Bill Freeman wrote:

Paul,

I assume that Tracy is referring to the force tending
to separate the two shafts with a std spur gear pair.
If we know the RPM and the pitch diameter of the gears
we can calculate the tangential force with the equation

   FORCEt = (33,000* HP)/V

  where V is pitch line velocity.   The radial force (tending
to separate the shafts, i.e. load the bearings and split the
housing is found from the tangential force, by

    FORCEr  =  FORCEt * tan phi

    where tan phi is the tangent of the effective angle of gear
     tooth contact, typically 17.5 or 23 are used when
    considering the friction effects on 14.5 or 20 deg gears.

So, if we have a 3" pitch diam gear running at 6500 RPM, the
pitch line velocity is 1625 ft/min.  Assuming a 200 rated HP
motor cruising at 75% power, we have:

       FORCEt = (33,000*150)/1625    =  3046 lb  (tangential)

But we know that the radial force is related to the tangential
by the above equation, so we have:

     FORCEr =  (3046* tan 23 deg)  = 3046 * .4245  =  1292 lb

So we can expect the bearing load to be well over 1000 lb,
JUST DUE TO THE GEARS TRANSMITTING TORQUE.
Add the gyro loads, imbalance loads etc, and you need a good
bearing at the end of the shaft.

This is quite independent of the prop loads.

Planetary gears (also known as epicyclic gears) will have these
same type of loads, but they are balanced inside the carrier and
the shaft does not see the bending load nor does the main
shaft bearing see the bending due to the gear tooth separation.
Without doing the math, I believe that the pitch line velocity,
plus the load sharing effect (4 or 6 planet gears) causes the
separtion loads to be much lower than a spur gear pair.

Hope this adds to the clarity, rather than obscuring the truth.

Bill

 
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