Calculating Yields

[sixshootertexan]

When calculating yields for a pressure tube should you use the wall thickness of where the threads are or the actual tube thickness. I'm wanting to use 1.125 x.095 2024. With a 50000 yield it has a working pressure of 3400psi with a 4:1 SF and a 2400psi working pressure with a 3:1 SF. I plan on using 3000psi in the gun. I don't have the thread size planned yet but if it decreases the wall thickness to .065 should I use that thickness?

I could use 4130 an be well into range for a 3000psi fill. The design I have in my head I could save 1 1/2 - 2 lbs of weight if I use 2024.

[rsterne]

If you have the O-ring inside the threads, where the tubing is full thickness (IMO the best way to do it) then you don't need to use the thinner wall when calculating the yield (or burst) pressure of the tube wall.... If you seal the end plug outboard of the threaded portion, then the threads are pressurized, and you have to use the remaining wall thickness for your calculations.... I would make sure that you use at least a 3:1 safety margin on steel where it is threaded, as the sharp corner at the bottom of the threads will create a stress riser and a possible fatigue point.... For aluminum you might even want a higher safety margin if the threads are pressurized, but I don't know enough about it to give you a suggestion.... QVTom would be a good guy to talk to, he uses a lot of threaded end plugs.... Personally I would just put the O-ring inboard of the threaded portion, where the wall is full thickness, and then the only load on the threads is a shear load (on the threads themselves) and the end load on the tubing wall outboard of the thread bottom (the plug trying to pull the end off the tube).... Don't forget to do the calculations for those as well.... The load is the end plug area time the pressure....

Bob

[sixshootertexan]

The orings will be inboard. I'm just not comfortable with the numbers I'm coming up with in the 2024.  Just to save a pound of weight on a gun that will most likely be used only on a bench it's not worth the uncertainty I'm having with the 2024 numbers.  I'll be making up a drawing to give to my coworkers to run in Solid Works to verify the numbers I come up with.

[rsterne]

For a 1.125" OD x 0.095" wall, with a yield strength of 50K, I get MSWP of 3400 at 3:1 SF and 2500 at 4:1.... I'm not an expert on threads, but a cursory look gives me a safety factor of 4.2:1 at 3400 psi using 1" OD x 24 tpi threads for the end force on the remaining tubing wall (0.063") with an end force of 2334 lbs (O-ring inside the threaded section).... Even a 1/4" of threads seems to be plenty, but I wouldn't use less than 1/2" engagement minimum on a 1" OD....

Bob

[sixshootertexan]

Well the 1.125"x.095" 2024 is not easy to find online. I could get through work but I don't like doing that if I can keep from it. Plus it's twice the price as 4130 and almost half the tensile. So 4130 is what I'm going with. We calculated 1"-20 threads at 2500lbs of end force. It was nowhere near failure at a 25:1 SF.

[rsterne]

There are two failures to look at where the tube is threaded, and assuming the O-ring is inboard.... One is the shear force on the threads (usually not an issue) and the other is tensile failure in the tubing wall from the plug pulling on that thinner portion.... The failure would, of course, occur right at the inside edge of the plug threads.... The loaded area is the circumference of the tube times the wall thickness outboard of the groove for the threads.... If the tubing is 0.095" wall and the threads 1" OD with a 1.125" OD tube, then the tubing has .063" wall remaining, for a total area of (1.000 x PI) x 0.063 = (3.14 x 0.063) = 0.198 sq.in.... At a yield of 50,000 psi, that would be 9891 lb.f, which with 2500 lbs. of load is just under 4:1 SF....

Bob