Have I got a magnetic challenge for you! 5

[Greavis]

Hello,

I'm working on a private project and I have (what seems to be) a simple requirement but I've run into some troubles. First, let me explain the requirement and then I'll explain what I've done and why (I believe) it isn't working.

### The requirement ###

Given a normal magnet with a surrounding field as shown in Figure A (if it doesn't show below go here ->

http://www.egretendeavors.com/shield_q.gif)

the requirement is to apply a shield to one side of the magnet such that the field does not extend past it and is, thus, "flattened" on that side. See Figure B.

Here is the trick (and the key requirement): It is very important that the width of the shield (w) be as low as possible. Other pieces of metal will come very close to the magnet and would, normally, be very impacted by its magnetic field. But, because of the shield, the req is to have the other pieces of metal not be impacted at all (as if there wasn't a magnet right next to it). More than anything that is the biggest requirement (hence the minimum shield width).

The magnet materials and shield materials are flexible. Meaning I am able to use whatever will work to flatten the field on one side with a minimum shield width.

### What I've done so far ###

I'm currently testing with Neodymium block magnets that have these specs:
* Pull Force: 20.07 lbs
* Surface Field: 5755 Gauss
* Brmax: 12,600 Gauss
* BHmax: 38 MGOe

For details of the magnet go here ->

http://www.kjmagnetics.com/proddetail.asp?prod=B88X0

I'm also using some magnetic foil (0.01 inch thick) shielding with these specs:
* Magnetic saturation of 21400 Gauss
* Maximum permeability of 4000 (I assume Gauss)

In order to achieve the desired goal I've had to apply 20+ layers of the foil to one side of the magnet (usually around the same size of the magnet but I've also done layers extending past the magnet). The problem is that by the time I've added 20+ layers the total width of the shield is about 1 inch (because of bends in the foil). The magnetic field is weak at that point -- not because of the shielding -- but, I fear, because of the forced distance from the magnet (i.e. The field is inherently weak at that distance and it has very little to do with the shielding).

So, because of the width of the shield, my method defeats the primary requirement (i.e. of keeping the shield width low).

Even though I'm new to the magnetic world I would have thought that my shielding would have dampened the magnetic field more than it does. Any ideas of what I am doing wrong? What are the key factors to achieve this requirement? Am I using the wrong materials? For the foil (shield) I thought magnetic saturation was key but should it have a higher permeability instead? Are there certain key shapes of the foil that need to be used (e.g. "The foil should extend past the magnet at a distance twice it's length", etc...)?

I'm relatively new to the magnet world so any help is greatly appreciated.

Thank you!
- Greavis

 

[LowEnergyParticle]

Greavis,

1) SReid's suggestion above to look into Halbach Arrays is a very good one. In a Halbach Array, many small magnets (instead of one large magnet) are assembled together in a pattern of orientations that effects cancellations on one side of the array. You end up with something astonishingly like a magnetic monopole. Here's a site with some pictures that may help you visualize:

http://www.matchrockets.com/ether/halbach.html

2) If your application allows it, I suggest you rotate the magnet in your sketch shown in your original posting 90 degrees clockwise. This will put the North pole against the shield, and the South pole out in space producing flux. (You could also rotate the magnet 90 degrees couterclockwise, and put the North pole out in space.) The advantages are:
A) The shield will be extremely effective,
B) The flux field will be symmetric relative to the North/South axis, and
C) The flux density produced around the South pole (or North, whichever one you put out in space) will be greater than without the shield in place.

Of course, your application may not be able to work with a pole out in space like that.

David

 

[Greavis]

David, that's great. I can use other magnets if necessary so I'll entertain what you said as well (where the poles are on the sides, not the length).

Between all of the posts I should be able to put together something that works... thanks again!

 

[IRstuff]

The Halbach array seems to be a more complicated version of the hard drive head actuator. The iron backing of the magnet mounting does the same thing as the lateral magnets in the Halbach array, by terminating the fields of the hidden poles, thereby shielding the backside.

see:

http://www.etel.ch/cms/default.asp?Id=372

versus:

http://en.wikipedia.org/wiki/Halbach_array

TTFN

FAQ731-376

 

[MrEvolve]

Try using different types of materials such as 1/8" thick piece of brass "May" do the trick. Im new to the magnetic inovation world myself. Im currently trying to find a way to increase resistance because pull to me means nothing really for what im working on. Im a machinist from Michigan I am experimenting w 2x 2" dia 2" 1/2 inch long neodyme boron & Ni rare earths. they have a pull of 259lbs and when held 1" apart they have a resistance of only 70lbs. any tipps would be helpful... Thank you

 

[sreid]

Make iron (low carbon steel) pole pieces to make a horse shoe magnet. It will have double the pull force.