Maximum forward reduction is obtained when the sun gear is the input because a smaller input gear yields lower output speed.

Maximum forward reduction is obtained when the sun gear is the input because a smaller input gear yields lower output speed.

• A. True
• B. False
• C. Only when carrier is input
• D. Only when ring gear is input

Answer: (A)

In a planetary gear set, how fast the output turns compared to the input depends on which component you drive and which you hold fixed. Driving the sun gear while keeping the ring gear fixed makes the planet gears mesh in a way that the carrier must rotate much more slowly than the sun. The reduction factor in this arrangement is largely set by the ratio of the ring gear teeth to the sun gear teeth; since the sun is the smaller gear, Nr/Ns is large, leading to a large reduction at the carrier. That’s why driving the sun yields the greatest forward reduction—the output speed drops the most relative to the sun input.

If you instead drive the ring or the carrier, the kinematic constraints produce a smaller effective reduction for forward motion, because the input gear is larger or the power flow through the planet gears is arranged differently, resulting in a less dramatic slowdown at the output.