To achieve high reduction ratios, a series of reduction gears are commonly placed inline, either plain small driver/large driven, or planetary. For a planetary gearbox to achieve 1000:1 reduction, four sets of gearboxes are needed (typical ratio per gearbox being 4 to 6). Driving the ring gear of a planetary gearbox in the opposite direction of rotation from the sun gear can achieve very high reduction such as 1000:1 in only one gearbox.
The ring gear housing of a planetary gearbox is driven in the opposite direction of rotation from the sun gear by means of a fixed idler gear on the input side. The ring gear housing is mounted and rotates on bearings on the input shaft (design shown) or bearings on the outer race of the housing or output shaft (designs not shown). To achieve a very high reduction ratio (i.e. 1000:1), the ring gear on the input side is smaller than the ring gear on the output side. For this particular design, the input sun gear is 12t, the input ring is 38t (the input idler or fixed planet is 13t, R = S + 2*P), the output sun gear is 13t and the output ring is 41t (the output planets are 14t each), achieving a reduction ratio of 1026:1.
In a planetary gearbox, (R + S)*Wo = R*Wr + S*Ws, where Wo, Wr, and Ws are rotations of the output, ring and sun respectively. Since our Wr ring rotation is not fixed as in a standard planetary gearbox, Wr = -(Ri/Si)*Wi, where Ri and Si are the input ring and sun gear, and Wi is the input rotation (signed negative for reverse direction). Combining and solving yields Wi/Wo = (R + S)/(S – Ri*R/Si).
Note: this design very likely has already been published is some form, and is likely not a new invention, though I’ve never seen it used in place of many stacked reduction gears.
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