What power flow will result from a planetary gear set if the carrier is the input member, the ring gear is held, and the sun gear is the output?

What power flow will result from a planetary gear set if the carrier is the input member, the ring gear is held, and the sun gear is the output?

• A. Minimum forward overdrive
• B. Maximum forward reduction
• C. Maximum forward overdrive
• D. Minimum forward reduction

Answer: (C)

In a planetary gear set, the motion of each member is linked by how the gears mesh, and holding one gear fixed changes how motion is transferred to the others. When the ring gear is fixed and the carrier is driven (the carrier is the input) while the sun gear is the output, the sun gear’s speed becomes multiplied relative to the carrier’s speed. This happens because the planet gears must roll around the fixed ring and mesh with both sun and ring, so the sun is driven by the carrier through the planet gears with a speed ratio roughly equal to the ring teeth count divided by the sun teeth count. Since the ring usually has more teeth than the sun, this ratio is greater than one, giving the sun gear a speed higher than the carrier’s. That’s an overdrive, and because the ring is fixed, you get the maximum possible forward overdrive for this arrangement.

In a planetary gear set, the motion of each member is linked by how the gears mesh, and holding one gear fixed changes how motion is transferred to the others. When the ring gear is fixed and the carrier is driven (the carrier is the input) while the sun gear is the output, the sun gear’s speed becomes multiplied relative to the carrier’s speed. This happens because the planet gears must roll around the fixed ring and mesh with both sun and ring, so the sun is driven by the carrier through the planet gears with a speed ratio roughly equal to the ring teeth count divided by the sun teeth count. Since the ring usually has more teeth than the sun, this ratio is greater than one, giving the sun gear a speed higher than the carrier’s. That’s an overdrive, and because the ring is fixed, you get the maximum possible forward overdrive for this arrangement.