shaft must turn at the same speeds, thus producing a
Helical gears in mesh have more tooth contact area in
direct drive or ratio of 1 to 1. If no member is held
contact and operate quieter than spur gears.
stationary and no two members are locked together,
Another gear common in the planetary gear system
then the system will not transmit power at all. The input
of automatic transmissions is the RING gear or internal
shaft may turn but the output shaft will not. The most
gear. In this gear the teeth face toward the center of the
common method of holding the members of a
gear instead of outward. The teeth can be of the spur or
planetary gear system is through the use of clutches or
helical configuration.
bands.
There are several combinations of speed/torque
Planetary Gears
ratios available through a planetary gear system.
The planetary gear set consists of three separate,
Figure 2-60 shows a chart containing six different
conditions that can result in the planetary gear system
but interconnected, rotating members or gears. Figure
by holding or turning the various members. For
2-59 shows a planetary gear set. Notice that the outer
example, the column under condition 1 shows that
gear, or ring gear, is an internal gear because the gear
holding the sun gear while turning the pinion carrier
teeth point inward toward the center. The inner gear is
causes the ring gear to turn faster than the pinion
called the "sun" gear. The gears between the internal
carrier. When the pinion carrier is turned, the pinion
gear and sun gear are called "planet pinion" gears; they
must walk around the sun gear because the gears are
are held in place by the planet pinion carrier. This gear
meshed with the sun gear. The pinion gears are also
system is called the planetary gear system because the
planet gears can rotate and at the same time revolve
around the sun gear.
CONDITION
1
2
3
4
5
6
Drums, hubs, and shafts can be used to put torque
SUN GEAR
H
H
T
IR
I
T
(power) into any one of the three members and, at the
same time, hold other members so the gear ratio
PINION CAGE
T
L
L
H
T
H
through the system can be increased or decreased. In
RING GEAR
I
T
H
T
H
LR
addition, by the proper turning and holding
arrangement, the system can reverse rotation.
H - HOLD
I - INCREASE OF
T - TURN
SPEED
ASf02060
Only one of the members can be the input, another
R - REVERSE
L - REDUCTION OF
SPEED
the output, and one must be stationary. If any two
members are locked together, the entire planetary gear
Figure 2-60.--Chart of the speed ratio combinations in a
system is locked out, and the input shaft and the output
simple planetary gear system.
PLANETARY
SUN GEAR
PLANETARY
PINION
PLANETARY
PLANETARY
GEAR
PINION
RING GEAR
CARRIER
ASf02059
Figure 2-59.--Members of a simple planetary gear system.
2-47
