Sent: Monday, March 30, 2009 4:30
AM
Subject: Re: [PSUBS-MAILIST] Shrinkage
of plexi windows.
Hi Shelley and Rick,
When I was building my sub I pondered how to deal with the Plexi an
lot. What I settled on is just what I thought was the best and easiest
for me. There is no substitute for a properly machined cone and
properly machined housing, machined after all welding is done. That
isn't easy.
If view port lenses was flat and even thickness then a face contact
O-ring would be good but you would have to also machine the flat area of the
viewport housing and groove after welding.
Using an O-ring on the sides of the viewport housing, contacting the OD
of the lens, was an option but again, there is warpege in the housing after
welding. Also you have to take into account the coefficient of
expansion of the Plexiglas. In order to leave enough room for it to
not be crushing itself while the sub is outside on a hot day,
you'd have to leave room for expansion. Then in cold water the gap
would open up to much for an O-ring not to be extruded.
The same if you put a chamfer on the outer edge of the lens
and made a three point seal with an O-ring just under the retaining
ring. When the lens cooled and shrunk, the gap would be too great to
trust.
Kittredge just glued his in with SekoFlex, and after all my
pondering, I decided to machine the lenses with a clearance gap between
the lens OD and housing ID and glue them in also. I spread
on the urethane sealant and pushed the lens in almost all the way
to leaving a bit of a urethane bed for the lens to mate against.
I know with the annuals filled, I don't have expansion room, but it seems to
work.
I did the shrinking and later annealed my lenses. My fifteen inch
lens shrunk about an eight of an inch in diameter on the first cycle and
didn't shrink hardly at all in the second annealing cycle.
So far it's working fine.
Dan H.
----- Original Message -----
Sent: Monday, March 30, 2009 1:41
AM
Subject: Re: [PSUBS-MAILIST]
Shrinkage of plexi windows.
Hi, Shelley . . .
I'm coming in on this late so I may be off-base. There's
something about SikaFlex that rings alarm bells in my head. It's a
fantastic boatbuilding goop but, for viewports, I don't think you want
something that flows under pressure. Again, you may have done all
the research required and SikaFlex may be fine.
I believe O-rings in grooves are the way to go. Some of the
pros may have opinions on this.
Years ago I uploaded images of O-rings under pressure/deformation to
the PSUBS site. I can't find any of my uploads but I'm
sure they're still in there somewhere.
Cheers
Rick in Vancouver
----- Original Message -----
Sent: Sunday, March 29, 2009 6:55
PM
Subject: Re: [PSUBS-MAILIST]
Shrinkage of plexi windows.
Thanks Jon. A little more gap would be acceptable, but not much
more. I'm kinda picky on things fitting right.
I'll be using polyurethane (Sika) so it's probably not that
critical, but I definitely don't want them to be sloppy.
I'll keep everybody up on what happens through the process, and
maybe I'll give Greg a call. He's done it before and may be able to give
me a good estimate of how much shrinkage to expect. Thanks for looking
that up. Frank D.
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