I've been monitoring, with interest, your progress on silicone-rubber
torque damper design. Although I hope everything works out for you, I
wonder how well the silicone will hold up under long-term use. If I
remember my physics, the energy of the engine providing during the
peak torque will be stored within the rubber (somehow done by
stretching the polymer bonds within the molecular structure of the
rubber). This energy is then released when the torque is at minimum
value. On the following sketch, the quantity of energy
stored/released in represented by the areas on the graph; the area of
the peaks must equal the area of the valleys.
Just to keep the math easy, let's say that the area stored/released
by the rubber is 10% of the overall energy output of the the engine
(as represented by the compared areas on the graph).
A 200 HP engine puts out 150 KW of energy. This would demand that
the coupling would store/release about 15 KW of energy continuously.
Now we know that rubber compounds are not 100% efficient at storing
energy in their elastic bonds. The elasticity of rubber is measure a
number of ways. There are durometer numbers (related to hardness
and elastic-modulus) but it doesnt seem that these values relate
completely to the high-speed energy handling capabilities of
polymers. A number that seems to be more useful is the “coefficient
of restitution” used to compare golf-balls and such. Coefficient of
Restitution relates to the velocity response of deformed elastic
medium. It is interesting that a glass marble has a very high
coefficient, as do hardened steel ball bearings. Kids toy
“super-balls” have a high coefficient, as does the soft mushy
“silly-putty” that kids something play with. If memory serves, most
very-bouncy things still have only a coefficient of about 90% .
Bottom line is that it is possible that 10% of the energy
stored/released by the elastic- coupling will be retained as heat
withing the rubber compound … at that will be about 1.5 KW or 5000
BTUs of heat.
I think it is likely that you could have a heat buildup problem
resulting in rapid coupling degradation
RBerglund
Silicon is good to 500 F and there is a lot of aluminium to carry
away the heat and dissipate by the rotating coupling.
I am not worried.
Paul Lamar
It's all theory until the flight test is done. Then we will know if
these couplings are a pipe dream or bullet proof. I'm looking
forward to the results.
Mark LaPierre
Interesting conversation! Perhaps the heat capacity and thermal
conductivity of the silicone should be considered, and that of the
housing itself. That would tell us how a material's temperature changes
when it absorbs energy, as well as how those materials would be able to
conduct away the heat generated from the compression and release of the
elastomeric material. We might be able to work out what a steady-state
temperature might be for a given energy input. Not that I am doubting
Paul. But it seems to be a question on peoples’ minds. Actually, it
was on MY mind too! I was wondering how one might size the coupler
safely. Might be fun to actually generate some numbers. I will look
into it.
Gene Kahn
The time to climb coupling will be the acid test as it is roughly 3
times the HP
and only 15% larger. We have room on the engine part for radial cooling
fins if
necessary..
Paul Lamar
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