Have I got a magnetic challenge for you!
Have I got a magnetic challenge for you!
(OP)
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 -> ht tp://www.k jmagnetics .com/prodd etail.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
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 -> ht
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





RE: Have I got a magnetic challenge for you!
1 - Make your shield taller extending above an dbelow the magent (is it allowed?)
2 - Better yet - make your shield wrap around the top and the bottom of the magnet like a backwards C.
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RE: Have I got a magnetic challenge for you!
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RE: Have I got a magnetic challenge for you!
Every single modern harddrive comes with a two-pole head actuator that has opposing poles mounted on a flat piece of steel. There's almost zero field leakage to the other side of the steel plate, which is only about 0.1" thick.
Using a mumetal shield would probably work even better. Most mumetal shields I've seen are quite thin.
TTFN
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RE: Have I got a magnetic challenge for you!
IRStuff, perhaps this would apply as well since harddrives are completely enclosed within their shield (at least I would assume so). Also, if what you say is true, harddrive shielding is 10x thicker than what I'm using now. Perhaps I need thicker material or, in theory, only 10 layers of the foil I'm using. The foil has the obvious advantage of being able to shape/cut it.
Thank you, both, for your replies.
RE: Have I got a magnetic challenge for you!
The only thing you can do, and you are indeed doing, is to use a ferromagnetic material as your shield. However, contrary to what you expect, this will attract the flux lines and will cause a major part of them to pass through the shield (the fraction of captured flux depending of course on the thickness, permeability and saturation of the shield).
The consequence of this is that your magnet will lose most of its power, so now the question becomes: what is this magnet for and how strong you need it to be?
To say the same in other words: fully shielding a magnet is easy, just enclose it in a ferromagnetic box, but of course this makes it useless!
prex
http://www.xcalcs.com : Online tools for structural design
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads
RE: Have I got a magnetic challenge for you!
el is used as a cover, but more for rigidity than for magnetic shielding. The flux shorting bars are only slightly wider that the magnets themselves.
TTFN
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RE: Have I got a magnetic challenge for you!
My understanding was redirecting the magnetic field wouldn't cause the magnet to loose it's strength -- I'll have to look into that further -- but, even so, some loss is OK as long as the above requirement is satisfied.
Thanks for your post.
RE: Have I got a magnetic challenge for you!
prex
http://www.xcalcs.com : Online tools for structural design
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads
RE: Have I got a magnetic challenge for you!
http://en.wikipedia.org/wiki/Halbach_array
RE: Have I got a magnetic challenge for you!
So, now that we are fully clear about the situation, do you have any ideas about why my previous solution didn't work? Do you believe it to be just the shape of the shield or am I, in general, doing it correctly but the material needs to change (do I need a higher permeability or saturation or both?)?
These are the areas where I lack the experience. To be honest I haven't had a chance to try the backwards "C" configuration yet (as mentioned by electricpete) -- and I plan to -- but my intuition tells me that if 20+ layers on one side doesn't do it then I have a hard time see how fewer layers wrapped around three sides will (but what do I know?!).
In short, I'm just looking for some more practical tips on how to achieve the desired result without spending tons of money on wasted foil patterns that don't work (or foil materials that won't do the job) if a little up-front knowledge can point me in the right direction.
Thanks to everyone for their contributions.
RE: Have I got a magnetic challenge for you!
TTFN
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RE: Have I got a magnetic challenge for you!
RE: Have I got a magnetic challenge for you!
http://www.magnetic-shield.com/index.html
They have material that is useful in high magnetic fields (such as you have). Their evauation kit would be usesful if you have some way to measure the effectiveness of the shielding.
RE: Have I got a magnetic challenge for you!
To wrap the shield on three sides of the magnet (but why not all around?) will help, but won't change too much if you are interested at a zero field only in the middle of shield's outer face.
I have however to insist on my main point: you need to define what you have to do with the magnet. What you get with the magnet + shield arrangement is something similar to a single fairly long magnet with distant N & S poles (anf this is normally not what you seek in a magnet arrangement), except that the flux lines will extend much shorter beyond the pole faces as they are bent by the shield. Is really this what you need?
prex
http://www.xcalcs.com : Online tools for structural design
http://www.megamag.it : Magnetic brakes for fun rides
http://www.levitans.com : Air bearing pads
RE: Have I got a magnetic challenge for you!
"Zero" for me would be if I put a piece of metal right on (or next to) the shield it won't be effected by the magnet at all. Whether that is really zero or some small value I don't know but that is the desired behavior.
Why not wrap it all the way around? Well here is the theory. The shield needs to "hide" one whole side of the magnet -- not at just one point -- but, at the same time, still be a functional magnet (even if that means at a loss of strength). If I only wrap the shield around like a backwards "C" then the field lines would still be strong on the opposite side of the shield and, thus, so would the magent as a whole (or at least stronger I should say). That was the idea as I understood it. Again, I'm new to this so I could be missing some obvious things (which is why I'm posting
If wrapping the whole magnet in a shield is what I have to do to block one side (and the magnet still has some life left) then that's what I'll do.
So, in essence, the first priority is to block one, whole side of the magnet.
The second priority is to maximize the magnet's strength.
Naturally, if blocking one side "kills" the magnet altogether then that defeats the purpose; if I have to simply reduce the strength on one side to keep the magnet alive then that will have to be the solution.
Does that help answer your questions?
RE: Have I got a magnetic challenge for you!
But I appreciate the link, though, thanks.
RE: Have I got a magnetic challenge for you!
You might start by simply trying cold rolled steel. Cheap and has a high saturation.
RE: Have I got a magnetic challenge for you!
From what you've discussed so far, I see no difference between what you have and the moving coil head actuators used on a hard drive. You have two magnets, side-by-side, flat, about the size of a 2032 battery. One has N up, the other S up. They are both mounted on a flat substrate that provides both structural and shielding functions. I suggest that you spend $40 or so and buy and hard drive and some Torx drivers and have at it. Near as I can tell, the actuator magnet structure duplicates your drawing, except that the exposed face away from the shield has N and S poles perpendicular to the open face.
TTFN
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RE: Have I got a magnetic challenge for you!
I'll look into the hard drive example as well (I never knew they were like that)... thanks again.
RE: Have I got a magnetic challenge for you!
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
RE: Have I got a magnetic challenge for you!
Between all of the posts I should be able to put together something that works... thanks again!
RE: Have I got a magnetic challenge for you!
see:
http://www.etel.ch/cms/default.asp?Id=372
versus:
http://en.wikipedia.org/wiki/Halbach_array
TTFN
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RE: Have I got a magnetic challenge for you!
RE: Have I got a magnetic challenge for you!