Wow! Ooops, take #3

On Friday, April 3, 2015 at 5:34:23 PM UTC-4, Dave Nadler wrote:
On Friday, April 3, 2015 at 5:25:34 PM UTC-4, howard banks wrote:
On Thursday, April 2, 2015 at 2:44:59 PM UTC-6, Dave Nadler wrote:
Yikes.
http://ad.easa.europa.eu/ad/2015-0052-E

Based on Mr Nadler's description, failure more or less inevitable.
Too small a radius will reduce the part's fatigue limit by somewhere
between one third and one half roughly speaking. A 90 degree non-radius
would result in a more or less infinite reduction in the fatigue limit
(which is the stress on a part below which it should have an infinite
fatigue life; in the real world all sorts of things reduce this limit,
as we are seeing).
Rough machining can be even more insidious. Each piece of rough machining
that you can see by eye is more or less the same as an already existing
early fatigue crack. Its root radius at a microscopic level is effectively
infinite with a corresponding reduction in the fatigue limit.
Very bad news especially when it happens at a designed in place of
inherently high stress.
All they had to do was to add some hard chromium plate on any wear
surface that ran around the radius and failure would have been even
earlier.
Pretty basic stuff. For it to have been repeated, as seems to have
happened after a known problem, amounts to extreme carelessness.

To be clear: The failed part I examined in fall 2013 was a "take #2" part.
The part was redesigned for "take #3", "resolving" the 2013 AD.
The new "take #3" part failed, leading to the most recent AD.
I have no idea what the failure of "take #3" looks like...

It is a bit surprising that 3 iterations of this part have failed...

But it is not an easy problem!

Hope that is clear,
Best Regards, Dave

I'm NOT an "ME", I'm a field service guy with a "ME" background (as well as spending a lot of time fixing "good enough bits" on many different types of machines.....).

My link before was to show that there are quite a few factors to consider (not that anyone here know all the facts or can direct the final decision).
Just pointing out that some failures have been hit before, thus a research can find a suitable resolution.

"Thus that ignore history are doomed to fail in the same manner"..... or close to that....

On Thursday, April 2, 2015 at 1:44:59 PM UTC-7, Dave Nadler wrote:
Yikes.
http://ad.easa.europa.eu/ad/2015-0052-E

Excellent paper on torsional vibration in aviation drive trains.
http://www.vibrationdata.com/tutoria..._vibration.pdf

Not always a simple as it appears at first.

Cheers,
Craig
7Q