[PSUBS-MAILIST] Minn Kotta 101 - Thruster Reliability

Fri Jul 28 22:23:21 EDT 2023

 Sean, I am willing to add another oil to the test as well as WD-40.  These are not long of test so I don't see any issue in adding another oil.  But, given the large viscosity dependency on temperature of the Shell DIALA S4 ZX-IG  (kinematic viscosity of 35 cSt at 0°C, 7.4 cSt at 40°C, and 2.2 cSt at 100°C.), I to would expect a strong dependence on operating temperature.  If I remember correctly, thrust goes as a square of rpm. The 101 factory prop is matched to the rpm that the MK 101 make with air inside.  With viscosity, the drop off in performance will be significant particularly in cold water until motor warms up.  We might be solving one problem (arcing and low run times) at the expense of what HP the thruster can put out. What I don't have a feel for is how the motor fluid temperature will vary with ambient water temperature.  For sure if we use this oil, we will need to add an RTD to the motor to track internal motor temperature.  I say we try it and add an internal RTD to the motor and one for ambient water temperature.
The prior testing, I did on bollard thrust was with air filled OTS MD 101's with different props.  It will be interesting to see how much thrust we lose by going to oil compensation.
River, can you send me the name and part number for the silicon oil you reference that you use in your ROV's. 
 Cliff
    On Friday, July 28, 2023 at 08:39:10 PM CDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Cliff, the SHELL MORLINA S2 BL 5 oil that you propose to test is listed as a bearing and circulating oil by design intent, and while it is likely to imply a higher kinematic viscosity, I wonder if there might be merit to choosing a purpose-specific electrical oil and then dealing with the increased viscosity, versus a lubricating oil with additives and then having to deal with insufficient dielectric insulation, as the latter property could well be contributing to contamination where it breaks down in an arc gap. While a higher viscosity oil would lower motor RPM, it could be that the resultant speed reduction reduces the tendency of the brushes to build an oil journal gap to a greater extent than the higher viscosity increases it.

Specifically, I might look at SHELL DIALA S4 ZX-IG, which is both biodegradeable (as it leaks from seals), and has gas absorbing properties. The viscosity of this one appears to be strongly proportional to temperature - I'm curious if the motor has a substantial warming effect on the compensation oil, or if that volume consistently approximates the seawater temperature? In any case, I wonder if it would be possible to also test this as a candidate?

Sean

-------- Original Message --------
On Jul. 28, 2023, 18:40, Cliff Redus via Personal_Submersibles < personal_submersibles at psubs.org> wrote:
Jon, from my perspective we don't have a good enough understanding of how these 101s work to start a pSubs standardization effort.  I think we need to keep experimenting on multiple fronts to sort out reliability issues.

Sean, likewise, for the same reason, I don't think we have these 101's working well enough to start hammering out functional specifications.  To me it would be better to find which set up makes these 101 the most reliable then perform a series of test to document performance envelope.

Below are my key notes from this thread.

Notes:
1)    Sean, River and others have questioned the efficacy of using WD-40 for pressure compensation purpose on the basis of its known properties.  Particularly it being a penetrant as opposed to an oil that has lubricity.
2)    Carsten is using silicone fluid - ELBESIL OIL B 1, a low-viscosity volatile methylsiloxane with a viscosity of 1 cSt.   “Also The glue of the permanent magnets can be affected” by the WD-40.
3)    "Handbook of Fluid-Filled, Depth/Pressure-Compensating Systems for Deep Ocean Applications" AD894795 recommends as a general-purpose pressure compensation oil that meets a lot of their requirements Tellus 11, ISO name Morlina 5, that has a kinematic viscosity 5 cSt. This can be purchased now as Shell Morlina S2 BL 5 which is a special low viscosity, solvent refined mineral oil blended with zinc free additives, to provide extended performance in the high-speed spindles of machine tools.
4)    River on his deep diving (6000m) ROVs is using Univolt transformer oil which has a kinematic viscosity 5 cSt, or silicone fluid. 
5)    Fischer Scientific has a silicone fluid  5 cSt at $294 per gallon
6)    Sean points out “Clearco makes a line of polydimethylsiloxane oils (silicone oils) with available viscosities down to 0.65 cSt, which is thinner than water.”
7)    Hank on his small motors drains and fills his thrusters each dive season.  “The oil is dirty but no sludge.  My feeling is the low amps from the small motors reduces wear on the brushes, compared to the 101's.  Also, my springs may be stronger relative to surface area.”
8)    Hank, “For me oil is reliable, but I don't like the mess.  I have two motors with oil (30lb). and two with air compensation.  I have a good feeling about the air system.  I am using a first stage regulator with the spring removed, giving me a 2 pound over pressure. The regulator gets psi from the first stage regulator on the supply tank.   The 2 pounds is a guess.  To relieve the pressure, I have added an adjustable relief valve that will vent both motors.  I think the problem with air compensation has been the regulator choice.  I had no luck with the Parker valve or the second stage scuba.  My theory on the scuba regulator failure is the lack of over pressure and the length of lines going to the motors.  The scuba second stage does work well on my arm, but it is mounted to the arm with a large junction box.”
9)    Hank, “I am working on a magnetic coupler end cap this winter.  I have had very good luck with WD-40 and have two 55 lb motors still running after more than 20 years.  The only difference is, I use much smaller motors.  My oil goes black right away and I do get a very small amount of water by season end.  I drain the dirty oil and replace.”
10)  Alan “One thing I gleaned from that reading (AD894795) is that there is always an internal over-pressure of around 5psi used. That is why I am working on an over-pressure regulator.”
11)  "Handbook of Fluid-Filled, Depth/Pressure-Compensating Systems for Deep Ocean Applications" AD894795, Page II-222.4.1 Individual Case “There is a need for filtering devices in fluid-filled motors to prolong life of bearings and wear surfaces.  Particle contamination may be continuously circulated in an individual case.  Especially with d-c motor, the problem carbon brush wear contaminations enhanced by the limited fluid volume available to dilute the carbon.  Bearing wear may increase and the dielectric strength of the fluid will decrease as carbon contamination ensues.  Carbon buildup in a fluid also increases the possibility of d-c arcing and grounding.  Another additive effect of carbon contamination is the d-c commutation tends to degenerate.”  
12)  From an article on “Brush life in DC motors” under erosion, “Sparking increases with current loading and motor speed. Brush life decreases with increased sparking.”
13)  Alec, Hank and I all saw the 10W-40 turn black very quickly. 
14)  Alec and I both use accordion shaped bladder for thermal expansion on our 101’s.  At the end of each dive these were expanded past the volume you would expect due to thermal expansion of the 10W-40.  Hank with his small motors found some but not a lot bladder expansion.  This leads me to think the larger current passing between the armature brushes and the commutator is causing more arcing in the 101 and this more gas generation.
15)  Sean and others have noted that a difference between air filled and oil filled motors is the journaling affect that is dependent on the rpm and fluid viscosity.  The journaling is causing the brushes to lift off the commutators to some degree overcoming the brush spring load.  Arcing then is occurring across this thin film of oil. The result is loss of performance and accelerated wear on brushes.
16)  River, “I suspect that the bushing wear is caused by the low viscosity of the WD40 making it a poor choice for lubrication. I've always been told WD40 is a penetrating oil, not a lubricating oil.”

We have a number of ongoing projects including Alan James brushless air/oil pressure compensated thruster design.  Alec is planning on trying a single mechanical seal on a 101 and use as a 1-atm can and live with whatever depth the 101 housings and o-rings can withstand.  I have a pressure test chamber that I can test under dynamic conditions this modified 101.  Hank has just successfully tested to 60 ft his new air compensation setup and is planning on changing the small MKs from oil to this same air compensation system.  His tear down showed minimal damage to commutator and not a lot of brush debris. He is also going to do some experimenting with a home brewed magnetic coupler on a 101 this winter. 
Before I abandon oil compensation on these 101s, I would like to use my bollard test rig in my pool to test three different pressure compensation fluids, 1) a low viscosity silicone fluid, 2) Shell Morlina S2 BL 5 and 3) Univolt transformer oil (5 cSt).  As a control I will run the MK 101 with air and no pressure compensation (1 atm).  For each case I will run the thrusters for 48 hours at high maximum control voltage with prop installed.  Before and after each test I will weigh the brushes to measure the wear per unit time.  The test rig already has bollard force measurement, rpm measurement, current and voltage to the motor.  I will also add a motor pressure measurement.  If arcing causes gas to build up, the plastic accordion bladder will expand so the pressure is an indirect measurement of the gas buildup in the motor.  The test rig as it now sets is run off three 12V deep cycle 100 Ah batteries wired in series.  To get the capacity to 48 hours I will need to add three battery chargers that will offset the amps being pulled by the motor under load.  
What I am looking for is if one of these fluids is substantially better at minimizing arcing and minimizing brush wear.  At the end of each test, I tear down, take pictures and will replace brush set and use emery cloth on the armature commutator.   For these full-load 48-hour test, I don’t see the need to replace the lip seals each time. 
For which ever fluid comes out best, I would do another set of tests at high, medium and low shaft speeds to see how the change in current and rpm affects the thruster performance as well a brush life.
If I cannot find a compensating fluid that works for these brushed 101 motors, I will abandon oil compensation and switch back to air compensation unless Alec or Hank come up with a 1-atm system that works.  Decision will also depend on how well Alan's new brushless thruster design works.
Cliff

On Thursday, July 27, 2023 at 08:33:07 AM CDT, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote: 

I would like this group “think tank” to come up with a viable and accepted solution for all compensation modes; air, oil, and 1-ATM.   It would provide a known standard to build to while still allowing for enhancements by those who want to try other alternatives.  I’m sure that all of collaborating on this project will end up with good results. 

Jon
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