[PSUBS-MAILIST] motor pod solution

Mon Nov 27 13:46:28 EST 2017

 Brian,Shouldn't matter, the two faces will seal it.Hank
    On Monday, November 27, 2017, 10:39:46 AM MST, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Hank,        I have a tab running all the way around.  It could be the edge surface is not good. 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: Sun, 26 Nov 2017 11:44:47 +0000 (UTC)

 Brian,I still think your okay, the end cap may just be siting uneven on the o-ring.  Easy fix if it is, just add a few stoppers so the end caps can't go past.  I would start by marking where the end cap is  by scribing a line and then remove the end cap and measure to see if it was even.  I thought you might have squeezed one part of the o-ring too much, but there is no way you  could do that. I bet if you weld some stopper tabs on the outside of the cylinder, it will be good.  By the way, I am pretty sure all sub builders have had to chase down an annoying leak of some sort.  I know I have.Hank
    On Saturday, November 25, 2017, 9:50:12 PM MST, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 could be !

--- 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 20:29:00 +0000 (UTC)

 Brian,That is not so good using the o-ring as a stopper.  Maybe your extruding the o-ring causing the leak?Hank

    On Saturday, November 25, 2017, 12:30:13 PM MST, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Hank,     Nothing stops the amount of squeeze, I can squeeze the heck out of it.       Gremlins ! 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 18:19:55 +0000 (UTC)

 Brian,To me it seems you have a working set up that just needs tweaking.  Is it possible that your rods are not tightened equally and the end cap is not even against the o-ring.  A 1\4 inch o-ring has a ton of forgiveness, that makes me wonder if your getting enough compression on the o-ring.  What stops the end cap from continuing to squeeze the o-ring?  Hank
    On Saturday, November 25, 2017, 10:48:36 AM MST, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Thanks Sean,  I remember now about the back up rings, I've never had occasion to use them however.  The other thing is I'm using a 1/4" o ring on the 14" dia , since it is not really doing a traditional o ring seal exactly would it be better to use a o ring with a square cross section in this case?    Also, the only deformation is the pressure of the squeeze I am applying to keep the whole thing together as the unit is filled with oil ( wd40)  .  I think the lightness of the oil is also a factor because despite my efforts I'm still getting oil coming through my seal some place, could be a nick on one of the faces.  You can't see in the picture but I have two 7/8" stainless round bars running through the length of the motor pod .  They are welded to the inside of one of the end caps and go all the way through and come out through fittings with o rings on the opposite end cap.  There are threads on the end where I'm able to compress the two end caps together ( if you can visualize that )  I'll see if I have a pic of that .     Brian 

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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 11:47:40 -0500

"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

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On Nov 25, 2017, 09:11, Brian Cox via Personal_Submersibles < personal_submersibles at psubs.org> 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 mechanicalseal & a bearing, the back section housing a bearing & the wiring port, & a centraltubular section. The front & back sections have an o-ring groove & o-ring for sealingon 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 Iwould probably have to cut in to it to part the sections. Cheers Alan

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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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