John,
Good question!
....
I suppose you could try for more planet gears with a smaller diameter with
the bigger sun gear ( reduced ratio) - but the speeds would then be the
killer.
Hope that helps!?
George (down under)

George,
Just to clarify things.
At the same output speed of the planetary, meaning at the same propeller
speed, the velocity of all the gears is the same when using  the same
size Ring gear.
It matters not what the ratio is and at what speed the engine turns. All
three gears, sun gear, the planet gears, and the ring gear have the same
pitch line velocity at the same output rpm, provided the diameter of the
ring gear is the same.

Rolf Pfeiffer

Rolf,
Point taken Rolf - how do you explain that a smaller gear rotates it's
diameter more times than the bigger gear at the same speed, perhaps 'duty
cycle' OR could we say that the circumference of the smaller gear repeats
itself faster than the bigger gear, proportional to the number of teeth.
However your right speed isn't the right word here, speed would be correct
if different sized wheels were independently driven by the same source, but
gears are interlocked so speed is the same but (duty)cycling is faster on
the smaller gear.

Then again if bearings were placed at the inside of the sun gear and the
outside of the ring gear, wouldn't the hearings themselves be running slower
than the smaller planet gear, maybe, maybe not, but the number of bearings
are less, the surface is less and the duty cycle is still greater - can you
shed some light on this for future reference.

I have probably been thinking 'speed' as an over simplification!
George ( down under)

George,
It is generally accepted that it is the "pitch line velocity" of gears
that determines suitability, wear and life. V (fpm) = d (in) * pi * n
(rpm) / 12.
Of course you are correct, that a smaller gear having fewer teeth will
have a more frequent contacts with the mating gear, which also has some
effect.

I am sure you remember the story of the question of where to put the
light dynamo on a bike having one large and one small wheel. The small
wheel turns faster, so, if I put the dynamo there, I get more light at
the same bike driving speed. Right? No, it is the same on either wheel.
But that's on old story. However, the condition is the same within the
gear box.

It does not matter how big or small the sun and planet gears are, pitch
line velocity remains constant if the ring gear size and output rpm does
not change. With a higher number of planets, each has to carry less
torque, and each contact point on the sun gear and ring gear is also
loaded less. Since the torque has a higher impact on life than frequency
of contact, more planet gears are better. However the cost goes up and
overall friction losses too, which are not that high though.

A pitch line velocity of 5000 fpm is the general recommended maximum
velocity. There are many factors that determine allowable velocity and
gear life, such as material of gear and mating gear, hardness of each
gear and gear surface, smoothness, toque transmitted and of large impact
is of course lubrication.
As a side note, apparently at extreme high pitch line velocities such as
50 to 100,000 fpm and high pressure high volume lub oil feed, direct
metal contact between teeth can be avoided allowing very high hp to be
transmitted with practically no wear. However, this is of very little
practical use.

Regarding you comments about the bearings 'inside the sun gear and
outside the ring gear', sorry, you lost me there.

Rolf

Rolf,
You quite obviously know your gears and I appreciate your comments.
On the bearings, I was trying to mentally compare the bearings speed
between the much larger ring gear when it's the driver (when the planetary
is held captive) and that of the planet gear bearings. The same comparison
with the sun gear in a different configuration.

On another point what would you consider ( if you were developing a new
planetary)  the best material  (for our purpose, yet cost effective) to use,
as well as hardness and heat treatment.

George (down under)



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