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Thread: Coil Spring Paradox

  1. #41
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    Quote Originally Posted by megafiddle View Post
    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.-
    The premise here is why I do not like comparing a coil spring to a torsion bar, but the comparison is relative albeit confused in the out come of more torsion bar and more coils is not more twist as it is with a torsion bar. A longer torsion bar is not stronger or weaker than a shorter torsion bar, but the amount it will twist under a load does change, longer will twist more before reaching its max load, and shorter will twist less before the max is met. Coil springs are similar in that more coils increases stroke length before stacking, and less coils will have less travel before stacking. That is the comparison. But crossing over the torsion bars increase in twist as it increases in length to consider a coil springs lack of twist to be a paradox, is comparing apples to oranges, both fruit, both springs, but not the same attributes.

    CAW
    “If it ain't broke, don't fix it' is the slogan of the complacent, the arrogant or the scared. It's an excuse for inaction, a call to non-arms.” Colin Powell

  2. #42
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    Quote Originally Posted by CAWalter View Post
    The premise here is why I do not like comparing a coil spring to a torsion bar ...
    Ok, suppose the mechanism of energy storage is completely unknown.

    You have said that the twist in the spring wire is created when the spring wire is formed. And I agree insofar as the geometric shape is concerned (like the staircase rail).

    I also agree that the wire will untwist in its shape as the spring is compressed. But that also means that the spring wire will twist even further as the spring is extended.

    Same problem. It applies to both compression and extension springs.

    How would you explain an increase (or decrease) in twist in the spring wire with no overall rotation at the ends of the wire?

    -

  3. #43
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    Quote Originally Posted by megafiddle View Post
    How would you explain an increase (or decrease) in twist in the spring wire with no overall rotation at the ends of the wire?

    -
    Each coil is an entity unto itself, it needs nothing else to display its tension.

    A torsion bar must be restrained on each end to display the tension.

    Since a coil spring is a continuous collection of coils which are holding the ends of each preceding coil, restraints are not needed, in fact each coil as said before will exhibit the same tension as the whole spring coil.

    Imagine this, take a piece of wire that is straight, and while holding it in your hands attempt to twist it. Now wrap it around a cylinder so the straight piece of wire is is now a single coil. You can now, by holding the ends twist the wire and feel its resistance.

    If a straight torsion bar is held at only one end and rotated, the other will twist without generating any tension. By being formed into a coil restraints are not needed.

    If in a fantasy, a coil spring when compressed showed an accumulation of rotation at the ends it would have zero tension.

    CAW
    “If it ain't broke, don't fix it' is the slogan of the complacent, the arrogant or the scared. It's an excuse for inaction, a call to non-arms.” Colin Powell

  4. #44
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    Perhaps considering a magazine spring will help. It is similar to a coil spring and is said to have X number of coils in descriptions. Note nearly all the tension of a mag spring coil is the result of the straight side bars twisting the round area, so the straight wires are the coils restraints, and force the 180 degree bend to twist. If that twist could somehow twist the straight sides and transfer the twist to the next coil there would be not tension, it would just collapse. But it doesn't because the twist is contained.

    CAW
    “If it ain't broke, don't fix it' is the slogan of the complacent, the arrogant or the scared. It's an excuse for inaction, a call to non-arms.” Colin Powell

  5. #45
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    I did say that I would photograph a flat wire spring, and I finally got around to it.

    There are three different but related subjects here, and this only addresses the first two:

    1) whether the operating mechanism of a coil spring is torsion
    2) how the spring wire rotates in relation to some reference
    3) why the torsional twist does not accumulate over length

    flatwire_spring_relaxed.JPG

    flatwire_spring_compressed.JPG

    The first photo is a relaxed flat wire spring. The second photo is the same spring compressed. I think it should be obvious that the spring wire undergoes a change in twist as it is compressed. Compare a single coil from each photo.

    The twist of the relaxed spring wire is only geometric, i.e., it is only in shape, and not due to torsional strain. The twist of the compressed spring wire, or more properly, the lack of geometric twist, is due to torsion. The spring wire is being torqued into a flatter untwisted geometric shape. This is independent of the type of spring wire. It is just more visible with wire with a rectangular cross section. With the featureless surface of round wire, the twisting is not obvious at all. I make a distinction between torsional twist and geometric twist because a spring wire can have a twisted shape with no stress present. The twist in the relaxed coil is simply the result of the fabrication process. For example, flexible couplings are machined out of a solid tube, and are similar in shape to a flat wire spring.

    I discovered something else. While vacuuming my workbench, I noticed that the vacuum hose was resistant to twisting. The hose was very flexible in its ability to bend in any direction, but it did not like to twist along its axis. This is very similar to the flexible shafts used on rotary tools. They bend easily, but do not twist easily. This makes them useful for transmitting torque from one end of the shaft to the other.

    Using the hose as a giant piece of "spring wire", I wound it around a large cardboard tube, forming a large coil "spring". This spring resisted compression. The hose had the flexibility to do everything except twist. If torsion were not involved in the mechanism of a coil springs operation, the vacuum hose spring should have simply collapsed. But it didn't. The hose resisted doing the one thing that it did not want to do: twist. So, there is more evidence that coil wire twists as the spring compresses, and the coil spring mechanism is torsion.

    Not as apparent in the photos, is the rotation of the spring wire itself with respect to the spring's axis. The horizontal surface of the flat wire remains perpendicular to the spring axis at all points along the wire, and under all states of compression. This is easily confirmed by simply examining an actual flat wire spring. This is also true of a round wire spring, but requires indicators of some type on the surface of the wire in order to observe it. The spring's axis is just one point of reference. It's interesting because all points on the spring wire remain nonrotating with respect to it.

    -


  6. #46
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    Is the spring truly rotating or is there different stretching of the inside of the spring vs the outside or is this the same thing?

  7. #47
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    Quote Originally Posted by Mark75H View Post
    Is the spring truly rotating or is there different stretching of the inside of the spring vs the outside or is this the same thing?
    Inside stretching of the spring wire vs outside stretching of the wire would amount to bending of the spring wire. Torsion springs operate on bending, and are basically coiled up leaf springs, with a single round wire "leaf". These torsion springs are closed wound, with a lever arm at one or both ends. As the lever arm is rotated, the spring wire bends into a larger or smaller radius coil. Little or no torsion of the spring wire itself is involved. The actual torsion is about the axis of the coil, not about the spring wire's axis.

    Compression springs, e.g. a recoil spring, do increase slightly in diameter as they are compressed. But this is due to the relaxed coils sitting at an angle (non perpendicular) to the coil axis. When compressed down, the coils no longer fit within the circumference of the relaxed spring. I imagine there could be some slight bending involved, but the decreasing angle of the compressing coils pretty much accounts for all of the diameter increase. In the case of the torsion spring, pretty much all of the diameter increase is due to bending.

    If you imagine taking a portion of one of the relaxed coils above (like a half circle), and deforming it to look like one of the compressed coils, you would need to grab the ends of the relaxed portion and twist them in opposite directions.

    -

  8. #48
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    It would seem you have come to grips with the fact that a torsion bar (as was first used in example) and coil spring are different forms of stored energy, no paradox. As the torsion bar only stores energy when twisted, and the ends must be contained. While a coil compression spring only stores energy when compressed. The flat wire spring gives a great view of exactly how a round wire spring works and shows what I was saying about a spiral stair railing and its relationship to the center axis. Twisting isn't a result of a compression spring it is the mechanism of a coil spring to store energy. The twist is contained by the coils resulting in stored energy.

    CAW
    “If it ain't broke, don't fix it' is the slogan of the complacent, the arrogant or the scared. It's an excuse for inaction, a call to non-arms.” Colin Powell
    Last edited by CAWalter; 26th January 2019 at 11:03.


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