Message-ID: <40F2A948.60601@chartermi.net>
Date: Mon, 12 Jul 2004 11:07:52 -0400
From: "ericruttan@chartermi.net" <ericruttan@chartermi.net>
Reply-To:  ericruttan@chartermi.net
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Three candidates for Turbo Failure
References: <list-310767@logan.com>
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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=
=20
and the turbine will touch the housing.  If a turbine touches the housing=
=20
the shaft will snap at the weld.

This was new to me.  I spoke to a professional turbo rebuilder who explai=
ned=20
that this type of failure is very rare, and probably do to an out of bala=
nce=20
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=20
junkyard all the time, and never had them fail.  I used to laugh at peopl=
e=20
who paid $100 to rebuild when you could get junk for $25 that worked grea=
t.

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=
=20
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 ou=
t=20
of balance condition.

Rebuttal please?

PS

I disagree with an over speed, as the turbo was in limits, the best we ca=
n=20
tell.  The evidence of an over speed is a wheel coming apart due to=20
centripetal forces, which john does not mention, and there is no evidence=
 of=20
in the photographs.

I disagree with heat, as there is no melting or warping, or any other=20
indication.

I have not posted this on canardaviation.dmt.net yet.  I await your learn=
ed=20
examination.

Bulent Aliev wrote:

> Both of John=B9s turbos were =B3Junkyard direct=B2 . Mine was also and =
I noticed
> considerable play while pushing up and down on the wheel. I=B9m 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 RP=
M
> with the cutting wheel able to play in the chuck? I rebuilt my turbo an=
d
> there is no end play at the wheels.
> Bulent
>=20
>=20
>>>>...
>>>>... 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? =20
>>
>>   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 i=
s
>>closer to being a brazing (in principle,
>>anyway).
>>
>>My somewhat semi-random thoughts ...
>>
>>Dale R.
>>COZY MkIV-R13B-NA #1254
>>
>=20
>=20
>=20
>=20
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