Motors for propellers on deep-dive submersibles - sealed?

[mrcharly-YHT]

I was reading about this submersible https://en.wikipedia.org/wiki/DSV_Limiting_Factor, when it occurred to me; are the motors sealed?

Having a watertight seal that functions at 8000m depth would be incredible.

[Kim]

No idea, but if I were trying to design such a thing, I'd go for an induction motor with the stator completely potted, and the rotor happily spinning in seawater.  Bearings left as an exercise for the reader.

[mrcharly-YHT]

No idea, but if I were trying to design such a thing, I'd go for an induction motor with the stator completely potted, and the rotor happily spinning in seawater.  Bearings left as an exercise for the reader.

I assumed something similar.

Bearings could be just plain bronze and nylon; there is no problem with cooling.

Just can't find any info on it.

[Diver300]

The other option is to seal the motor and fill it with a really light oil. That would probably keep the bearings happier than seawater would, but without needing a pressure seal to the outside air.

For scuba diving tank pressure gauges were sometimes allowed to fill with sea water years ago. More recently they were full of glycerine. A lot are just sealed and full of air but that is only for 100m depth or so.

[Diver300]

8000m is 800 bar. Diving pressure gauges work at 300 bar and are free to rotate and don’t leak, so I would imagine 800 bar isn’t too much of a leap.

[Regulator]

Isn't this partly why DSVs/bathyscaphes are relatively limited in their mobility?

[mrcharly-YHT]

The other option is to seal the motor and fill it with a really light oil. That would probably keep the bearings happier than seawater would, but without needing a pressure seal to the outside air.

For scuba diving tank pressure gauges were sometimes allowed to fill with sea water years ago. More recently they were full of glycerine. A lot are just sealed and full of air but that is only for 100m depth or so.

How would you seal it, though? Any solid housing is going to distort under the pressure.

Maybe a membrane on the casing to take up the compression of the oil.

This article talks about the challenges, but they are only considering down to a few hundred feet https://www.alldrivesandcontrols.co.uk/motors-deep-sea-application

[Goldcrank]

No, no they are not.

"Good afternoon,
Our motors are not sealed against water ingress." - submersible vehicle motor company

[Feanor]

On the electromechanical systems I used to work with in the oil industry, they were rated for a max working pressure of 20,000psi at the bottom of a borehole full of hot sometimes conductive mud.

On hydraulically-operated equipment, the downhole motor and pump sat in the hydraulic reservoir, which then had a floating piston with the other side open to the borehole.
Thus the entire hydraulic system was equalised up to the external hydrostatic pressure.

So the hydraulic pump was generating 3000psi *relative to hydrostatic*.
This means there's no pressure differential across the instrument body between inside and out.

[Efrogwr]

No idea, but if I were trying to design such a thing, I'd go for an induction motor with the stator completely potted, and the rotor happily spinning in seawater.  Bearings left as an exercise for the reader.

I assumed something similar.

Bearings could be just plain bronze and nylon; there is no problem with cooling.

Just can't find any info on it.

"Cutless"* and stave bearings are often used on surface vessels as propellor shaft bearings just as you state.


* I think its a trade name.

[ScumOfTheRoad]

I have a book about the NR-1  the pet project of Rickover. Deep diving nuclear submarine.
If I recall correctly the actuators etc. with through connections use pressurised oil - it is slightly over the external pressure.
I am not sure how accurate this is, could be misinformation. The book did not discuss the propellors.

[Diver300]

The other option is to seal the motor and fill it with a really light oil. That would probably keep the bearings happier than seawater would, but without needing a pressure seal to the outside air.

For scuba diving tank pressure gauges were sometimes allowed to fill with sea water years ago. More recently they were full of glycerine. A lot are just sealed and full of air but that is only for 100m depth or so.

How would you seal it, though? Any solid housing is going to distort under the pressure.

Maybe a membrane on the casing to take up the compression of the oil.

This article talks about the challenges, but they are only considering down to a few hundred feet https://www.alldrivesandcontrols.co.uk/motors-deep-sea-application

You need some flexibility in the casing to allow the pressure to equalise, but the amount of flexibility is tiny. Most of what is being allowed for is the thermal expansion of the oil, and the compression of any air that remains inside. I've seen quite small diaphragms used.

One diving depth gauge I had didn't have diaphragm, and relied on the flexibility of the case. That caused a small pressure difference as the oil heated up which meant it was a somewhat inaccurate depth gauge but it otherwise worked.

[rogerzilla]

I'm sure lignum vitae will come into it somewhere.

[mrcharly-YHT]

On the electromechanical systems I used to work with in the oil industry, they were rated for a max working pressure of 20,000psi at the bottom of a borehole full of hot sometimes conductive mud.

On hydraulically-operated equipment, the downhole motor and pump sat in the hydraulic reservoir, which then had a floating piston with the other side open to the borehole.
Thus the entire hydraulic system was equalised up to the external hydrostatic pressure.

So the hydraulic pump was generating 3000psi *relative to hydrostatic*.
This means there's no pressure differential across the instrument body between inside and out.

So you'd fill the motor with non-conductive oil, and have a piston or diaphragm to equalise pressure?

What about bearings? Just the same?