Which gear arrangement is foundational to most automatic transmissions and provides multiple gear ratios through engagement of sun gear, planet gears, and ring gear?

• A. Bevel gears
• B. Worm gears
• C. Planetary
• D. Spur gears

Answer: (C)

Planetary gear sets are the backbone of most automatic transmissions because their sun gear, planet gears, and ring gear arrangement lets a single gear train produce several different output speeds without changing the overall layout. The sun gear sits in the center, planet gears mesh with it and orbit on a carrier, and the outer ring gear has internal teeth that mesh with the planets. By using clutches or brakes to lock or drive the sun, ring, or carrier, you control which part is fixed, which is driven, and which is output. When one element is held stationary while another is driven, the planets rotate and the arrangement yields a different gear ratio. This setup makes it possible to achieve multiple forward gears (and reverse) efficiently in a compact design, which is why it’s foundational to automatic transmissions. Bevel gears change direction between non-parallel shafts, worm gears provide a high reduction with self-locking characteristics not geared for multiple selectable ratios in automatics, and spur gears offer fixed ratios without the three-part interaction that enables multiple ratios in the same space.