Re: [PSUBS-MAILIST] Wet Exit Removable Front Dome CAD Models

[irox <irox@ix.netcom.com>]

Hi Brent,

a CAD drawing is worth a 100 paragraphs.  Thanks for drawing
that up.

>From reading the Stachiw book, here are the problems I spot:

1) You have an O-ring grove on the main pressure bearing surface.
This looks to be taking up at least a quarter, if not a third of
the pressure bearing surface.  This will lead to the Acrylic
extruding into the O-ring grove when it is under pressure.
This will lead to stress fractures along the trim.  While
this probably won't kill you when it starts, it will mean
replacing the view port lenses earlier than expected.

2) The edge of the dome.  As the pressure on the face of
the dome increases, the base will expand.  This may
push the thin point edge of the dome in to the gap 
between the view port seat and the retaining ring.  All
the localized pressure build up on the edge may cause
stress fractures on the outer lip.  Which will result
in replace the dome sooner than expected.

3) The inside rim extending up inside the dome.  The
dome contract under pressure, shrinking is size in
slightly extruding into the cabin. I would be concerned
that the dome pressing against the inside rim may cause
it to deform and start to suffer from notch cracking.

4) The O-ring on the inside rim.  This is another area
which notch cracking may occur.

5) The lip of the dome.  The type of dome you've drawn
is a sectional spherical dome.  These can come with two
types of lips (according to Stachiw), conical or squared.
I don't see anything about the style of lip you've drawn
on the dome.

You have three seals, the outer most isn't effective
since water can leak in view the retaining ring and
the frame.  The metal retaining ring looks expensive
to manufacture, and looks like it will eat expensive
view port Acrylic.

This could be designed to just use one O-ring and be
much simpler.

On top of this you've got 3 extra hull penetrations which
hold the retaining ring on.  I'm assuming this is some
think you are supposed to use to release the view port?
You have to push these three things out of the hull,
against outside pressure, if you are successful the
hull starts floating before you are ready to exit.

What if one of the pins jams, catching the thread on
the O-ring groove?  If you've already successfully
removed a pin, then you have no way to stop the flooding,
no way to finish removing the dome, no way to escape
from a submarine which is flooding.

If you start preparing for a wet exit by removing a
pin causing the hull to start flooding, then whatever
emergency which prompted the decision for a wet exit
goes away, you can't cancel the wet exit now, you
must follow though and abandon the sub.

So, in short, this is going to be expensive to make,
it's probably not going to facilitate a wet exit when
the time come, it's probably going to damage the dome
over time causing it to start cracking and need replaced
long before it's scheduled maintenance.  This design
could be improved by making it much simpler:

 - view port frame could be made cheaper and better
using less cuts and eliminating all (expensive) curved
cuts on the cross-section.

 - Use one O-ring, instead of two O-rings and one gasket.

 - Don't add the extra three thru-hulls/flood-holes.
Design it like a hatch, fasteners on the inside, not
outside. 

 - Use a standard lip for a sectional spherical dome.


You really need to read the Stachiw book when designing
anything related to view ports.  Compared to the cost of
making a view port frame and acquiring the acrylic dome,
the Stachiw book is very cheap.

Cheers,
  Ian.

-----Original Message-----
>From: Brent Hartwig <brenthartwig@hotmail.com>
>Sent: Aug 30, 2008 1:49 AM
>To: PSUBSorg <personal_submersibles@psubs.org>
>Subject: [PSUBS-MAILIST] Wet Exit Removable Front Dome CAD Models
>
>
>Greetings Subbers,Jame Frankland and I spent a fair bit of time discussing and working up ideas for his removable front dome this last winter. I've wanted a hatch type configuration from the start. But first we needed to work thru the concept of copying some features on Nemo's front dome first. So we would like your opinions on the last model I worked up of are combined ideas at that time. I've not been available to refine the models until now.
>
>As it is in the below link to the models and drawings, I would like to add more thickness to the seating ring.  Also we might need to make some changes to make it easier to fabricate, and size things to easily available materials. 
>
>I've been informed that O-rings used to seal acrylic viewports is perhaps not the best idea because of perhaps stress concentrations in the small area the acrylic touches the O-ring.  To deal with that issue I've been thinking about doing some FEA work on two new configurations. One being to use one large soft O-ring for the domes bottom surface to seat on. The other to have two smaller soft O-rings under that said surface. Any thoughts. I have one more sealing configuration, but I'll need to work up a model to better define it.  
>
>And yes I'm aware that the dome James now possesses, has a vertically machined surface on the outer perimeter.  
>
>http://www.frappr.com/?a=viewphoto&id=2384531&pid=10393161
>
>
>Your resident possibility thinker ;)'
>
>Regards,
>
>Szybowski




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