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The, or my, theory of the day.
I believe that both of john's turbos were out of balance. Although some wobble is unavoidable and allowed for in the oil bearing, too much and it and the turbine will touch the housing. If a turbine touches the housing the shaft will snap at the weld.
This was new to me. I spoke to a professional turbo rebuilder who explained that this type of failure is very rare, and probably do to an out of balance condition, causeing the turbine to touch.
But the question becomes How, or why, did it happen?
I raced turbo chryslers for a while and we used to race turbos out of junkyard all the time, and never had them fail. I used to laugh at people who paid $100 to rebuild when you could get junk for $25 that worked great.
So this is my theory of the day.
John was running way to rich.
That is important because he mentioned "gunk" on his prop after the first failure. I theroize it came off the turbine when it stopped suddenly.
The unburnt fuel residue is building up on the turbine and causeing an out of balance condition.
Rebuttal please?
PS
I disagree with an over speed, as the turbo was in limits, the best we can tell. The evidence of an over speed is a wheel coming apart due to centripetal forces, which john does not mention, and there is no evidence of in the photographs.
I disagree with heat, as there is no melting or warping, or any other indication.
I have not posted this on canardaviation.dmt.net yet. I await your learned examination.
Bulent Aliev wrote:
Both of Johnıs turbos were ³Junkyard direct² . Mine was also and I noticed
considerable play while pushing up and down on the wheel. Iım sure at very
high RPM the wheels get in some harmonic vibrations and it is matter of time
before they snap at the root. Just imagine your Dremmel tool at high RPM
with the cutting wheel able to play in the chuck? I rebuilt my turbo and
there is no end play at the wheels.
Bulent
...
... It's the turbine wheel that failed, not the compressor wheel. On
each occasion the weld broke right at the base of the turbine wheel, the
wheel came off and blocked the exhaust outlet.
The *weld* broke? Does that mean that the shaft is
still intact? Someone else mentioned the term "friction weld"; I'm not
familiar with it. This is something different than
"interference fit"?
Gotta wonder what kind of stresses would cause that weld
to break. Is the shaft keyed? Maybe it should be - although
that would complicate balancing it. Perhaps balancing is
part of the problem. Maybe at umpteen thousand rpm, +/- .5
gram isn't close enough.
Two senarios come to mind: (1) Turbine wheel heats up
faster than the shaft; develops a small amount of clearance
at the end opposite the weld; starts wobbling a little, too
much for the weld to absorb; (2) Turbine gets a little
wobbly at high speed; after sustained high speed operation,
the vibration overcomes the weld.
I don't quite understand why the only thing holding that
turbine wheel on the shaft is a *weld*. Isn't the turbine
Iconel or similar? Is the shaft also Iconel? If it isn't, the joint is
closer to being a brazing (in principle,
anyway).
My somewhat semi-random thoughts ...
Dale R.
COZY MkIV-R13B-NA #1254
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