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Old 20th October 2006, 10:22 PM   #17
Dream Team - School Guru
Join Date: Aug 2006
Posts: 2,177
Default RE: Help - Water in the hole!

Hello Hatori,
Well, you are preaching to the choir here.
I've been telling the guys at Starboard for a number of years that they are not using the O'ring under the vent plug in the way O'rings are designed to be used.
I've designed a number of both high and low pressure sealing systems for use aboard US Navy ships, and the designs all followed the well know and published "How an O'ring works" and "How to design a proper O'ring groove" to ensure that the O'ring seals properly.This engineering data is readily available from Parker Seal and other major O'ring producers.
You never want to "squash" an O'ring! That's not how they work.
They are only supposed to be "compressed" from their full diameter by 15%. They are designed to fit into an O'righ groove that's depth is about 85% of the cross sectional diameter of the O'ring that's going to be used.
The inside and outside diameters of the O'ring groove have a 5 deg. draft to them and the 15% compressed O'ring is designed to "just touch and seal lightly on the mid point of the O'ring groove.
Then, when you get a vacuum, the O'ring "creeps" to the inside corner of the O'ring groove and seals becuase the vacuum is pulling it into full contact with the corner. The more the vacuum, the tighter the O'ring packs into the corner and the better it seals.
When you have pressure, the pressure pushes the O'ring to the outside corner of the O'ring groove and the more pressure you have the better the O'ring seals.
It's a "dynamic" sealing system and the O'ring groove must be designed quite carefully so you get the right compression (15% approx.) and have the right diameters so the O'ring can do it's job.
Simply crushing the O'ring between too reasonably flat surfaces is a whole different type of "seal design" and that design "REQUIRES" a "flat washer like" slightly compressible "gasket" to seal the mating flat surfaces.
So, you and I are definitely on the same page here.
Not sure how we will get the folks at Starboard to see things differently though.
As far as the Goretex vents, those have been used pretty successfully in fresh water, but in salt water the microscopic holes in the Goretex membrane that allow air to pass through, but not water, tend to get plugged up with salt and the overall function of the membrane is degraded to the point that it either won't release the pressure, or it fails completely and allows water into the core of the board.
So, I'm in the process of finding some fiber "gaskets" to send to Starboard so they can spec. them, as well as modifying some vent plugs with a correctly designed "O'ring" groove.
I will do some testing to demonstrate the far better sealing and durability characteristics of the both these sealing systems, then pass the data along to Starboard to assist them in making a decision (hopefully) to change the way they seal up their boards.
Hope this helps,
Roger is offline   Reply With Quote