[PSUBS-MAILIST] Oil Compensator

Wed Apr 5 19:49:00 EDT 2017

Hank I realize that the orientiation of the off the shelf lip seal set is
backward when the internal pressure is greater than ambient.  First off, I
wanted to see how well the seal set worked without monkeying with them.
The springs around the lip seal are tight enough that this 4 psi does not
open them in this situation.  The other reason I left them was that the
boat could be sitting in the water over night without the pressure
compensation on.  I wanted to make sure the thruster would not flood in
this case.  When I get a chance I will talk with Parker guys on this
application and see what hey recommend.  Minn Kota guys will just say that
we are trying to use this lower unit for something it was not designed
for.  This seal arrangement is a work in progress.  I am reluctant to go to
a higher order seal system like double mechanical cartridge seal as it
defeats the KISS theme for these cheap DIY thrusters.

Cliff

On Wed, Apr 5, 2017 at 5:45 PM, hank pronk via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Cliff,
> I am confused as to why you would want a 4 psi internal pressure pushing
> against a spring that is trying to keep the seal tight to the shaft.  I
> would think the seal should be reversed with a 4 psi internal pressure to
> assist the spring  in keeping it tight to the shaft.  This will also apply
> to an oil filled motor, in the case of oil filling, the 4 psi would be
> beneficial in keeping the seal tight.
> Hank
>
>
> On Wednesday, April 5, 2017 3:50 PM, Cliff Redus via Personal_Submersibles
> <personal_submersibles at psubs.org> wrote:
>
>
> *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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