How to demagnetize a cast iron transmission case?
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  1. #1
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    Default How to demagnetize a cast iron transmission case?

    Rebuilding a transmission and the cast iron case has been magnetized by a scrapyard crane magnet.

    Tried wrapping 12 wraps of #2 lead cable around it, 200 amps of AC current from my welder for up to 10 seconds and it doesn't seem to diminish the magnetism.

    Any ideas?

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    Heat it up 300df or so...Phil

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    I don't think that is a viable option, since I have the other parts I want demagged too. Like bearings with plastic cages.

    The Curie temp for iron is 1400 degrees F, would 300 F even do anything?

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    That's not really how degaussing works. You need an AC current that initially produces a magnetic field stronger than the magnetization, that then slowly tapers off. Think of it as wiggling the magnetic domains out of alignment (definite oversimplification).

    Use a variac and a coil with more wire. Something like 20 amps into a lot of turns and a gently ramping down over the course of a few seconds should work. I've never degaussed anything that large but have experience removing the magnetic field from CRTs.

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    I don't know if this will help, had to weld a new shaft onto a magnetic conveyor roller a few years back, the magnetism was screwing with the arc big time, an old guy suggested wrapping the ground lead around the roller, said if it makes it worse, reverse the direction of the wrap. All I can say is it worked.

    Thinking along the lines of a degaussing coil, maybe make a hoop with ground lead and pass the case thru while someone is running a long bead???

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    To demag something with AC, you must taper the magnetic flux to zero. Sure, you can use a variac, but not everyone has one. So instead you remove the part from the magnetic flux. Distance will taper the flux to insignificant very nicely. With your AC welder shorted, move the transmission case through the coil and out the other side by a couple of feet.

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    Quote Originally Posted by gbent View Post
    To demag something with AC, you must taper the magnetic flux to zero. Sure, you can use a variac, but not everyone has one. So instead you remove the part from the magnetic flux. Distance will taper the flux to insignificant very nicely. With your AC welder shorted, move the transmission case through the coil and out the other side by a couple of feet.
    That is what I need to know, I really don't what to buy a bunch of 'stuff' to demag 1 case and parts.
    How long should this passing through the hoop take?

    I think I can impale the trans on a wood pole and an engine stand and roll it through the hoop.

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    Well that worked!!

    Clamped a 2x6 wood gangplank to my workbench, set the case at the end. Wrapped my hoop up, cable tied it together and took about 15 seconds to traverse the hoop the 1.5 feet ahead of the case to the 2.5 feet after.

    No more magnetism!

    Thanks all!

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    I think as fast as you can move the case will be fine. If that doesn't do it, you probably need more amp*turns. A tighter coil may be helpful as well.

    On edit: Looks like we were typing at the same time. Thanks for closing the circle and glad it worked.


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