Are pendulum absorbers the same as the counterweights on the crank of say an O-360-A1F6?
Yes. Lots of folks refer to pendulum absorbers as "counterweights", including the manufacturers. I prefer to be more precise with the terms I use, a habit pounded into my thick skull by an engineering mentor. Here's the deal.
Set the wayback machine for the 1930's. Everyone is trying to develop larger, more powerful radial engines, but cranks break and props fail. The cause is found to be vibratory twisting of the crankshaft. Along come three guys named Solomon, Sarazin and Chilton, who develop different devices all operating on the same principle...a pendulum to counter torsional vibration. Chilton's patent describes what we know today as a "counterweight", but how did it get the name?
In the case of a
radial, the pendulum absorber is in fact located 180 degrees from the crankpin, and replaces one or both
fixed counterweights. A pendulum arranged this way is both counterweight
and absorber. Visit a university library and dig out the SAE Transactions from around 1938 or 1939. The engine designers were really excited; the new device meant they had a way to control an entire vibratory order, not just a frequency, and do so
without adding any new weight to the engine. It was one of many developments which helped win WWII.
I think it was Continental who first published (again in the SAE Transactions) experiments with applying torsional pendulums to a small flat engine. The difference, as compared to a radial, was that the pendulums were applied in matched pairs, at 90 degrees to the adjacent crankpin.
A flat engine has no counterweights; each piston assembly is countered by its opposing twin. The pendulums counter nothing but themselves.
Ahh, but the name. Imagine an entire generation of mechanics trained on big radials. That dangling thing on the radial crank? That's a counterweight. Says so right there in the textbook. Now along comes new postwar flat engines with more cylinders (i.e. longer, more flexible cranks) and more power; they need pendulums. Never mind that a flat motor doesn't need a counterweight. It's got dangling things on the crank, so they're counterweights, right?
No, but we're stuck with the name, however wrong.