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Re: [PSUBS-MAILIST] fabrication question

Hi, Gene!
        Re: 'Side seal' -  OK,  . .suppose you have a chunk of pipe,
standing vertically, with the top end cut off and ground or filed off as
flat as you can get it, but not machined. Further suppose it is a couple of
feet in diameter and will be used as your entry hatch coaming. First off,
take a thick 'O' ring - really thick - say, 3/4" to 1" and a slight stretch
fit over the coaming ( 'slight' about 2" to 3" - so 'O' ring about 23" ID)
roll it onto the coaming and down a couple of inches. Now find a piece of
flat plate of a thickness withstand the pressure  you expect to encounter
and cut a hatch out of it - about one inch bigger than the outside diameter
of the pipe that you are using as a coaming and which has the 'O' ring
rolled onto it, like the ring on a con . . .well, you get the picture! ( I
should have mentioned that being a good waterman you coated the 'O' ring
liberally with silicon grease - enough that touching the 'O' ring will
leave a mark in the grease. ) Now put a weight on the flat plate to hold it
in position, since you haven't welded a hinge on, yet. Now roll the 'O'
ring up until it contacts the flat plate underside - since the plate is
flat, it will touch evenly. Likewise the ring stretched over the pipe - the
pipe is round and the 'O'ring is under some tension so it will touch
everywhere. Now increase the outside pressure by the method of your choice.
What's happenin'? Since the ring is sealed to both the underside of the
plate and the outside of the pipe, pressure will attempt to force it into
the crack between the plate and the pipe edge, which is not perfectly flat,
since we finished it by grinding or filing. As you increase the pressure,
the ring will force its way between the two pieces, forcing them apart,
until it finally blows into the inside - the area of lesser pressure - or
so one might think !  .But the area of the flat plate is a swillion times
bigger than the pressure - affected area of the ring, so the ring can't
push them apart, not even close! All it can do is try to intrude into the
minute gap between the two pieces - but the gap is small and the ring is
enormous ( compared to the gap) and the energy required to force the ring
through is much greater than the outside pressure can supply . . . Voila! a
perfect, bubble-tight seal!
        "Yeah, great"  you're thinkin' " So I'll have my wife's brother out
there with his scuba gear rolling the ring in position and holding it there
'til I get a pressure seal!"
        Here's how it's done: Say you're going to use a  3/4" 'O'- ring,
(about 50 - 60 durometer or harder)  - take a piece of 3/4" by 3/4"  square
stock, roll it into a ring that is a slide fit over your coaming/pipe and
weld it closed. Slide it over the coaming until the top of the square stock
is 5/8" from the lip /edge of the coaming. Weld the flat stock onto the
coaming on the underside of the flat stock. Intermittent weld is ok 'cause
you're not trying to make any kind of a seal - just provide a 'ledge' for
the ring to sit on. Now take a piece of 1/8" thick flatbar, 1 and 1/4"
wide. Bend it around the 3/4" squarestock with its edge flush with the
lower edge of the squarestock and tack it into position on the underside of
the squarestock - with the tack welds contacting both the 3/4" material and
the 1/8th stuff. Stuff in your 'O' ring and if you've done every thing
right . the 'O' ring should stick up about an eighth higher than the
coaming edge - this to get a quick pre-load - and the 1/8th band that
protects the ring should leave a 1/4" gap to the outside to give the 'O'
ring some place to go when the first pressure comes on. 
        Caveat: you need an internal latch that is spring-loaded, cause the
flat hatch will move down fractionally as the pressure comes on it. A heavy
bungee cord works well and has the added advantage of burping if you
surface with an over-pressure.The hinge should be set with the plate flat
on the lip - since what you will quickly have is a metal to metal contact
as soon as the pressure comes on, with the 'O' ring acting not as a 'face'
seal as it does initially,( courtesy of the energised latch) but as a 'Side
seal".
If you can make out this convoluted explanation, I'm proud of you . . .
Phil Nuytten