600 psi you're looking at 4200 pounds of tensile strain. Solid 1/4 inch rods should fail at 6800.

That's not even considering any fatigue that may have been imparted when the threads were formed!

Also note that even if the rods are solid in the mid-section you will still calculate by the thread depth on the ends, it's a matter of 'weakest link'.
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Re: Pressure Equation Confusion

Unread postAuthor: grumpyoldman » Tue Apr 22, 2014 9:46 am

Fnord wrote:Do *not* use those threaded rods. What is the inside thread diameter of 10-24? Something like 0.15?

If your ultimate strength is 70kpsi then you're only looking at 2400 pounds before they(both) snap, and at 600 psi you're looking at 4200 pounds of tensile strain. Solid 1/4 inch rods should fail at 6800.

That's not even considering any fatigue that may have been imparted when the threads were formed!

Also note that even if the rods are solid in the mid-section you will still calculate by the thread depth on the ends, it's a matter of 'weakest link'.


It's nice to know things like that. :)

How about two 1/4" high-strength 1144 carbon steel rods? The yield strength 125,000 PSI. I found them at McMaster. Link: http://www.mcmaster.com/#standard-steel-rods/. Item number 6628K23

I could also thread it for 1/4-32. The minor diameter for that thread is .2117.

How did you calculate how many pounds would be needed to break the rod? It would be nice to have that equation for future use. :)
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Re: Pressure Equation Confusion

Unread postAuthor: Fnord » Tue Apr 22, 2014 10:35 am

How did you calculate how many pounds would be needed to break the rod? It would be nice to have that equation for future use.


Tensile, or ultimate strength, is how much force one can stretch a material with before it breaks.
If you have a 1 square inch section of 70kpsi material, it should break at around 70,000 pounds. Divide it up based on the cross section of your rod.

Two things are important,
1 You should use the yield strength rather than ultimate strength (this is where it will start to deform).
2 There are often other factors that play into the strength of your material, which is why mcmaster says their numbers are for reference only.

Beware of steels that do not have much difference between yield and ultimate strength, this means you have a hard, but brittle alloy!
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Re: Pressure Equation Confusion

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Re: Pressure Equation Confusion

Unread postAuthor: Technician1002 » Tue Apr 22, 2014 10:43 am

Before threading carbon steel rods, be aware it can be a very hard steel and may end up reaming out your threading die instead. Threading tools are for mild steel only such as maluable iron pipe.

To thread high carbon steel, it will either need annealed changing its strength, or cut with a die grinding tool. Do not use a threading die on high carbon steel.
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Re: Pressure Equation Confusion

Unread postAuthor: GonzoInferno » Tue Apr 22, 2014 11:14 am

Grumpyoldman, is this the same or another t-shirt adventure?
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Re: Pressure Equation Confusion

Unread postAuthor: grumpyoldman » Tue Apr 22, 2014 1:20 pm

Thanks Fnord. You're a virtual fountain of knowledge. Now I can figure out what rods to use. :D

Technician1002 wrote:Before threading carbon steel rods, be aware it can be a very hard steel and may end up reaming out your threading die instead. Threading tools are for mild steel only such as maluable iron pipe.

To thread high carbon steel, it will either need annealed changing its strength, or cut with a die grinding tool. Do not use a threading die on high carbon steel.


Thanks Tech. I bought a lathe over a year ago but I haven't tried threading with it yet. I'll probably get the die grinding tool, I wouldn't trust my lathe threading.

GonzoInferno wrote:Grumpyoldman, is this the same or another t-shirt adventure?


Believe it or not, this is the same t-shirt launcher adventure. It's taking a lot longer than I originally thought. I'm having to learn a lot of new stuff, which is not a bad thing. I'm actually enjoying it.
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