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Adjustable spring?

  1. Aug 27, 2012 #1

    Does anyone know about a spring type in which the spring constant can be adjusted?

    I am building a model which will be used in a wave tank experiment, where I want to be able to adjust det stiffness of pretensioned mooring lines with adjustable springs, if they exist.


    Best regards from Norway
    Anders S
  2. jcsd
  3. Aug 27, 2012 #2

    Simon Bridge

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    You mean like pneumatic "springs"? The elastic coefficient depends on the pressure. There are mechanical systems where you add or remove springs when you want to adjust the stiffness - or you can use springs whose stiffness depends weakly on extension - so, for the extensions used while working are small compared to the variation in k.

    Note: if it is adjustable, it is not really a "constant".
  4. Aug 27, 2012 #3
    Thanks for the tips, Simon, I haven't seen the ones in which you add/remove springs in order to adjust the stiffness.

    I've checked pneumatic springs, which can be adjusted, but they does not have a proportional relationship between force and displacement, which is what I am looking for.

    I don't want to argue, but if the stiffness of the spring does not vary with the displacement, it's stiffness is called a spring constant. Therefore the type of spring I look for, would have one spring constant per setting.
  5. Aug 27, 2012 #4

    Simon Bridge

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    Technically there is no such thing as a spring with a "spring constant" that is the same for all extensions. Hooks law is only valid for "small" displacements. What count's as small depends on the spring. We are kinda stuck with the term though.

    So if your extensions are only a few millimeters and you have to extend the spring 10s of centimeters to noticeably change k, then that would do ... you'd adjust k by winding the spring tighter or looser. Which reminds me - the return springs on self-closing gates are adjustable by winding the coils.

    The multi-spring version is very common but I've only ever built them.
    It's easy - you just have several springs and hooks and exploit that springs in parallel add their rates. The details depend on what you want to do exactly.
  6. Aug 28, 2012 #5
    That's true, Simon.

    One could also connect springs in series to reduse the springs stiffness, in addition to the parallell ones. With a lot of small springs, this apporach could work.

    Winding the coils like twisting the spring about it's longitudinal axis? (if we see the spring as a strait rod).

    Do you know of any good links for these two spring concepts?
  7. Aug 28, 2012 #6

    Simon Bridge

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    For the adjustable-by-winding kind - go to a hardware store and ask about gate springs.
    You'll see what I mean - and yes: they wind about the longitudinal axis. You get them in a kit that includes a simple torque wrench and ratchet. As the springy bit in a mooring line - you will need to rig something that will maintain the relative angle of the ends (they will want to untwist) while allowing the spring to extend and compress enough for your experiment.

    The other kind - you usually decide on the loosest you want the springs and that is your unit. The only industrial applications that comes to mind are bungee cords and trampolines... though I've built others. If you want a very linear relationship between force and extension, then you'll not want to use rubber :)

    Another way is just to clamp a long spring off to shorter lengths - you don't have to attach the spring by it's ends. You can imagine a kind of washer at one end wound into the coils with a cord running off that instead of the end of the spring. If you want a tighter spring, you can just screw the washer further down by turning the spring.

    If this is small scale, then you will probably have to build your adjustable springs yourself no matter what you choose.
  8. Aug 29, 2012 #7
    Yes, it is 1:50 scale, which gives under 500 N in max tension in the spring.

    Thank you very much for your help, Simon.

    I will be sure to check out the twist springs.
  9. Aug 29, 2012 #8
    Back in the days of vacuum tubes and massive analog instrumentation, a different spring was designed for every instrument mounted in the airplane. This resulted in thousands of spring part numbers.

    The Russians had a different idea and mounted all instruments with the same spring. It was a helical spring wound in the shape of a cone. The spring rate was a function of deflection. So each spring was compressed a different amount depending on the spring rate required.

    You can do something similar with belleville washers.
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