Try this on for a theory, guys. Looks to me
like the o-ring is too big
for
the groove, not necessarily "won't fit" but,
fills the groove TOO FULLl.
The
silicon and the teflon both increase in size
very substantially when
heated,
and the teflon appears to have expanded to
perfectly fit the groove in the
photo. If you wind up with the groove 100%
filled with teflon and silicon
rubber
before it is fully expanded due to thermal
expansion it will generate
essentially
UNLIMITED forces, it WILL get the expansion.
This is what this looks like
to me.
This is a phenomenon that I have seen to AMAZING
damage, and
breaking a chunk of cast iron is well within the
capabilities. I do a lot
of
analytical work on electronics and mechanical
stuff for thermal expansion
damage possibilities and we have seen this many
times. I have seen
trapped silicone rubber bend a 2" thick steel
lid between 1" dia. studs on
3" centers.
The key is to ensure that the fully expanded o-
ring takes up LESS
volume than the groove. I would like someone to
drop a teflon
encapsulated
o-ring in boiling water and measure the section
diameter. If this hot
diameter
converted into cross sectional area isn't some
significant number smaller
than
the section area of the groove, you are in
trouble. This doesn't account
for
the axial growth of the o-ring, too.
I suspect that you need one step smaller section
diameter for these rings.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
The outer slot has an area of .13 by .070 or .0091
square inches.
so the radius squared would be .0028966.... working
backwards.
The square root of that is .0538.
So the largest section diameter to fill the outer slot
would be double
that or 0.107640. That lends credence to Bill's theory
as a 3/32
section diameter ring is not .096 section diameter but
is about
0.11 section diameter or about 14 thou larger (.014).
And then you have the O-ring tolerance on top of that.
Some might be 0.005
under and some might be 0.005 over. Also any crude in
the slot will not
help matters. Make sure the slots are spotlessly
clean.
Perhaps Mazda themselves ran into Bill's contention
back around 88 and
that is why some housings cracked in this area.
Perhaps they reduced
the section dimensions on the stock O-rings slightly.
This is why we call them experimental airplanes.
Bill, do you happen to know what the coefficient of
thermal expansion
is for red silicone?
The bottm line is measure your water O-rings and make
sure they are
less than .093 +/- .005 section diameter. Particularly
the outer
one.
Paul Lamar
Hi Guys;
I'm in full agreement with Bill's synopsis, and
can personally attest to it by identical experiences in
designing Pharmaceutical equipment.
Paul your numbers are good ones, you are close, but for
practicality this is what we practice when retrofitting
exotic seals (Rings) in general...................
I'll use the 13B existing gland design:
Brand new groove is.............130 x .070 = .0091 area
Lapped groove is................130 x .067 = .0087 "
Ideal o-ring (.094d)+ max tol (.005)...... = .0077 "
This will give you a net area of .0010 or 11.5% for
MAXIMUM thermal expansion of the seal.
So the question remaining is, What is the thermal
expansion of these composite seals?
The manufacturer NOT the Rep is supposed to have this
information thru the Lab(s) who performed the tests.
The other downside is that this type of seals vary in
diameter throughout the same seal.
Butch
Yes there is some variation around the ring and they also appear to be oval
in some areas. Hang in there I will heat one up tonight.
So the one to use then is the .093 as I guessed.
Paul Lamar
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org