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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 Hi Brent. Nice drawings. 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. Frank D.
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