Planetary transmissions use planetary gearing to transmit power and to make speed and directional changes. Hydraulic clutches control the rotation of the planetary gear components allowing the planetary gearing to serve as a direct coupler, a reduction gear, or a reversing gear.
Planetary gear sets are compact units. The countershaft has been eliminated and the input shaft and the output shaft rotate on the same axis. A planetary gear set permits gear ratios to be changed without actually engaging or disengaging gears. As a result, there is little or no interruption of the power flow.
In planetary gear sets, the load is spread over several gears, decreasing the load on each tooth. The planetary system also spreads the load evenly around the circumference of the system, eliminating sideways stress on the shafts.