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.
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.
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