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
Paul:
Do you remember what size rings you ordered for my engine? I think
you got them from McMaster Carr?
--
Perry Mick
Duckt N7XR
http://www.ductedfan.com
I'll have to look that up. The very first batch I got was from All O-rings.
Bad place as they were trying to obscure source and part numbers.
I think I did get some from McMaster Carr but not sure
if yours was among them. The last batch I got directly from Creavey
and I think those went into your engine.
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
Paul,
Sounds like a 3/32" O-ring would be very bad, esp with a bit of crud
build-up in the groove, or even sealant. Using sealant sounds like a
bad idea in that this fills the expansion volume and will either force
the sealant out when heated or break something. Non-curing sealants
probably extrude when heated the first time.
As to having a CTE on the red silicone rubber O-ring material,
not specifically, but some of the silicone rubbers have a CTE of
100 up to 260 ppm/F which compares to about 7 ppm/F for steel and
about 12 ppm/F for aluminum. PTFE (Teflon) has a CTE of 78 -106
ppm/F from 100C up. These things can really move when heated and,
if trapped, can do amazing damage.
ppm is parts per million so that is 260/1000000 or .00026 per degree
F times the diameter. Is that correct?
So 150 F would be .039 times .1 or about 4 thou larger for a .1 section
diameter O-ring. Is my math correct Bill? This sounds too small of an increase
to explain what is happening. Perhaps Tom's Oring was larger than the slot
or he used RTV to glue it in filling the slot. PL
Calculate the cross sectional area of the groove and the cross sectional
area of the cold o-ring. (Like Paul did above) Then increase both
dimensions of the o-ring volume by say 100ppm/F with a temp rise of
140F and see if you run out of room. If so, this is your problem. I
believe this is what is seen in the extruded O-ring picture.
Typically, O-ring grooves are wider than they are deep and often are
relatively square in cross section, leaving room for thermal expansion.
You need to do accurate calcs on the cross sectional areas and avoid
sealant, altho this may just extrude out of the cracks when heated. The
rubber is too strong to easily extrude thru the fine cracks (near zero crack
width) and can crack something if fully trapped.
Bill Freeman
Your numbers are correct, for a 0.1" diam section the area increase is about
8%, which should cause no problems for a properly sized O-ring with a reasonable
groove clearance.
Anyway, Tom's pix indicate seriously oversized section diameter, didn't even
get to heat up, failed due to hydraulic loads like I thought, but due to the
bolt torquing loads. Exactly the same effect, different driving force.
Bill Freeman
Yep I think your right. Well Tom send me a piece of that O-ring.
Paul Lamar
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