Which arrangement yields a direct drive, i.e., a 1:1 speed ratio, in a planetary gear set?

Which arrangement yields a direct drive, i.e., a 1:1 speed ratio, in a planetary gear set?

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

Answer: (C)

In a planetary gear set, the speed ratio depends on which gears are driven, which is held, and how the planet gears transmit motion between them. A true 1:1 drive occurs when the output is linked to the inputs in such a way that no speed change is produced. Driving both the carrier and the sun at the same speed, with the ring as the output, creates that direct connection. When the two inputs are synchronized, the planet gears simply carry that motion to the ring without creating either a reduction or an multiplication, so the ring turns at the same speed as the inputs. The other arrangements involve either driving only one gear or holding a gear, which imposes a constraint that results in a gear ratio other than 1:1 determined by the gear tooth counts.

In a planetary gear set, the speed ratio depends on which gears are driven, which is held, and how the planet gears transmit motion between them. A true 1:1 drive occurs when the output is linked to the inputs in such a way that no speed change is produced.

Driving both the carrier and the sun at the same speed, with the ring as the output, creates that direct connection. When the two inputs are synchronized, the planet gears simply carry that motion to the ring without creating either a reduction or an multiplication, so the ring turns at the same speed as the inputs.

The other arrangements involve either driving only one gear or holding a gear, which imposes a constraint that results in a gear ratio other than 1:1 determined by the gear tooth counts.