> The 3 gears on the planetary set are marked so that they are in a
> position relative to one another when the sunwheel is engaged.
> Why is this so? In other words, why is their position critical?
I'm not sure I can explain this clearly, so bear with me.
The number of teeth on the annulus is not a multiple of the number of teeth
on the sun gear. Thus, if you draw an imaginary line from the center of
rotation of the sunwheel, through the center of each planetary gear, it will
hit both the sunwheel tooth and the annulus tooth in different positions.
The two gears that are joined together to create each planetary gear are
offset to match this "phase shift", but only in a certain position (since
they also have different numbers of teeth).
Some positions of the marks are simply impossible (the teeth won't mesh);
but in some cases there is enough accumulated clearance to allow them to
slide into engagement. That will pull the planetary carrier and sun gear
off-center to some extent, leading to vibration in operation. It also
causes most of the load to be carried by one planetary gear instead of
evenly divided between 3 of them, so increased wear and even potentially
tooth breakage could ensue.
> If the gears were not marked for some reason, how could proper
> alignment be determined?
If the gears were not marked for some reason, I think I'd want to look for
another set of gears. At any rate, I can't think of a guaranteed method of
lining them up, short of cutting apart an annulus so the tooth engagement
could be examined in situ. However, you might be able to do a setup without
the housing, and check that everything is running on the same center, which
_might_ be good enough.
Suggested annual donation $11.47