RE: [PSUBS-MAILIST] motor seals

["Wade Carson" <wacarson@interchange.ubc.ca>]

Gary,

I'm not exactly sure that I understand your idea but could you use ceramics
rather than PVC?  It seems to me that ceramics would be much stronger and so
you could probably make them thinner and keep the magnets closer together.
I think that ceramics are generally not used for too much because they are
so brittle but they can be much stronger than steel and are safe if used in
compression applications.  I have absolutely no idea what they might cost
though.

I know what you are talking about with the rare earth magnets.  I had one on
a chair once and walked past it no closer than six inches.  I had a steel
nut in my pocket and the magnet actually leapt from the chair onto my
pocket.  Scared the heck out of me because I wasn't looking at it at the
time.

Wade

-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org]On Behalf Of Gary R.
Boucher
Sent: Tuesday, August 15, 2000 9:21 AM
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] motor seals


     I have an idea, not an original one I will admit, but I have some
thoughts on magnetic coupling.  I played around with the idea of using
magnetic coupling to make my scanning sonar have no seals, leakage, etc.  I
purchased several rare-earth magnets from a surplus vendor.  Some of the
new permanent magnet motors use these and most people can not imagine how
powerful they can be.  I literally hurt my hand a couple of times when I
got two of them in close proximity and they came together with my fingers
in the middle.
     I believe there may be a way to engineer a coupling with these
high-strength magnets.  A couple of thoughts on that; these magnets are so
powerful that when I took a piece of aluminum plate 3/16 inch thick, tilted
it on a 45 degree incline, and released one of the flat magnets it creeped
slowly down the aluminum.  This was due to the fact that the very strong
mag flux moving through the aluminum (conductor) created currents in the
metal opposing the flux created by the magnet.  This effect will drain
energy out of any coupling system where metal is used between the
magnets.  The faster the rotation, the more energy lost.  If the magnets
are placed on either side of a flat plate for the coupling you have the
additional problem of using a flat, non-metallic plate to withstand the
water pressure.  I believe that a tube of non-metallic material perhaps PVC
or fiberglass (not composite - carbon fibers conduct!) could be used.  This
could withstand the pressures because of its geometry.  Also, I believe
that the rotating components could be "potted" in an epoxy or even
fiberglass or composite.  With the rotating components, you would not have
to worry about small currents as you would with the housing, since both
systems are turning at exactly the same RPM.  Metal could also be used for
these components.  What ever was used for the rotating magnet holding
system, you would have to balance it to minimize vibration.
     If both motor and propeller magnets are rotating at the same speed
there will be very little loss of power except for the drag of the water
around the prop side magnet.

Gary Boucher