Re: Rare earth magnet question

On Mon, 09 Apr 2007 12:42:41 -0500, Ignoramus26085
<ignoramus26085@xxxxxxxxxxxxxxxxxxxx> wrote:


SNIP

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

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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