Currently, the final gear reduction stages in virtually all helicopter main rotor drives consist of one or two simple spur planetary stages. These simple planetaries are composed of a sun gear, multiple planet gears (typically between 3 and 6) and an internal gear that is generally the fixed member of this epicyclic system. While this proven concept provides relatively good power transmission efficiency, it has a tendency to generate high noise levels due to the limited contact ratio of the individual gears and the multiple, often simultaneous,tooth contacts of the several planet gears.
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