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Thread: Coil Spring Paradox

  1. #21
    Join Date
    29th August 2017
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    Quote Originally Posted by brickeyee View Post
    Stress is the application of force.
    Strain is the reaction of the material resisting that force.
    It does NOT imply motion in any way.

    It is often evidenced by motion, but not required.
    In that case, the motion is either imperceptible (but still non zero), or the material is perfectly rigid. And neither apply here.

    When you tighten the bolts on an engine head, the bolts will stretch in length. You may not be able to measure it, but the strain is definitely there, as the elastic modulus of grade 8 bolts is not infinite. And even if not measurable, it can certainly be calculated. Diamonds are about 5 times more rigid than steel, but are still not perfectly rigid, and will exhibit non zero strain under stress.

    When a coil spring is compressed, there is obvious strain of some form or another. I maintain that the primary form of strain is torsion, with significantly less forms of bending and linear shear. This torsion is difficult to imagine, even if you are observing the spring undergoing compression. The reason is that a round wire is featureless on its surface. But a flat wire coil has a rectangular cross section, and if you observe a flat wire spring undergoing compression, the torsion is visible, although it may not be understood as such. I am working on some illustrations to show exactly this. The spring wire really does twist.

    -

  2. #22
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    Quote Originally Posted by megafiddle View Post

    When a coil spring is compressed, there is obvious strain of some form or another. I maintain that the primary form of strain is torsion, with significantly less forms of bending and linear shear. This torsion is difficult to imagine, even if you are observing the spring undergoing compression. The reason is that a round wire is featureless on its surface. But a flat wire coil has a rectangular cross section, and if you observe a flat wire spring undergoing compression, the torsion is visible, although it may not be understood as such. I am working on some illustrations to show exactly this. The spring wire really does twist.

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    I thought your premise when you started this discussion was that there is NO torsion in a coil spring as it's compressed. Now you're saying that a coil spring does twist (which is torsion). You had me confused before, but now you have me really confused.
    Hawkmoon
    On a good day, can hit the broad side of a barn ... from the inside
    Last edited by Hawkmoon; 16th November 2018 at 22:22.


  3. #23
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    26th December 2015
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    Quote Originally Posted by megafiddle View Post
    In that case, the motion is either imperceptible (but still non zero), or the material is perfectly rigid. And neither apply here.

    When you tighten the bolts on an engine head, the bolts will stretch in length. You may not be able to measure it, but the strain is definitely there, as the elastic modulus of grade 8 bolts is not infinite. And even if not measurable, it can certainly be calculated. Diamonds are about 5 times more rigid than steel, but are still not perfectly rigid, and will exhibit non zero strain under stress.

    When a coil spring is compressed, there is obvious strain of some form or another. I maintain that the primary form of strain is torsion, with significantly less forms of bending and linear shear. This torsion is difficult to imagine, even if you are observing the spring undergoing compression. The reason is that a round wire is featureless on its surface. But a flat wire coil has a rectangular cross section, and if you observe a flat wire spring undergoing compression, the torsion is visible, although it may not be understood as such. I am working on some illustrations to show exactly this. The spring wire really does twist.

    -
    I was thinking if the coil had a dab of paint, the torsional rotation would become apparent.

  4. #24
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    Quote Originally Posted by Hawkmoon View Post
    I thought your premise when you started this discussion was that there is NO torsion in a coil spring as it's compressed. Now you're saying that a coil spring does twist (which is torsion). You had me confused before, but now you have me really confused.
    My premise is not no torsion, but no proportional twist from one end to the other.

    I believe the model of a coil spring as a coiled up torsion rod is correct. By proportional twist, I mean that the spring wire does not exhibit a twist that increases end to end with spring wire length. In the case of the torsion rod, the twist does increase with length. The further you get from one end, the more twist you will see. This doesn't happen when the rod is coiled up. There is no apparent progression of twist as you progress along the length of the spring wire. Nor do you see more and more twist as more coils are added.

    Something you said earlier has some relevance to the paradox:

    "... Now, take the end out of the vise, clamp into the vise an eye or bushing that's just large enough for the torsion rod to go through it, and insert the rod so the exact midpoint is at the bushing. Now we get two guys, each holding a wrench on one end of the rod. Each one turns his wrench clockwise, from his perspective, which means they are working against each other. Since the two guys are exactly equal in strength (because I said so), each guy turns his wrench through an arc of 10 degrees. Now, if each guy went 10 degrees and they went in opposing directions, the total rotation is 20 degrees, but the rotation at the center eye bushing is zero. So there's no movement at the center point, but it's obvious that the rotational stress is present and uniform everywhere in the rod - including at the center."

    If you look at a cross section through any point along the length of the coil spring wire, you will see no rotation as the spring is compressed. It's as if every point is the center, with the torsion occurring on either side of that point it in opposite directions.

    More about this in my next post below.

    -

  5. #25
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    Quote Originally Posted by Mark75H View Post
    I was thinking if the coil had a dab of paint, the torsional rotation would become apparent.
    Yes, or you could attach little pointers to the coils. I actually did this, and the coil wire at that point did not rotate.

    It's important to specify a reference point. If you stood the spring up vertically, and compressed it vertically, we can choose the central vertical coil axis as a reference. In my experiment, the spring was vertical, and the pointers remained horizontal. So the wire did not rotate with respect to the vertical axis.

    And this is true for every point on the coil with the exception of an open coil at the spring end. The end of an open coil will rotate, and there is also some bending present. This is independent of the rest of the coil though.

    -

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