O-rings:
How they are supposed to work, and why they sometimes fail. The results
of my participation in our own and the Shuttle mishap on O-rings
engineering design practice, and the modes of failure.
1) O-rings are round in cross section, which is why they are called "O"
rings.
2) There are other rubber seals, even of the same materials, that are
NOT round, and do not physically function in the same manner as an O-ring.
3) An example of such a seal is right on the end of most oil filters.
The seal is a rectangular, or square cross section rubber like
(neoprene, typically) material. As are O-rings. But these are crimped
in place on three sides, and a small amount sticks out as the sealing
face against a machined surface. The crimping physically seals three of
the sides, and the compression against the block, or holder, the first
pressure seal. There is no dynamic energization as a result of
increasing pressure, as an O-ring design. It is strictly a compression
seal, like your head gasket and valve cover.
4) The TRUE O-ring is designed in accordance, at least in military and
commercial applications, against JANNAF (Joint Army, Navy, Air Force)
standards. The dimensions of the o-ring groove, relative to the
application, the clearances between the walls and the o-ring, the
compression forces, an the amount of pressure it is rated at, along with
the temperature operating range are variables in the design and result
in a different product.
Typically, the O-ring is in a square groove, and does NOT fill the
groove. It is sealed at the low and high pressure side operating on
opposite sides of the ring, and groove, "across" the top. -|_O_|- (close
as I could get, imagine a top that squeezes the o-ring, but does not
touch the opposite metal.
The instant pressure is applied, the true o-ring physically moves across
the o-ring groove and the pressures try to force it out the small gap on
the low pressure side. Of course it gets squeezed into that groove
"gap" harder, the more the pressure increases, sealing all the harder.
Which is why it is a good design.
It can be "killed", of course, by poor material choices, incorrect
clearances or clamping forces, damaged grooves or rings, environmental
extremes beyond design parameters.
Most of you know about the Shuttle disaster. This 3 back-up ring design
was based upon the very successful Titan rocket design with only one
O-ring in each 10 foot DIAMETER joint, sometimes 7 on each side of each
launch. In the shuttle case, only one parameter was not adequately
monitored, the O-ring material temperature. It was so cold on launch
day that icicles were hanging from the joints.
Need you imagine just how flexible these huge rings were when they were
so cold. On energization, they simply did not have the plasticity to
deform around the groove opening, and seal tighter the harder it was
pushed. It leaked, just a little. Well, a little leak of 6,000 deg
gases does not bode well for a rubber ring.
So, how does this relate to Tigers.
Here is how. If you do NOT have a SQUARE o-ring groove, you cannot use
an O-ring. The FoMoCo 6880 had such a groove between the adapter and
the block (flat surface), and does use an O-ring for this seal. The
O-ring does NOT fill the groove, per design practice, and does not leak.
The gasket on the filter is a 3 sided captured square rubber seal, not
an o-ring, and also does not need another gasket. There is another
O-ring on the enter holding bolt for the block mount.
Older right angle adapters did NOT have O-rings, and relied on flat
surface seals, much like your head gasket or valve cover.
Well, you wanted to know about O-rings. Those were the opening remarks
of the tech eval team on the first day. 4 months later, we REALLY
understood the issue.
Steve
Tom Witt wrote:
> I know that this has been touched on before and that Jim Boynton has an
>article relating to the matter at Tigers United, but my problem has a bit of a
>twist. My oil filter adapter (part # E5TE-6884-BA) came off a late 70's thru
>early 80's Ford Van. The O-Ring slot is square cut. The van this came off of
>simply had a similar sized round O-Ring with silicone put on it. I went to the
>dealer (taking the adapter) and asked for the proper replacement. I was given
>(without looking anything up) a very thin, round O-Ring FCM 87147S91 and was
>told that "it would work." I questioned the fact that the O-Ring was so thin
>(and round), and that it slopped around in the square groove, but was told "it
>would work."
> Well, after I bought it ($4.59 at Ford, O-Rings everywhere else --- .99
>cents) I still had my doubts and opted not to use it. To add insult to injury
>my wife took it back while out shopping and Mr. Parts Expert "it will fit"
>wasn't bright enough to refund her the $4.59, but instead gave her back $1.43
>on the fiber washer price! I know it's only three bucks, but it is the point
>that counts. As far as I'm concern this guy has now made two mistakes on a
>simple transaction. So that I might properly correct "Mr. Parts Expert" could
>anyone tell me if the Ford Motorsports oil filter adapter has a square cut for
>the O-Ring and if, in fact the O-Ring itself is square(ish). From what I can
>see in the pictures it appears to be and I suspect that the FMS-M6880A50 might
>be what I need.
>
> By the way, is it just that I'm a walking crash test dummy for Murphy's law
>or do you, like my wife and I, find it takes the time of nearly an eight hour
>day each and every week just to correct for people unable to do their job
>properly. I know that we all make mistakes, but for us this nonsense happens
>ALL the time.
>
>Tom Witt B9470101
>
>
>
--
Steve Laifman
Editor
http://www.TigersUnited.com
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