[PSUBS-MAILIST] Oil Compensator

Wed Apr 5 17:50:24 EDT 2017

*I covered this in my Psub presentation at the 2016 PSub convention.  But
will hit the highlights.*

*The Minn Kota 101 lower unit (MK part number 2886289) had a shaft seal on
the end of the shaft that is connected to the prop.  The seal arrangement
consists of two identical lip seals oriented to see external pressure.  The
inner lip seal lands on a shoulder in the Brush End Assembly.  This cast
aluminum part does three things. The first is hold the DC brush assembly,
the second to hold a brass or bronze journal bearing, the third is to
provide a cavity to land two lip seals.  The stock MK-101 lower unit the
two lip seals separated by a cardboard spacer.  This seal arrangement is
design for a low differential pressure on the order of maximum 30 fsw or
about 13 psig.  Antidotal experience is that this seal assembly has
withstood three times this depth.  *

*For the last two years I have been operating with this stock seal
arrangement on the four thrusters on the R300.  I use a WATTS R364-02BSS/M1
REGULATOR part number 364-02BSS/M1 to do pneumatic pressure compensation of
my four thrusters. I supply pressure to the regulator from the LP pressure
side of a scuba first stage regulator and have it adjusted to supply
pressure at 4 psi above ambient water pressure. The scuba first stage
regulator is supplying pressure to this reducing regulator at 50 psi above
ambient water pressure.  This is a relieving style pressure reducing
regulator meaning that  when you ascend, pressure vents from the regulator
body. The deepest I have had this arrangement is 160 ft.  To date I have
had no thruster flooding but I don’t have a lot of hours on the units.*

*As both Sean and Alan have pointed out this two lips-seal arrangement is
correctly orient for low pressure applications like the original intent of
the MK 101 units but not ideal for a pressure compensated deep diving
thruster.  As Alan pointed out, the 1 atm air initially between the two lip
seals is an issue as well as the cardboard spacer.  The outer lip seal will
see more pressure that it was designed for which could lead to it failure
with the lip rim on the ½” shaft being pushed in which could flood the 1
atm space and damage the cardboard spacer.  At this point the inter lip
seal is the only seal holding and it is seeing a 4 psi differential
pressure with the pressure inside actually being higher than ambient.  I
have not torn down my 101s to see if the space between the lips seals was
flooded.  *

*I hope to do some testing of different seal arrangements for the MK 101
lower unit in my test chamber at some point. I just have not had time and
since my thrusters have been behaving, not strongly motivated.  *

*The idea of using this pressure reducing / relieving regulator came from
Hugh Fulton as he has installed the system on the Q-Sub.*

*Cliff *

On Wed, Apr 5, 2017 at 3:29 PM, Jon Wallace via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Hi Alan,
>
> Yeah, that white paper pretty much just states fill it with oil and you're
> good to go.  It doesn't address some of the issues that have come up on the
> list regarding heat and expansion, inter-seal cavity, etc.  I understand
> the drive to find a perfect solution and that minn-kota oil compensation is
> not perfect, I'm just trying to understand the real-world ramifications of
> using it as that white paper suggests and most psubbers have implemented.
> Given that water is such a good heat sink, are the heat and expansion
> issues of the oil really that serious?  I know Alec wrote something about
> his experience with this some time ago but am having a hard time finding
> that thread.  Pretty sure SNOOPY is still using simple oil compensation
> regardless.
>
> Jon
>
>
> On 4/5/2017 4:02 PM, Alan James via Personal_Submersibles wrote:
>
> Jon,
> Cliff has been doing a lot of work on Minn kota modifications & has the
> hands
> on experience. If he's not tuning in it would be worth contacting him
> privately.
> The Minn-kota seal set up is not ideal, with the gap between the two seals
> meaning one or the other seals will fail when their pressure rating is
> exceeded.
> The outer seal seems to serve a double purpose, being the first line of
> defense
> against water ingress & also a sacrificial seal that will see all the
> abrasive muck stirred up by the prop.
> Noticed in this kit that the outer seal has a shield to protect it.
> Historically in deep diving set ups they have had dual seals, but with
> either ambient pressure between them,
> or a cascaded pressure system, and normally mechanical seals. The outer
> seal was regarded as sacrificial.
>    Maybe you could pack between the seals with grease to reduce the air
> gap.
> I am not sure whether you could simply reverse the inner seal or not as
> they need a shoulder that the pressure
> will push them up against.
> Cliff is using a small relieving air regulator to compensate his motors.
> The regulator is fed air from your tanks
> & is set at an overpressure of about 4psi. The regulator is orientated
> upside down as the relieving air feeds
> out through the handle portion & the orientation will stop water entering
> when this valve opens.
> This system was suggested by Hugh, & Cliff is trialing it. It has the
> advantage that you could either air or oil
> compensate. If your motor ran out of oil it would just be replaced with
> air above ambient pressure.
> I am not sure at what pressure the relieving valve opens.
>    There is this very good Psub resource from an expert in modifying Minn
> kotas for underwater use.
>
>
>
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>
>
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