[PSUBS-MAILIST] Minn Kotta 101 - Thruster Reliability

Fri Jul 28 20:40:37 EDT 2023

 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 ofusing WD-40 for pressure compensation purpose on the basis of its knownproperties.  Particularly it being a penetrantas opposed to an oil that has lubricity.

2)    Carstenis using silicone fluid - ELBESILOIL B 1, a low-viscosity volatile methylsiloxane with a viscosity of 1 cSt.   “Also The glue of the permanent magnets can be affected” bythe WD-40.

3)    "Handbook of Fluid-Filled,Depth/Pressure-Compensating Systems for Deep Ocean Applications" AD894795 recommendsas 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 lowviscosity, solvent refined mineral oil blended with zinc free additives, toprovide extended performance in the high-speed spindles of machine tools.

4)    River on his deep diving (6000m) ROVs is using Univolttransformer oil which has a kinematic viscosity5 cSt, or silicone fluid. 

5)    FischerScientific has a silicone fluid  5 cSt at$294 per gallon

6)    Seanpoints out “Clearco makes a line of polydimethylsiloxane oils (silicone oils)with available viscosities down to 0.65 cSt, which is thinner than water.”

7)    Hankon 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 smallmotors reduces wear on the brushes, compared to the 101's.  Also, my springs may be stronger relative tosurface area.”

8)    Hank, “For me oil is reliable, but I don't like themess.  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 firststage regulator with the spring removed, giving me a 2 pound over pressure. Theregulator gets psi from the first stage regulator on the supply tank.  The 2 pounds is a guess.  To relieve the pressure, I have added anadjustable relief valve that will vent both motors.  I think the problemwith air compensation has been the regulator choice.  I had no luck withthe Parker valve or the second stage scuba.  My theory on the scubaregulator failure is the lack of over pressure and the length of lines going tothe motors.  The scuba second stage does work well on my arm, but it ismounted to the arm with a large junction box.”

9)    Hank, “Iam working on a magnetic coupler end cap this winter.  I have had very good luck with WD-40 and havetwo 55 lb motors still running after more than 20 years.  The only difference is, I use much smallermotors.  My oil goes black right away andI do get a very small amount of water by season end.  I drain the dirty oil and replace.”
10)  Alan “Onething I gleaned from that reading (AD894795) is that there is always an internalover-pressure of around 5psi used. That is why I am working on an over-pressureregulator.”
11) "Handbook of Fluid-Filled, Depth/Pressure-Compensating Systemsfor Deep Ocean Applications" AD894795, Page II-222.4.1 Individual Case “There is a need for filteringdevices in fluid-filled motors to prolong life of bearings and wearsurfaces.  Particle contamination may becontinuously circulated in an individual case. Especially with d-c motor, the problem carbon brush wear contaminations enhanced by the limited fluid volumeavailable to dilute the carbon.  Bearingwear may increase and the dielectric strength of the fluid will decrease ascarbon contamination ensues.  Carbonbuildup in a fluid also increases the possibility of d-c arcing andgrounding.  Another additive effect of carbon contamination isthe d-c commutation tends to degenerate.”  

12) From an article on “Brush life inDC motors” under erosion, “Sparkingincreases with current loading and motor speed. Brush life decreases withincreased sparking.”

13) Alec, Hank and I all saw the10W-40 turn black very quickly. 

14) Alec and I both use accordion shapedbladder for thermal expansion on our 101’s. At the end of each dive these were expanded past the volume you wouldexpect 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 currentpassing between the armature brushes and the commutator is causing more arcingin the 101 and this more gas generation.

15) Sean and others have noted that adifference between air filled and oil filled motors is the journaling affect thatis dependent on the rpm and fluid viscosity. The journaling is causing the brushes to lift off the commutators tosome degree overcoming the brush spring load. Arcing then is occurring across this thin film of oil. The result is lossof performance and accelerated wear on brushes.

16) River, “I suspect that thebushing wear is caused by the low viscosity of the WD40 making it a poor choicefor lubrication. I've always been told WD40 is a penetrating oil, not alubricating oil.”

We have a numberof ongoing projects including Alan James brushless air/oil pressure compensatedthruster design.  Alec is planning ontrying a single mechanical seal on a 101 and use as a 1-atm can and live withwhatever depth the 101 housings and o-rings can withstand.  I have a pressure test chamber that I cantest under dynamic conditions this modified 101.  Hank has just successfully tested to 60 fthis new air compensation setup and is planning on changing the small MKs from oilto this same air compensation system. His tear down showed minimal damage to commutator and not a lot of brushdebris. He is also going to do some experimenting with a home brewed magneticcoupler on a 101 this winter. 

Before I abandonoil compensation on these 101s, I would like to use my bollard test rig in mypool to test three different pressure compensation fluids, 1) a low viscositysilicone fluid, 2) Shell Morlina S2 BL 5 and 3) Univolttransformer oil (5 cSt).  As a control Iwill run the MK 101 with air and no pressure compensation (1 atm).  For each case I will run the thrusters for 48hours at high maximum control voltage with prop installed.  Before and after each test I will weigh thebrushes to measure the wear per unit time. The test rig already has bollard force measurement, rpm measurement, currentand voltage to the motor.  I will alsoadd a motor pressure measurement.  If arcingcauses gas to build up, the plastic accordion bladder will expand so thepressure is an indirect measurement of the gas buildup in the motor.  The test rig as it now sets is run off three12V deep cycle 100 Ah batteries wired in series.  To get the capacity to 48 hours I will needto add three battery chargers that will offset the amps being pulled by themotor under load.  

WhatI am looking for is if one of these fluids is substantially better atminimizing 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 onthe armature commutator.   For these full-load 48-hour test, I don’t seethe need to replace the lip seals each time. 

Forwhich ever fluid comes out best, I would do another set of tests at high, mediumand low shaft speeds to see how the change in current and rpm affects the thruster performanceas well a brush life.

IfI cannot find a compensating fluid that works for these brushed 101 motors, Iwill abandon oil compensation and switch back to air compensation unless Alecor Hank come up with a 1-atm system that works. Decision will also depend on how well Alan's new brushlessthruster 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_______________________________________________
Personal_Submersibles mailing list
Personal_Submersibles at psubs.org
http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20230729/f66e5bd3/attachment-0001.html>