----- Original
Message -----
Sent: Sunday,
September 06, 2009 8:00 AM
Subject: RE:
[PSUBS-MAILIST] No Machining Hatch Sealing
Concept
Just
a clarification with Seals and O?rings etc. On a hatch don?t use
anything that is new and unproven. Sorry Brent. Same goes
for Domes. Sorry Alan.
When
looking at any Seals book you see O?ring grooves with Corner radiuses and
tapers. A common fallacy is that the bottom of the groove needs to be
radiused. Not so. These tolerances etc were done before tip
tools were invented. The angle which states zero to 5 degrees, or some
such, is because inspection would reject an otherwise acceptable groove
because of slight machining anomalies. An industrial acceptable groove
must have a tolerance on it. This should allow for slight tool wear
which will occur on corners of the older used high speed steel ( HSS ) and
also for slightly inaccurate tool setting. ( How do you set a tool so it is
guaranteed parallel to zero tolerance.?) When a turner is grinding a
tool for making an O?ring groove they will do so typically on a
grindstone. Not all machine shops have Tool grinders. Now they
are almost a thing of the past with tip tools and specialist Tool sharpening
services available. The Taper also does not need to be there. It
shows a maximum of 5 degrees and this is not a target. It is a
tolerance. You can?t get a negative angle in a lathe on a groove unless you
have tool bend (which can happen) or a cross slide out of tolerance unless
you deliberately want one and then of course it can be machined.
(Cross slide angle to bed has a tolerance as well and should have a concave
tolerance but not convex tolerance) The angle has no effect. There is
an exception. You can have negative angles such as a dovetail groove for
O?ring retention without any adverse effect. E.g. Kittredge
hatch. Corner O?rings and machining are OK and that is a 45 degree
angle. The important things are clearance, squeeze and surface
finish. Metal to metal is the most desirable with an elastomer as the
bubble tight backup. IF there is a clearance such as a piston to
cylinder type of seal with an O?ring then the corners should be ?broken?
with a small radius so as not to cut the O?ring when it squeezes between the
diameters. When machining it is normal practice to ?break all
corners/edges? . In relation to the bottom corners and radii, if a tool has
sharp corners they won?t last as long as it cannot get rid of the heat so
tips are made, or ground, with a built in radius. Heat removal,
longevity and aiding surface finish. It is one thing to machine a
groove, it is another to machine high production items with a groove to a
tolerance and surface finish. Swarf can damage surface finish. Swarf control
is addressed in the modern tip designs. It is not necessary to have a
radius in the bottom of a groove. Just pay attention to breaking the
edges without too much radius. Radii in corners are a good thing
though to eliminate stress multipliers.
Corner
O?rings. The books give dimensions which are OK if the fluid can
energise them but in low pressure or pulsation applications the sizes should
be amended. Normal O?ring to cavity ratio is 80% but for a pulsing
situation or low pressure situation then we find 90% is a better ratio.
O?ring hardness and surface finish equals friction. Harder
seals for greater pressure but for most the standard 70 durometer is more
than adequate.
Therefore
the hatch situation is a typical static seal application. Tried and
proven! It will cost very little extra to put in an O?ring
groove as opposed to machining it flat and then adding other
?inventions?.
I
have had supposedly experienced people try and understand o?ring grooves and
the machining tolerances and wrongly interpret them. I resigned off a
standards committee because a bunch of people sat and listened to a ?would
be engineer? who got up and lectured on O?ring grooves and drew a round
bottomed groove!!, and the others all listened to him. They even put
it in the first issue of the standard!! Chs,
Hugh
From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of
ShellyDalg@aol.com
Sent: 05 September 2009 03:17
To:
personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] No
Machining Hatch Sealing Concept
A
couple of thoughts though.
This
rendering shows basically a flat gasket trapped between two ridges. It won't
operate like an "O" ring. It operates like a flat gasket. It would probably
seal at low pressure but loses it's sealing capacity at greater
pressures.
If
a guy wanted to weld rings instead of cutting grooves ( for an "O" ring
) it seems it would be a lot easier to use square stock. The grooves
specified by the Parker "O" ring book call for rounded corners at the
bottom. A TIG weld would give you that radius. I considered that option when
first building my hatch. I just didn't think it would be nearly as strong as
a machined groove. For the $300 cost of the machining versus buying,
welding, and other work associated with the welded rings approach, It's
pretty cheap for a nice clean means of sealing the
hatch.
I
don't think the mating surfaces need to be perfect, as the "O" ring will
seal up a fairly large crack, and as the pressure increases the crack gets
smaller. Eventually the two mating surfaces are touching and the "O" ring is
forced into an almost rectangle shape. It still seals.
I
think it's pretty well established that an "O" ring is the only way to go,
but it's good to keep looking at different ways to skin a
cat.
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