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How
are magnets rated?
Magnets
are characterized by three main characteristics.
These are known as the:
1.
Residual Induction (given the symbol Br, and measured
in Gauss). This is an indication of how strong
the magnet is capable of being.
2.
Coercive Force (given the symbol Hc, and measured
in Oersteds). This is an indication of how difficult
it is to demagnetize the magnet.
3.
Maximum Energy Product (given the symbol BHmax,
and measured in Gauss-Oersteds). This is an indication
of what volume of magnet material is required
to project a given level of magnetic flux.
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What
are the properties of commonly used magnet materials?
Here
are the three important properties that characterize
magnets for some of the most common magnet materials
used today.
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| |
|
Material
|
Br
|
HC
|
BHmax
|
|
Flexible
|
1,725
|
1,325
|
0.6
|
|
Ceramic 1
|
2,200
|
1,900
|
1.1
|
|
Ceramic 5
|
3,950
|
2,400
|
3.6
|
|
SmCo 18
|
8,600
|
7,200
|
18
|
|
SmCo 26
|
10,500
|
9,200
|
26
|
|
NdFeB 35
|
12,300
|
11,300
|
35
|
|
NdFeB 35
|
13,050
|
12,500
|
41
|
|
How
can I use this information?
Given
a magnet size, you can estimate how much magnetic
flux different materials will project at a given
distance or you can use this information to compare
one material to another.
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Examples:
For
example, How much more flux will a Neo 35 project
as compared to a Ceramic 5 of the same dimension
at a given distance?
Simply
divide the Br of Neo 35 by the Br of Ceramic 5
(12300/3950) to get 3.1. This means that the Neo
35 would give you 3.1 times the flux a Ceramic
5 the same size would at a given distance.
Given
a certain flux required at some fixed distance
from the magnet, you can use this information
to estimate what magnet volume will be required
for different magnet materials.
For
example, what volume of Ceramic 5 magnet would
give the same flux as a Neo 35 magnet at a given
distance? Simply divide the BHmax of Neo 35
by the BHmax of Ceramic 5 (35/3.6) to get 9.7.
This means that the volume of the Ceramic 5 magnet
would have to be 9.7 times that of the Neo 35
magnet to give you the same flux.
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What
is a magnetic assembly?
A
magnet assembly consists of one or more magnets,
and other components, such as steel, that generally
affect the functioning of the magnet.
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How
should I assemble magnets to my device?
If
a magnet needs to be fastened to a device, you
can use either mechanical means, or adhesives
to secure the magnet in place.
Adhesives
are often used to secure magnets in place. If
magnets are being adhered to uneven surfaces,
you will need an adhesive with plenty of 'body'
so that it will conform to the uneven surface.
Hot glues have been found to work well for adhering
magnets to ceramics, wood, cloth, and other materials.
For magnets being adhered to metal, 'super-glues'
can be used very effectively.
We
can supply Flexible magnets with an adhesive already
attached to the magnet: all you need to do is
to peel off the liner and attach to your product.
As
with all adhesive applications, it is very important
to ensure that all surfaces being bonded are clean
and dry before bonding.
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What
are the maximum recommended operating temperatures
for different magnet materials?
The
maximum temperature that a magnet may be effectively
used at depends greatly on the 'permeance coefficient'
- which is a function of the magnetic circuit
- the magnet is operating in. The higher the permeance
coefficient (the more 'closed' the circuit), the
higher temperature at which the magnet may operate
at, without becoming severely demagnetized. Shown
here are approximate maximum operating temperatures
for the various classes of magnet material. At
temperatures close to those listed here, special
attention may be needed in order to ensure that
the magnet will not become demagnetized.
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| |
|
Material
|
Approx.
Maximum Operating Temperatures |
|
°C |
°F |
| NdFeB |
140 |
284 |
| SmCo |
300 |
572 |
| Ferrite |
300 |
572 |
| Alnico |
540 |
1,004 |
| Flexible |
100 |
212 |
|
Why
is the maximum temperature a magnet can operate
at not a set value?
Magnets
function at different levels of efficiency given
different circuits that they operate in. The more
closed the circuit the magnet is operating in,
the more stable it is, and the less effect temperature
will have on it.
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Can
I machine magnets?
Magnets
can be machined. However, hard magnet materials
- as opposed to the flexible or rubber type magnet
materials - are extremely difficult to machine.
Magnets should be machined using diamond tools
or soft grinding wheels, and in the unmagnetized
state as far as possible. In general, it is best
not to try to machine hard magnet materials unless
you are familiar with these specialized machining
techniques.
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How
much does it cost to machine magnets?
The
factors which determine cost to machine magnets
are:
Quantity
- the larger the quantity, the lower the cost
since setup charges must be amortized over the
quantity, and special tooling can be created to
machine larger quantities;
Material
- SmCo materials are more costly to machine since
they are very brittle, flexible materials are
very inexpensive to machine because of their physical
characteristics;
Shape
- complex shapes are more expensive than simple
shapes; and,
Tolerances
- the closer the required tolerances, the more
expensive it will be to machine the magnets.
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What
can I use to block a magnetic field?
Only
materials that are attracted to a magnet can 'block'
a magnetic field. Depending on how thick the blocking
piece is, it will partially or completely block
the magnetic field.
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Tips
on handling and storing magnets
Always
take care! Magnets can snap together and injure
personnel or damage themselves.
Keep
magnets away from magnetic media - such as floppy
discs , credit cards and computer monitors.
Store
magnets in closed containers, so that they don't
attract metal debris.
If
several magnets are being stored, they should
be stored in attracting positions.
Alnico
magnets should be stored with "keepers"
(iron or magnetic steel plates that connect the
poles of the magnet) since they can easily become
demagnetized.
Magnets
should be kept away from pacemakers!
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What
are eddy currents?
These
are electrical currents that are induced when
a magnetic field moves in relation to an electrical
conductor, which is placed within reach of the
magnetic field. In turn, these eddy currents create
a magnetic field that acts to stop the relative
motion of the original magnetic field and electrical
conductor.
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What
are some good magnetic reference books?
Permanent
Magnet Design handbook, by Lester Moskowitz, a
385-page book aimed at the technical layperson,
price approximately $150.
Permanent
Magnets and their Applications, by Dr. Peter Campbell,
a 203-page book aimed at the technical person,
price approximately $40.
The
Driving Force, by James Livingston, a 310-page
book aimed at the non-technical reader - a very
well written and interesting book on the history
of magnets and some of their more exotic applications.
Price approximately $20.
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What
do magnets cost?
The
costs of different magnet materials vary significantly
from one to the other. Here is an approximate
guide as to what magnets cost.
| |
|
Material
|
BHmax
|
Relative
Cost
($ / pound)
|
Relative
Cost
($ / BHmax)
|
|
Flexible
|
1
|
$1.00
|
$0.60
|
|
Ceramic
|
3
|
$2.00
|
$0.50
|
|
Alnico
|
5
|
$20.00
|
$4.30
|
|
SmCo
|
20
|
$100.00
|
$6.00
|
|
NdFeB
|
40
|
$50.00
|
$1.40
|
|
Note:
the costs shown here are relative costs based
on high volumes of magnet materials that have
no special machining or other characteristics.
On
a cost-per-pound basis, Neodymium magnets seem
very costly. However, on a cost per BHmax basis,
they do not s | 
C
and above), the Samarium Cobalt types can be stronger
that the Neodymium-Iron-Boron types (depending
on the magnetic circuit).