[PSUBS-MAILIST] Oil Compensator

Wed Apr 5 18:48:50 EDT 2017

Jon,I don't know whether you have followed, but I have made a brushless 2000W thrusterwhich I trialed in a pool for 30 hrs. The glue on the hall sensor board melted& it stopped because the timing went out. Apparently the glue was designed to meltso people could adjust the timing. The thruster got hot because the compensating oilwas being forced away from the motor through centrifugal force, & up the wiring tube.If I had an overpressure on the system I could have prevented this, but didn't think itwas needed in a few feet of water. I was running this motor with 2 lip seals with ambientpressure in between, but have purchased a mechanical seal for the next version.   It had a comparable or better thrust to Watt ratio than a lot of the advertised ratingsof commercial thrusters.   I think it is a matter of time before Minn kota put out a brushless thruster if they haven'tyet. Most of the commercial ROV & submarine thrusters are now brushless.   If there was enough room to fit a mechanical seal on a Minn kota it would be a bigimprovement. The white paper suggests grinding the Minn kota shaft to a specificfinish, with matching bearing & seal, however a mechanical seal is more tolerant ofthe shaft surface finish & you could avoid this.   Re the heat; one concern is transferring heat off the windings, which oil compensatingwill do 100 times better than air. However Minn kota motors would have large gauge windings that can cope with the heat.Below is a 2010 thread by Carsten dealing with expansion.AlanAlan and the other gentlemens, 

our expierence with oil filled motors, batterys or even propeller shafts is that the oil expand if the motor is runing longer time. 
Simple because its warm up. We had that problem on Eurosub on the first dives. 
And on Euronaut we filled the bladder complet up and the get pretty hard in the hot summer just by sun heating the sub. 
A full filled bladder with over pressure creates a lot of interal 
pressure in the wrong direct - a lot of force for the bladder itself and hose work. 

Therefore our bladder are only half expand during the filling. But still free of any air. 
An other point is that the bladder has to be not to small.

To create a slightly overpressure on the motorseal you just install the bladder below the motorcasing. 
But we found out that on trolling motor (electric outboards) this is not nessesary. There seals are good for some 
douzend feets at least - so you can install the bladder else were. 
Filling is simpler if the bladder is slightly over the motor - you can vent the system direct on the filling point.

Another point is that we install now one bladder per motor  - in the earlier sub (Sgt.Peppers) we install a central
one for all motors. Hard to find a leak after a dive with all the T-crossing piepings and motorseals.. 

vbr Carsten

      From: Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
 Sent: Thursday, April 6, 2017 8:31 AM
 Subject: Re: [PSUBS-MAILIST] Oil Compensator

 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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