Pete C. wrote:

Bob La Londe wrote:

"Bob La Londe" wrote in message
...

So what's the

spproscribedsp

prescribed method? I expect I need a pretty consistent radius

bottom. What's the best way. Make a rough semi V lathe bit, and then
make a decent round one to finish?

LOL.

Silicone o-ring for modest heat. Upto about 325 degrees.




Most O-rings I look at are seated in square grooves. I believe you need
the extra space afforded by the square groove to allow the O-ring to
deform and seal properly. For a piston though, you should probably be
looking at cup seals and the like on McMaster.com.

Correct. Most O-rings are intended to be seated in nearly square
(rectangular) grooves. Rubber is essentially like a fluid ... it is
nearly incompressable. When pressure is applied it DEFORMS, but does not
compress significantly. That's why it's used for some kinds of
one-sided-mold stamping operations. It applies a uniform pressure just
like a hydraulic fluid.

Assuming a square groove with a round O-ring, the groove should be about
the same width as the O-ring's diameter, and bit less in depth than the
ring's diameter. Then, when the cover-plate (or whatever) moves to put
pressure on the O-ring, it deforms, swelling sidewise to press against
the sides of the groove as well as the bottom. It effectively makes a
three-point seal in the groove, with one seal against the closing element.

A round-bottom groove doesn't allow the ring to deform properly, and it
will "mushroom" out of the groove, deforming oddly, since there's no
other place for it to deform into.

Now, it's true that a round bottom groove CAN work, for low-pressure or
unusal applications, where the O-ring is not overly deformed, but it's
not what's usually recommended. Check most any o-ring manufacturer for
specs.

Dan Mitchell
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