Magnetics Business & Technology - Summer 2017 - 22

SPONTANEOUS THOUGHTS

a Column by Dr. Stan Trout

Permanent Magnet Mistakes, Part Seven
Stan Trout, Spontaneous Materials
We conclude this series with the seventh and final blog, describing the many types of mistakes made with permanent magnets. My
intent is to help engineers in the future avoid mistakes made in the
past, and not to embarrass anyone.
The last two mistakes on my list are:
* Ignoring appropriate mechanical considerations, such as using
a magnet as a structural part, believing that chips and cracks are not
normal for brittle materials, or ignoring the effect of magnetic debris.
* Treating magnets as toys
Most engineers are surprised when I tell them that the mechanical properties of any grade of magnet are measured only once.
They are measured once so that they may be included in the datasheet. Since mechanical properties almost never come up in the
conversation, there is no point in measuring them again.
Permanent magnets are almost never put in a design because
of their mechanical properties. They are only included because of
their magnetic properties. Mechanical properties are only included
in datasheets for the sake of completeness, and little else.
This means that the mechanical robustness of a design must
come from the components other than the magnet. Applying stress
to a permanent magnet is asking for trouble.
In the early days of SmCo magnets, many people referred to
them as jewels. They certainly had a nice appearance, perhaps too
nice. It is difficult to maintain a magnet's perfect appearance all the
way through the process into their ultimate use. Chips and cracks
happen all the time to magnets, and they should not be considered automatic disqualifiers. The best approach is to include a note
on the magnet drawing to spell out what amount of chipping or
cracking might be acceptable. This is a good topic to discuss at the
beginning of a project, otherwise the buyer and seller will need to
argue out this point later when chips and cracks inevitably occur.
Suppliers have been known to ship nearly perfect magnets early in
a business relationship and then gradually relax the physical standards later. Caveat Emptor!
Most magnetic devices have what is called a gap. This unique
region, where the magnetic flux is available in the air, not buried
inside the magnet or the return structure, is important. Without a
gap, most of the magnetic devices we know would not work. Gaps
also have a problem in that they are natural collection points for
any loose magnetic material. For example, any dust that contains
enough iron will ultimately find its way to the gap and accumulate
there. Where it is often difficult to remove. Debris in this collection
point has the effect of reducing the efficiency of the device and
may eventually cause it to fail. It is important to keep the environment around devices clean, to preclude anything that might collect
in the gap.
As children, one of the first things we notice about magnets is
that they can attract and repel each other, depending on how we
configure the poles of the magnets. It is a cause for great fascination. Likely, the first magnets we handled actually were toys, just
large enough to demonstrate the powerful effect of magnetic
fields, and small enough not to be hazardous.
Before we had rare earth magnets, making magnets safe for children or adults to handle was a relatively easy task because the
materials available at the time were all relatively weak. Once rare
earth magnets came on the scene, we faced a conundrum. Rare
earth magnets are especially hazardous when they are large and

Magnetics Business & Technology * Summer 2017

free to move. Anything in their way, as they fly together, is in serious danger. As we shall see, making them smaller creates a different
problem. They become a swallowing hazard.
Perhaps a personal story would be illuminating. As a graduate
student, I received several SmCo5 one-inch cube samples to use
in my research. Before I could store them and while my back was
turned, one of my lab mates decided to play with these samples. It
didn't take very long. I heard a loud snap and when I turned around,
I saw my lab mate was bleeding from a gash in his hand and my
magnets were shattered. It was a good lesson for both of us in the
dangers of isolated magnets and their sharp edges. Large magnets
are not toys.
More recently, the US Consumer Products Safety Commission has
banned small spherical NdFeB magnets being sold as toys. Typically, these magnets are 3 to 5 mm in diameter. They certainly show
the strength of sintered NdFeB magnets very well. The concern is
the report of several cases of children swallowing a few magnets.
Should the magnets snap together while passing through the digestive system, severe damage may result, even death. Ignoring the
safety concern, one Colorado company has persistently fought the
ban of these small spherical magnets, and may prevail in court.
Allowing these small magnets back into the marketplace as toys
would be truly unfortunate.
Now that I have completed the series, I would like to ask my
readers two questions. 1) Did you learn anything new that you
would care to share? 2) Are there any mistakes left that I should
cover? Please email me your comments.
I wish you a mistake-free future!
Dr. Stan Trout has more than 35 years'
experience in the permanent magnet and
rare earth industries. Dr. Trout has a B.S. in
Physics from Lafayette College and a Ph.D.
in Metallurgy and Materials Science from
the University of Pennsylvania. Stan is a
contributing columnist for Magnetics Business & Technology magazine. Spontaneous
Materials, his consultancy, provides practical solutions in magnetic materials, the
rare earths, technical training and technical writing. He can be reached at strout@ieee.org.

www.MagneticsMagazine.com

http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2017