Figure 4-29.Planetary gearset.
A planetary gearset (fig. 4-29) consists of three
members-sun gear, ring gear, and planetary carrier
which holds the planetary gears in proper relation with
the sun and ring gear. The planetary gears are free to
rotate on their own axis while they "walk" around the
sun gear or inside the ring gear.
By holding or releasing the components of a
planetary gearset, it is possible to do the following:
Reduce output speed and increase torque (gear
Increase output speed while reducing torque
Reverse output direction (reverse gear).
Serve as a solid unit to transfer power (one to one
Freewheel to stop power flow (park or neutral).
Figure 4-30 shows the simplest application of
planetary gears in a transmission. With the application
shown, two forward speeds and neutral are possible.
High gear or direct drive is shown. The clutch is
holding the planet carrier to the input shaft, causing the
carrier and sun gear to rotate as a single unit. With the
clutch released, all gears are free to rotate and no power
is transmitted to the output shaft. In neutral, the
planetary carrier remains stationary while the pinion
gears rotate on their axis and turn the ring gear. Should
the brake be engaged on the ring gear, the sun gear
causes the planetary gears to walk around the inside of
the ring gear and force the planet carrier to rotate in the
same direction as the sun gear, but at a slower speed
(low gear). To provide additional speed ranges or a
reverse, you must add other planetary gearsets to this
A compound planetary gearset combines two
planetary units into one housing or ring gear. It may
have two sun gears or a long sun gear to operate two
Figure 4-30.Simple planetary gear application.