Rare earth magnet question

I have been reading about searching for meteorites. I live in a good area for it, with lots of open desert and few plants. Lots of sand dunes, and
lots of sandy washes.
I want to make a device that I can haul behind my ATV, like a rolling magnetic nail sweeper. However, I want to make one on steroids with six inch tires, and a row of very powerful rare earth magnets. I want it to be three feet wide. I want a VERY powerful rare earth magnet about every six inches. I want it to pick up meteorites and metal from at least three inches, which would be the axle height.
Anyone have a clue as to what sized magnets I should be looking at? There's millions of these magnets on ebay, but I'd just like to make one big mondo one and see if it's going to work.
Help appreciated.
Steve
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Steve B wrote:

Steve Nooooooo!
Wind yer own electromagnet so you can turn it off when your ATV goes 'wheels up' on locating a chunk of railroad. If you don't feel like winding, just bolt a bunch of A/C clutch coils facing downwards:
http://bernardembden.com/xjs/comclutch/19coil_test_old.JPG
--Winston
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wrote:

Indeed. Lots of road maint guys build rigs with electromagnets..so they can dump the picked up items when the rig is full.
Unless you can pull that big chunck of scrap off that rare earth magnet with a plastic crowbar.....
Gunner
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Lazarus Long
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Aluminum alloy won't work?
If the OP is raking the desert out by El Centro and finds some metric wrenches, masterlinks and B9E sparkplugs, they're mine and I'd like them back.
Fred
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Steve B wrote:

Good points have been made about an electromagnet instead of permanent magnet. Picking up stuff from 6" takes a fairly mighty unit, I think it'd end up by the time you've got the power supply, switchgear and magnet together you might have quite a load, and anything of respectable voltage makes a heck of an arc when you open the relay. 240v DC is impressive..
Another approach might be to get a magnet that's made to hang from the bumper of a yard truck, check out your local truck terminal to see an example. The ones I've seen are about 48" wide, they pick up debris from a paved surface pretty well and probably come in different sizes.
How about a permanent magnet roller with a belt over it? Let the thing roll on the ground, run the belt over a small roller above/behind the magnetic roller, and a hopper under the belt- sort of like a lawn sweeper. Look up magnetic conveyor roller and magnetic separator, you'll see what I'm talking about. Notice some belting has paddles or ridges to help carry debris away from the magnet, otherwise it just slides on the belt.
John
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Steve, I have plenty of tiny neodymium magnets, and know that they are fun, but they are also so strong that pulling them apart is not easy. Try buying a 1x1x1" magnet and stick it to a steel block (try to make sure that it does not slam into the steel or else it may break). Then pry it off. You will be able to pry it off, but it will be difficult. Now imagine that you have a magnet as big as you described!
i
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I was thinking of the one inchers, because of cost factors, and because of the strength of the ones I have that are even smaller than a penny. I could do the whole thing for under $100.
Steve
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This would be a really fun thing to watch progress of.
I actually have some strange stone that I found 30 years ago and suspect of being a meteorite. I will try to find it and research a little bit.
i
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We are desert rats. We have found all sorts of things out there from pioneer junk to arrowheads to WWII ordinance. And all the time, I could have been dragging one of these, and snagging a meteorite here and there. We're in a good spot with lots of flat areas and few plants. I'll keep everyone posted as it goes.
Steve
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Would be interesting. Effects of old projectiles jumping and slamming into neodymium megnets may also be interesting.
A long time ago we were working in a field and my friend found a WWII mortar shell.
Realistically, though, magnets' pulling power declines very quickly as a function of distance, so even very strong magnets may not be able to pull out something buried a couple of inches in the sand. I would experiment on a small scale first.
i
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We have found lots and lots of WWII ordinance. Mostly 50 cal. BMG casings. I don't know if they are metallic, as they are all stored away now pending moving. The bullet does have a steel armor piercing core. We do find live rounds occasionally.
Steve
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On Mon, 09 Apr 2007 12:42:41 -0500, Ignoramus26085
SNIP

Rare Earth magnets store a lot of magnetic energy in a small volume but their normal shape is such that the field strength falls off very rapidly with distance. It's a complex subject but VERY roughly speaking the main part of the useful pickup field strength is around a volume centred on a semicircle joining N pole to S pole. For your 3" working distance the magnet length should be in the region of 6" !!!.
However this is only necessary if you aim to reach a large fraction of the possible maximum flux density. If you are content with somewhat lower values the effective magnet length can be increased by suitably placed pole pieces.
The very simplest arrangement would be a series of short bar magnets and pole pieces producing a
N-S-N-S-N-S........ field.
Pole pieces could be 6" long lumps of mild steel of the same cross section as the magnet and.arranged as below. (NS or SN is a magnet, PP is a pole piece, 1/2PP is half length) 1/2PP-NS-PP-SN-PP-NS-PP-SN-1/2PP
However this would be pretty inconvenient as most of the pickup would gravitate towards the rare earth elements. This would make it difficult to remove and probably damage the magnets, It's better to convert the series of artificial "bar" magnets to a series of "horseshoe" magnets. Pickup then sticks to the ends of the horseshoe extensions and is safely distanced from the expensive magnets.
The extensions that form the horseshoe shape should be mild steel plates typically 4" to 6" long, the same width as the 6" long pole pieces but about 1/4 to 1/3 the cross sectional area and located at the centre of each 6"pole piece.
For a test rig, make up a single horseshoe from one magnet,two 3" pole pieces and two 5" extensions mounted at the end of the 3" pole pieces. Buy a pair of large magnets and try them out in the combinations
1 magnet 2 magnets side by side NN SS 2 magnets on top of each other.NSNS
This will tell you whether a larger magnet area or larger magnet thickness is the best way to go. Because of the very large leakage flux in this arrangement I would expect largest magnet area to be the best choice. For the two magnets side by side test it is not necesary to double the cross section of the pole pieces - just sandwich the two magnets between two plates (1/4"++ thickness) and insert the assembly between the 3" pole pieces. The magnets will object to being same polarity side by side and you may have to temporarily glue them in position.
You could also experiment with the 3" pole piece length. Increased length reduces the useful field strength but the reduced field is effective over a longer distance.
This is not the only possible arrangement. The basic requirement is to have about 6" of pole piece in series with the rare earth magnet. The pole piece can be split and the magnet located at any convenient split position. One alternative arrangement is a single pole piece bar extending the width of the vehicle. At 6" intervals a magnet is located between the lower side of the bar and a downward extension pole piece.
These arrangements should produce the right sort of field shape but 3" is a very long working distance and it may be difficult to achieve sufficient pickup power.
Jim
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I once had a welder that had a magnet that was the size of a shoebox. We used it to keep tanks from rolling around in the back of a truck. (dumb, I know) He ran of with some stripper and left that and some other gear. (actually went missing)
It was a Mongo magnet, and I wish I had not been parted from it. It would be the perfect thing. Mount it on a 3' x 3" x 1" piece of plate, and the whole thing would be a magnet.
Steve
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Magnetic force varies with the square of the distance.
So if you want the magnet to pull one pound at 3" then it will pull
9 lbs at one inch. 900 lbs at 0.1 inches 90,000 lbs at 0.01" inches.
Its not going to happen with any magenet you can physically lift.
And if you can physically lift it you would never ever get stray metal unstuck.
Sorry to be a spoil sport, but the physics just don't work.
Paul
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wrote:

Incorrect, it varies with the cube of the distance. (which makes the rest of your argument even stronger)
i
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Ignoramus3938 wrote:

While magnetic field falls off in an inverse-cube relation with distance, apparently magnetic force between a magnet and another object falls off even more rapidly. Eg, inverse 4th power between magnets, inverse 7th power between a magnet and a bit of steel. At least that's what the page http://isaac.exploratorium.edu/~pauld/activities/magnetism/forcebetweenmagnets.html says, near the end.
--
-jiw

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Steve B wrote:

After reading all the replies so far (Mon 1603) I havent seen any discussion of "pole peices" or the magnetic path. I think you can do a lot with less magnet strength by aranging the rest of the magnetic path to minimize the air gap. As is done in some magnetron magnets. Too complex a subject to try to define here. :-) ...lew...
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Very interesting discussion, on these rear earth magnets.
Now, I have a question not exactly along the lines of the discussion, but pehaps at a tangent.. A friend of mine has told me that these magnets can be wound with a coil, as in and an electromagnet, with amazing results to their apparent strength, to increase or diminish it. Is this so?
Thank you,
Flash

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Flash wrote:

It is so.
--Winston
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As many others have pointed out, direct use of big magnets is not going to be practical.
What will work is a large pulse induction metal detector. meteorites of any size are going to give a huge signal.
One can purchase such units, or build them. for more information than is healthy, go to:
<http://www.gi.alaska.edu/~jesse/treasure/misc/howdetector.html
and <http://www.findmall.com/list.php?34 (PI Technology Forum).
Joe Gwinn
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