I think if you look at the engine cutaways or blueprints, you will see
in the drive train a torsion shaft that functioned as a torsion bar to
keep the gears engaged in the same direction and soak up some of
the inevitable torque variations around the revolution of the crank.
Engines that didn't have that were a bit fussier to run and had some
blanked out RPM ranges where you got extreme gearbox wear.

That was a problem with the ill fated Cessna 175 with its GO-300
engine. It performed a LOT better than the 172 that it shared an
airframe with, but got a reputation for eating gearboxes. The
gearbox also got a reputation for being as expensive as the engine
to overhaul. The main problem was running them in the same RPM
range as the O-300 they were used to from their 170/172. The
GO-300 was good from 2800 to 3200 RPM. Below 2800 it was
best to go all the way to idle. Then the gearbox only transitioned
once from driving to driven. People tried to run them at 2400 RPM,
which just "felt" better to an old 172 pilot. At that RPM the gearbox
would beat back and forth from driving to driven every time a cylinder
fired. A few hundred hours of that would destroy a set of
gears! :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

"Morgans" wrote in message
...

"Charlie" wrote

You don't often see that because any torsional resonance problems get
amplified at an even-multiple ratio, like the cam's 2:1 ratio. Note the
final drive ratio of almost any reduction gear train & it will be some
odd number like 2.17:1, 2.85:1, etc.

Perhaps an even larger factor is to get different teeth meshing together
in a cycle, each time around. it is better to not have the same mesh,
time after time, although what you say about harmonics is also a factor.

In the really big "warbird"engines, the gearboxes had to be built so
hell-for-stout to deal with the HP levels, the harmonics were not as much
of an issue because the resonance was hard to achieve with the components
being so stiff.
--
Jim in NC