Suitable Aluminum tubing for 7/8 tubed guns--Prod, Disco, Maximus, QB???

[mackeral5]

There is a post for Marauder sized tubes.  Bob gave a specification--- 2024-t3 aluminum 1.25 OD .095 wall. 

https://www.gatewaytoairguns.org/GTA/index.php?topic=168575.0


What about for 7/8 od tube guns such as QB78, Prod, Discovery, Maximus, Fortitude?  Is there a suitable aluminum tubing for these guns? 

[Sbak]

aircraft spruce, they offer certificates on their tubing by testing to destruction. A little pricey, but well worth the piece of mind.

[rsterne]

I would use 2024-T3 aluminum of the same dimensions as the Disco tube (7/8" OD x 0.065" wall).... Use the certified yield strength to calculate the MSWP, using at least a 3:1 safety margin.... Using the nominal values for 2024-T3 of 50 Ksi Yield and 70 Ksi Tensile, at 3,000 psi using Lloyd's tube calculator I get 2.7:1 to Yield and 3.7:1 to Burst.... You need to pay attention to the valve screws, of course.... I would use three 8-32 high-tensile SHCSs....

Using 3,000 psi might be pushing it for long cycle life, due to fatigue.... Do your own math, don't rely on mine, I'm not an engineer....

Bob

[mackeral5]

I've used Aircraft Spruce for items in the past.  They have great service. 

Bob, thanks for the input.  I would love to see someone come up with a solution, but your tone seems to be pretty guarded---which tells me it likely isn't the best idea for my intended application of 3kpsi.  Especially considering 3k fills are sometimes 3200 fills, if only by accident.  Again, thanks for weighing in.   

Sounds like someone with a Disco or Maximus that wanted to stick with 2k fills may be well served by the 7/8 .065 2024t3. 

[rsterne]

I personally would be OK with it.... but some guys insist on a 3:1 safety margin using the yield strength, so I recommend that.... I am not aware of a better grade of aluminum that is stronger, staying with the stock dimensions (necessary to fit the valve inside), and is commonly available as tubing.... 7075-T6 is stronger (73Ksi yield and 83Ksi tensile), but usually only available as bar stock....

Bob

[mackeral5]

I personally would be OK with it.... but some guys insist on a 3:1 safety margin using the yield strength, so I recommend that.... I am not aware of a better grade of aluminum that is stronger, staying with the stock dimensions (necessary to fit the valve inside), and is commonly available as tubing.... 7075-T6 is stronger (73Ksi yield and 83Ksi tensile), but usually only available as bar stock....

Bob

Again, thank you for your sharing your view.  This is definitely interesting to me.....

[PikeP]

Using Barlows Formula for working pressure, for a 2024-t3 .875" OD and .065" Wall tube @ 3000 psi I get a safety margin of 1:3.4 Burst and 1:2.4 Yield...which is perfectly acceptable in my book.. I personally wouldn't go below 2~ yield or 3~ burst.


-Matt

[WhatUPSbox?]

I don't have a Disco, is the wall thickness effectively thinner in the root of the thread for the fill assembly?

[rsterne]

Yes, however the O-ring that seals the front fill plug is behind where the threads are.... Therefore only the portion aft of the O-ring, where the tube is full thickness, is pressurized....

Bob

[WhatUPSbox?]

The threads are still taking the axial mechanical load, the o-rings only seal the gas but are not structural by themselves.

[mackeral5]

Another member will likely try this out on his Maximus, I will wait for him to start a thread/post details.  I am recommending he convert to disco valve/retention since he is making a new tube.....

Based on this thread I picked up a 3' stick from Aircraft Spruce, I will likely convert my huma reg'd QB78 from an SS Cothran tube to Aluminum, at some point down the road.  Coupled with a CF-sleeved FX liner it should make for a very light gun....

[BigBird]

Curious, what are the tolerances for 2024 tubing?  Is it the same drawing proccess as steel?  I know 7/8" DOM steel that I have used was about .741 ID and some smaller .370 tubes are .004 smaller, etc.  Probably not good for strength to have to reduce either metal further.

[rsterne]

Stan, you are correct, the threads take the axial loads, but they are not subject to hoop stress, providing the O-ring is inboard of the threads.... Proper threading tolerances (and the choice of the thread used) are critical, whether the material is steel or aluminum....

Dan, tube wall tolerances are one of the reasons for the generous safety margins used.... The ASME requirements for pressure vessels is a 3.5:1 safety margin to burst, but only 1.5:1 to yield (under the worst conditions).... Since fatigue considerations become important if the safety margin to yield is less than 2:1 (and cycle life drops), I have my own personal limits.... I will use a material that is 2.5:1 to Yield and 3.5:1 to Burst for a reservoir, providing all the other criteria (thread, valve pinning, etc.etc) is also done properly.... Others use 3:1 to Yield, while some feel comfortable with 2:1 to Yield and 3:1 to Burst, depending on their comfort level, and confidence in their calculations and construction techniques.... I have used 2:1 to Yield for the bearing load for screw heads in the holes in a tube on occasion, because that then becomes a visual indicator for a "slow" failure, should you ever see distortion of the tube wall there.... which I never have.... Some CO2 guns converted to HPA have shown such distortion of the tube wall, an indication they are being pushed beyond their limits, IMO.... I have been quite unpopular for pointing out examples of that at times.... 

Bob

[Madd Hatter]

Yeah Bob, you're such a Deby downer! J/k

[subscriber]

Just remember that aluminum has poor fatigue resistance compared to steel.  A factor of safety that is good for steel is too low aluminum.  Still OK to use aluminum, if you replace the pressure vessel after so many fill cycles.

2024 Al is good for pressurized air vessels, but does not have the best corrosion resistance.  The latter also will affect part life; as corrosion sites can trigger cracks.

Here are some reference papers, indicating that 2024 aluminum fatigues, even when stressed well below the yield strength.  This, while steel has almost an infinite fatigue life, if loaded below yield stress:
https://digitalcommons.unf.edu/cgi/viewcontent.cgi?article=1721&context=etd
https://pdfs.semanticscholar.org/d11a/2728a7af795053983a666e649ddb6d5cb2c6.pdf

[rsterne]

You are correct, subscriber, aluminum does have lower fatigue resistance compared to steel.... However, with 2024-T3 having a yield strength of 50 Ksi, with a 2.5:1 safety margin the maximum stress is 20 Ksi.... You will note on your diagram that works out to 10^8 cycles to failure.... That is 100,000,000 fill/refill cycles, from empty to MSWP.... and even more if you are merely topping up the bottle from 2/3rds the fill pressure.... Drop the safety margin to yield down to 2:1, and the life is about 1/50th as great.... Having said that, this is exactly why that aluminum PCP reservoir I mentioned failed in serious competition use, because the engineers didn't allow for it....

Bob

[subscriber]

Thanks for the perspective, Bob

The thing with even that very large number of cycles is that it is contingent on the surface not being scratched, dinged or corroded.  An extruded aluminum tube that is full of internal striations won't last nearly as long as one that is drawn or bored and polished to a near mirror finish.  Nor will one that has corrosion from internal condensation.   

To achieve the rated fatigue life, the material needs to be "flawless" without any inclusions, or its fatigue life will be severely limited.  Samples are tested, but is the whole production quantity screened?  Ditto for the chemical composition and state of temper down the full length.  Thus, it behooves the home builder to do their own testing.  And to avoid buying cheap tubing scraps off ebay...

This is one reason why air tanks over 2" in diameter are retired after 10 years of service, regardless of the number of cycles.  Presumably to cover the uncertainties that could accumulate over time.

My point was that for the same level of confidence, the factor of safety on an aluminum pressure vessel needs to be higher than for a steel one.  Exactly how much higher is the next question.  Perhaps anything safe to "only" a million fill cycles is already close enough to infinity, considering how long that would take to achieve.  So, arguing that we might have only 25 million cycles in hand, rather than 100 million would seem silly.

In the grand scheme of things, fatigue of aircraft wings seems much more scary, because how many cycles and how severe the flexing will be over so many years is much harder to predict.

[rsterne]

And if aircraft were built to a 3:1 safety margin, they would be too heavy to fly.... I think that 1.25:1 or maybe 1.5:1 (to maximum G) is more typical.... Still, with enough cycles, even a commercial aircraft can become a convertible....



Bob

[MJP]

I personally don't encourage people to use aluminium for airtube material. Seamless crmo tubing is not that heavy in the sizes we use. Specially if rated for 3000psi fill.
If the gun feels too heavy with a steel tube then you are out of shape and should do something about that first.

Every cycle that you fire the gun does pulsate the cylinder, don't know if it affects the lifespan of the tube but still.
Stay safe and use steel, no need to risk for a couple of hundred grams of weight reduction.

Marko

[mackeral5]

I personally don't encourage people to use aluminium for airtube material. Seamless crmo tubing is not that heavy in the sizes we use. Specially if rated for 3000psi fill.
If the gun feels too heavy with a steel tube then you are out of shape and should do something about that first.

Every cycle that you fire the gun does pulsate the cylinder, don't know if it affects the lifespan of the tube but still.
Stay safe and use steel, no need to risk for a couple of hundred grams of weight reduction.

Marko

I just saw this reply, thank you for weighing in.  I dont know if I will ever build an aluminum tubed gun, but if I do the contributors to this thread provided some good information.