For the design concept shown in Figure-3(c), the 2-
stage planetary gear train can be redrawn as Figure-
6. Sun gear 2, carrier 4 and ring gear 5 for 1st
planetary gear train, and sun gear 4, carrier 8 and
ring gear 7 for 2nd planetary gear train. Ring gears
5 and 7 are fixed, sun gear 2 is the input link, sun
gear 4 and carrier 4 are connected together, and carrier 8 is the output link. The reduction ratio of the
planetary gear reducer can be obtained as bellow: