stainless steel axle magnetism

[SuperSlab]

[GT6]

Yes, this is becoming a deep and expensive swamp full of dragons. Haven't decided whether to throw in the towel on the SCC motor, bite the bullet and buy some TiN coated Ti axles, or go with ball bearings and Ti axles (or even carbon - what the heck, if your walkin' on thin ice, then you might as well dance*)

cheers
Scott

* Jesse Winchester: Do It

[HO RacePro]

Let me add a couple of points...

One, there are two sources of drag on a magnetic axle: the radial load against the bushings/bearings, and what is known as "eddy current braking". Rotating an electrically conductive axle inside a magnetic field induces currents inside the axle that create their own magnetic field, acting against the outside magnetic field and causing a braking effect. Note that the axle does NOT need to be magnetic, only electrically conductive. This braking effect is proportional to the diameter of the axle. A large diameter axle would experience a more power full braking effect.

Two, it is possible to shield the rear axle from the motor magnets by placing a ferromagnetic material -- like steel -- between them. That steel plate "short circuits" the magnetic field. This may be your best solution.

Ed Bianchi

[SuperSlab]

[HO RacePro]

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

[SuperSlab]

I might have done that if the Piranha motor was unique or vastly superior. At this stage it just seems like too much trouble for too little benefit. Not keen on adding weight behind the motor in any case. The proxy my car is intended for is so competitive I do not want to do even the smallest things that could be detrimental.

[Kevan]

Simple solution, put a piece of steel shim the same height as the motor between the motor and axle, 0.5mm would be overkill and reduce the magnetic effect greatly.

[Brumos RSR]

[F1Fan]

[Kevan]

Titanium nitride...not nitrate

[RichD]

The magnetic effect would in fact be proportional to the square of the distance, so doubling the distance would reduce the attraction by 75%. If it was me and I wanted to use the Piranha 25K motor I would be tempted to use an inline setup.

[SuperSlab]

[Brumos RSR]

Let us know you're findings please.

[GT6]

[SuperSlab]