Planetary Gears

     A planetary gearset consists of a sun gear，several planet gears，a planet gear carrier,and a ring gear. A simple planetary gearset is shown in Fig. 9.8.

    An automatic transmission will commonly use two or more planetary gearsets. Byholding or releasing the components of a planetary gearsets，it is possible to reduce outputspeed and increase torque，increase output speed while lowering torque, reverse outputdirection，serve as a solid unit to transfer power, and a freewheel to stop power flow