Re: [PSUBS-MAILIST] motor seals

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

:Hi Gary,
>You could have some problems with the motor bearings in the long term. An
>electric motor is not normally designed to take an end load on it's shaft as
>would be applied if you had strong magnets pulling on one end.

I don't know what the bearing configuration of a trolling motor happens to 
be.  But I know that the motor in that case can take a shaft loading 
because the propeller generates its thrust coupled through the shaft.  I 
was proposing to use a trolling motor as opposed to a motor that did not 
have a thrust bearing.  I would "speculate" that a trolling motor can take 
shaft loading of a pulling form because the motors can be operated in 
reverse thrust.


>Second is, as you mentioned, there are some problems to be solved if you
>want to not lose too much energy on the prop shaft side. As well as the
>thrust from the prop working on your Teflon or whatever washer, you also
>have the magnet load working against the washer in the same direction.

It might be possible to construct a bearing system that can contain the 
thrust and is sealed at both ends.  This system would be reasonably easy to 
pressure compensate fluidly with the same oil that the bearings rotated 
in.  Rather than two bearing-seal combinations you could have just one.

>You
>mention that there would be some loss of energy caused by the rotating
>magnets and the effect of the resistance of the separating plate. All these
>losses are starting to add up !

There would be no noticable loss in the plate because I would definitely 
not use a conductive material here.  The rotating magnets would be far 
enough from conductive media where that too would be an insignificant loss.

>  The oil in itself could be 10%.
>You can get mechanical seals that are good for 14 Bar or 200PSI from
>Johncrane and that is the first site I checked although the smallest was
>35mm ID. I'm sure you can get smaller sizes.

Compare this to oil inside the motor compartment which this system would 
not use.  I would guess that oil inside the rotating internals of a motor 
would drain as much as a quarter to a half of the motors power depending on 
the operating RPM.

>Ceramic plate for the 'between magnet ' plate. Not wise I would have
>thought.

The ceramic should work but if you read back in the thread, I suggested 
that you do not use a plate, but a hemisphere or a tubular arrangement.

>  Even a small pressure differential could possibly crack it.
>Polycarbonate is great, you can bend it double and it won't break. Acrylic
>will shatter.

I would not use a polycarbonate for the same reason that I would not want 
to have a submersible with lexan windows.  It cold flows under 
pressure.  Acrylic should prove to be a much better material if one wanted 
to use plastic.  The trick with acrylic is to configure it so as to not put 
the stresses on it that will cause it to crack.  But a tubular geometry or 
a hemisphere say 0.5 inches thick will take a great deal of pressure.

I would be totally in favor of high pressure seals as you suggested if:
  (1) it did not require a great deal of retrofit for the motor.
  (2) they did not have a tendency to create an elevated friction of the shaft.
  (3) they did not slowly allow a creeping of the excluded fluid into the
       internals of the motor through long operation.

This is an area that I am high interested in as I am thinking of adding 
external motors to my sub.  Ken Martindale probably has the most expertise 
in this area.  What I would like to see is someone to come up with a 
retrofit of an existing trolling motor.  I think it should be one of the 
high thrust models.  But if the retrofit is explored, tested, and 
documented then others can simply buy one and do the modifications or have 
them done.  If you need extra thrust use two or three.

>Good luck,
>Karl.
>----- Original Message -----
>From: Gary R. Boucher <protek@shreve.net>
>To: <personal_submersibles@psubs.org>
>Sent: Wednesday, August 16, 2000 1:02 AM..
>Subject: Re: [PSUBS-MAILIST] motor seals
>
>
> >      The magnets will not be "slowed" by rotation unless there is some
> > conductor in the proximity to allow the fields to generate current.  The
> > motor will not have to "work harder" to do anything.  The only headache is
> > trying to create the drive shaft going to the propeller.  If done properly
> > there will be very little energy loss with the majority of that loss going
> > into the bearings and any portion of the coupling that has to rotate in or
> > through the water itself.
> >
> > GB
> >
> >
> > >The loss through heat may be a problem, but something else to consider is
> > >that not very small electromagnetic fields get generated too, slowing the
> > >magnets down and making your motor work harder to get up to the speed you
> > >want generating more fields and causing more headaches than it's worth.
> > >
> > >Anthony
> >
> >