Using magnets as bearings

[Russell]

I have started dabbling in model aircraft again and have dug out an old model that I thought had a damaged and therefore out of balance propellor.  So I knocked up a device similar to this:



which is just a pointy ended shaft held between two neodiddlydum magnets.

It works very well such that a piece of sellotape 10x10mm is enough to unbalance a propellor.  However when spinning the shaft (2mm piano wire) on its own it spins very freely for a few seconds and then stops fairly abruptly as if there is a magnetic brake effect.  It nearly always stops in the same place.  It could be a mechanical issue with shaft straightness or point accuracy but I'm inclined to think it's a magntic effect.

Anyone any thoughts?


NB:  I don't have cones on mine as the bore of my propellor is 2mm and is a snug fit on the piano wire.

[Brucey]

IIRC Piano wire has a reasonable coercive force so may well retain enough field such that it will always 'settle' in the same place when it stops spinning.

Two things strike me

1) that (due to reluctance effects) the wire will tend to find 'the centre' of the field, but if the thing is out of balance the wire may run out of true slightly, (which might be best seen using a high magnification camera?). and
2) the effects of gravity could presumably be minimised by having the shaft vertical instead of horizontal?

It is very nice though!

cheers

[Russell]

2) the effects of gravity could presumably be minimised by having the shaft vertical instead of horizontal?

Yes but.  The idea is to use gravity to find an out of balance, ie one blade heavier than the other.

[Russell]

Ah, as I was playing with my shaft  I realised you probably meant having the shaft vertical to isolate gravity when checking the shaft trueness.

Anyway the same effect occurs, horizontal or vetical.

[Davef]

You could make a tiny turbo trainer for a mouse. I am guessing it is the same eddy current principal in action.

[Russell]

Just seen this in an instruction manual:

Handle  the  balancer  shaft  with  care.  It  is  made  of  soft  steel  (for  high  magnetic  attraction)  which  will  bend  easily   if   handled   roughly.

Piano wire is hardened is it not, so will be different from a magnetic point of view?

[Brucey]

yes, hence earlier comment about coercive force.

cheers

[Wobbly John]

Just seen this in an instruction manual:

Handle  the  balancer  shaft  with  care.  It  is  made  of  soft  steel  (for  high  magnetic  attraction)  which  will  bend  easily   if   handled   roughly.

Piano wire is hardened is it not, so will be different from a magnetic point of view?

Although piano wire is mechanically hard, I belive it is magnetically soft (it will become magnetised)...

[Brucey]

a soft magnetic material has both a high relative permeability and a low coercive force. Piano wire has one but not the other.

cheers

[Russell]

Thanks for the ongoing comments.  I don't know if it achieved anything but I heated up the piano wire to cherry red and let it cool and swapped out the magnets for the smallest I could get away with with the largest gap (prop supported at the end that touches) and I believe it works well enough for balancing the props I intend to use.  The mass of the prop is sufficient to over come the magnetic 'braking' effect.  It is sensitive enough that a piece of bluetac much smaller than a pin head will unbalance a 6" prop.

[Brucey]

if I wanted to lower the coercive force of piano wire I would have done the same thing.  It'll be (mechanically) much softer now too.

cheers

[aidan.f]

If you are setting up eddy currents in the conductor you will get a braking effect.  However I don't see that being significant in this case as there is not a large cross section of conductor rotating in the field