Machine Design

In a planetary final drive reduction unit of an agricultural tractor, the sun gear has $Z_s = 12\text{ teeth}$ and the stationary ring gear has $Z_r = 60\text{ teeth}$. When input drives the sun gear and output is taken from the planet carrier, the gear reduction ratio is $1 + \frac{Z_r}{Z_s} = 1 + \frac{60}{12}$. The reduction ratio is: