[PSUBS-MAILIST] Oil Compensator

Wed Apr 5 18:45:31 EDT 2017

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 PSubconvention.  But will hit the highlights.The Minn Kota 101 lower unit (MK part number 2886289) hada shaft seal on the end of the shaft that is connected to the prop.  The seal arrangement consists of twoidentical lip seals oriented to see external pressure.  The inner lip seal lands on a shoulder in theBrush End Assembly.  This cast aluminum partdoes three things. The first is hold the DC brush assembly, the second to holda brass or bronze journal bearing, the third is to provide a cavity to land twolip seals.  The stock MK-101 lower unit thetwo lip seals separated by a cardboard spacer. This seal arrangement is design for a low differential pressure on theorder of maximum 30 fsw or about 13 psig. Antidotal experience is that this seal assembly has withstood threetimes this depth.  For the last two years I have been operating with thisstock seal arrangement on the four thrusters on the R300.  I use a WATTS R364-02BSS/M1 REGULATOR partnumber 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 firststage regulator and have it adjusted to supply pressure at 4 psi above ambientwater pressure. The scuba first stage regulator is supplying pressure to this reducingregulator at 50 psi above ambient water pressure.  This is a relieving style pressure reducing regulatormeaning that  when you ascend, pressurevents from the regulator body. The deepest I have had this arrangement is 160ft.  To date I have had no thrusterflooding but I don’t have a lot of hours on the units.As both Sean and Alan have pointed out this two lips-sealarrangement is correctly orient for low pressure applications like the originalintent of the MK 101 units but not ideal for a pressure compensated deep divingthruster.  As Alan pointed out, the 1 atmair initially between the two lip seals is an issue as well as the cardboardspacer.  The outer lip seal will see morepressure that it was designed for which could lead to it failure with the liprim on the ½” shaft being pushed in which could flood the 1 atm space anddamage the cardboard spacer.  At thispoint the inter lip seal is the only seal holding and it is seeing a 4 psi differentialpressure with the pressure inside actually being higher than ambient.  I have not torn down my 101s to see if thespace between the lips seals was flooded. I hope to do some testing of different seal arrangements forthe MK 101 lower unit in my test chamber at some point. I just have not hadtime and since my thrusters have been behaving, not strongly motivated.   The idea of using this pressure reducing / relieving regulatorcame 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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