Dimensioned Drawing of Bolt?

AR/M4 AR Technical Discussion
1

My curiosity has gotten the better of me wrt some of the stresses the AR bolt sees on firing. I want to run a few hand calculations followed possibly by some FEA to see the location and magnitude of the stress concentrations on the bolt. I’ve already got a good SWAG at where they’ll be but I want a little more detail.

Problem is I can’t find a dimensioned drawing that either doesn’t suck, of unknown provenance or both. Anybody know where I can find one?

BTW can you tell things are slow at work right now?

2

You have the like 8 page pdf with the whole carrier? I can email it if you want. Carrier itself is for the AR they use in the tanks or something, but the rest of the dimensions apply.

3

I’ve only been able to find smudged up crap that you can’t read all the dimensions and treatments.

4

I think you can make out the dimensions, I can give you a transcript of the NOTES if you need them.

5

I would be very interested in seeing an FEA analysis of the bolt. Its complex, asymmetrical shape (and asymmetrical loading) would be quite challenging to analyze by hand.

There was a post a while back quoting some armory researcher (IIRC) as saying that the fatigue cracks which cause lug failure often originate at tiny, stress-concentrating corrosion pits at the lug root. It would be nice to know the stress level before it is concentrated by surface defects.

Perhaps I could supply some measurements of the LMT enhanced bolt, and your FEA mesh could be modified to reflect them. This would give an idea of the efficacy of the changes LMT made.

The diameter increase in the area of the cam pin hole that ARPerformance made could also be analyzed.

6

Comparisons to different geometries would be good.

I’ve gotta start small and work my way up though. Hand jam some simplified numbers to give me some sanity checks and go from there. It’s been nearly 10 years since I’ve had to do structural analysis on anything so I’ve basically gotta reteach myself to do it. That’s what I get for becoming a MS Word Engineer instead of a real one I suppose.

Hell, before I even get to FEA I’m gonna have to draw the thing up in CAD, which is a whole other can of worms. I may get down in the weeds and decide I’d rather go fishing or ride the tractor or something lol…

The other thing that occurs to me is that static analysis stands a fair chance of not even giving accurate results. The time from firing pin hitting primer to peak pressure and back to ambient is so short, you can basically consider it an impact load which is a much more complex problem.

7

https://www.m4carbine.net/archive/index.php/t-63182.html

8

Ho-lee Toledo. Many thanks, unclemoak. I never get the limits of this forum.

In an impact loading, we know the momentum and energy of the “hammer” and (as you say, nova3930) it’s a problem to calculate the forces. But here, even though the forces have a fast rise time, we know the load at every point of time because we know the chamber pressure curve.

And I don’t think that a dynamic analysis is necessary, because the 100 microsecond time scale of the load is long compared with the deformation time scale of the system. But that’s an intuition, and of course it would be nice to really know.

9

Unclemoak, that picture says “Fig. 4a”. Is it from some kind of article on AR15 analysis (I hope, I hope) ?

10

Pictures and thread or both interesting, especially the report that was posted.

11

Again, I’m digging back nearly a decade, and it may be different but from what I recall of my coursework, even knowing what the impulse looks like, you still have issues because of the rate at which the materials can react to the impulse. The gist of what I remember is that even if a structure can handle a static load, if you apply the same load dynamically fast enough, the stresses will be locally higher due to material reactions and could result in a failure.

12

You’re right that dynamic loading can increase stress, but only if the system doesn’t have time to reach equilibrium as the load is applied. My intuition is that the lug roots will be in equilibrium on the time scale of 100 microseconds which is the scale of the chamber pressure rise.

My sanity check here (as you aptly put it in the original post) is to imagine plucking the lugs like a tuning fork in the fore and aft direction (find the resonant period). I think the frequency you would get would be much higher than 10,000 Hz (a period of 100 microseconds). If the load is applied at a much longer time scale than the resonant period, a dynamic analysis shouldn’t be necessary.

It’s fun to discuss these issues as applied to our favorite machine.

13

It’s a moot point anyway. Reading into the lit a bit, I think a dynamic analysis is at the least beyond the resources I have available to me and possibly even beyond my skills as an engineer. Reading more into the link that was posted earlier they’ve already gone deeper than I probably can in that analysis.

I will at least claim victory in that my SWAG of the stress concentration areas was correct.

14

Just to address some of the things brought up on the other thread:

The explanation I have heard on the M-4 that makes the most sense is as the gas port erodes, the bolt is still unlocking(unlocking sooner because the bolt carrier pressurizes quicker) while under substantial residual pressure and this loads the lugs in torsion instead of compression. As the lugs were not designed to endure a twisting motion, this tends to break the two weakest lugs first, the ones beside the extractor.

The rifle length does not suffer from port erosion, the port is further down the barrel and the gas tube is longer for the trip back too with more volume to pressurize at a much lower pressure. It takes several milliseconds longer for unlocking to begin and residual pressure has dropped almost to nil before the bolt begins to open.

The gas from the port has to travel from the port to the bolt carrier cavity. The time it take depends on the size of the port, the diameter of the gas tube and the distance it has to travel. For a rifle length with a standard diameter port it takes about one milliseconds to built to maximum pressure and the carrier doesn’t start to move any appreciable distance (0.050") for another millisecond. By this time the pressure in the chamber is low enough that the bolt thrust is less than the piston thrust and the bolt is actually being pushed into the chamber. On a rifle the peak pressure in the carrier cavity occurs about a millisecond after shot ejection.

In a carbine length gas system, the time to peak pressure is only about .75 that of a rifle, but the cavity pressure tends to be higher, especially if you are running a 16" barrel. The bolt is still essentially unloaded at time of unlocking, but the bolt velocity is higher.

When the gas port erodes and all of them do, the carbines do it faster because of the high pressure and hotter temperature gas, the flow through the port becomes more efficient, and the times decrease and the pressures increase. You will never have much bolt load at the time of unlocking because the time to get the gas back to the cavity, even with an eroded carbine gas tube, is still very large compared to the when the P-T curve starts to drop.

The bolt velocity causes a majority of the problems.

As to why the Army is just now noticing an increase in bolt failures is related to the 1999 change to the ammunition specifications raising the average maximum chamber pressure of M855 Ball and M856 Tracer from 55,000 psi to 58,700 psi and the absolute maximum from 61,000 psi to 64,700 psi.

15

This study

http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf

shows about .25ms between the expansion chamber pressure reaching its peak and carrier motion of .050" (by comparing Fig. 4 with Fig. 5).

What is your source?

16

http://www.dtic.mil/dtic/tr/fulltext/u2/704342.pdf, http://www.dtic.mil/dtic/tr/fulltext/u2/880431.pdf, and the on you linked to.

From the linked report:

The bullet passes the gas port at T = -0.25.
Peak pressure in the cavity occurs at T = 0.625.

Time from uncovering the gas port to peak pressure:

0.25 + 0.625 = 0.875 or approximately 1 ms.

Peak pressure occurs at T = 0.625.
Bolt carrier motion 0.050" at T = 1.125

Time from peak pressure to 0.050" bolt displacement:

1.125 - 0.625 = 0.5 ms I was off by a half a millisecond.

17

Also, you can model the gas system as two orifices, one for the gas port and one for the gas tube. It’s pretty accurate for rifle pressures as you can adjust the orifice coefficient to match the experimental data. For other lengths, it probably has some errors, I have not compared the two orifice model applied to carbine length gas tubes to the model used in the above referenced reports calculating carbine length gas systems.

For timing in the two orifice model, I assumed choked flow through the gas port and tube (gas velocity does not exceed Mach 1) and a restriction coefficient of 0.62 which, again, matches experimental data for the rifle, but might not be totally accurate for other lengths, but I think it is pretty good

18

It’s odd that in Fig. 4 of the same paper, if we subtract the receiver motion from the carrier motion, we get a relative displacement of .050 at t = .85ms or thereabouts.

Your geometrical constructions for interpolating the graph were great. I usually use my thumb and forefinger.

19

The gas velocity may be less than Mach 1, but since the bullet passes the port at -.25ms, and the carrier pressure 1 foot away starts rising at t=0, the shock velocity must be about Mach 4.

20

It’s still about a half a millisecond, give or take a few microsenconds.