Which gear type is primarily used in a propulsion system creating torque and speed reduction?

The correct choice focuses on the fundamental purpose of a reduction gear, which is specifically designed to reduce speed while simultaneously increasing torque in a mechanical system. In propulsion systems, maintaining adequate torque while controlling speed is essential for optimal performance and efficiency. Reduction gears serve this purpose by utilizing gear ratios that allow for a decrease in output speed relative to the input speed, thereby boosting the torque output.

In comparison, planetary gears can also reduce speed and increase torque, but they are typically used in more complex applications where compactness and multiple speed ratios are required. Spur gears can transmit torque and change speed; however, they are generally used for direct connections and do not specifically create a reduction unless configured in a specific manner. Bevel gears, while effective for changing the direction of rotation, don't inherently reduce speed and torque in the same straightforward manner as reduction gears do. Therefore, the design and function of reduction gears make them the most appropriate choice for such an application.