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Thread: Action Spring Analysis

  1. #11
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    Quote Originally Posted by rsilvers View Post
    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.
    More questions!

    I've read about 'shock loading' and 'shock resistance' with regard to springs. Can you explain that phenomenon to me in a way that I can understand?

  2. #12
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    Quote Originally Posted by Clint View Post
    Don't forget to measure the rifle spring in the A5 configuration.

    That is a real good one.
    Good idea. I updated the chart.

    According to the software, the Colt rifle spring is 26.3 %MTS in a rifle - slightly better than a Colt carbine spring in a carbine (which was 27.9).

    In an A5, the Colt rifle spring is reported as 28.4.

  3. #13
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    Quote Originally Posted by DiabhailGadhar View Post
    Out of curiosity how is the machine not just reading force applied (weight) instead of stress?
    It is just reading force. The force at a certain length is one of the inputs the software uses.

  4. #14
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    Interesting info, thanks for posting.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  5. #15
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    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.
    Last edited by kartoffel; 12-09-11 at 12:51.
    Oh no, not another lube thread! Read this first: Lubrication 101.

  6. #16
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    What have you been cutting on that mill? Looks like sawdust all over the place.

  7. #17
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    Pinewood Derby

  8. #18
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    Quote Originally Posted by rsilvers View Post
    Good idea. I updated the chart.

    According to the software, the Colt rifle spring is 26.3 %MTS in a rifle - slightly better than a Colt carbine spring in a carbine (which was 27.9).

    In an A5, the Colt rifle spring is reported as 28.4.

    Double check those lengths and forces on the A5.

    Looks like a dup.

    Also it would be good to verify the force at the recoil length compared to what the software says the force is.

    If they agree, then the stress number is supported.

    Also with both forces and the stroke you can show the work / energy that the spring can store.
    Last edited by Clint; 12-09-11 at 22:02.
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  9. #19
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    Quote Originally Posted by rsilvers View Post
    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.





    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.

  10. #20
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    this is really cool, I like here!

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