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You may be right, Bulent.
But I think your postulation about some deposite on the wheel does not
invalidate the overspeed possibility and if a fact - then the two would sort
of join together and spell doom for the turbo.
The reason I believe you can not rule out overspeed is that it causes the
exact same problem - turbine wheel separation. With the pressure and mass
flow that John was at (it was even higher than I had orginally calculated as
he was at a higher rpm) without anyother considerations, he was to the right
of the efficiency iso lines. Then if you consider the additional mass flow
caused by the 1/2" hole in his intercooler and any possibly increase
attributed to the BOV, that could easily have pushed him further to the
right on the compressor map into the point where small increases in mass
flow results in much higher rpms. So, I that is why I think you can not
rule out overspeed.
Overspeeding ends up with the same results - a loss turbine wheel. Any out
of balance caused by imbalance in the wheel (due to a deposit building up)
or I believe more plausible - a worn bearing - is acerbated by the
additional rotation speed leading to a "wobble" in the wheel's plane of
rotation. Then due to the small clearances between blades and housing, it
doesn't take much wobble and you have contact. At 100,000+ rpm the shortest
degree of contact and it is over, the turbine wheel is junk. So I believe
it is likely (as a turbo bearing housing becomes more worn with usage) and
as you said both turbos were "used "with ???? miles on them before going
into the aircraft, that it becomes more susceptible to such a failure
mode. It may survive longer it 1. It is not under long periods of boost 2.
The rotational speed is kept below some limit. Neither of those conditions
are likely to be met in aircraft usage.
I do agree that any imbalance (whatever the cause) in the rotating assembly
would end up with the same failure, so can not rule out imbalance due to
other factors such as something being deposited on the wheels. But, with
the 1700F+ temperature of the exhaust, I guess I would be surprised that the
gas/oil mixture would survive long enough to build up a residue.
I'll offer up a theory that the tarry substance was actually manifestation
of oil leaking into the turbine housing around a worn shaft. At boost,
sufficient exhaust gas heat would burn it up, but on descent or non boost
conditions sufficient oil survived the temps to coat parts of the exhaust.
But ultimately the worn bearing and high rotational speeds and longer boost
times caught up with the turbo and it died.
So, it would seem we could have several plausible conditions for this mode
of failure - separation of the turbine wheel from the shaft.
1. we could have a condition whereby the turbine was not in the "overspeed"
territory but operating at a lower (safer speed) and a condition existed
that build up a deposit on the turbine wheel until its out of balance state
caused it to touch the housing and fail.
2. A worn bearing ultimately resulted in failure due to wobbly and wheel
contact (Possibly acerbated by periods of long boost at high speeds - but,
not a true overspeed condition)
3. Overspeed of the rotating assembly resulted in failure (due to operating
too far to the right on the compressor map possibly acerbated by the leak in
John's intercooler and/or BOV valve).
4. Defective weld of wheel to shaft (Pretty remote chance - I believe -
especially with two turbos)
5. Some combination of the above. But, I believe that in all cases higher
turbine speed would acerbate the condition and lead to quicker failure -
whether true overspeed or not.
Good addition to the theory stack, Bulent.
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From: <ericruttan@chartermi.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Monday, July 12, 2004 11:07 AM
Subject: [FlyRotary] Re: Three candidates for Turbo Failure
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
>>
>
>
>
>
>
>> Homepage: http://www.flyrotary.com/
>> Archive: http://lancaironline.net/lists/flyrotary/List.html
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