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Discussion Starter · #1 ·
Hallo from Bonn !

Any ideas how to practically degauss a gearbox Mainshaft ?

Would pole switching a DC current thru the length while at east - west orientation help ?

I dont have a degauss-coil but perhaps one can be hired ?

This is not for my motors but my work shop asked my advice !

Miles
 

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Hallo from Bonn !

Any ideas how to practically degauss a gearbox Mainshaft ?

Would pole switching a DC current thru the length while at east - west orientation help ?

I dont have a degauss-coil but perhaps one can be hired ?

This is not for my motors but my work shop asked my advice !

Miles
Pole switching a DC current ? Why not an easier approach of using the mains voltage of the power grid which is AC.

In standard CRT applications, degaussing of the tube is achieved with an AC current flow (mains voltage) through a wire positioned around the back of the tube in series with a PTC (resistor with positive temperature coefficient). At start up, the resistance of the PTC is low allowing a maximum current flow that in return heats up the PTC that increases its resistance and thus reduces the current flow untill reaches an equilibrium by the time the tube is degaussed.

The same principle can be used for a 'gearbox mainshaft', however I would let the current flow through the shaft itself. However, for safety reasons use a transformer to reduce the voltage to a safe value as the mains voltage from the power grid has a potential that is lethal.

If you need a constant magnetic field, add a series resistor (without tempoerature coefficient) with a sufficient power rating to control current flow. Always use a step-down transformer to a low secondary voltage below 30VAC (anything above 42VDC is concidered lethal). A transformer transforms AC power into a lower AC voltage (or higher) but as it maintains power (minus efficiency), AC current increases with a lower voltage. This increases the magnetic field.

Last but not least. These are experiments that must be prepared with care and performed in a controlled environment by people who know what they are doing. With other words, I cannot and will not accept any liability. This is at your own risk.
 

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Discussion Starter · #3 ·
As always the most informative site on the Net ! Thanks Raymond.

The DC idea was that AC tends to flow over the surface & DC thru the conductor !
This was internet info ; it is a long way back to my Physics Studies!!!

The shaft is magnetised during milling , & the idea is to avoid it attracting debris.

Thanks again
Miles
 

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The DC idea was that AC tends to flow over the surface & DC thru the conductor !
That is called 'skin effect', which may be described as an impedance that increases with rising frequencies. However, you may neglect this effect for low frequencies up to the MHz range. It certainly doesn't play a role at mains frequencies (and audio frequencies unlike what many sales agents use as argument to let you buy far too expensive cables).

This does not apply to Maxwell laws, that applies for high 'dI/dt'. That may have influence at mains frequencues but only at very high currents.

Bear in mind that pulsing a DC voltage results in a signal with AC components, but I won't go into the maths behind it (Fourier transformation). In the frequency domain this results in a base frequency and higher harmonics that all contain energy. Ideally you'll need one base frequency and no harmonics (sinusoidal source, i.e mains voltage). You cannot use a DC current since this results in a constant magnetic field and thus magnetizes the object.
 

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Discussion Starter · #5 ·
Thanks !
I did understand it all , but there is no way that i could deliver the information myself !

PS. My E28 is ready for the holland meet . Is this now 06.04 ? E34 is in for service & valve clearance & finally the Chain tensioner !

Miles
 

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If I might ask, what would the technical reason be for doing this?
 

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Discussion Starter · #7 ·
Iron filings should exit with the oil & not be attracted to the critical parts of the engine / gearbox.

On my m/cycle I have a magnetic sump plug to help in the eventuality .

With aircraft / boats you cant use a magnetic compass if the frame / motor is magnetized. With small problems compensation magnets must be mounted in the compass to get it to point to approx north on all courses .

Here the problem has occured following milling of the main shaft during ear box rebuild .

mm
 
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