Machine Screw Pull-Out Capacity

[Gerrha]

I am a retired scientist and off/on for many years I planned to build a small liquid fuel rocket engine. My son recently got me interested again so I am back working on the project. This engine will never leave the ground. It is for fun and also for my son and I to learn many new things along the way. As a starting point, the design will have a combustion chamber with a round injector plate on the top. They will be bolted together. These parts are fairly small, less than 3" in diameter, so the screws also need to be small as in #4 through maybe #8. Our current design would use 8, 6-32 socket-head cap screws with a thread engagement of 1/2". The material is copper, and the chamber pressure is 500-psig. The thing will be operated remotely, so if it blows up no harm will be done, but the point of the project is to learn what is required for a proper design. This bolt arrangement has become a question for me. It looks good on paper, but from an engineering consideration, is our bolt arrangement adequate or not?

I have searched many sources and cannot find this type of information. However, it must be out there somewhere. Could someone please help us with this?

 

[dgeesaman]

It's hard to give crisp answers because I can't visualize what you're showing.

The primary questions are:
1) will the applied load put the screws be in shear or pure tensile/tear-out?
2) how flat and consistent will the mating surfaces be under the screws?
3) what strength is expected for the screw material and for the copper?
4) when you say 1/2" thread engagement and screws, those keywords have me visualizing a tapped hole in solid copper that is deeper than 1/2" and has the screw engaging 1/2". Please confirm this is correct as opposed to a screw/nut setup.

I can offer that for standard steel bolts screwed directly into tapped holes in soft materials like copper, I would aim to have (1.5x to 2.0x screw diameters) of thread engagement. This follows rules of thumb that say the full tearout strength is reached in 1.0-1.25x for high strength materials and more engagement is required for lower strength materials. 1/2" seems like more than enough there.

 

[Gerrha]

Thanks for your reply. Your questions are all things I have thought about as well and this is what I can answer.

1. Pure tear-out tensile. There should not be any shear.
2. The surfaces will be flat and very smooth because there will be o-rings involved to keep the combustion gases where they are supposed to be. The surface finish will be specified at Ra 16.
3. The screws planned as of now, will be stainless steel A286 with 160K psi tensile strength and 120K psi yield strength. I have assumed 10K PSI yield strength for the copper.
4. You are correct in that the plan is for blind tapped holes in the solid copper injector plate that are deep enough to provide 1/2" thread engagement. This depth can be adjusted if needed. Also, the tap used will be selected to provide 75% thread. For the planned 6-32 screw, this is a tap drill of 0.1065" diameter.

Thanks!

 

[lucky-guesser]

I had a similar conundrum a few years back on a project, couldn't find any data so just ended up having to do my own tests.

Agreed with dgeesaman, if you have 1/2" engagement I would suspect that would be plenty. Wouldn't surprise me if you broke the bolt before getting it to pull out. Assuming pure tension loads of course.

 

[Gerrha]

There must be some sort of guidance out there.

There is an o-ring seal between the combustion chamber and injector plate. The OD of that o-ring is 2.637" and the chamber pressure at maximum engine capacity is 500-psig. That will equate to about 2750-lbs of force pushing those two plates apart. There must be some way to determine the number of 6-32 screws (4, 6, 8, 12?) that will be safe under those conditions or maybe not safe at all and a larger screw is needed.

 

[Stick.]

Screw thread Calculations - Roy Mech

roymech.org

That covers the basics of calculating the capacity of a screw and tapped hole. I can't say if copper requires any special considerations not covered there, but that may be worth looking into.