The gear sets used in most automatic transmissions are referred to as

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

Answer: (A)

Planetary gear sets are used in most automatic transmissions. They stack a sun gear in the center, planet gears that orbit on a carrier, and a surrounding ring gear. The beauty of this arrangement is that the same set can produce several different gear ratios simply by deciding which member is driven and which is held stationary. By using clutches and brakes to lock or release different parts, a transmission can switch between multiple speeds and reverse without needing separate, bulky gear trains. This compact, strong arrangement makes it ideal for automatic transmissions, where smooth, reliable shifting across several gears is essential.

Spur gears are straightforward straight-cut gears used for simple speed changes but would require larger, more complex assemblies to achieve the same number of ratios. Bevel gears change the direction of motion between shafts and aren’t used to provide the full multi-speed train in automatics. Worm gears can give high reductions but are inefficient and bulky for an automotive transmission, and they don’t support the smooth, multiple-ratio shifting that planetary sets enable.