[PSUBS-MAILIST] Oil Compensator

Thu Apr 6 08:42:18 EDT 2017

Pressure can help, of course, but your sealing arrangement should provide a positive seal even in its absence. The spiral action can be avoided by careful attention to surface finish and eliminating machining marks on your shafts. This doesn't eliminate the problem though. Some seals are designed to be hydrodynamically lubricated by their encapsulating fluids, lifting the seal lip away from direct contact with the shaft (thin film lubrication). The bias pressure ensures that any leakage across the seal is of oil out (which has only environmental consequences) instead of seawater in (which can ruin your day in a hurry). It also allows you to monitor either the compensation pressure or the displacement of the compensator (via either a displacement transducer, which would give you rate information, or a limit switch, which would just tell you if it had bottomed out) so that you can alarm on loss of compensation.

As for the air / oil debate, I would look carefully at the dielectric constants of each versus your intended operating depth. Oil behaves pretty much identically at all pressures. Gases increase in density at depth, reducing their effective isolation voltage. Probably not an issue at PSub depths, but for a much deeper diving sub, were I to consider gas compensation, I might look at something like sulfur hexafluoride instead of air, but then you're still dealing with venting the excess gas on ascent. Oil is messy and consumes more power, but provides additional lubrication and an excellent heat transfer mechanism.

Sean

On April 5, 2017 10:33:08 PM MDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Greg,
>I use 2nd stage octopus regulators on my ambient sub, but feel this is
>better
>because you can adjust the regulator to your desired pressure over
>ambient. 
>With the second stage regulator you might be getting 1psi above ambient
>if 
>you have it 2ft below the thrusters. Industry standard seems to be
>around
>4-8 psi above ambient. Most of the cheaper commercial compensators rely
>on a spring whose force varies depending on it's extension which is
>relative
>to the oil volume, whereas with Hugh's method you should be able to
>keep
>a narrower range.
>I don't know why the commercial units work in that range. Obviously the
>more
>pressure you have the less chance there is of water getting in. I
>believe mechanical
>seals like a bit of pressure to help oil their contact faces. Also I
>have read that
>you can get an unpredictable pumping action from microscopic spiralling
>on the 
>prop shaft, that can pump water in against an over-pressure.
>Alan
>
>Sent from my iPad
>
>> On 6/04/2017, at 3:54 PM, james cottrell via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>> 
>> Alan/ Cliff,
>> 
>> That's interesting- but I don't understand why it's better than a
>good working second stage? This is what I did-
>> 
>> Three trolling motors (one vertical and port/starboard pushers) had
>clear vinyl tubing connected to the tube/pipe where the wires come out.
>> Each clear piece of tubing ran to the "inlet" side of a small water
>filter housing (the kind with a clear plastic removable sight glass).
>> The water filter housing served as a junction box so each motor could
>be removed individually.
>> The tubing them came out of the "outlet" side of the filter housing
>and went to a "T" fitting.
>> One piece of tubing carrying the wires went from the "T" to a
>penetrator in the hull.
>> The other piece of tubing went downward to a set of two "T" fittings
>that served as a manifold connecting all 3 pieces of tubing together.
>> Finally one Large piece of tubing went to a scuba second stage
>mounted low on the hull (connected where the mouthpiece usually goes).
>> 
>> When the sub descended, air was "inhaled" into the motors. When the
>sub ascended, the air in the motors "Exhaled" back out of the reg. Very
>simple.
>> 
>> The only reason I went to oil was that it seemed to be all the rage
>at that time.  Having used both methods, I have to say that I like the
>air better. It was easy to set up and take apart without a lot of mess
>and best of all- no worries about a leak.
>> 
>> Greg C
>> 
>> From: Alan via Personal_Submersibles
><personal_submersibles at psubs.org>
>> To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org> 
>> Sent: Wednesday, April 5, 2017 8:58 PM
>> Subject: Re: [PSUBS-MAILIST] Oil Compensator
>> 
>> Greg,
>> Cliff's regulator is not a divers second stage reg. It runs direct
>from the first stage
>> & you can dial in the overpressure you require. The excess pressure
>above set
>> pressure escapes through the handle of the regulator but only in the
>"relieving" model.
>> They go for around $150-
>> I contacted the guys from Fugu sub a few years ago.
>http://www.fugusub.com/fugusub.html
>> about compensating, am pretty sure they use Minn  Kotas. They
>preferred air 
>> compensation. They operate out of Florida, so warm shallow waters.
>> Alan
>> 
>> Sent from my iPad
>> 
>>> On 6/04/2017, at 12:24 PM, hank pronk via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>>> 
>>> Cliff,
>>> What is the cost for a air compensated set up?
>>> Hank
>>> 
>>> 
>>> On Wednesday, April 5, 2017 6:12 PM, hank pronk via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> 
>>> 
>>> Steve,
>>> Yes I need a bushing (bearing) that holds the prop shaft in the
>water.  It would be like a cutlass prop shat bearing on a boat.  
>>> It would have to be long enough to hold the prop strait and true. 
>Also it would need to act as a thrust bearing.
>>> Hank
>>> 
>>> 
>>> On Wednesday, April 5, 2017 6:00 PM, Stephen Fordyce via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> 
>>> 
>>> Hi Hank,
>>> By "finding a bushing for the propeller shaft that is lubricated by
>the water" do you mean the bearings which will sit outside the pressure
>housing?
>>> 
>>> I started looking into ceramic bearings for just this application,
>and they were interesting - ie. some of them have no metal parts and
>little need for lubrication.
>>> 
>>> Cheers,
>>> Steve
>>> 
>>> On Thu, Apr 6, 2017 at 9:47 AM, hank pronk via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>>> Sean,
>>> I think one of us should develop a magnetic coupler in kit form. 
>Maybe if there is an interest with potential sales it could become a
>reality.  I have come close to starting and even bought the magnets. 
>My stumbling block is finding a bushing for the propeller shaft that is
>lubricated by the water.
>>> Hank
>>> 
>>> 
>>> On Wednesday, April 5, 2017 5:29 PM, Brian Cox via
>Personal_Submersibles <personal_submersibles at psubs. org> wrote:
>>> 
>>> 
>>> I've decided to go with the   PSF - 20 cSt Silicone Bath Fluid for
>my motors.
>>>  
>>> http://www.clearcoproducts. com/pure-silicone-low- viscosity.html
>>>  
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs. org wrote:
>>> 
>>> From: Alan James via Personal_Submersibles
><personal_submersibles at psubs. org>
>>> To: Personal Submersibles General Discussion
><personal_submersibles at psubs. org>
>>> Subject: Re: [PSUBS-MAILIST] Oil Compensator
>>> Date: Wed, 5 Apr 2017 22:48:50 +0000 (UTC)
>>> 
>>> Jon,
>>> I don't know whether you have followed, but I have made a brushless
>2000W thruster
>>> which 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 melt
>>> so people could adjust the timing. The thruster got hot because the
>compensating oil
>>> was 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 it
>>> was needed in a few feet of water. I was running this motor with 2
>lip seals with ambient
>>> pressure 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 ratings
>>> of commercial thrusters.
>>>    I think it is a matter of time before Minn kota put out a
>brushless thruster if they haven't
>>> yet. 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 big
>>> improvement. The white paper suggests grinding the Minn kota shaft
>to a specific
>>> finish, with matching bearing & seal, however a mechanical seal is
>more tolerant of
>>> the shaft surface finish & you could avoid this.
>>>    Re the heat; one concern is transferring heat off the windings,
>which oil compensating
>>> will 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.
>>> Alan
>>> Alan 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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>>> 
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>> 
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