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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