Re: [PSUBS-MAILIST] Coved O-ring Groove

Cliff,

Thank you for your insight and information. I was looking for that software from Park for my self and Jim K. last night. My drawings were not meant to show what the O-ring would look like under normal loads, but for much higher pressures then it was meant for. I've seen a number of other O-rings in the gland that showed a lot more extra space then I expected there would be. So I'll have to find the balance between the two using the software.

I was conflicting between whether there should be room for the O-ring to compress into the gland, or whether I should fill that gap with a sealant like Paul Moorhouse did on the Alicia subs main pressure hull large flange seal. I guess I didn't like the idea of the hatch surfaces being metal to metal, since at that point there would no long be any more pressure being applied from pressure on the hatch to keep adding sealing pressure to the O-ring as the sub went deeper. In any case it's best to just keep the sub with in the healthy operation range of the O-ring that is installed. But understanding just how different parts of a sub will likely fail when over loaded is useful.

Regards,
Brent Hartwig

Date: Fri, 22 Feb 2008 06:46:54 -0800
From: cliffordredus@sbcglobal.net
Subject: Re: [PSUBS-MAILIST] Coved O-ring Groove
To: personal_submersibles@psubs.org

Brent,

A couple of observations on your O-ring drawings. The shape of rectangular machined grove to accept the gland is off, i.e., typically the ring has more play than shown. You can use the Parker inPHorm O-ring software to get the exact recommended dimensions of the grove based on specified operating conditions. The software also will tell you the type of gland material, i.e., Viton vs BunaN. Your drawings shows quite a bit more extrusion than would occur if the correct gland hardness was specified. Gland hardness is given as a Durameter (Shore A) number. For my boat, which has a design depth of 300 ft, the 1/4 inch diameter gland has a Durameter value of 75. Also, the hatch surfaces are normally metal to metal. Your discussion on how the pressure is distributed is correct.

Cliff

Great works are performed not by strength but by perseverance.
Samuel Johnson

Cliff Redus
Redus Engineering
USA Office: 830-663-6445
USA mobile: 830-931-1280
cliffordredus@sbcglobal.com

----- Original Message ----
From: Brent Hartwig <brenthartwig@hotmail.com>
To: personal_submersibles@psubs.org
Sent: Thursday, February 21, 2008 11:36:44 PM
Subject: RE: [PSUBS-MAILIST] Coved O-ring Groove

I worked up three more drawings to show what I was thinking a O-ring in a coved gland or square gland with silicone or urethane filling the square corners of the gland, would look like being under great pressure. Since I think there would be far more pressure coming down on the O-ring from the hydrostatic pressure on the hatch's outer surface then would be coming in from the space between the hatch flange and seat. Also I would expect that the more exterior hydrostatic pressure is applied to the hatch that it would further help seal the O-ring against the seating surface, keeping it from extruding to the low pressure side of the seal. Any thoughts?

http://www.frappr.com/?a=viewphoto&id=4001713&pid=9275457

Regards,
Brent Hartwig

From: brenthartwig@hotmail.com
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Bottomed Out Hatch Flange O-ring Failure
Date: Sat, 2 Feb 2008 15:37:06 -0800

I've been thinking about this for some time now, and thought it was time to talk about it. There has been a fair bit of discussion on O-ring failure in a hatch, by means of extrusion. I don't think this would be the mode of failure for a hatch O-ring, and here is why. When the hatch is shut and under water, the exposed surface area on the O-ring between the two metal mating surfaces is vastly smaller then the surface area on the top of the whole hatch pushing down on it. This means that if the pressure is to great for the O-ring, the hatch mating surfaces will bottom out and then the water will just start to flow past the O-ring without extruding it. Also if the hatch bottoms out there will be no place for the O-ring to extrude into. At this point I think the O-ring would just rather let the water pass between it and the flat flange mating surface, rather then extrude. Any thoughts?

I made up a model to better show what I'm thinking the O-ring would look like when fully compressed into it's gland by a bottomed out hatch flange.

http://www.frappr.com/?a=viewphoto&id=4001713&pid=9115314

Here is a really great link I found today showing a lot more modes of O-ring failure then I'd seen any place else.

http://www.allorings.com/failure.htm

Not to long ago Frank D. asked about whether he should cut a coved or square O-ring groove in his hatch, and I don't remember if any one answered him? I would think the cove would make for less room for a O-ring to deform into to help keep the metal hatch surfaces from bottoming out. While watching the Nova show on the Alicia sub called "Underwater Dream Machine", I noticed that they put a lot of silicone type sealant in the O-ring groove that is in the large unboltable flange, that keeps the two large steel pressure hull sections together. I'm guessing they did this to fill the extra space of a square type O-ring groove, that the O-ring doesn't fill when in a non compressed state. So there will be more space between the flanges at depth, to keep them from bottoming out. Any thoughts, or should I add more O2 to my snow cave?

Here are two pictures showing snow through the windows of my house. One shows snow up against a window in the the spa room, and the other shows that the snow outside my shop window is completely blocking the view out the window. So when I say snow cave, I mean it. This is not normal for around here.

http://www.frappr.com/?a=viewphoto&id=4001713&pid=9115443

Regards,

Brent Hartwig