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Planetary Gearbox Design Selection

Planetary Gearbox Design Selection

Planetary Gearbox Design Selection

(OP)
I am an engineer working with a planetary gearbox reduction that needs to lie in the 18-20:1 reduction range.
I am limited strictly in the diameter of the part, and will be using two stages in a fixed ring configuration.
I have determined that there are about 8 arrangements that will fit my requirement for a common ring gear.  
However, I am having difficulty making an educated choice on what to use.
I'd like to lower noise as much as possible through good contact.  The box is loaded on either side of the tooth flank through its motion depending on the point of travel, so even approach and recess are ideal.
These are all formed gears.
My parameters are my tooth numbers.  The module floats slightly depending on the ring gear tooth numbers.
I seem to have difficulty keeping the sliding ratio down when I attempt to bring the contact ratios into reasonable ranges.  
Given the fixed OD, is it better to go toward a smaller or higher module to improve my operating parameters?

RE: Planetary Gearbox Design Selection


Did you try long addendum gears?
Something like 50 percent in the sun
and 25 percent in the planets.

RE: Planetary Gearbox Design Selection

(OP)
I'm not sure I get what you mean.  I'm using long addendum already, these are plastic gears and I'm using 95% of pointed tooth length.
What are you referring to by 50% sun and 25% planets?
Also, is there any reason to use a different combination of teeth to achieve the end ratio?
i.e.
Case I: 1st stage 15 Sun 39 Ring (3.6:1)
        2nd stage 9 Sun 39 Ring (5.33:1)

CaseII: 1st stage 11 Sun 37 Ring (4.36:1)
        2nd stage 11 Sun 37 Ring (4.36:1)

There are six other cases however what criteria can I use at this point to choose which will give me better meshing properties in the end?

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