Dan,
That clears things up a bit, I sort figured that was the case but I wasn't
sure. One reference you made to the 45 threw me off.
When I first went about designing my sub
concern for the ability for someone outside the sub to be able to open the
hatch was important to me and drove my design process. So much so that it
actually kind of messed up my whole design and got me off on a tangent.
I'm glad to hear about your solution. Trying to make hatch dogs accessible
from the outside just adds another layer of difficulty to an already complicated
area. Probably a thread about this concern stuck in my mind.
So on your hatch veiwport the conical part of the
acrylic is touching the metal directly, which is not a problem.
As long as we're on the subject, did you anneal
your Acrylic before and after machining ?
Best Regards
Brian
----- Original Message -----
Sent: Wednesday, December 21, 2005
05:35
Subject: Re: [PSUBS-MAILIST]
viewports
Brian,
The viewports I was describing are the ones that
are a flat disk type. The 45 degree chamfer I was referring to, is
not for a conical viewport. The lenses I was describing have straight
sides, but on the external side of the lens, the otherwise 90 degree edge is
chamfered at 45 degrees, to accept an O-ring in what becomes a cavity because
of the big chamfer. All of my flat viewports are made like
this.
There is one conical viewport in the hatch cover
on "Persistence". It's viewport housing does not have a flat seat for
the lens to sit against but instead it is cone shaped. Of course the
small hole end of the cone is inboard and the big end is outboard.
This is the only viewport in my sub that isn't urethaned in place also.
The cone hole in the housing is a complete taper from one side the
other. The lens is machined with the same matching taper, but on the big
diameter of the lens, the outboard side, some of the taper part is machined
away so the lens has straight sides for about a quarter of an inch. It
is this part of the lens, where the cone does not come to a sharp edge, that
creates the cavity for the O-ring seal.
Two totally different types of lenses and
housings but both sealed with O-rings in a 45 - 45 - 90 degree shaped
cavity. Only, as I said in the last posting, I also sealed the flat
lenses with urethane.
I do like the idea of the conical lens in the
hatch cover not being urethaned in because my hatch cover latches aren't
accessible from outside the sub. If anyone has to drag me out of it, the
best way to access the inside is through this easily removed conical
viewport. My crew knows this.
Hope this explains it better. If not you
may be able to see some of my description in pics of "Persistence" during
construction. I know PVHO standards show both types of lenses and more.
(for those not Familiar with PVHO, it is Pressure
Vestal for Human Occupation standards)
Dan H.
----- Original Message -----
Sent: Tuesday, December 20, 2005 10:54
AM
Subject: Re: [PSUBS-MAILIST]
viewports
Dan,
That is
some really good insight, something you wouldn't necessarily find in a
book. Thanks, I only hope I can return the favor some day
to somebody in the future.
What about the viewports that do not have
that 45 degree taper, just squared off. Don't you have some
viewports like that? I've been thinking of maybe using that type
because of the ease of machining them and just having a small tapered one in
the hatch.
On the flat viewports does your o ring also go
in that little chamfer space? And all the same would apply as far as the
size of the lens and the placement of the urethane sealant.
Thanks
Brian
----- Original Message -----
Sent: Tuesday, December 20, 2005
05:57
Subject: Re: [PSUBS-MAILIST]
viewports
Brian,
The Acrylic people can tell you what the
thermal expansion of acrylic is. I talked to them when I was doing
mine but I forget the details right now. It's pretty great, as most
plastics are. On a small diameter viewport, it's not a big deal
because there aren't that many inches of acrylic to expand and contract,
but a large diameter lens changes dimension quite a bit.
My first design followed the
recommendation in PVHO for a flat lens with a 45 degree chamfer on it's
outer edge to contain an O-ring seal. The O-ring hits in three
places. One is on the 45 degree chamfer on the lens, the other is on
the viewport housing and lastly on the cover ring that holds the O-ring
and lens in place.
The PVHO standards also call for a flat
urethane cushion to be placed between the lens and the housing. I
guess that's so any out of flatness in either the lens or housing can be
cushioned.
The problem was that the hot, 130 degree,
diameter and the cold, 30 degree, diameter varied so much that a viewport
housing made to fit the lens when at it's largest, will be big enough to
leave a big clearance gap when the lens is cold. The gap between the
lens and housing when cold was more then the O-ring manufacture
recommended. I talked to the O-ring engineers and they said that a
three point contact application as this application is in PVHO actually
forces the O-ring to extrude at a lower pressure. They said,
this coupled with a gap that could grow quite large is not a
good design. If the O-ring did extrude in this design, it may
also take the urethane disk with it and blow the whole thing into the
sub.
So what did I do? I did what worked for
Captain Kittredge for years. I machined my lens smaller to create an
1/8 inch gap all around it circumference and I made the housing so it was
about fifty thousants deeper then the lens was thick. When I
installed the lens, I imbedded it in urethane sealant. This created
a flat bed of urethane for the lens to rest against and a 1/8 inch
ring of Urethane to seal the gap around the circumference of the
lens. For good measure, I put the O-ring in the 45 degree chamfer I
had around the edge of the lens also. The O-ring is sort of
redundant, but if the urethane ever separates, the O-ring will still keep
the seal and as long as the urethane fills the gap, the O-ring can't be
extruded.
I know I don't have the expansion room I was
first trying to have. But, I don't have the lend pressing against
bare metal and I have a good seal. You can't knock
success and Captain Kittredge had success with this method for thirty
years.
Dan H.
----- Original Message -----
Sent: Tuesday, December 20, 2005
1:02 AM
Subject: Re: [PSUBS-MAILIST]
viewports
Maybe make the lens and then measure it hot
and cold, then machine the viewport housing to the hot dimension and get
the right size o ring for whatever the gap there is.
Bri
----- Original Message -----
Sent: Monday, December 19, 2005
20:03
Subject: Re: [PSUBS-MAILIST]
viewports
Yep! Plexiglas is a trade name for
Acrylic.
Some one wrote that you should get a copy
of PVHO. It's a good idea if you can get a look at it. In
there, you'll find the answers to most of your questions.
The one thing that PVHO didn't answer for
me was how to deal with thermal expansion of the acrylic lens.
It's a great set of standards to design with if your viewport is in a
pressure chamber in a building, but a sub viewport can be exposed to
130 degrees F in or better in the hot sun and 30 degrees F in cold
water. Mine has already seen that extreme. If I followed
the PVHO exactly, my external O-ring seal would now be stuffed in the
annulus between the lens and the viewport housing after my deep
water test in cold water.
Dan H.
----- Original Message -----
Sent: Monday, December 19, 2005
9:32 AM
Subject: Re: [PSUBS-MAILIST]
viewports
"Cast" Acrylic is what to use - correct
?
Brian
----- Original Message -----
Sent: Monday, December 19,
2005 04:51
Subject: Re: [PSUBS-MAILIST]
viewports
Joe,
Look up Acrylic in a search, find a
manufacture you like and either E-mail or call them with an
inquiry about where the nearest stocking distributor is in your
location. That works for most anything your looking
for. Get a few distributors names and you can bounce pricing
between more then one if you like.
Isn't the internet a great thing?
;-)
Dan H.
----- Original Message -----
Sent: Monday, December 19,
2005 12:54 AM
Subject: [PSUBS-MAILIST]
viewports
Looking for a plastic supplier for
viewports, in the USA
Thanks
Happy holidays
Joe
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