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ElectroMagnet Force Calculations?

  1. Aug 15, 2012 #1
    I was toying with the idea of making an iron core electromagnet to retrieve a broken portion of my driveshaft from a recess in the end of the crankshaft on my boat motor. The mechanic at my work said I wouldn’t be able to do it, so I figured I would entertain the idea.

    The driveshaft is somewhere between 5/8 and 3/4 of an inch in diameter. The crankshaft has a female splined hole that is about 1-1/4 inches deep and accepts the male splines or the driveshaft. About 3/4 of an inch of the the driveshaft is broken off in the crankshaft, leaving a 1/2 inch recess.

    I have a basic understanding of electricity, the relationship between volts, amps, watts and resistance and the ability to use a multimeter.

    Here are some things I know about iron core electromagnets.
    *The more windings, the greater the magnetic force.

    *The more amps flowing through the windings, the greater the magnetic force.

    *Once I figure out my power supply, I can obtain a suitable amp rate for my
    windings through the use resistors and/or a load such as a light.

    Here are some things I don’t know.
    *What direction is the force the strongest? Toward the end of the iron core?

    *How does the size and composition of the core effect the strength of the magnetic force?

    *Is it a reasonable task to build an electromagnet that can create 10-25lbs of pull, if so would that force be able to act on a slug of metal that’s not much more than a few cubic centimeters?

    *Would I be better off to build a big magnet that will not fit in the recess but may have more overall force, or make a small one that can fit into the recess and directly contact the piece to be removed. How much will the force diminish over that 1/2 inch gap?

    *Is the metal of the crank around the slug I’m trying to remove going to deflect or redirect the magnetic force in any way?

    *Do these examples have an iron core? http://www.solenoidcity.com/electromagnet/E-28-150p1.htm

    Any input would be great, thanks.
  2. jcsd
  3. Sep 1, 2012 #2
    Definitely make the magnet touch the object. The force is the strongest just above the surface at the end of the core, and decays very very fast (something like distance cubed). The force calculations are complicated and if you have a good power supply with current limiting maybe you can build something. It you fold the magnetic field back onto itself and have some king of air gap this would work best (like in a horseshoe magnet). But what I would do instead is using a strong conventional magnet, you will never get the field strength of these with a home made electromagnet. If you worry about turning it on and off you could put it in some kind of bucket of a non magnetic material like brass. When you pull the magnet out of the bucket the things attached to the outside fall off.
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