Tapped axial holes on tube

[sleepdrifter]

I have to design a fixture that is a roughly a 23" ID tube w/ a 1" WT that will have a tapped bolt pattern around the flat base portion of the tube. I've been trying to find how I can calculate what kind of stress would cause the part to fail. My biggest concern is that I'm aligning with 1/2" clearance hole BP so I'm aiming to use 1/2"-13 tapped holes and the slimmest margin I have from major diam of tapped hole to my part OD is roughly .175".

What sort of equations should I be looking at to confirm that I'm not going to shear the threads off since I don't have much material between the tapped hole and OD of the part? Less worried about shearing threads off and more so a major deformation of the thread area due to a thin wall thickness. I've abided by the rule of thumb 1.5D of wall thickness for steel but how can I mathematically determine if my design is prone for issues?

 

[sleepdrifter]

@MintJulep

With a 1/2-13 bolt, using a lubricant with a k-factor of 0.18 and a torque of 80 ft-lbs I'm looking at a clamping load of about 10,698.76 lbs. I'd risk pullout at any thread engagement length less than 0.75in. Ontop of this, I need to factor in the load on the bolts from the pressure vessel, right? Which is roughly 1,000lbs. Can you enlighten me on any concerns with the fasteners having such a high strength for their application? Aside from being able to cut cost on less strong fasteners, what am I missing

Thanks for pointing this out.. up to this point in my career I've been so used to just pulling torque from a torque table and calling it a day (working on extremely overengineered things where failure wasn't much a concern). I'm now engineering things that if they fail could cause serious damage which is why I'm trying to learn how to properly conduct analysis on these designs.

@goutam_freelance @Stress_Eng

Thank you both for the detailed analysis. I'm going to digest this as I have time to so I can understand exactly what's going on in this analysis. I'll also conduct a simple FEA to the best of my ability to see how it lines up with what you two have provided.

 

[LittleInch]

There seems to be a thought here that your bolts are actually being torqued up to create a clamping force. Your design needs zero clamping force as the sealing is being done by your o ring surely?

You could hand tighten the bolts and it would be fine.

 

[sleepdrifter]

There seems to be a thought here that your bolts are actually being torqued up to create a clamping force. Your design needs zero clamping force as the sealing is being done by your o ring surely?

You could hand tighten the bolts and it would be fine.

You know, I've never had this properly explained to me and I've been reading up on old textbooks in my spare time to try and find the answer. For an axial load, do I really need a clamping load? In my head I need to resist the bolts backing out, but that's about it. Please help me understand.

 

[MintJulep]

Can you enlighten me on any concerns with the fasteners having such a high strength for their application?

There is a big mismatch between fastener strength, tapped material strength and joint capacity vs. load. That's not in and of itself a problem, but in the context of the overall design it's leading to a sub-optimal outcome isn't it?

Your entire design seems dictated by external constraints, and not functional needs.

Q: What size fasteners do you need? A: I'm given 1/2" holes.

Q: How many fasteners do you need? A: I'm given 26 holes.

Q: What' wall thickness do you need? (your original question) A: I'm given customer geometry that forces holes towards the edge.

You explicitly stated that you don't want a broader design review, but...

1. Why do you need an inch thick shell for 20 psi?
   1. Why can't use use a much thinner shell with a formed flange for mounting?
2. How do you imagine making the shape that you've drawn?
3. Fundamentally, it's a pressure vessel.
   1. Where's your safety relief?
   2. Why do you need a flat top instead of the typical (for good reasons) elliptical top?
4. See other member's comments about the o-ring seal.

 

[MintJulep]

Thank you both for the detailed analysis. I'm going to digest this as I have time to so I can understand exactly what's going on in this analysis. I'll also conduct a simple FEA

1. A few scribbles on a post-it show that the hoop stress is trivially small, and thus the deflections and bolt bending will also be trivial.
2. Given any choice at all, nobody would purposely design a pressure vessel with a flat end. Your seemingly arbitrary choice of shell thickness bludgeons the stress reasons for this into submission. But at the same time, you've implied that lighter would be better.

This thing is so far from efficiently using material that an FEA is a real waste of effort a this point. You'll get a pretty picture that 98% blue with 2% red, and the red will be entirely dependent on applied boundary conditions (bolt preload), not global loading.

 

[Stress_Eng]

Preloading has a big part to play in fatigue. Does fatigue need to be considered? Does the part see any cyclic loading? Preloading is also to stop the joint from gapping. Sufficient gapping will alter bolt loading (all bending on bolts, no load sharing in joint).

Edit - When it comes to hoop stress, I think you may have three components to consider. One is hoop stress in cylinder due to circumferential strain, and the other two are from bolt loading, consisting of thread radial load and, if analysed as a socket, local socket wall hoop stress due to peak distributed socket load.

 

[MintJulep]

Why preload is important.

Why bolt preload is important