[PSUBS-MAILIST] Motor modification

Mon Dec 12 13:47:45 EST 2016

Rick,All chatter aside, if you want a total leak free motor  then you must go air compensated.  I have not seen a leak free oil compensated Minn Kota.  The reality is, who cares if a bit of water gets in.  Change the oil often and call it the cost of doing business.  In your case, with rotating thrusters, you have little choice but to go with oil compensated motors with hose compensators.  Just keep an eye on them, drain them often.  Build it so the oil change is fast and easy with the motor on the sub.These motors are cheap and reliable, we are over thinking this! ;-)  my motors are 20 years old.Hank 

    On Monday, December 12, 2016 11:25 AM, Rick Patton via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Hi Sean,
I would rather use the"hose wrapped around the motor" idea rather than a bladder basically as the hose is a much cleaner look and you don't have to figure out a way to secure the bladder from flapping in the breeze and you said the hose idea was fine in your #1 line but my concern is that though all the things you listed are true, I would think that the volume transfer after diving and warming up the oil in the motor would be (just throwing out numbers without data to support it) something like 5% inward and 95% outward from oil heating and armature rotation? so I would think based on that, that the hose could easily collapse enough " if pre crimped" to give in to the 5% inward flow but I can't see how the hose could expand enough to allow for the 95% outward flow without first spitting out the factory seals?  Sorry to beat this to death but I must be missing something. Would be nice to hear from others who have only used the hose method without a bladder to see if they have had any prop seal problems.
Thanks 
Rick
On Mon, Dec 12, 2016 at 6:18 AM, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

The direction of oil flow is somewhat indeterminate. Heating of the motor, shaft, etc., will displace oil, while heating of the housing will consume oil. Cooling of the shaft will consume oil. Compression and deflection of the housing due to external pressure will displace oil on descent, and consume it on ascent. Contraction / expansion of any air bubbles in the system will do the opposite. Leakage past the seal(s) will consume oil. Heating of the oil (operating viscous friction) will displace oil.Using a hose is fine, provided that:1) it is soft and doesn't strongly resist external pressure collapse, and
2) as with any bladder, is only initially filled part way (pre collapsed), so that you have compensation capacity in both directions.Wall flex, as in diameter expansion when full, should never come into play. If you are hitting that limit (completely full tube), then you need to increase the tube volume until you don't.The more air you can get out of the system, the better, as you then don't need the compensation oil to compensate for the volume change as that air expands / contracts under pressure changes. Draw a vacuum before doing the oil fill if you can, but be mindful of not exceeding the capability of your seals. This is another reason why positive bias compensation pressure is helpful - if you have a high point in the line to vent from, you need only crack that open to vent any air until oil seeps from it.Sean

On December 10, 2016 1:41:15 PM MST, Rick Patton via Personal_Submersibles <personal_submersibles at psubs. org> wrote:
Sean,
Good data to chew on. From what you and the others have said, it sounds like a partially filled airless bladder without hose would be the best way to go. It sounds like the major volume of oil that moves after diving would be in an outward direction, not inward and though the hose idea would allow for "some" inward flow, it just doesn't seem to have the wall flexing capability to accommodate the outward volume.?It's snowing as we speak here in Hawaii
Rick

On Sat, Dec 10, 2016 at 10:03 AM, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs. org> wrote:

You can make the hose behave like a bladder by crushing / crimping it to initiate its collapse, and then only filling it to half of its undistorted volume, but then its rigidity or "memory" will make it act like it has bias pressure in the wrong direction - actually pulling the pressure down a bit from ambient, unless you specifically use a really soft tube. You can demonstrate this with a sample of hose, and you don't need an expensive vacuum pump to do it - just pull a vacuum on it with a large syringe, or cap the ends and take it diving. If it doesn't collapse easily with a relatively low external pressure applied, it isn't good for compensation. You want to avoid negative pressure in your compensated space, for obvious reasons. The tube idea works best with a tube which has no structural rigidity, in which case it's just acting like a bladder anyway. Which is why I prefer the bladder embodiment -  encas! ed, itis essentially a sm! all lowpressure hydraulic accumulator, with the precharge side open to sea if compensating at ambient, or biased with gas pressure (or spring pressure) if adding some bias pressure.If you fill a rigid PVC tube completely (rigid in the sense that it resists collapse due to external pressure, even if it is "flexible"), the overpressure when the oil expands will either require the first seal to resist that pressure (which it is oriented the wrong way for in the default configuration), or will force oil past that seal into the inter seal space. If the housing subsequently contracts, oil is demanded from the compensation system, and if the tube can't deliver it, the oil pressure drops, loading the shaft seal if the inter seal pressure has been allowed to build up, which will prematurely wear out the seal.Ergo, best practice with regard to oil compensation is to provide compensation capacity in both directions, so that the compensation pressure is a true function of ambient pressure and neither bottoms out nor hits maximum expansion in operation.  You should also avoid having uncompensated void spaces between seals, as any leakage across a seal will change that pressure and you can't control it. This may be immaterial from a motor protection perspective, because compensation oil, if under biased pressure, will only leak away from the motor housing, but overpressure in the inter seal volume will eventually push oil past the second seal into the water, which is environmentally irresponsible.If you follow my earlier recommendation of using slightly biased pressure in the motor housing, and unbiased pressure between the seals (necessitating two separate compensation bladders), any oil leaking past the first seal (which will occur with increasing severity as that seal wears out) just transfers its volume to the unbiased compensation bladder. By monitoring the change post dive, you have an indication of seal condition. A loss of bias pressure (compensation failure) would indicate a serious leak which you could alarm on if you monitored that, as would monitoring the extremes of bladder displacement with limit switches.  If you want to be really fancy (Cliff?), you could encase the bladders and monitor the entire range of their travel with  displacement transducers to give you real-time seepage monitoring. Similarly, if the sum of both bladder volumes decreases, you know you are losing oil past the second seal to the water. A periodic analysis ! orobservation of the oil in the unbiased bladder will tell you if you are exchanging oil for water at the second seal at constant volume.Sean

On December 10, 2016 11:42:44 AM MST, Rick Patton via Personal_Submersibles <personal_submersibles at psubs.o rg> wrote:
Hey Alan,
Been tied up for a couple days since this post but thanks for the heat/expansion/volume data as it makes it much easier to comprehend. Based on what I see here, is it safe to assume that due to the amount of expansion of oil, just wrapping the motor several times with the clear PVC hose and connecting the other end to the second barbed fitting on the motor would not work due to the fact that the hose would not expand enough to allow for the volume of expanded oil where as a bladder half filled would work better? Also would like to hear from the others out there who have used only the clear hose to see if they have had any issues with leakage? 
Thanks allRick
On Wed, Dec 7, 2016 at 1:42 PM, Alan James via Personal_Submersibles <personal_submersibles at psubs.o rg> wrote:

Rick,Sean's beat me to it, but I'll add that you can usually look upyour compensating oil's coefficient of thermal expansion,&calculate how much it will be. Engine oil is abou! t .0007.Thatwould mean that if you had a liter of oil (1000cc) & the temperaturewent up by 50 degrees C, then the oil would expand another 35cc.Not a lot given  that you would not require a lot of oil in the thruster.Getting the air out could be a biggerproblem & it would expand more.In a declassified military document on compensating I read , it talksabout & demonstrates how to pull a vacuum on a thruster to get allthe air out.Another issue that I mentioned earlier is that all seals leak oil tolubricate themselves, so having a reservoir makes sense.What I am doing is enclosing my motor wiring in a hose & fillingthe motor & hose with oil. In to this I will have a T to a relieving regulator(PR 364) that will pressurize the system with air at 4psi above ambient.Carsten & Emile had all sorts of problems about ! oilexpanding & leakingthen contracting & sucking in water. Will re post their emails if Ican find them.Regards Alan

      From: Rick Patton via Personal_Submersibles <personal_submersibles at psubs.o rg>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.o rg> 
 Sent: Thursday, December 8, 2016 10:38 AM
 Subject: Re: [PSUBS-MAILIST] Motor modification

Alan,
I am embarrassed to say that I am still not getting this concept. You mentioned in your email about the clear hose that people wrap around there Minn-Kota motors! andmention about collapse pressure and also mention about lighting systems. Maybe we are talking abou

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