[PSUBS-MAILIST] motor pod solution

Sat Nov 25 11:47:40 EST 2017

"Backup rings" does not refer to redundant o-rings, but rather to contoured polymer bearing rings which sandwich the sealing ring in the (slightly wider than standard) groove. Backup rings help to resist extrusion of the o-ring. More important in dynamic applications, but can be useful in static ones as well, in which case you generally only need one backup ring on the low pressure side of the groove.

Where redundant o-rings can be useful is when assembly alignment is improved with two rings in two separate grooves, or where a "cushion" ring is installed to prevent metal mating parts from bottoming out against each other. Not necessary though, and can even cause problems with trapped volumes between seals.

Choice of cross-sectional diameter is a tradeoff. Smaller rings obviously require less machining and required space for the groove, and the smaller rings have more consistent physical properties. Larger rings tolerate compression set better, seal gaps with looser tolerances more consistently, and are much more tolerant of scratches / defects on the sealing surfaces.

Cross-sectional diameter does not need to bear a relationship to the part size, but as mating parts get larger it becomes more difficult to maintain the tolerances required to reliably seal a small series o-ring, and you have to be more careful about not twisting it during installation.

Honestly though, if the mating faces of your existing parts are machined surfaces with appropriate surface finish and flatness, there is nothing wrong with your existing seal arrangement (face seal with o-ring slightly stretched over the inner lip. The only problem with face seals for this application is the dimensional stability of the assembly. Under increased pressure, the face seal or gasket will deform more, shortening the assembled length. With the radial seal arrangement, the seal is independent of the mechanical load because the parts bear directly on each other.

Sean

Sent from ProtonMail mobile

-------- Original Message --------
On Nov 25, 2017, 09:11, Brian Cox via Personal_Submersibles wrote:

> Hi Sean,    One problem is that the motor and gear box just barely fit in the 14" pipe so I can't really increase the material on the inside of the pipe.  At present I have a 1/8" inner lip welded to the inside which mates up with the end cap.  When you say back up rings would those rings be multiple rings in the same groove or separate grooves?   or either way?   So I would need to slide a larger diameter end cap over the casing having the o rings in the casing itself ?  Sean, as a general rule would you say that the larger the piece you're working with the larger the o ring cross section should be?  Just as a mater of tolerances.
>
> Brian
>
> --- personal_submersibles at psubs.org wrote:
>
> From: "Sean T. Stevenson via Personal_Submersibles" <personal_submersibles at psubs.org>
> To: personal_submersibles at psubs.org
> Subject: Re: [PSUBS-MAILIST] motor pod solution
> Date: Sat, 25 Nov 2017 08:33:48 -0500
>
> I would use a radial seal for this, not a face seal. Turning a groove is generally cheaper than milling one, and a radial groove permits the use of backup rings. The casing side just needs a smooth seal surface and an entry chamfer. The mating part would have a guide chamfer and the seal groove. What sort of clearance do you have on the inside? You mentioned that there is already an inner pipe section?
>
> Sean
>
> -------- Original Message --------
> On Nov 24, 2017, 19:14, Brian Cox via Personal_Submersibles < personal_submersibles at psubs.org> wrote:
>
>> I could probably do that on one end but the other I have the propeller shaft coming thru the end cap , where I have a stainless 2" pipe fitting , that is where the bearing and seals are located.
>>
>> Here's a pic of what I have:
>>
>> Brian
>>
>> --- personal_submersibles at psubs.org wrote:
>>
>> From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.org>
>> To: Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org>
>> Subject: Re: [PSUBS-MAILIST] motor pod solution
>> Date: Sat, 25 Nov 2017 01:24:11 +0000 (UTC)
>>
>> Brian,
>> Make a steel ring 1\4 thick and use a 1\8 inch thick aluminum plate for a cap.  If you use a steel cap then make the cap and ring, then clamp together and drill the holes, bolt the two parts together. Then weld the ring to the housing with the cap bolted on.  The two parts will be perfectly mated for a gasket.   You can do the same with ss except it is a bugger to drill the ss.  No machining needed !
>> Hank
>>
>> On Friday, November 24, 2017, 6:12:57 PM MST, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>
>> Hank,  I still think I would have to machine it flat.  Then I would have to paint it also.  The edges seem to be problematic with painting them, even using epoxy paint.
>>
>> Brian
>>
>> --- personal_submersibles at psubs.org wrote:
>>
>> From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.org>
>> To: Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org>
>> Subject: Re: [PSUBS-MAILIST] motor pod solution
>> Date: Sat, 25 Nov 2017 00:59:38 +0000 (UTC)
>>
>> Brian,
>> Why don't you just drill and bolt flat plates onto a ring with a gasket?.  Or you can send your material up to me and I can machine them.
>> Hank
>>
>> On Friday, November 24, 2017, 5:36:02 PM MST, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>
>> Alan,  What kind of material are you using aluminum ?  Steel?
>>
>> Brian
>>
>> --- personal_submersibles at psubs.org wrote:
>>
>> From: Alan via Personal_Submersibles <personal_submersibles at psubs.org>
>> To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
>> Subject: Re: [PSUBS-MAILIST] motor pod solution
>> Date: Sat, 25 Nov 2017 13:21:23 +1300
>> Brian,
>> my latest thruster design is in 3 sections; a front section housing the mechanical
>> seal & a bearing, the back section housing a bearing & the wiring port, & a central
>> tubular section. The front & back sections have an o-ring groove & o-ring for sealing
>> on assembly. The 3 sections will be press fitted together then drilled,tapped &
>> screwed. With the press fit I am regarding the central section as disposable as I
>> would probably have to cut in to it to part the sections.
>> Cheers Alan
>>
>> Sent from my iPad
>>
>> On 25/11/2017, at 12:27 PM, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>
>>> Hi All,        Been wrestling with my motor pods !   The way I decided to mate the two end caps to my motor casing was not the ideal method.  In fact I don't even remember ever seeing an arrangement like what I have.  I'm squeezing a 1/4" o ring between two 1/4" pieces of pipe and then have side rings inside and out to keep the o ring from expanding out of that area.  Surprisingly it has worked, probably only because there is next to zero pressure.  So not wanting to invite a bad situation of oil leaking into the ocean I decided I need to have a proper sealing scheme.  So that was going to mean I do a hatch style sealing arrangement with  14" dia stainless steel rings and a o ring groove etc..  So getting quotes on everything is was beginning to really add up , especially because the 14" pipe is just too big for my lathe , which means I would have to farm that part out !   The rings themselves were gong to be $600 for each motor pod !  So What I think I'm going to do is to just weld a 1/4" flat ring on both sides ( end cap and casing) regular steel,  put them together and then just weld a bead around edge and seal it that way.  Then if I need to get in there I will just grind off the bead and open it that way.  It will also be rather easy to weld it back together where I previously ground out the weld on that lip.
>>>
>>> Brian
>>
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