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Thread: Underwater robots
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face BY : Mike Kendall email (remove spam text)



I don't think the oil slowing down the motor may be that much of an issue.
I would use the Shell Diala as it is$27 for 5 gallons and it has the proper
inhibitors in it. If the stator is fixed and the permanent magnets are the
rotating element, then the curved magnets may have pieces of plastic
inserted between them.  The combination of the magnets and plastic would
make a continuos disk.  This "disk" would be mounted on the prop shaft
between two Delrin bushings (self lubricating in the water).  I have seen
some of the "supermagnets" made from ceramics/rare earth sold on ebay pretty
cheap.  They are recycled from computer disk drives and other electronics.
There is even a web page where a gentleman in Colorado built a brushless
wind generator turbine from them. The majority of the friction would be from
the laminar fluid flow but negligable on performance due to the low rpm and
the case of the u/w robot being shaped around the disk assembly.  I've got a
book written by a Alfred T. Forbes in new zealand called "the homebuilt
dynamo" that uses one of these designs. The NeFeB magnets have flux lines
that extend longer than traditional magnets and should extend through the
case between the magnets and the fixed stator windings. The u/w robot case
could have a molded section for the bushing to be pressed into it and molded
threaded holes that don't  penetrate through the case.  The second delrin
bushing could be then pressed into a crossarm, slid over the shaft and
secured into the threaded holes.  As I own a few fiberglass boat  molds, my
first choice for the case would be fiberglass. The part of the case that
forms the barrier between  the rotor magnet disc and the fixed stator
windings would have to be a different material that is thinner and attached
with a compression o-ring to the main case, probably the real reason for the
incompressible fluid inside the case as it allows for the thinner material
between the fixed stator and rotor disk containing the magnets.  The larger
the diameter of the disk the lower the rpm and higher the torque.  A dead
spot causing stalling is non-existant. Check out Hugh Piggot's web sites out
of Scotland for brake drum wind generators.  Everything down to his xerox
machine runs off of power from his homebuilt designs.  You have to be
careful about the oil.  For example, I have used the dummy load oil to cool
ferrite baluns before that have teflon dielectric windings.  The oil won't
bother ferrites or teflon, but will permeate powdered iron cores and change
their properties.  Also, you have to be certain that the correct potting
compounds and such are used.  I've seen mineral oil cause rubber insulated
wires turn "spongy" over time in sonar stuffing tubes for transmitters and
cause shorting.  I would even be tempted to look into encapsulating all my
circuit boards in potting compound.  If you don't have a vacuum machine to
pull the bubbles out of the epoxy potting compound, possibly a circuit board
potted with a spin casting machine?  Spin casting is some pretty cool stuff
and can be done in a garage setup. All said, I wonder if the disk could be
mounted 90degrees apposed to allow a better hydrodynamic shape?  Then, it
would require a delrin worm gear?
Regards,
Mike
{Original Message removed}
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