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  • #16
    These guys can make just about anything, I believe. You might want to rekindle the Ti flame!

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    • #17
      Titanium is notoriously hard to work. It is one reason it is not a commonly used metal.

      That said, the Soviet Union built submarines with hulls made entirely of titanium. I read this was discovered by an allied spy who gained access to a Soviet shipyard while wearing shoes with special rubber-based soles, which picked up titanium metal shavings during his visit. A real-life gumshoe!

      Ed Bianchi

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      • #18
        Originally posted by HO RacePro View Post
        There are not a lot of good non-magnetic materials that could be used for axles. Aluminum and brass are too soft. Hard-drawn 304 stainless steel tubing comes close, it has very little ferro-magnetism, but bends too easily. Your trick of filling it with carbon fiber was a good way to improve on that.
        Ed, I don't think a material needs to be ferromagnetic to suffer from eddy current losses. I think it just needs to be able to conduct current. Changing magnetic fields induce a current in conductive materials, and the internal resistance within the material dissipates energy. Right? I would link to eddy current Wikipedia page, but for that obnoxious link ban.

        Maybe making an axle out of gold would reduce intercanal resistance/losses? It could also be that different geometries make a difference, say a cylinder vs. a tube?

        Originally posted by HO RacePro
        I believe you and Rich deserve your own channel.
        I would add your own name to the list, Ed. But otherwise, I could not agree more!!

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        • #19
          Axle,

          You are right. A material only has to be conductive to experience eddy current braking.

          I remember an exhibit on eddy current braking at the Franklin Institute in Philadelphia. They had a disk of solid copper -- maybe two feet in diameter -- with an electromagnet coil in close proximity. You could turn the disk with a crank and turn the magnet on and off with a switch. Without the magnet the disk spun freely. When the magnet was on you had to grunt to keep it turning.

          I haven't been to the Frankly Destitute for decades. I'd be interested to know if that exhibit is still there.

          Please note that the magnetic field does not have to change to create eddy current braking. A permanent magnet can be used for an eddy current brake. All you need is a conductive material moving through a magnetic field.

          A more conductive material will produce more braking. The ultimate brake material would be pure silver, which has the highest conductivity of all metals.

          If you want to see eddy current braking in action, try dropping a supermagnet down a piece of copper tubing. While the magnet won't stick to the tube, since copper isn't ferromagnetic, the magnet will drop very slowly down through the tube due to eddy current braking.

          A similar effect will make a permanent magnet float above a superconductor. Because a current induced in a superconductor doesn't lose energy over time, the magnet won't be able to fall at all.

          I have played around with the concept of using eddy currents to float a model maglev (magnetic levitation) train over an aluminum rail. I'd use spinning magnets to generate lift as well as propulsion. Extra credit if you can figure out how.

          Ed Bianchi

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          • #20
            kinda funny- i used to cut big slots in the back of my motors to bury the axle right into the magnets in the corner of the motor so that i could get the angle i wanted. i don't know if my axles were constrained by eddy currents or not, but it worked the b4lls.

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            • #21
              Eddy current braking -- like all other braking -- is a drag force applied at a radius. Big radius, big torque. Small radius, small torque.

              The amount of eddy current braking your axles saw was minimal. You probably created more drag by the extra load of the magnet pulling the axle against its bushings.

              Ed Bianchi

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