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Thread: Extractor Spring Question

  1. #21
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    Quote Originally Posted by 5shot View Post
    The number of coils has nothing to do with the "Stiffness" or spring rate. You could take a 10 coil spring and cut it in half to make 2, 5 coil springs, and they will be just as stiff as before. Coil spring pitch, wire diamter and the heat treating process are what determine how stiff a spring will be. Too many unknowns to make a blanket statement about them.
    Sure it does. Spring rate is the amount of weight needed to compress a spring a certain amount (lbs/inch). If you want a spring to require more lbs to compress, you use fewer coils.

    If you take a 10 (active) coil spring and put a 1 lb weight on it, and it deflects 1/2 of an inch, and then modify it to a 5 (active) coil spring, it will now only deflect 1/4 of an inch with the same weight on it.

    So yes, wire diameter and outer-diameter are a factor, but if they stay the same and you only change the number of coils, a 5 coil spring will compress more easily than a 4 coil spring.

    Todd.K is correct. There is too much in a spring to just specify it by the number of coils. I am going to try out those new Colt springs as I suspect they have the most actual testing.

  2. #22
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    ......
    Last edited by 5shot; 07-28-10 at 15:54. Reason: I was wrong

  3. #23
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    Quote Originally Posted by 5shot View Post
    The force to compress a spring is defined by:

    f=-kx

    f is the force, k is the spring constant and x is the distance of the compression. The number of coils plays no part. x is the diference between the free length and the compressed length - just because you cut a few coils off means nothing.
    The number of coils plays a role because the calculation for F requires knowing the spring constant K.

    K can be calculated with this tool:

    http://www.engineersedge.com/calcula...ring_k_pop.htm

    Notice when you change the number of coils, the value of K changes.

  4. #24
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    You're right
    Last edited by 5shot; 07-28-10 at 15:48.

  5. #25
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    I have not found a reason to worry about the spring with an O-ring installed. If you install an extra power spring and O-ring run it to make sure it does not have too much tension to clip over a rim before you put it into service.

  6. #26
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    Stress in a spring is caused when you compress it. The more you compress it, the more stressed it is. That will eventually lead to fatigue failure in this kind of spring.

    The claim is that an o-ring reduces stress on a spring and makes it last longer. But I don't see that as possible because the o-ring will not result in the spring being compressed less because the spring has to be compressed a fixed degree just to get over the rim.

    The o-ring will only add extractor force.

    There is one exception I can think of - if the extractor is bouncing then it may be compressing the spring more than needed to get the extractor over the rim. An o-ring would help. And so that would make the spring last longer.

    As for which spring will last longest, you want the stress/strain ratio as low as possible. Once you get over about 40% with typical materials, then things get bad. There are also materials, heat treat, etc.

    What I would like to do is get the drawing for a USGI spring, a new Colt spring, an ISMI spring, and a Wolff spring - and measure them all and do the calculations and report the stress/strain ratio of each.

    I think the way to go would be whatever spring has the lowest stress/strain ratio and combine it with an o-ring.
    Last edited by rsilvers; 07-29-10 at 12:21.

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