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Using magnets to transfer power through metal?

  1. Jul 19, 2011 #1
    Hey guys. A friend and I are planning on making a scuba diver tow device for his trip to the florida keys. We could just put a trolling motor exposed to the water, but at depth I have reason to believe water would push past the seal. So what we are hoping to do is weld up a thin wall steel tank, and have magnets transfer the power from the motor, enclosed inside, to the propeller on the other side. This, in theory, should work, right? would the magnetic field be weakened traveling through thin steel?

    Another possibility would be putting the trolling motor exposed to the water, but adding pressure to the dry side of the seal.

    If you have a good idea, let me know.


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  2. jcsd
  3. Jul 20, 2011 #2
    Are you trying to have something turn inside the steel tank and the magnetic field will then turn the prop outside? The B field will depend on the Magnetic susceptibility of where your at. So it will be weaker in the steel but in the water you would use the magnetic constant of water and it shouldn't be that much weaker. Are you worried about water leaking in through the seals, how deep are you going. In my head it sounds like you are trying to make magnetic gears where they dont touch but the field pushes on the other one. How would you make that?
  4. Jul 20, 2011 #3
    Yes, we would have two "magnet wheels". I'll make them with rare earth, or neodymium magnets. The motor will spin a flywheel on the inside of the tank, and there will be a 2nd one outside the tank, which will have a shaft going off it to turn the propeller.

    We're not planning on going too deep, my friend does have some scuba tanks so it'll deffinatly go beyond snorkeling depth a few times. I'm mainly concerned with the amount of time it'll spend in the water I want a really good seal.
  5. Jul 20, 2011 #4
    The magnetic wheel should work if you have a good design.
  6. Jul 21, 2011 #5
    we would use as many magnets as we can on each wheel
  7. Jul 22, 2011 #6
    to any one that has an answer to this i would be grateful,,, im a billet designer who has asked me to machine T6106-T6 billet aluminum boxes to store money in,,,they are beautiful polished boxes with an inside distance of 1.200 inches,,,each box will hold 30,000$ cash in hundred dollar bills,,,in the lid i have .300 of wall thickness and in bottom i have .190 wall thickness,,so i was wondering what size of neodymium magnets i should purchase so i can machine a pocket in the bottom .125 deep and a .250 deep pocket in the top,,,i want approx 10-12 lbs of pull from lid to bottom with the box full of hundred dollar bills,,i have an account with applied magnets so i can order them,, so does anyone have the knowledge to tell me what size (round-square-rectangle-doesnt matter) but the .125 and .25 thickness i have to stay with to achieve 10-12lbs of pull through 300-100$ bills,,,,i can machine any shape pockets i need to press the-mags into,,,just dont have any idea what stength to buy,,,the will be pressede flush into lid and bottom insides,,,,thank for anyone who can help,,,
  8. Jul 22, 2011 #7
    what do you what the magnets to stick to. Because they wont stick to aluminum.
    But then I guess you could machine a place in the lid and put a piece of steel in it.
    It would be pretty tricky to figure out the magnetic field of a bar magnet. In classical E&M you do everything with currents. You could determine it experimentally or maybe bring the magnet next to an electron gun and see how much it deflects the beam and then use the Lorentz force to get an approximation of the B field at that distance. Or take a balance and set the magnet on one side and have it sticking to steel but with something in between them and then see how much weight you have to add to one side to pull the magnet off the steel.
    Just some ideas.
    Last edited: Jul 22, 2011
  9. Jul 22, 2011 #8
    ive machined several items from billet and attached magnets,,,i use non-ferrous micrometers so i can cutter comp a pocket in that allows a .001 interferance press fit,,i always put a dab of JB-WELD on the back side with a small groove milled into the aluminum,,,180lb push/pulls are the strongest ive worked with,,,i just completed two muliti-faced cylinder that have neo"s in pokets,,i spin one rotor and it spins the adjacent rotor which is 1.0 inch smaller in diameter,,,,just like gears,,,the big one is driven by a single DC-power source which rotates the smaller one faster,,,i also machined in a circular fan style pattern of 30-.125 thick pockets for 30 mags-that are press fitted into the top of the driver-rotor, i take energy through coils off the smaller rotor and i am adapting a very large alternator style coil over the top of the driver rotor,,,this way i have multiple points of energy coming out of one driver-rotor,,,i call it my "neodymium magnetic rotormotor",,,still in small scale,,,but,,once ive comleted this i will scale all my designed pieces up,,,,with a slight press fit and a dab of JB-WELD,,i havent had one come lose yet,,,,as long as the machine work is very high quality,,,that ,,,so far,,,has held them in place,,,,,,
  10. Jul 22, 2011 #9


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    One altenative to this is to have the coils of an electrical inductive motor sealed (water tight) within the perimeter of a cylinder (perhaps made of composite material), with the magnetic rotor (and it's bearings) mounted inside the water filled cylinder. The cylinder could be otherwise hollow using an inducted fan to propel the water.
  11. Jul 22, 2011 #10
    thnx,,good ideas,,ur idea of embedding a small piece of iron is excellent,,,never thot of that,,,ill never forget the first time i tried to measure the physical-dims of one of those 180's,,,i used a 300$ starret pair of what i thought was all stainless and brass calipers,,,,it completely trashed the tiny steel gears inside of the rack gear mechanism,,,expensive screw-up,,,,ur idea of using weights is good,,im now wondering if i could take about 1.25 worth of phone book and use 2-of my 180lb'rs,,,see what it feels like and just guess frm there,,,it doesnt have to be perfect,,,,these are expensive money storage boxes and im trying to give them somthing unique in a way of holding the lid on without any external hardware,,,right now they look like polished,,profiled silver bars,,,they turned out more cool than i thought they would and other than magnets im not sure how else to go about hiding holding fixtures,,,,im a descent machinists,,,,love science,,but im no-scientist,,,,thanx for talkin to me,,,im not advertising,,,but if u ever want somthing in the 3inch by 6inch by 3inch range designed or made from billet,,,just holler bak,,,i use solidworks connected to 3D-tool path mappping,,,i can do some cool stuff,,,thanx again for your help,,,,
  12. Jul 23, 2011 #11
    That seems like a good idea, but at the moment we're more or less aiming to take a trolling motor off a store shelf and use it. I have some electric scooter motors at my disposal as well, but I don't yet know if they operate in a suitable RPM range.

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