[PSUBS-MAILIST] motor seals

["Gary R. Boucher" <protek@shreve.net>]

     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