In the described planetary gear arrangement, a single rotation of the carrier causes the ring gear to rotate: a) less than one revolution, b) exactly one revolution, c) one revolution plus the sun gear teeth, d) two revolutions. Which option is correct?

In the described planetary gear arrangement, a single rotation of the carrier causes the ring gear to rotate: a) less than one revolution, b) exactly one revolution, c) one revolution plus the sun gear teeth, d) two revolutions. Which option is correct?

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

Answer: (C)

In a planetary gear set, how the ring gear moves when the carrier is driven depends on which gear is held fixed. If the sun gear is held stationary and the carrier is turned, the planet gears roll around the sun while meshing with the ring gear. This motion drives the ring gear in the opposite direction to the carrier, and because the ring has more teeth than the sun, the ring cannot keep up in speed. The result is a slower rotation in the opposite sense, described by the relationship ω_RING = - (N_S / N_R) ω_C, where N_S is the sun gear teeth and N_R is the ring gear teeth. Since N_S < N_R, the magnitude is less than one turn for each full turn of the carrier, and the direction is reversed. That combination—rotation in the opposite direction with reduced speed—matches what’s meant by reverse reduction. The other possibilities would require different constraints (for example, rotating in the same direction or rotating exactly one revolution per carrier turn), which doesn’t occur in this arrangement when the sun gear is fixed.

In a planetary gear set, how the ring gear moves when the carrier is driven depends on which gear is held fixed. If the sun gear is held stationary and the carrier is turned, the planet gears roll around the sun while meshing with the ring gear. This motion drives the ring gear in the opposite direction to the carrier, and because the ring has more teeth than the sun, the ring cannot keep up in speed. The result is a slower rotation in the opposite sense, described by the relationship ω_RING = - (N_S / N_R) ω_C, where N_S is the sun gear teeth and N_R is the ring gear teeth. Since N_S < N_R, the magnitude is less than one turn for each full turn of the carrier, and the direction is reversed. That combination—rotation in the opposite direction with reduced speed—matches what’s meant by reverse reduction. The other possibilities would require different constraints (for example, rotating in the same direction or rotating exactly one revolution per carrier turn), which doesn’t occur in this arrangement when the sun gear is fixed.