I apologize Paul.. I didn't read Steve's post carefully enough. I
think I've got it now. The "creep" or distortion he's talking about is
strictly on the wear surface, right?
Due to high power operation.
I take that to mean that the wear surface might be getting too hot and
thus becomes weaker, and then the seals chew into it. Does that sound
right?
If it's heat weakening the base metal, I wonder what's primary factor
driving that. Combustion gas exposure or friction? Lack of cooling?
Under oiling? Mismatch between the side seal material and the surface
coating? A combination?
Regards,
Matt-
I have no idea. I am waiting to hear from Steve on what he means
when he says creep.
I suspecting the base metal is merely deflecting and not weakening.
The term weakening is not in the lexicon of the mechanical engineering.
The terms are yield... a permanent deformation of the material.
and deflection.... a non permanent deformation.
The bottom line is to duplicate the Mazda test rigs.
Pay particular attention to the plating tester. Chrome on steel
was far and away superior to anything else Mazda tried.
Paul Lamar ...No rotor no motor.
Matt,
I suspect what Steve is saying is that the aluminum is yielding and the
coating is failing ie the substrate is failing to support the surface
coating.
It really doesn't matter why unless your designing a new aluminium.
George ( down under)
Hi guys--
I apologize for any confusion regarding the references to creep in relation
to our aluminum alloy rotary engine end housings. Neither of us was a
trained engineer--basically self-taught hot rodders and mechanics who
studied the literature on various issues when fixing stuff that broke or
when working up new designs. So I apologize if any of our references are
inaccurate. But in terms of evaluating all the various problems with our
rotary engine aluminum end housings built and tested in the 1980's and
1990's, we used the standard reference literature. That is the case
regarding the creep references.
These included Marks Standard Handbook for Mechanical Engineers and other
references. Here are a few excerpts from Marks r.e. creep.
"... In metals, creep is a plastic deformation caused by slip occurring
along crystallographic directions in the individual crystals, together with
some flow of the grain-boundry material."
"After complete release of load, a small fraction of this plastic
deformation is recovered with time. Most of the flow is nonrecoverable for
metals."
There are many types of creep. Although elevating temperatures makes the
problem worse, this viscoelasticity can occur at lower temperatures...some
of the softer metals even at room temperature. But as temperatures increase
the problem of atomic diffusion goes with it. Stresses for given creep rates
and temperatures is shown on Table 5.1.6 in Marks. Steel, duraluminum and
other metals are also in other exhibits and within the narrative. Marks also
provides key refernce paper titles/authors regarding creep...from as far
back as the 1930's.
Regarding our housings, what we referred to as creep deformation was only
one of the problems associated with the long term flight tests of aluminum
alloy housings sand cast using 356, 390 and 713 aluminum.
1. Fatigue cracking around the locating dowels for rotor housings
2. Failure in the main bearing support area
3. Distortion over time from various combined sources including
creep/thermal expansion and combustion pressure
5. Added complexity and expense of applying the sealing grid wear surfaces
to the aluminum
6. Exfoliation experienced with plasma sprayed housing wear surfaces was
usually not a problem; it generally resulted from other issues related to 1
thru 4
Combustion pressure was believed to be a major design issue. Subsequent
design modifications and dyno and flight testing helped somewhat but
ultimately we concluded that even in improved, modified form, housings of
aluminum alloy using materials available at that time was not the best
material for long term service life. Our experience with the Mazda factory
cast iron housings was very good in all respects except weight. It seemed
that the rigid cast iron housings, together with the steel insert in the
throchoids, provided a very solid structure with none of the problems
involved with aluminum. But due to the weight of the cast iron, to attain
the best compromise of power-to-weight and longevity, it was determined
thin-wall steel housings with nitrided wear surface would be best. We
researched investment casting of these and found it was simply was too
costly. That is when we turned to furnace brazing.
For doing the first set of these housings test housings, design methodology
was very basic. But Everett spent considerable time researching and
computing stress-deflection (compression) of columns from combustion
pressure and a stress analysis (elastic strain) of stress in the end
housing wall produced by combustion pressure of 1,000 psi. Deflection rates
were calculated for a 1" square plate fixed on all edges and a uniform load
over a small circular area at the center for steel .120 thick, equivalent
cast iron .200 thick and aluminum .200.
After the prototype housings were furnace brazed they were machined and
surface lapped, then shipped for nitriding. After nitriding we finished and
final lapped prior to assembly.
Weight of furnace brazed steel housings taken during assembly of the power
section were:
Center housing--9.75 lb
End housings--10 lb each
By comparison, our cast aluminum housing weights--
Center housing--8.75 lb
End housings 7.5 lb.
Steve Beckham.
Thanks for the clarification Steve.
I think Everette got carried away on the 1000 psi combustion pressure.
It is more like half that. Initially it can be 1000 psi but the area or more
importantly the span of unsupported side wall is very short. As the rotor
moves the pressure drops of drastically while the span increases.
--
Paul Lamar ...No rotor no motor.
Steve,
Thanks for the great information. If you or anyone plan on producing
steel housings, add my name to the list of first purchasers and for long
term dyno testing.
Doug in Japan.
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