by HO RacePro » Fri Jun 28, 2019 3:32 pm
The shield will still help even if you have to install it in two parts, above and below the axle, with a narrow gap in between. The magnetic field will be directed across the gap, largely bypassing the axle.
Magnetic fields are a bit like electricity, except that open air is much more "conductive" of magnetic flux. If the magnetic field is deprived of a good, ferromagnetic path for its flux it will spread out into the air, following those field lines we are all familiar with from iron filings and bar or horseshoe magnets. If you provide a ferromagnetic path from the north pole to the south, the flux will happily follow it, with little additional leakage into the air.
The steel can of the traditional can motor does carry a lot of the flux flowing between the outer poles of the motor magnets. Trouble is, the can metal is thin enough it becomes "saturated". Too much flux is trying to flow through too little steel, so it leaks out into the surroundings. That is why your rear axle is experiencing the magnetic field of the motor. If the steel can was made of thick enough metal that would provide all the shielding required. The axle would not see any magnetic effects.
So, how thick does your shield need to be? Dunno. A sixteenth may be enough. More would probably be better. A little experimentation should quickly point you in the right direction.
Ed Bianchi