Free A.S.E. Study Guide
An ASE Study Guide for the Automotive Technician
Basic Planetary Gearset
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A planetary gear set consists of three gears: the ring, sun, and pinion
gears. These three gears remain in mesh at all times. The transmission is
able to change the gear ratio by holding one member and driving another.
This leaves the third gear to be the driven gear. There are a limited number
of gear ratios a single gear set can achieve and most manufacturers apply
a second planetary gear set to create what’s called a compound planetary
gearset.
Ring Gear: The ring gear has internal
teeth and remains in constant mesh with
the pinion gears. This is the outer gear
of the set.
Sun Gear: The sun gear similar to
our own solar system is located at
the center of the planetary gear set.
This gear also remains in constant
mesh with the pinion gears. Do not
confuse the pinion gear with the
planetary gear set.
Pinion Gears: The pinion gears
are located between the ring and
sun gear and remain in constant
mesh with both. This gear consists
of several smaller gears contained
inside a carrier called a planet
carrier. This carrier contains the
pins that the pinions or planet gears
turn. As mentioned earlier the
pinion carriers planet gears remain
in mesh with the sun gear as well as
the ring gear at all times.
For instance if we hold the
ring gear while driving the sun
gear the pinion gear would be
the driven gear. Each of these
gears has a holding device so
it may be held or driven
according the desired gear
ratio.