[PSUBS-MAILIST] pressure comp motors

[sfortt@ozemail.com.au]

hello to all at psub. i have been watching on the sidelines of psub for
many years now and with no experience or expertise this area i remain quiet
on those sidelines. till now. I have read the white paper (to follow) on
the psub sites that indicates the ease of pressure compensation of minkota
motors. this email is therefore addressed to the authors of this paper (pat
regan aka Captain Nemo and dave of dc marine). i want to pressure
compensate a minkota motor and need alot of help to do so. i have no idea
how or where to start. i am leaning towards air compensation but open to
all options

                                                  in anticipation thank you
   shayne




Contributor:    Pat "Captain Nemo" Regan and Dave at DC Marine in Arizona

Last update:    29 Nov 2000

Synopsis:       Discussion about Minn Kota Motors and compensation.

Disclaimer:     You are resposible for your own safety. Even though
                this information may work or seem resonable in some 
                cases you need to approach the subject carefully and 
                even aquire the help of a Marine Architect before applying
                into one of your designs.


Body:


Just got off the phone with Dave at DC Marine in Arizona; my MK RT-101/S 
should be here by Monday; installation to follow.

Dave is a very experienced and knowledgeable tech, who has supplied 
many stock motors and built many custom propulsion systems for owners 
of small submarines and ROV's around the World; and his website at:

http://www.dcmarine.com

Dave's pretty busy most of the time; but he said he'll try to answer 
technical questions on an "as able" basis.  Today we got into a good 
rap over the phone, and he clarified for me some things that have been 
talked about at PSUBS lately.

1. MK (Minn Kota) rates their motors as watertight to one additional 
atmosphere beyond surface pressure: a depth of 33 feet.  People have been 
known to operate them to about 100 feet without modifications to the lower 
end unit, and the failure rate at that depth is about 4 to 5 percent.  
Still, the company won't stand behind that, so 33 feet is the specified 
depth limit for seal integrity, according to MK.   And no, doubling the 
number of seals won't ensure operations at greater depths, because pressure 
sufficient to implode the outer seal will also be enough to implode the 
inner seal, too.  (Since they are side by side on the shaft, there is 
no cumulative increase in their depth resistant integrity.  When the water 
gets past one, it will immediately bring it's pressure to bear against 
the other, which will at that time be operating alone, and have no 
greater strength than it's failed counterpart.)

2. Yes, MK motors run fine and have excellent depth compensation 
qualities when the lower end is filled with transformer oil.  The fluid 
is of a fine enough viscosity that reductions in armature RPM are not a 
problem.  Dave has used this method on many motors he's built for customers, 
some of whom are operating ROV's at 300 FSW; for these units, Dave also 
polishes the armature shaft to a 4 micron finish, and substitutes a matched 
bushing and seal in place of the stock ones.  But for shallow ops, no 
compensation is necessary; and at moderate depths, the oil alone is 
sufficient.  Just make sure the case is completely topped off and free 
of any air voids (impart a vacuum to the case before filling).  There's 
another fluid that can be used: it's called by the nickname "Jesus water"; 
Dave couldn't recall the exact name of it; and he hasn't been able to 
find any for a long time; but if anyone knows what this stuff is and 
where to get it, it sounds like it might be another option worth considering.

3. As some of us already know, air compensation works fine; Dave agrees, 
but thinks the oil filled method is a better in terms of simplicity, 
reliability, lubrication, and such.  This time around, I'm sticking 
with air compensation; but I might try oil in the future, when I'm not 
facing a deadline to get the boat into the water like I am now.

4. Lately, there's been some discussion at PSUBS about continuity 
between the three sections of the lower end housing being possibly 
disrupted by paint or O-rings at the juncture points.  According to Dave, 
this shouldn't be a problem, because the three sections are joined by the 
armature shaft which passes through bronze bushings in each component.  
Maybe there's more to this "gremlin" than meets the eye.  I don't know.

5. Of course, if a motor does get flooded, it's got to be disassembled 
and serviced.  The methods Ron, Herve, and others mentioned sound good: 
open it up; rinse it with kerosene or the like; dry it thoroughly, and 
reassemble.  Carsten's idea of a little oil inside as a preventative 
measure to inhibit rust is good, too.

6. If you do have to disassemble your lower end unit, the critical part 
of reassembly is BRUSH TIMING with the armature.  This was a little 
complicated, so I'm waiting for a better detailed description of how 
it's done.  The key is getting the correct alignment between the case, 
brushes,  and the armature; and then tightening the case through-bolts 
down to a spec of 32 to 35 inch pounds.  If I get time, I'll try to 
put together a more detailed description of the process; or you might 
ask Dave.  But reportedly, if you get this aspect of it correct, she's 
going to run real sweet; while without it, she might not.

7. Those dealing with a complete MK unit which has the composite 
shaft should understand taking it all apart is not as easy as it is 
with the metal shaft.  The metal one can simply be unscrewed from the 
lower end case mounting collar; the composite one is still 1-1/8" 
tube and threaded, but it's put in with locktite, and to get it out you 
might have to chuck it up in a vise and go through some trouble.  Then, 
you might have to order a special tap from MK that will chase the 
threads in the lower end collar, which will probably be filled with 
locktite and composite residue.  Something to be aware of before 
tearing into a composite shaft trolling motor setup....  

Anyway, Dave's a good guy to buy from or talk tech with, so give him 
a call or drop him an E sometime.

Hope this helps.

Pat