## Large Threaded Inserts

Dear Y'all

I have a large pressure vessel 12'' ID 1.5'' wall that I use for combustion experiments (UCF). The cylindrical vessel has two inserts on each side that are attaches through large threads (12''-4 UNC), 12 inch nominal diameter an 4 threads per inch, about 2.5'' deep. I want to calculate the maximum axial load (pressure in vessel) the material is 7075 Aerograde aluminum. Hope someone can help me.

Kindest Regards, Jaap
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 Blog Entries: 7 Recognitions: Gold Member Homework Help Science Advisor Hi Jaap, Can you provide a drawing for this? Please show as much detail as possible. Also, what temper is the 7075? Is there any cyclic stress on it?
 Recognitions: Science Advisor 12-4! Wholly molley. That's a big thread. Cool! To add more questions, since this is a combustor test, what is the expected temperature range? Even the 7000 series has a pretty steep temperature degradation curve. Also, are these threads manufactured by a company like Rosean or Fairchild or are they your manufacture? How are they inserted and locked in place?

## Large Threaded Inserts

The threads are standard Unifified Course. It is not a combustor. I will monitor a freely propagating flame using a High speed camera so I don't expect high temperature effects (only for a very short time). The design was not mine but I had it made.
Threads are made by a standard (good) machinist. Since the ASME boiler and pressure vessel code manual does not advise such large hreads it is hard to find inormation. I just need to know within a certain factor what strength to expect.
See attached PDF
Attached Files
 Document1.pdf (194.3 KB, 17 views)
 Recognitions: Science Advisor It's tough to tell from your print, so I'll ask for a couple more details. I need to make sure that this is what you are doing: - Aluminum pressure vessel with pilot ID's on both ends. - Steel threaded inserts inserted into the ends (How are they retained? Threads? Press fit?) - End caps screwed into threaded inserts. Does this represent what you are doing or is the vessel end cap being screwed directly into the aluminum housing?
 Goest I think you may have underestimated the strength of 7075 AL This is what I got from Matweb http://www.matweb.com/search/Specifi...snum=NALIMEX06 Tensile Strength, Ultimate 360 - 540 MPa / 52200 - 78300 psi Tensile Strength, Yield 260 - 470 MPa / 37700 - 68200 psi This puts the pressure rating over 5000 (ASME BPV VIII div 1 UG-27) . It needs to be about 3500. The holes on the top are NPT fittings going into swagelock plumbing. Th vessel is seemless to. The Threaded inserts are threaded straight into the vessel. Goest you are correct the inside diameter is more like 11.75. There are two 8''OD 2.5''thick fused quartz windows clamped between two gaskets. there is a endplate that bolts into the threaded insert. There is a steel reinforced, concrete filled cinderblock blast wall that surrounds the vessel. I will do a hydrostatic test according to ASME BPV section 5. Any advise? How do I calculate the threads load capability? That is my biggest issue. Thanks for all y'alls help guys Who can stamp this vessel for me?
 Recognitions: Science Advisor The shear of the internal thread is going to be, most likely the weak spot of the assembly. In that case, the maximum shear force can be calculated through the following: Shear Area: $$A_{s}=\pi*d_{min}*n\left[\frac{1}{2n}+\frac{1}{\sqrt{3}}\left(d_{min}-D_{2,max}\right)\right]$$ where $$A_s$$= Shear Area $$d_{min}$$= Minimum Major Diameter $$n$$= Threads per Inch Engaged $$D_{2,max}$$= Max Pitch Diameter of Internal Threads The calculated shear force for the internal threads is then: $$F_{max}=\frac{1}{2}S_t*A_s*L_e$$ where $$S_t$$= Shear Strength of the Material $$A_s$$= Shear Area from above $$L_e$$= Length of Engagement One note I would impress is that the design criteria is for threaded fasteners and the UN series threads (FED-STD-H28). I can not comment on the applicability of this to such large threads or something that is not, technically, a bolt or nut. It should get in the area you need to be though. With an ample safety factor you should have your bases covered. BTW...those numbers look good for 7075 Al. Just remember what I mentioned in regards to the temperature effects on the material properties IF you ever go to a higher temperature.
 Ok lets try this; n = 4 St = 37700 psi L = 2'' (Female Threads) Effective pitch diameter = 12.0 in Pitch, p = 0.25 (4 threads per inch) Minimum major diameter, d = 12.16238 in Max diameter, D = 11.83762 in L = 2 in Then from equation (1) A = Pi * dmin * n * [1/2n + 1/Sqrt(3) * (d-D)] A = 47.76 in^2 Then from equation (2) Fmax = 1/2 * S * A * L Fmax = 47.76 * S = 1.8 Mlbf When I cannot use the full length of engagement (see Goest's comment) then: L = 3/4 Fmax = 0.675 Mlbf This last number would result in a pressure rating of about 5970 Psi. Does this look OK?
 Recognitions: Science Advisor I don't have a good gut feel for threads that large. But it does look like you have it right. Just double check your units. I would then divide that final pressure by a generous factor of safety before you go any further. I would also yield to Q for any other recommendations. Pressure vessels are his neck of the woods.
 Blog Entries: 7 Recognitions: Gold Member Homework Help Science Advisor Hi jaap. Regarding the strength of the material, per ASME code, you should be using the allowable stress, not the yield strength. I had a brief look through Part D (properties) but without having the ASTM number, it’s hopeless. Section D doesn’t use common terms such as 6061 or 7075 since these are too vague to pin down properties with. They use the ASTM number and various types/grades and UNS numbers to properly identify the specific type of material. Once the material properties are known, they apply a safety factor to them and say, “don’t exceed this stress level”, which they call the “allowable stress”. However, I can find values for 6061-T6 in the piping code (ASME B31.3) which is around 12 ksi for allowable stress. The BPV values for 7075-T6, if you were to identify the material, would be very close to the 6061 values in the piping code, which is how I arrived at the values for stress allowable that I gave above. In this case, you’re interested in allowable shear stress. Shear stress is about 2/3 of yield, so don’t get confused. See for example: http://www.roymech.co.uk/Useful_Tabl...ar_tensile.htm So I’d suggest using about 8000 psi as an allowable shear stress. I’m not exactly sure how the Code handles shear stress on threads though, I’m not heavy into pressure vessel design and usually the shear stress on bolt threads is not the limiting factor, it's the tensile area. All this gives you a very conservative pressure containing component of course, and an absolutely miserable value for pressure. However, that’s what the code would tell you to do. As for who can code the vessel for you, that’s impossible. It has to be designed, built and tested by a National Board certified manufacturer. If you’re intent on using the vessel, I’d suggest as a minimum, doing a hydrostatic test on it. Pressurize it with water to 2 times the pressure you intend to use the vessel for. Hold the pressure for at least 5 minutes, and I’d suggest raising the pressure in 10% increments till you get to this hold pressure. Once the pressure test is complete, disassemble it and verify dimensions haven’t changed (which would indicate yielding of the material). You shouldn’t have any leakage either of course. Either would be cause for rejection.
 There is a big difference between in 60 and 70 series in Al thats why we picked it out. How much water should I use for a hydrostatic test and how dangerous is that? Jaap
 "I’m not heavy into pressure vessel design and usually the shear stress on bolt threads is not the limiting factor, it's the tensile area." I am not sure what you mean with this?

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Hi jaap,
 There is a big difference between in 60 and 70 series in Al thats why we picked it out.
I just checked, and you're right. There's quite a difference, I thought they were closer. 7075 is almost twice the strength of 6061.
 How much water should I use for a hydrostatic test and how dangerous is that?
You just need enough water to fill the vessel and any piping, then a small, high pressure hand pump or booster pump (Haskel booster) to get it up to the pressure you're looking for. It's a whole lot less dangerous than a pneumatic test. I personally wouldn't be too concerned about standing next to it, given the test was conducted properly. Worst case is something ruptures, but with only water inside, there's very little energy release. You get a crack, a little deformation, and that's it. ASME code prefers the hydraulic test over a pheumatic one and puts different factors in depending on the type of test.
 I am not sure what you mean with this?

Regardless, the biggest concerns for your vessel are the shear stress area of the thread, hoop stress in the cylinder and bending in the head. Bending on the head can also be calculated using either ASME BPV code calcs or you can use Roark's.

Remember you don't want to use yield strength, you need a safety factor of at least 1.5 on yield and 3.5 on ultimate. Of course, you'll need to find out the temper of the material used to figure out the strength of the material. That's another missing piece of information.

Another bit of missing information is how the pressure is going to be developed inside this and what the maximum pressure it could see is given a failure of other components that create this pressure. When considering what pressure you need to rate a vessel at, you need to consider the system it is going into. It would be nice to understand how you are safing this vessel too, do you have relief valves on it?

- At the very least, I'd suggest doing the hydrostat test on this, and because no stress analysis has been done, I'd suggest doing it to 2 times the intended MAWP (ie: relief valve set pressure or maximum possible pressure the vessel could be exposed to). If this vessel is going to be used more than about 1000 times, fatigue is an issue (especially for aluminum) and I'd suggest doing this test on a regular basis (or just tossing it out).
- A better option would be to have someone go over the design and calculate stress and THEN do the hydrostat test.
- The best option is to ditch the entire thing and have an ASME certified company make you a stamped vessel. They'll do the hydro test to certify it.
 The thing at a University is that we would like to build our own stuff. We do have a burst disk that can be set at a pre determined pressure. The maximum pressure is created by a single combustion event of the gas inside. (Like a single piston engine stroke). There is no other system component that could cause the pressure to rise even more (which is good). For the hydrostatic test is it ok to fill it with water and then to top it of with a unregulated air bottle through a needle valve? At the event of rupture the needle valve opening will immediately choke. The material is 7075-T6 Thanks for all your help!! Ever have a question about shock waves make sure to let me know!!
 Blog Entries: 7 Recognitions: Gold Member Homework Help Science Advisor I can understand you'd like to build this yourself for economic and other reasons, but pressure vessels are governed by ASME code and almost all states have laws requiring such vessels to be coded. If you have a burst disk set at 3500 psi, I'd suggest you do the hydrostatic test at 7000 psi. The highest pressure you can get in a cylinder is around 6000 psi, so you might be able to use that but I wouldn't recommend it. Putting any kind of gas into the system defeats the purpose of doing a hydrostatic test, regardless of how you might control it. See if you can get a manual hand pump. Grainger has hydraulic hand pumps for as little as \$264 that will go up to 10,000 psi. Don't know if it's good for water, but if you have to use oil, no big deal. Another option is to have someone do the pressure test for you. Try calling one of these pressure test outfits: http://www.thomasnet.com/nsearch.htm...sec=prodsearch