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  • reciprocal rotary motion to one way rotary motion

    Discussion in 'The main mechanical design forum' started by itali89, Jun 12, 2015.

    1. itali89

      itali89 New Member

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      hello,

      so i am working on a way to be able to pump a fly wheel using a pull cord (like a lawnmower)
      however instead of only grabbing onto the generator when I'm pulling the string i want to have it so it keeps up adding speed while the string is recoiling. that way i can use the energy of the spring and not waste it.

      does anyone have any ideas?
       
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    3. K.I.S.S.

      K.I.S.S. Well-Known Member

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      The energy of the spring isn't being wasted - it's performing its function of retracting the cord. If you want the spring to assist in increasing the rotational velocity of the flywheel then you'll need a spring that has more stored energy (when torsioned) than required to simply retract the cord - this means that the same amount of energy will be required when retracting the cord to achieve the same required RPM of the flywheel as it would be without the assistance of the spring.
      There may be a specific 'time to full speed' application that you're thinking of which requires such a boost regardless of the input energy required, but keep in mind the above.
      If you are aware of what I've just stated and you do have such an application then it's possible that a bi-stable spiral wound torsion spring in combination with a simple gearbox (the RPM of the flywheel can never exceed the RPM of the spring arrangement) might help, but it would become increasingly ineffectual as the flywheel reaches its maximum RPM.
      K.I.S.S.
       
    4. Dana

      Dana Well-Known Member

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      As said above, the spring's energy is going into retracting the cord. If you want it to do any more work, you'll need a stronger spring, which means it will take more energy to pull it. There is no free lunch.
       
    5. K.I.S.S.

      K.I.S.S. Well-Known Member

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      That's what I meant to say...:)
       
    6. s.weinberg

      s.weinberg Well-Known Member

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      A. I have to add my voice to the common sense listed above. No free lunch.

      B. As far as the mechanism itself, it's fairly simple. Simply size your wheel such that it is in-line with the spring's line of travel at both ends of the string pull/retract. Inertia will carry the wheel past the neutral in-line position, and your retract will carry it the rest of the way around full-circle back to its starting position.
       

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