Question on Pressure Rated Aluminum Tubing

[grumpyoldman]

Hey guys,

The aluminum tubing is 6061-T6 Drawn (not Extruded), 2 inch OD, .065 Wall, ASTM B210. It's rated at Max 610 PSI @ 72 degrees F on McMasters web site. Link: http://www.mcmaster.com/#standard-metal-tubing/=o8ypk2

Considering the launchers that are made with this will be sold to the public, what would be a safe operating pressure? Thanks.

[Gun Freak]

Considering the pressure they put as max already has a safety factor included, I would halve it again to give it at least a 4x safety factor. That would mean about 300 psi is probably the highest you should go with the max pressure. Besides most people don't have access to such pressures anyway... especially for high volume like this pipe!

[jackssmirkingrevenge]

It's more likely that an endcap will blow out than the pipe will fail.

Remember a pressure chamber is only as strong as its weakest part.

[grumpyoldman]

Thanks GF and JSR. I was thinking 300 PSI would be good too.

[Gun Freak]

Like JSR said, you need to be confident in all of the joints too. That is especially critical because they are often the weakest points. Are you going to be welding them?

[jackssmirkingrevenge]

Thanks GF and JSR. I was thinking 300 PSI would be good too.

sounded like you're thanking me and your significant other there

[Gun Freak]

The abbreviated form of my username is rather unfortunate... oh well

[grumpyoldman]

LOL. Sorry about that Gun Freak. I wasn't thinking about the abbreviation for girl friend. I don't think you're my type anyway.

[Gippeto]

Smallish point...welding will destroy the heat treat (T6) and alter the pressure handling capability significantly.

Would suggest an oring sealed plug with good grade socket head cap screw in shear...preferably with the head of the screw bearing on the tube wall...this increases the bearing area and reduces bearing stress.

Low head socket head cap screws have larger heads.

Along these lines...

[grumpyoldman]

Smallish point...welding will destroy the heat treat (T6) and alter the pressure handling capability significantly.

Would suggest an oring sealed plug with good grade socket head cap screw in shear...preferably with the head of the screw bearing on the tube wall...this increases the bearing area and reduces bearing stress.

Low head socket head cap screws have larger heads.

Along these lines...

Thanks for the great idea. I'm just afraid the walls might be too thin (.065 inch) to put screws in. Could the screws rip through the aluminum tubing? Do you think it would hold under a constant 300 PSI?

I had a similar idea with making end plugs with o-rings on them because of the weakening aspect of welding. But I was thinking about connecting the two ends together with a couple of threaded rods like the drawing below.

If the cap screws would work and not sufficiently degrade the integrity of the tubing, I would prefer to use the cap screws. I could also put washers under the head of the caps screws to further reduce the stress.

I saw that McMaster has the low head, self-locking socket screws They call them Button Head socket cap screws. Link: http://www.mcmaster.com/#socket-head-cap-screws/=o9fffn. They're toward the bottom of the page.

[Gippeto]

In my example, the threads are in the plug...the hole in the tube matches the diameter of the head. The heads of the screws are ...rectangular? squared off?...not rounded as with the button head screws.

Bearing area is calculated by hole diameter x tube thickness. The tube shown is for a pcp which handles 3000psi with a 3x safety factor at yield.

Lunch is over, will be back to this.

[Gippeto]

Start with the tube.

Material properties of 6061 T6:

http://asm.matweb.com/search/SpecificMa ... m=MA6061t6

Handy working pressure calculator:

http://www.engineersedge.com/pipe_bust_calc.htm

Fill in the data, use 40000psi (yield strength) as material strength, a safety factor of 5....and get a working pressure of 600psi...that'll work fine for your desired 300psi max WP.


Calculate load on the plugs...pi*r*r multiplied by 1500psi (300psi x sf5)

Making the assumption that your tube is 2" on the od, 3.14*.935*.935 = 2.75in2 x 1500psi =4125 lbs force...a good bit of force.


Now you know the force that needs to be held back, lets find out what kind of bearing area we need. Note from material properties that bearing yield strength (By) is 56000psi.

lbs force = By * area

4125 lbs / 56000psi = .07366 in2

.07366 in2/ .065" =1.133 in

Typical fastener might be low head socket head cap screw with a head diameter of .312".

1.133"/ .312" =3.63

We would need a minimum of 4 of the 10-32 screws with a head diameter of .312" each.


Also note that under bearing yield, edge distance/ pin diameter = 2. The holes should be a minimum of 2x their diameter to the edge (tube end). Typical is center of the hole to edge...using edge of the hole to tube edge adds safety factor and is IMO good practice.

Screws...need some screws.

Minor diameter of a 10-32 screw is... .1517"

http://www.carbidedepot.com/formulas-tap-standard.htm

A pretty good screw to use:

http://www.grainger.com/Grainger/UNBRAK ... crew-5PRV9

Calculate area from minor diameter.... .01868in2

Multiply this by 60% of the tensile strength... .6 * 150000psi =90000psi

.01868in2 * 90000psi = 1681.2 lbs force...this is the force required to cause a shear failure in a single screw.

4125 lbs force / 1681.2 = 2.45 screws required to handle the force (this IS still at 5x our max WP) 3 screws gives us our sf of 5, but we need 4 to handle the bearing stress....we use no less than 4.

To get a reasonably flush fit with the screws, it is necessary to recess the screw heads into the plug slightly.

Hope this all makes some degree of sense...if not, let me know what needs clarifying.

Regards,

[grumpyoldman]

Thanks for all the useful info, Gippeto, especially how to calculate force, bearing area and shear failure.

There is still one thing I'm a bit hazy on. You say you can slightly recess the screws in the plug but don't you screw through the tubing first then into the plug? So the head of the screw will be touching the tubing? Sorry for being so dense.

[Gippeto]

The hole in the tube is the same size as the head of the screw...this allows the actual head of the screw to bear on the wall thickness.

If you just drilled a hole in the tube for the threaded part of the screw, the diameter would be MUCH smaller, and the bearing area reduced...and thus bearing stress (lbs force per unit of area) increased.

You can recalculate for that situation if you wish to, but bear in mind that a thread bearing on the tube wall does not present much surface area as they are essentially pointed...this creates very high lbs force per unit area...I would expect the threads to sink into the aluminum.

[grumpyoldman]

I think I know what you're talking about now. The screw head secures from side to side, that's why we needed to know the shear strength of the screw. Now I really feel dense.

Zoro Tools has a hundred priced at $10.63. A $40.00 difference. The description looks almost identical to the ones at Grainger except for tensile strength.

Zoro Tools description: Class 3A, Rockwell Hardness C39-C44, Min Tensile Strength 135,000 PSI, Meets ASTM A574/ANSI B 18.3
Link: http://www.zorotools.com/g/00090492/k-G2023034/

Using the same numbers we've been using, it would only change the amount of screws needed from 2.45 to 2.72.

They also have the same brand as Grainger but for $27.72 for a hundred. Around a $20.00 difference.
Link: http://www.zorotools.com/g/00092351/k-G1622853/