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)

Speed reduction in a planetary gear set depends on which gear is driven and which one is held. When the sun gear is driven and the ring gear is held, the planets transmit motion to the carrier, but the carrier ends up rotating much slower than the sun. The reduction grows with the ratio of the ring gear teeth to the sun gear teeth; since the sun is the smallest gear, driving it with the ring fixed produces the largest possible reduction. In other words, a smaller input gear (the sun) yields a much slower output speed under these constraints, giving the maximum forward reduction. If you drive the ring or the carrier instead, the effective gear ratio is different and the forward speed is not reduced as much.

Speed reduction in a planetary gear set depends on which gear is driven and which one is held. When the sun gear is driven and the ring gear is held, the planets transmit motion to the carrier, but the carrier ends up rotating much slower than the sun. The reduction grows with the ratio of the ring gear teeth to the sun gear teeth; since the sun is the smallest gear, driving it with the ring fixed produces the largest possible reduction. In other words, a smaller input gear (the sun) yields a much slower output speed under these constraints, giving the maximum forward reduction. If you drive the ring or the carrier instead, the effective gear ratio is different and the forward speed is not reduced as much.