Lug setback? What am I missing?

[Mram10us]

In my little brain, I think of sending a 230 otm at 3k FPS out of a rum and a 300 Norma. It seems the lugs, being the same size will feel the same amount of force from each.
I might be misusing the term force. I'll study up. I'm not trying to pretend to be an expert, just expressing my thoughts based on aeronautical experience. Weight, lift, drag and thrust make sense to me, but I'm sure to be misusing some terms we've been discussing. Appreciate the debate

 

[birdiemc]

In my little brain, I think of sending a 230 otm at 3k FPS out of a rum and a 300 Norma. It seems the lugs, being the same size will feel the same amount of force from each.
I might be misusing the term force. I'll study up. I'm not trying to pretend to be an expert, just expressing my thoughts based on aeronautical experience. Weight, lift, drag and thrust make sense to me, but I'm sure to be misusing some terms we've been discussing. Appreciate the debate

What's got me scratching my head is the fact that we used the same formula, I just converted to Newtons and ended up with completely opposite answers

 

[Mram10us]

Let me try one more time. Since the lugs did not change but the area of force on the bolt face did. Then there is a increase of force on the lugs. Image if you increased the area of the bolt face to a 50 cal would you expect the same lugs on a 223 to hold a 50 cal. Obviously no. Even though the pressure would be about the same.

Cohunt is exactly right and has done a very good job of explaining the difference in Force and Pressure. My hat is off to him. Sometimes we need to try and understand instead of arguing. Kirby Allen knows more about this and has more personal experience than most of us. After all his reputation depends on it.

I agree with you, but it isn't arguing to debate a subject. That is how we all learn. Never take advice from others until you back it up.

 

[Mram10us]

What's got me scratching my head is the fact that we used the same formula, I just converted to Newtons and ended up with completely opposite answers

I agree the psi on the bolt face would be different per square inch, but shouldn't the lug size be included somewhere in the formula?? My head hurts...

 

[birdiemc]

I just checked all Cohunts math and I checked mine again and figured out we said the same thing. There is more actual force exerted on the large bolt head than the smaller one.
Its because you are not placing a 60,000 lb
weight on the bolt, you are applying 60,000lbs to every square inch on the bolt...so back to your table example you're not placing a 1000lb weight on top of the 4ft square table, instead you're placing one 1000lb weight on every square inch of the table, which would effectively transfer 2,304,000lbs to the floor, vs a 1ft table which would only be 144,000lbs transferred to the floor.
4ft x 4ftx 144sq in per sq ft= 2,304,000
1ft x 1ft x 144sq in per sq ft= 144,000

 

[birdiemc]

I agree with you, but it isn't arguing to debate a subject. That is how we all learn. Never take advice from others until you back it up.

I rather enjoy these discussions, you always keep me doing math...last time I had 3 sheets of papers with formulas all over, my wife just looked at it shook her head and walked out the room lol

 

[cohunt]

I accidentally sent before I meant to.
Since you are talking about force, force is measured in newtons. So take 60,000 psi, that equals ~414newtons/mm^2.
.585 bolt face has area of .269 square inches or 179mm^2
.470 bolt face has area of .173 square inches. 112mm^2
Force=pressure x area
414 x 179 = 74106 newtons applied to .585 bolt face

414 x 112 = 46368 newtons of force applied to .470 bolt face

Hmmm I set out to back up what Cohunt was saying cuz it made perfect sense, but the math says otherwise

What do you mean? Isnt 74368 Newton's more than 46368Newton's? ?
In fact it backs up my numbers as 74368 is about 1.6x more force than 46368... and my 16800 pounds for the large bolt face is about 1.6x the 10700 of the smaller bolt face
Psi is actually pound force per square inch so it is a force per square inch measurement
We are just using different units--- BUT I think your math is wrong as
1 psi = 6895 n/m2 so 60000psi would = 413700000 nm^2

 

[sedancowboy]

OK Cohunt, Birdiemc and I have said the exact same thing. Birdiemc you did not come up with a different conclusion you simply stated the larger bolt face first instead of second. But we all came to the same conclusion that a .588 bolt face exerts about 60% more pressure on the lugs than a .473 bolt face. Be it in Newtons or PSI. The lug size is figured in to the formula the whole point of this discussion is that the area of the bolt lugs is the same size but with different forces on them because of different size bolt faces.

 

[Mram10us]

I rather enjoy these discussions, you always keep me doing math...last time I had 3 sheets of papers with formulas all over, my wife just looked at it shook her head and walked out the room lol

Ha. I have too many questions and would like to have more answers

 

[codyadams]

I agree the psi on the bolt face would be different per square inch, but shouldn't the lug size be included somewhere in the formula?? My head hurts...

It's hard for me to make sense of it too.....if I hadn't seen the first hand result from other people, I would have a hard time believing it, honestly when I first thought of it without doing research, I figured bolt thrust would be LESS, due to being spread over a larger area.....however, the term "psi" somewhat helps me understand....as there is more surface area for the forces of expanding gasses to be imparted onto the bolt, thus being transferred to the supporting areas of said bolt, the lug abutments.

I think the size of the lugs would be a helpful number in a calculation, if you were trying to figure the point at which the lugs would fail, or be "set back", which would also include the amount of supporting material, the materials composition, rockwell hardness, density, and so on....I have absolutely no clue however on what to do with that information to calculate the yield strength....

 

[birdiemc]

What do you mean? Isnt 74368 Newton's more than 46368Newton's? ?
In fact it backs up my numbers as 74368 is about 1.6x more force than 46368... and my 16800 pounds for the large bolt face is about 1.6x the 10700 of the smaller bolt face
Psi is actually pound force per square inch so it is a force per square inch measurement
We are just using different units--- BUT I think your math is wrong as
1 psi = 6895 n/m2 so 60000psi would = 413700000 nm^2

I took it to n mm^2 to have smaller number to work with. I never took physics just learned what I could through self education and never really encountered pound force so dont understand it as easily as newtons.
I should have deleted my post about scratching my head...cuz after redoing your math and mine I realized we both said the same thing....too much physics/math got me all confused

And I forgot to mention I used 3.14 instead of PI button, couldn't find it on my phones calculator...so my numbers aren't exact, mere approximations

 

[Mram10us]

Assuming the above, what kind of chamber psi does it take to push a 230 at 3k psi from each cartridge?

 

[cohunt]

I took it to n mm^2 to have smaller number to work with. I never took physics just learned what I could through self education and never really encountered pound force so dont understand it as easily as newtons.
I should have deleted my post about scratching my head...cuz after redoing your math and mine I realized we both said the same thing....too much physics/math got me all confused

But nm^2 would be a larger number not smaller. See my post above where 1psi =6895nm^2
Nope, you were right I was reading nm^2 not nmm^2, I misread your units

 

[David Emerson]

A member brought up reading about lug setback from Kirby Allen. He explained that a lapua sized boltface :
1. Doesn't show signs of over pressure till it's too late because of tougher brass and what I would assume would be the more area on the case head to absorb
2. the lugs feel MORE energy due to the bigger boltface

The first one, I will agree, makes a bit of sense since it's a bigger surface area, however, the ejector is still the same size. The second makes ZERO sense to me. What does boltface size have to do with lugs and lug abatements? The rearward force on the bolt should be the same regardless of bolt face size since the bolt lugs and abatements are the same size. What am I missing?

You left some smart remarks to me on a different topic. PRESSURE VS AREA DUH