Magnets and clutches

--Hmmmm. You might try asking that over at sci.physics.electromag...

There is no way to stop a magnetic field. You can alter the flux path with a ferrous metal however putting aluminum between a steel plate will do nothing to increase or decrease the strength. Magnets lose their attraction at 1/D squared.. in other words if we double the distance from the magnet it will only have 1/4 the attraction For many years Eddy Current clutches were used to do what you describe .

The only practical effect would be to isolate the steel from the magnets for as long as the magnets move in relation to the aluminum. Any paramagnetic effect of the aluminum will be negligible, if I'm even correct in remembering that Al is paramagnetic at all. Strong magnets working against aluminum _will_ generate eddy currents as a consequence of motion, and you can use that to generate drag. Strong magnets working against steel will generate an attractive force, eddy currents, and some hysteresis force that'll work even when the relative motion has ceased.

Without knowing why your clutch uses steel instead of something else, and how you propose to mount the aluminum, I couldn't say what the effect will be.

Seems to me it would WEAKEN the field. Conducting non-ferrous metals actually provide some shielding of varying fields.

It is. A bit. The right word to look for in wiki is susceptibility. You'll find that Al has one of 22.2·10?6. And materials with a susceptibility above 0 are paramagnetic.

Nick

Just came to my mind: Brass is halfway good for shielding magnetic fields, copper is better. But you have to make a closed case. Not applicable in your case. Materials out of my head, I might be wrong. Once made a shielding case for measuring cellulars and researched a bit into that. In the case of radio waves, things are a bit more complicated, because there are magnetic and electric fields.

Mu-metal is best for shielding pure magnetic fields.

Nick

I've done a spring loaded clutch. Check my web site under builders log. Might be of some interest.

DOC

Have robots. Will travel.

The magnetic field that represents a radio wave generates a current in the shielding material, whether it's aluminum or copper or whatever, and this current is grounded, effectively killing the field's effect on whatever the shield is protecting. A common example is shielded ignition wires used in aircraft to stop radio noise. I would expect that a magnetic field passed through an aluminum plate would have no loss of strength UNLESS the plate or magnet was rotating, whereupon an eddy current would be generated that would likely heat the aluminum somewhat. That heating would represent some loss of field strength through the aluminum. The current generated in a wire's shielding is created as a result of the AC nature of the field, which has the same effect as relative movement between the field and shielding.

Dan

If the field is varying then a conductor will block the variation -- the varying field generates eddy currents which set up opposing fields.