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In an automatic transmission utilizing a simple planetary gearset, engine power is applied to drive the sun gear while the ring gear (internal gear) is held completely stationary by a friction band. If the planetary carrier is connected to the output shaft, which of the following best describes the resulting rotational direction and speed of the output shaft compared to the input?

  1. A
    It splits the input torque equally into two different speeds for separate output shafts, allowing the wheels to corner.
  2. B
    It rotates in the opposite direction at a reduced speed, which is the standard configuration for reverse gear.
  3. It rotates in the same direction at a reduced speed, providing gear reduction for torque multiplication.Answer
  4. D
    It rotates in the same direction at an increased speed, creating an overdrive condition for highway cruising.

Answer

The output shaft rotates in the same direction as the input shaft at a reduced speed, providing gear reduction for torque multiplication.
In a simple planetary gearset, driving the sun gear while holding the ring gear forces the planet gears to walk around the inside of the ring gear. This causes the planetary carrier (the output) to rotate in the same direction as the sun gear (the input). Because the carrier has the largest effective diameter in this specific power flow configuration, it rotates more slowly than the sun gear, resulting in maximum forward gear reduction and increased torque.

Step-by-Step Solution

1
Identify the power flow assignments for the planetary gearset components described in the scenario.
The sun gear is the input, the ring gear is the held (stationary) component, and the planetary carrier is the output.
Assigning the correct operational status to each component is the mandatory first step in determining the gearset's dynamic behavior.
2
Analyze the mechanical interaction caused by holding the ring gear stationary.
As the sun gear rotates, it turns the planet gears. Since the ring gear cannot move, the planet gears must 'walk' around the inner teeth of the stationary ring gear.
This physical interaction dictates the movement of the planetary carrier, which holds the pins for the planet gears.
3
Determine the direction and speed of the planetary carrier.
The planet gears walk in the same rotational direction as the sun gear. Because the carrier follows the orbit of the planet gears around the largest diameter (the ring), it rotates at a slower speed than the sun gear.
This configuration results in forward gear reduction, which increases output torque while decreasing output speed.

Key Concept

Planetary Gearset Power Flow and Gear Reduction
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