Epicyclic gear stages provide high load capacity and compactness to gear drives. There is a wide variety
of different combinations of planetary gear arrangements [1 , 2]. For simple epicyclic planetary stages
when the ring gear is stationary, the practical gear ratio range varies from 3:1 to 9:1 . For similar epicyclic
planetary stages with compound planet gears, the practical gear ratio range varies from 8:1 to 30:1 .
This paper presents analysis and design of epicyclic gear arrangements that provide extremely high gear
ratios. Using differential-planetary gear arrangements it is possible to achieve gear ratios of several
hundred to one in one-stage drive with common planet gears and several thousand to one in one-stage
drive with compound planet gears. A special two-stage planetary arrangement may utilize a gear ratio of
over one hundred thousand to one.
This paper shows an analysis of such uncommon gear drive arrangements, defines their major
parameters, limitations, and gear ratio maximization approaches. It also demonstrates numerical
examples, existing designs, and potential applications.

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