[PSUBS-MAILIST] Motor modification

[Sean T. Stevenson via Personal_Submersibles]

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 -  encased, 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 about two separate things?
>>>> My question was based on wondering about that same clear tubing
>that
>>>> wraps around the motor pods to allow for when the oil gets hot from
>use and
>>>> expands.
>>>> Taking any air out of the equation, I got the impression that you
>could
>>>> completely fill a motor with dielectric oil (or any liquid for that
>matter)
>>>> and then take it to any depth and since you basically can't
>compress a
>>>> liquid, t! here would be no water ingress to the motor (except for
>the
>>>> small space between the two prop shaft 0 rings.) So now when you
>add a
>>>> slight internal increase in pressure from the oil being heated from
>use,
>>>> people use the clear tubing that wraps around the outside of the
>motor
>>>> (which is also 100% filled with the same oil) as a place for the
>oil to go
>>>> to expand rather that spitting out the prop shaft 0 rings. Am I
>correct so
>>>> far?
>>>> So my question was, the clear hose wall must be flexible enough to
>allow
>>>> for expansion before placing enough pressure on the prop shaft 0
>ring to
>>>> dislodge? And then if you don't get all the air out of the motor or
>clear
>>>> hose or bladder, don't you have the water pressure trying to
>compress that
>>>> bubble from not only the bladder or tube but past the shaft 0 ring
>as well?
>>>>
>>>> Rick
>>>>
>>>>
>>>> On Mon, Dec 5, 2016 at 11:03 AM, Alan James via
>Personal_Submersibles <
>>>> personal_submersibles at psubs.org> wrote:
>>>>
>>>> Some compensating thoughts.
>>>> On James Cameron's sub they used IV drip bags. I am not sure what
>they
>>>> were compensating.
>>>> These come with various means of hose attachment.
>>>>    With regard to the wrap around hose method; I did some
>calculations
>>>> on a standard pvc hose, & surprisingly
>>>> the colapse pressure was around 100psi. I am sure one of Nuytco's
>>>> technicians told me they crimped the hose
>>>> that was compensating their lights, to initiate the collapse of the
>hose
>>>> for compensation.
>>>>    The industry standard for thrusters seems to be around 4psi
>internal
>>>> overpressure,
>>>> but the compensators they use for this are relatively expensive.
>Hugh
>>>> came up with
>>>> the novel idea of using a releiving regulator set at 4psi (Parker
>>>> PR364). Cliff is using this.
>>>> This could be used for air or oil compensation.
>>>>    The seals need oil for lubrication, & I have read that they can
>use
>>>> about a teaspoon a day. Based
>>>> on that I would have some sort of reservoir.
>>>>    Depending on the motor it may be tricky to get all the air out.
>If
>>>> you have a bearing in a bore
>>>> followed by a seal, the air / oil would have to move through the
>bearing
>>>> which may be a sealed type
>>>>
>>>>
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