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Varying the strength of a spring or something similar

  1. Jun 29, 2017 #1
    I have an idea for an invention that focuses on a spring that supports weight. Is there a way to manually make a spring vary how much weight it supports? Or is there maybe something else other than a spring that could do the same thing I need the spring to do, but could vary how much weight it supports?
    I thought about having a lever hooked up to the bottom end of the spring (the actual metal tip end) that pulls it apart, causing it to sort of unravel a little bit. Does that cause springs to lose compression strength?
    Thank you.
  2. jcsd
  3. Jun 29, 2017 #2


    Staff: Mentor

    Instead of fastening the end of the spring, fasten the middle. That way, only half the spring would stretch/shrink when flexed. It would be effectively twice as stiff.
  4. Jun 29, 2017 #3


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    Staff: Mentor

    Building on what @anorlunda suggests, how about using a spring and a hollow threaded tube that have the same pitch, with the OD of the cylindrical spring equal to the largest ID of the tube? You can turn the tube to run it farther up the spring, exposing less and less of the spring. The part of the spring in the threaded tube is fixed, and only the part of the spring sticking out the top of the tube is still able to compress.

    Can the effective length of the spring change as the stiffness changes?

  5. Jun 29, 2017 #4
    Great responses, guys. I kinda need the spring to stay relatively long, so the length kinda needs to stay the same. It doesn't have to be a spring. I was thinking of maybe a pully system at first, but then switched to a spring. But maybe a pully system could work? It won't lift automatically, like the spring, but it will assist the lift, which is what I'm looking for. Do you think it would be possible for a pully system to have a switch that can change the difficulty without having to re-string the rope?
  6. Jun 29, 2017 #5


    Staff: Mentor

    Yes, there are pulley systems that have belts moving over variable diameter pulleys.

    But no more guessing. If you want more help, explain better what you are trying to accomplish.
  7. Jun 29, 2017 #6
    I want to make an object easier to lift or harder to lift by a simple adjustment by the user, while not taking away the range of motion. It's an exercise device, so range of motion is important. You mentioned variable diameter pulleys; what is the purpose of the variable diameter pulley? Those are used in vehicles to change speed? Can that translate to varying lifting difficulty?
    Thanks for the response.
  8. Jun 29, 2017 #7


    Staff: Mentor

    OK, that's better.

    Consider a simple deraulier gear on a bicycle. Instead of a wheel, think of a rope wrapped around the rear axle with its other end lifting a weight. Then shifting the gears makes lifting the weight easier or harder. It is called simple mechanical advantage. There are countless ways to make it.


    Edit: it is also the principle of the lever. To adjust a lever, just move the fulcrum.
    Last edited: Jun 29, 2017
  9. Jun 29, 2017 #8


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    The usual method is to vary the weight being lifted by adding or removing weights. In some machines the weights don't have to be physically removed just "disconnected" from the moving part by withdrawing a pin and inserting it in a different hole.
  10. Jun 30, 2017 #9
    Thank you. The derailleur gears are a great suggestion. For my application, I would need to think of a way to change gears without them being in motion. You mentioned a variable diameter pulley. I think that could work. That would be more difficult to construct, and I would need to think of a way to vary the diameter with the rope being in contact with the pulley at all times. Thanks again.

    And yes, CWatters, I had that in mind when thinking of my idea. My device would be able to be moved without having to move all the weights, so it cuts down on the weight of the device.
  11. Jun 30, 2017 #10


    Staff: Mentor


    Another simple way. Think of a see-saw with the user on one side and a counter weight on the other side. Just move the counter weight back and forth to adjust the force needed by the user.

    There are countless ways to design your exercise machine.

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