[PSUBS-MAILIST] Motor modification

Mon Dec 12 13:25:33 EST 2016

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, it is essentially a sm!
>>> all low pressure 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 ! or observation 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.org> 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 all
>>>> Rick
>>>>
>>>> On Wed, Dec 7, 2016 at 1:42 PM, Alan James via Personal_Submersibles <
>>>> personal_submersibles at psubs.org> wrote:
>>>>
>>>>> Rick,
>>>>> Sean's beat me to it, but I'll add that you can usually look up
>>>>> your compensating oil's coefficient of thermal expansion, &
>>>>> calculate how much it will be. Engine oil is abou! t .0007. That
>>>>> would mean that if you had a liter of oil (1000cc) & the temperature
>>>>> went 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 bigger problem & it would expand more.
>>>>> In a declassified military document on compensating I read , it talks
>>>>> about & demonstrates how to pull a vacuum on a thruster to get all
>>>>> the air out.
>>>>> Another issue that I mentioned earlier is that all seals leak oil to
>>>>> lubricate themselves, so having a reservoir makes sense.
>>>>> What I am doing is enclosing my motor wiring in a hose & filling
>>>>> the 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 ! oil expanding &
>>>>> leaking
>>>>> then contracting & sucking in water. Will re post their emails if I
>>>>> can find them.
>>>>> Regards Alan
>>>>>
>>>>>
>>>>> ------------------------------
>>>>> *From:* Rick Patton via Personal_Submersibles <
>>>>> personal_submersibles at psubs.org>
>>>>> *To:* Personal Submersibles General Discussion <
>>>>> personal_submersibles at psubs.org>
>>>>> *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! and mention about collapse pressure and also mention
>>>>> about lighting systems. Maybe we are talking abou
>>>>>
>>>>
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