Which of the following will produce a direct ratio from a planetary gear set?

• A. Ring gear input, carrier output, sun gear held
• B. Sun gear input, carrier held, ring gear output
• C. Carrier input, ring gear held, sun gear output
• D. Carrier input, sun gear input, ring gear output

Answer: (D)

Direct drive in a planetary gear set happens when the output rotates at the same speed as the input. This occurs when the internal constraints of the gears produce no relative motion that would change speed. If you drive both the carrier (the arm carrying the planet gears) and the sun gear together at the same speed and let the ring gear be the output, the planet gears have no differential motion to create a gearing ratio. The ring must rotate at that same common speed to satisfy the mesh between sun, planets, and ring, giving a 1:1, direct drive.

Other arrangements—where only one member is driven or where a member is held—produce speed changes (reduction or overdrive) rather than a direct 1:1 ratio.