On Thu, 09 Oct 2003 23:37:37 GMT, Peter Dohm
wrote:


I am a little curious about the specifics of how the 2:1 reduction ratios
are acheived on both the belt and hy-vo chain drives, as I learned a
number of years ago that evenly divisible ratios (such as 2.0:1, 1.50:1,
3.0:1, etc.) should be avoided in spur gear type reduction drives as
they will wear unevenly and require more frequent overhaul. The problem
occurs when the same gear teeth consistently transmit the power or
compression strokes of the engine, and can be mitigated by slightly
hanging the ratios; usually by one tooth on either the drive gear or
the driven gear. However, since the drive gear is fixed to the crank
shaft, the uneven wear problem can not be eliminated in a spur geared
system.

In the case of a belt or hy-vo chain system, it should be possible to
mitigate the wear problem to a similar degree if the number of teeth
on the chain is not divisible by those of either of the two pulleys;
although the crank shaft pulley is obviously the more important of the
two.

I couldn't find much on the Northwest-aero web site regarding the
"innards" of their PSRU, so I am curious.

Peter Dohm

Peter, perhaps Bruce Frank can tell you more about the cog belt
systems, I can only give you an overview. Bruce issues a newsletter
on the subject and has written about a guy who managed to accumulate
2,000 hour on his engine/psru (Ford 3.8L V-6 with 2 to 1 ratio cog
belt psru). At that point, he tore down the engine and also inspected
the psru. He replaced the belt, but did not actually discern much
wear on it. His impression was that it could have run much longer.

The 2 to 1 ratio drives don't seem to have any abnormal wear pattern
that has been reported.

The problem you describe may be specific to metal to metal gears
meshing.

Corky Scott