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  • Floating, but Heavy, Sphere

    Discussion in 'The main mechanical design forum' started by nitay, Jul 7, 2014.

    1. nitay

      nitay New Member

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      Hello,

      I'm working on a project for which I need a sphere, about 40-50mm diameter, which will be heavy enough to turn a turbine (100-200g +) but will still float on water.


      I've checked hollow steel spheres, but couldn't find a thick enough sphere (most spheres are 0.5-1.5mm thick). Styrofoam balls (polystyrene) are too light, and I couldn't find a supplier that gives freedom in choosing the foam density. Fishing buoys are too big.

      Does anyone has an idea on where can I get spheres as described?
      Please add links


      Thanks
      Nitay
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      Hi Nitay,

      If you take a solid sphere that is 50mm diameter - that will displace 65450mm^3 of water. Therefore the mass of water displaced is 65.45grams. So if you want it to float - then it has to be less than 65.45grams.

      If you have a hollow sphere - where the internal cavity is not open to be flooded - then the displaced volume of water is still 65450 mm^3. The weight in air of a 50mm steel sphere - with a 2.5mm wall thickness is 140 grams - so that will sink.

      However, (you could puncture the hollow sphere) - to let it flood - and below are some 2.5mm wall thickness spheres - which I think is what you said you wanted - but as I said above they will sink.

      What in my opinion you need is a 50mm OD x 1mm wall thickness - which will weigh 60 grams - and so will "just" float.

      http://www.fhbrundle.com/iron-spheres.htm

      Hope this helps.
       
    4. Erich

      Erich Well-Known Member

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      Lochnagar's analysis shows that what you are asking for is impossible.
      A 50mm sphere that weighs more than 65 grams will not float.
      You want 100-200 grams. Not going to work.

      To get on the right side of physics you have to change your requirements.

      1. Live with less mass.
      2. Go to a larger diameter so the density of a 200 gram ball is less than 1 gram/cc
      3. Replace water with Mercury.

      The last one is not serious but a much denser item will float in a denser fluid than water.

      Since we have no real idea what you are up to, only you can decide how to modify your requirements.
       
    5. nitay

      nitay New Member

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      Bah. I mis-calculated the sphere density. Thanks for the fix guys.

      So how much does mercury cost these days? :)
       
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