I wanted to know the force for each common spring at the L1 length of an AR carbine, so I put a digital scale on my mill and adjusted it so that there would be 6.902 space between the scale and the head. I then inserted each spring between the scale and mill, inside a clear tube.
I measured the wire diameter of each and the OD. I counted the number of coils. I input them all into spring design software, and it estimated the stress as a percent of the material’s tensile strength. I then sorted each spring from least to most stress while at full recoil. The lower the stress as a percent of MTS, the less likely it will be to fatigue (take a set). If you see any mistakes, please let me know.
Just to clarify, according to that data, there is no difference between the stress developed in 17-7 (Colt) springs and CS (Brownells) springs of the same geometry?
Reason I ask is that I’ve seen colt carbine action springs get noticeably weaker shooting a steady diet of M193 from a 6920 in as little as 3,000 rounds (A-B’d with a fresh spring and the 5-person group vote was unanimous that the new spring felt ‘stronger’).
I know Gunz has had very good luck using the Springco Blue, which I use currently as well. I think he’s at or past 10K rounds on hs Springco Blue in an SBR.
I realize that the above are calculations and I’m not disputing them; I’m NOT an engineer. However, is there a source for real-world testing on this?
I’m not one of the fanboy’s who thinks that he can get 100K or even 50K compression cycles from his action spring before replacement…but the CS springs from Springco do seem to last longer than the 17-7 Colt springs I and my fellow LEO’s have used before.
I’m interested to hear what you have to say…and thanks for taking the time to compile all this data.
That is pretty much what it is showing. It actually thinks CS and Music Wire are virtually identical, and 17-7 is a little better than either (if given the exact same inputs). I was not aware there was a case where 17-7 was better, but maybe it has to do with the Wahl stress correction calculation for this particular geometry. It sure seems like the original designers knew what they were doing.
Let’s assume that is true (and I would not accept it as true without the spring going on a scale setup like mine every 500 rounds and then a curve plotted), then one of two things may be happening:
One - it has so much force that the recoil of the BCG is not able to compress it as much, and so it is not stressed as much. And if one liked that effect, they could just make a 17-7 spring with more force.
Or two - maybe there is something to CS, such as being more shock resistant than 17-7. I did read that music wire and CS are both the top choices for shock resistance.
One would have to put it on a scale like I did when new, and then retire it when it reads 5.3 lbs - and then see how many lbs it dropped per 1000 rounds.
I actually do this to decide when to throw away a spring. I just threw away a CS one which was at 5.3 lbs. I am not sure what it started at though, so I have no proof it dropped a lot.
Out of curiosity how is the machine not just reading force applied (weight) instead of stress? what formula is the software using to compute the percentage? when taking the OD and wire diameter did you take multiple measurements over the length of the coil and then average them when compiling inputs for the computer? I’m not knocking at all what you did here, just a curios calibrator. In fact more of these types of post would make a Physical/Dimensional Tech (nerd) very happy… As always thanks for your time…!
Great work, rsilvers. Were the springs generally linear, or was there any surprise in spring force vs. length?
It’s kind of neat to see the SpringCo blue and red springs exerting ~7.5 and ~8.0 pounds of force, respectively, when compressed. I’ve always liked how pistol spring manufacturers will straight up sell you a “16 pound spring” without resorting to goofy color codes. Now we finally know what “red” and “blue” mean in this particular context.
So reading your data, I see that the Tubbs flatwire CS spring is stressed the most, but even at mid 40%, it still has a significant margin.
For comparison, what other stressed parts exists on an AR, and what are their stress levels as a percentage of tensile strength? For example, the lugs on a bolt? The pins in a FCG or the bolt overrun stop? the buffer retainer pin? The barrel at MAP? What are their margins?
Regarding buffer springs specifically, if you were to conduct these tests after 5,000 rounds after spring-set set in, would the resultant stress level percentages be the same, or less, or more?
The chart is interesting, but I have no idea if best engineering practices suggest you should to design in a margin factor of 2X or 3X or more for “stressed” parts.
