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There is a valuable lesson in problem solving here George. Very familiar
pattern that really does apply to our rotary opportunities.
Here's what I suggest. Don't tell your foundry guy you are discussing this
with someone, that will just get him defensive. Instead, ask him:"How do
you know the quantity of hydrogen in the alloy?". He's going to give you
some theoretical based statement. So tell him"Pretend I'm the hydrogen fairy and
the quantity just doubled in that bath, how would you know it's actually
double?" You'll likely find he doesn't really know. He'll say "We could blah
blah", but you ask him how often they MEASURE it and you'll start to appreciate
they have no facts that prove hydrogen affects their casting process. It's all
just theory.
All of us tend to make decisions based on theory. An important key to
success in problem solving is to just find a simple way to convert the theory to
facts. This is rarely done. So when someone says:" I think this will make things
better", you need to say :"Lets prove it....let's do the opposite and see if it
gets worse!"
So here's the deal, the foundry guy knows that the hydrogen level increases
if you raise the temp of aluminum. That's a theory, it's true. But it's not
significant. It's not the thing that affects the outcome. Just
because something is true does not mean it's significant. You have to also test the idea for significance.
Yes, familiar with nitrogen bubbling.
-al wick
Artificial intelligence in cockpit, Cozy IV powered by
stock Subaru 2.5
N9032U 200+ hours on engine/airframe from Portland,
Oregon
Prop construct, Subaru install, Risk assessment, Glass panel design
info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html
Al,
All good info, it's good to talk to someone in the know. We
have looked into the vent and riser potential and this has been
optimised. Shrinkage defects can be eliminated with the right number of
risers, in the right places especially in (as you say) thicker areas of thin
castings.
There is a strategy for eliminating Hydrogen from molten
aluminium and that is to purge with Nitrogen ( Nitrogen bonds to hydrogen
molecules - so I'm led to believe) through a ceramic wand ( immersed in the
aluminium) for approx 20/30 minutes. Have you tried this method to eliminate
the possibility of hydrogen?
George ( down under)
Oh, why didn't you say it was sand cast? 90% probability your porosity
was shrinkage porosity, nothing to do with hydrogen. You sure want to avoid
shrinkage in high stress areas. It's most likely to develop at inside
corners near thick sections of casting. A good foundry can take action to
minimize risk. Depends on details, but they can add vent, or riser as
needed. We also used special sand for optimum qualities. But it would likely
be unnecessary for your application.
I conducted a number of statistically designed experiments to optimize
the process. The goal being to reduce the likelihood of shrinkage defects.
Total blast doing that kind of stuff.
The nature of shrinkage porosity is that it comes and goes. So most
foundries have hard time identifying contributing causes and optimizing. Lot
of statistical noise.
Good luck in your endeavor!
-al wick
Al,
There's reasons for my enquiry, which involves castings
for Aviation use. My initial challenge was a multi use bellhousing to
accommodate Aussie PSRU and that of Tracy's unit.
It doubles as a engine mount and accommodates 5 (
maybe more) starter types and has to be light and strong. This requires
strong thin wall castings.
Usually thin walled castings require pressure injection
technology. This is expensive and not cost effective
because of the projected low demand, probably one
to 2 hundred (at most), in an initial 2/3 year period.
I settled on sand cast technology, but because of
the thinness of some of the pattern, the aluminium is heated beyond it's
recommended melting temps, to allow for easier running into thinner area
before the temps are reduced by the sand casting
process.
The initial trial did identify obvious porosity,
throughout the pattern although later trials, being carried out in the USA
have yet to identify any significant reduction in the projected strength
requirements.
My development partner in the States, Butch as he is
affectionally know throughout the Industry - is an Aviation
Engineer.
This design has been thoroughly tested on Finite Element
Analysis, was CAD designed, with myself making the pattern to exacting
tolerances, due to design restrictions and as Butch's exacting demands -
he's a hard man to please!
Although this took some considerable time ( approx 12
months) the pattern was completed and the prototypes done, by a very
competent foundry.
If I can quote Butch's recent remarks to me "
The Bellhousing arrived safe and sound (Excellent Packaging)..... Very
Robust to say the least, should be able to handle 800hp at least. It has
been Ultrasonically analysed for density and voids, point load tested and
torque twisting along both the horizontal and vertical
axis.
Needless to say it passed with "Flying
Colours!!
Do you see a pattern developing here? Research design
and testing by competent authority!! - even the
packaging!
To a unenlightened onlooker, on initially first seeing
this bellhousing, their response might be this design might not meet
what we normally accept as a bell-shaped design i.e. form not meeting
design requirements etc. etc.
This is the type of development work carried out by many
Experimental designers - but not necessarily communicated to everyone
to this degree. I won't say this is true in all Rotary installations, but
I will say there is much in the way of skilled and talented builders
involved in the process of the Rotary development.
The point I'm trying to make is, although I believe your
risk analysis is valid, I believe it is only valid when the information
you base your assessment is correct and complete. Often a valid assessment
can be completely turned on it's head when seemingly correct information
is found to be incomplete, therefore making the initial assessment
completely useless. I believe some assertions, on this discussion group,
have pointed to this possibility.
BTW I'm on the look out for any good foundries
around the East Coast Nth of Washington, who could carry out this
Bellhousing work ' Cost Effectively', for the US market, if you
know of any I would love to hear about it. One of the problems on
supply to the USA, is the 'Tyranny of Distance'.
George ( down under)
I only did hydrogen experiments with permanent
mold castings(thick wall parts), so unsure if it applies to other types.
But the experiments were conclusive. Hydrogen was absolutely trivial. It
was shrinkage porosity which dominates the mechanical properties.
Hydrogen porosity develops round voids, shrinkage voids tear.
I suspect the myth continues regarding hydrogen. I did those
experiments over 10 years ago. It gave us huge advantage over
competition. We focused on methods to reduce shrinkage defects. Ended up
out performing our competition. That was a blast. I miss those
challenges.
-al wick
Artificial intelligence in cockpit, Cozy IV powered
by stock Subaru 2.5
N9032U 200+ hours on engine/airframe from
Portland, Oregon
Prop construct, Subaru install, Risk assessment,
Glass panel design
info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html
I've had a look at Al Wicks approach and for me it
leaves a lot of unanswered questions. I have the benefit of being a (
now retired) Government Logistics manager, trained in Quality
Assurance, Occupational Health and Safety, Risk management and
of course procurement. I had a good deal of experience within the
medical logistics field.
This basic approach gives a basic guide
provided you get your facts straight and work on with the right
information - I can't see this being done with the Rotary. Perhaps he
has done quite well with the Subaru - who would know.
Al if your on here would you please elaborate on the
statement on Aluminium - the information to me is that Hydrogen is
indeed the major problem with non- injection cast aluminium.
Especially if it involves elevated thin pour castings - the elevated
temperature draws hydrogen from the air and releases it as bubbles in
the aluminium, the higher the humidity the greater the chace of
Hydrogen porosity.
As we all know porosity is the primary cause of
strength reduction in a cast aluminium piece. I understand there are
other causes of porosity, but am unsure of what they all
are.
George ( down under)
Ernest
Christley wrote:
Jim, Al is
not following his own process (I think I alluded to this
previously). First, you have to ask, "How many failures have
accurred due to a faulty CAS?" That's a
fair question. Do you know? Does anyone? If so,
Who? Seems there was a thread around that just a month or
two ago. Intuitively, I would say that CAS would be a
single point of failure, important enough to be
remediated. The text below is copy and pasted
from http://www.maddyhome.com/canardpages/pages/alwick/risk.html
The key phrase is the last sentence.
We are going to do an
FMEA. What is the goal we are trying to achieve with this process?
It’s to make sure we place our efforts on the facets which need
it. Put another way, it’s making sure we don’t waste time and
effort on insignificant items, while ignoring the truly important
items.
There are only three pieces to the
puzzle. In the case of CAS (just
my guess)
1) If the component failed, how serious
would that effect the airplane? catastrophic
2) What is the
probability of the component failing? Undetermined. Start with doing some research
at NAPA et al and repair shops around how many they
sell.
3) What is the likelihood that you would
notice the problem before failure? I'd
guess very VERY remote.
You may have heard
statements like “You have to replace component x on your engine
before installing into an airplane because it represents a single
point failure”. Meaning that if x fails, there is no backup
component. That statement is not meaningful until you assess all
three questions above.
Exactly. Al's
question is "... to what extent are "we" using his
methodology. My own guess would be "not much ...".
Single point(s) of failure in Tracy's ignition (and fuel control)
systems - if there are any - would be a case in point. As
would redundant fuel pumps powered by a single source, and
charging systems that are not sufficiently redundant and with
appropriate indicators. If one DOES have a single point of
failure (and there are inevitably many) we must be sure that that
component is sufficiently robust to give us all confidence that it
will NOT fail.
>> Homepage: http://www.flyrotary.com/
>> Archive: http://lancaironline.net/lists/flyrotary/List.html
-al wick
Artificial intelligence in
cockpit, Cozy IV powered by stock Subaru 2.5
N9032U 200+ hours on
engine/airframe from Portland, Oregon
Prop construct, Subaru install,
Risk assessment, Glass panel design
info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html
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