|
Brian,
If I were starting all over again, I think I would
use a different latching design, so I could lock and unlock the hatch from both
inside and outside the hull. True, it would be another penetration and more
complicated but it would be the fastest and best way to gain
access.
The conical viewport; yes the lens is mated
directly against the steel. I greased the steal first then installed the
lens.
Annealing, also yes; I first roughed out the
lenses, then shrunk them in an oven, then machined them to size and lastly
annealed them. The fifteen inch viewport shrunk about 1/8
inch in diameter in the shrinking process. PVHO has all the
calculations and tables for doing these processes.
It was recommended that I do the oven part with the
lenses laying on glass plates lightly dusted with talc. I stress, only
lightly dusted because where there were a few bumps of talk it imprinted
little dents into the soft lenses. If you dust the glass, stand
the plate up and tap off all the extra. Maybe there is a better way but
don't just lay the lenses in the oven on a racks because you'll have
only scrap for sure. They get soft.
Dan H.
----- Original Message -----
Sent: Wednesday, December 21, 2005 10:18
AM
Subject: Re: [PSUBS-MAILIST]
viewports
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
|