Tim Ward wrote:

"Peter Dohm" -KNOW wrote in message
-KNOW...
Tim Ward wrote:

"Norman Yarvin" wrote in message
...
In article ,
Badwater Bill wrote:

When I was the test pilot on the OMABP RV-6A project, we used the
Chevy Vortec V-6 engine, the PSRU was specifically designed to turn
the prop at tip speed below 80% the speed of sound. Jess Meyers also
used a reduction ratio number that was about equal to the square root
of 2 to eliminate harmonics that could have resulted in reversed
torque pulses reflecting back into the engine. By using a reduction
ratio of 1.41 (or close to it) he eliminated many sympathetic
harmonics that may have occured.

What, in order to have the ratio between the two be an irrational
number?
That's not actually going to help eliminate resonances, unless you get
lucky -- and you are about equally likely to get lucky with any number
of
about the same size, irrational or not.



--
Norman Yarvin http://yarchive.net

Just out of curiosity, how would you get any ratio to be an irrational
number?

Tim Ward

There is another word I don't recall. Basically, the idea is to even out
the wear in the gearbox--especially if it is a spur gear system--to give
dramatically longer life to the psru.

Peter

I can see how having the numerator and denominator of the ratio relatively
prime might be a benefit. Then the same configuration would only turn up
once every product of the two numbers revolutions.
That's kind of a convoluted sentence, but I don't know how else to put it.

Tim Ward

You are exactly right. The remaining problem with spur gear reduction units is
that the same portions of the crankshaft pulley will always take the power
pulses--unless there is also a clutch, fluid coupling, etc. Therefore,
planetary or epicyclic drives are usually preferred unless an offset is also
needed.

Supposedly, the problems are mitigated by a hi-vo chain drive for engines of six
or more cylinders, or by a cog belt drive. Both have the advantage of spanning
approximately half of the teeth on each pulley. Unfortunately, I doubt that
some of the belt drive designers know much more than I do.

At the moment, the Geshwender drive (which is back in production despite Mr G's
death) looks like the most reliable scheme for much more than 100 horsepower,
any may still be the best value in the long run.