Demagnetizing crank?

[Dennis Prince]

How do you demagnetize a crankshaft? I bought an EE crank that is in really good condition except that it is magnetized. Any ideas? Thanks, Dennis

[Mark Gregush]

Found this;

Demagnetize a Magnet by Heating or Hammering

If you heat a magnet past the temperature called the Curie point, the energy will free the magnetic dipoles from their ordered orientation. The long-range order is destroyed and the material will have little to no magnetization. The temperature required to achieve the effect is a physical property of the particular material.
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You can get the same effect by repeatedly hammering a magnet, applying pressure, or dropping it on a hard surface. The physical disruption and vibration shake the order out of the material, demagnetizing it.
Self Demagnetization

Over time, most magnets naturally lose strength as long range ordering is reduced. Some magnets don't last very long, while natural demagnetization is an extremely slow process for others. If you store a bunch of magnets together or randomly rub magnets against each other, each will affect the other, changing the orientation of the magnetic dipoles and lessening the net magnetic field strength. A strong magnet can be used to demagnetize a weaker that has a lower coercive field.


Apply AC Current

One way to make a magnet is by applying an electrical field (electromagnet), so it makes sense you can use alternating current to remove magnetism, too. To do this, you pass AC current through a solenoid. Start with a higher current and slowly reduce it until it's zero. Alternating current rapidly switches directions, changing the orientation of the electromagnetic field. The magnetic dipoles try to orient according to the field, but since it's changing, they end up randomized. The core of the material may retain a slight magnetic field due to hysteresis.

Note you can't use DC current to achieve the same effect because this type of current only flows in one direction. Applying DC might not increase the strength of a magnet like you might expect, because it's unlikely you'll run the current through the material in the exact same direction as the orientation of the magnetic dipoles. You will change the orientation of some of the dipoles, but probably not all of them, unless you apply a strong enough current.

A Magnetizer Demagnetizer tool is a device you can purchase which applies a strong enough field to change or neutralize a magnetic field. The tool is useful for magnetizing or demagnetizing iron and steel tools, which tend to retain their state unless disturbed.

[TXGOAT2]

It probably does not matter if it has some magnetization. I would NOT hammer on a crankshaft or apply heat to it. Do check it for cracks and straightness, etc. Any slight magnetization in the shaft will probably disappear in service due to normal heat and vibration and the normal field effects of the magneto magnets and coils. Using a screen in the transmission with a magnet in it will capture most metal floating particles and a magnetic drain plug can also be helpful.

[Dan McEachern]

The residual magnetism is no doubt the result of the crank being magnafluxed at some point without being demagnetized after inspection.
You can take the crank to a crank grinding shop and have them demagnetize it. A wet magnaflux machine has a coil that is used to demagnetize parts by passing the part thru the coil. Yes, the coil is energized with AC current.

[Dennis Prince]

Thank you all for your responses, I will look into the ac option, this crank is magnetized to where it will pick up a 5/16 washer, since there is no oil filter and the amount of metal that is on the magnet in the trans screen I want to get rid of the pull factor.

[TXGOAT2]

Demagnetizing (degaussing) with an electromagnetic device will do no harm.

[Luke]

If you had an old CRT television or computer monitor you didn't mind scrapping you could remove the degauss coil from that and probably use it to demagnetise a crank.

I've not personally used one to demag anything as large as a crank, but see no reason why it wouldn't do the job. I have demag'd many other things with one - essentially you need to:

(1) apply power
(2) move the coil near the object several times
(3) then draw it away and turn it 90deg...
(4) turn off power

Luke.

[Dennis Prince]

I just remembered that I have 2 journeyman and 2 apprentice electricians coming to dinner on Thursday, seems like a good time to use them.