[PSUBS-MAILIST] Motor Modification

Mon Dec 5 23:06:33 EST 2016

I can give you my experience with the seals on the Minn-Kota 101 I have on
the R300.  Much of this was covered in my Psub presenation at Watkins Glenn
earlier this year.

The OTS Minn-Kota lower unit (MK part number 2886289) consist of a center
section that contains the permanent stator magnets, the plain end housing
assembly (bow) that contains two radial roller bearing that support the
shaft and armature assembly, a brush end assembly that holds the brushes, a
bronze flange bearing and two lip seals with a carboard spacer between them
each oriented to expect higher external pressure than internal.  There is a
static o-ring seal between the center magnet section and the plain end
housing assembly.  On the brush end assembly, the dual lip seals are design
for approximately 10 feet submergence but have been known to work to a
depth of about 30 ft.  To modify this MK-101 for the four thrusters on my
boat, I reverse engineered the brush end assembly so that I could machine a
replacement  brush end assembly that beefed up the part so that I could
screw three 316-SS supports to hold a nylon MARLIN Wageningen nozzle 37.
This modify part was made of 6061-T6 and hard anodized.  This new part uses
the same two lip seals in the original part design and the same static
o-ring seal beween the brush end addemlby and the center section.  In the
nozzle that is welded to the center section that the two 10AWG conductions
exit from , I inserted a 316-SS adapter that would accept a Subconn BHB2M
bulkhead connector and a Swagelok 1/4" NPT to 1/4" tube fitting.  I use a
single pressure reducing/ releaving regulator that Hugh Fulton specified
for his Q-Sub for air pressure compensation of all four thrusters.  The
regultor is adjustable and I have mine set to hold a pressure of 4 psi over
ambient water pressure.

For both the original MK-101 brush end assembly and my modified version,
there is no barrier fluid other than air beween the two lip seals and their
is no compensation line.  As Alan mensioned in an earier post, it is likely
that this arrangement would cause the the outer lip seal to collapse
againsit the cardboard spacer beween the two lip seals because the initial
pressure beween the two seal is initially 1 atm.

On Jan 24th this year I took my boat to Lake Amistad and did a manned dive
to 155 ft. I had no issues with the thrusters either before or after the
deep dive. They worked great. When I was ascending from the deep dive I did
note that the pressure regulator was releasing air from the thrusters. When
I disassembled the thrusters after the dive, they were all dry.

As to Sean's idea of using a positive bias pressure inside the thruster and
filling the space beween the two seals with oil that has ambient pressure
via a compensation line, I think this could be done for both the original
MK-101 brush end assembly and my modified version.  Might be tricky to
machine the hole beween the two lip seals but I think it might be
possible.  Not sure how you would keep the oil place as there is no room
for a bladder. Might be able to use some kind of plug that would transmit
ambient water pressure to area between the seals.

Before I take my boat out again, I am going to call the  *Parker Hannifin*
guys and get their take how to modify this seal arrangement. In the spirit
of KISS,  I am leaning towards just removing the outbound lip seal and
relying on the pressure compensation system and a single lip seal. We don't
put a lot of hours on this seal so duty cycle is light.  A single lip seal
with pressure compensation might be all that is required.

Maybe we could get Jon to post the PP file on the MK 101 thruster project
to the PSubs web site.

Cliff

  Late year I reverse enginnered

On Mon, Dec 5, 2016 at 8:01 PM, Sean T. Stevenson via Personal_Submersibles
<personal_submersibles at psubs.org> wrote:

> Do the Minn Kota motors have a dual seal arrangement that would permit you
> to run a compensation line to the void between the two seals?  I am not
> familiar with their design. In designing mission critical sealing
> arrangements, I have always stuck to the rule of thumb of using any single
> seal to change fluid, or to change pressure, but not both simultaneously.
> Thus an ideal arrangement with dual seals would be to run bias compensated
> oil in the housing, unbiased compensated oil in the volume between the
> seals, and then the seawater outside. This way, leakage across the inner
> seal is immaterial because the fluid is the same, and leakage across the
> outer seal is unlikely because there is no delta-P.
>
> Sean
>
>
> On December 5, 2016 3:58:49 PM MST, Alan James via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>>
>> Rick,
>> one method Psubbers were using to compensate thrusters was to have two
>> nipples
>> in the thruster with a piece of PVC tube leaving one, wrapping around the
>> thruster
>> & joining the other. The idea being that the oil filled PVC tube
>> compressed to equalize the thruster.
>> It's easy to squeeze the tube with your fingers, but when you have water
>> pressure
>> squeezing evenly, it's collapse pressure is quite high. Also lights are
>> compensated like
>> this with the wiring running through oil filled PVC tu! be from the back
>> of the light fitting
>> to the through hull. Again, the tube needs to be crimped to initiate it's
>> collapse.
>>    With the addition of a bladder there is no need to worry about this,
>> as the bladder collapses
>> easily. If you semi fill the bladder then it can take any expansion. As
>> long as there is enough
>> oil in the bladder to cover the compression of any air at the depth you
>> are going, then
>> you could have quite a bit of air still left in your thruster. But the
>> ideal is to have
>> as little as possible.
>>    I know Cliff mentioned the problem of the dual seals when he was doing
>> his Minn Kotta
>> modifications. I think he built a new end section to the motor, but
>> aren't sure.
>>    I have heard that the Minn Kotta seals are rated to 15-30ft but have
>> been tested
>> to over 100ft in some cases. I have only looked at them from an obscure
>> parts diagram.
>> There may be such a small void between them that it would be a non issue
>> if the seaward seal
>> fails. Sometimes in thrusters with dual seal arrangements the seaward
>> seal is regarded
>> as sacrificial because it sees all the grit etc.
>>    Today I am emailing manufacturers to try & find a cheaper version of
>> the Parker PR364
>> relieving regulator that Cliff is using for compensation.
>> Cheers Alan
>>
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