All automatic transmissions rely on planetary
gearsets to transfer power and multiply engine torque to the drive axle.
Compound gearsets combine two simple planetary gearsets so load can be spread
over a greater number of teeth for strength and also to obtain the largest
number of gear ratios possible in a compact area.
A simple planetary gearset consists of three
parts: a sun gear, a carrier with planetary pinions mounted to it, and an
internally toothed ring gear or annulus. The sun gear is located in the center
of the assembly. It can be either a spur or helical gear design. It meshes with
the teeth of the planetary pinion gears. Planetary pinion gears are small gears
fitted into a framework called the planetary carrier. The planetary carrier can
be made of cast iron, aluminum, or steel plate and is designed with a shaft for
each of the planetary pinion gears. (For simplicaity, planetary pinion gears are
called planetary pinions.)
Planetary pinions rotate on needle bearings
positioned between the planetary carrier shaft and the planetary pinions. The
carrier and pinions are considered one unit - the mid-size gear number.
They planetary pinions surround the sun
gear's center axis and they themselves are surrounded by the annulus or ring
gear, which is the largest part of the simple gearset. The ring gear acts like a
band to hold the entire gearset together and provide great strength to the unit.
To help remember the design of a simple planetary gearset, use the solar system
as an example. The sun is the center of the solar system with the planets
rotating around it; hence, the name planetary gearset.
How Planetary Gears Work
Each member of a planetary gearset - the sun
gear, pinion gear carrier, and ring gear - can spin (revolve) or be held at
rest. Power transfer through a planetary gearset is only possible when one of
the members is held at rest, or if two of the members are locked together.
Any one of the three members can be used as
the driving or input member. At the same time, another member might be kept from
rotating and thus becomes the held or stationary member. The third member then
becomes the driven or output member. Depending on which number is the driver,
which is held, and which is driven, either a torque increase or a speed increase
is produced by the planetary gearset. Output direction can also be reserved
through various combinations.
Sun Gear |
Carrier |
Ring Gear |
Speed |
Torque |
Direction |
1. Input |
Output |
Held |
Maximum
reduction |
Increase |
Same as input |
2. Held |
Output |
Input |
Minimum
reduction |
Increase |
Same as input |
3. Output |
Input |
Held |
Maximum
increase |
Reduction |
Same as input |
4. Held |
Input |
Output |
Minimum
increase |
Reduction |
Same as input |
5. Input |
Held |
Output |
Reduction |
Increase |
Reverse of
input |
6. Output |
Held |
Input |
Increase |
Reduction |
Reverse of
input |
7.
When any two members are held together, speed and direction are the same as
input. Direct 1:1 drive occurs.
8.
When no member is held or locked together, output cannot occur. The result is a
neutral condition.
The
above table summarizes the basic laws of simple planetary gears. It indicates
the resultant speed, torque and direction of the various combinations available.
Also, remember that when an external-to-external gear tooth set is in mesh,
there is a change in the direction of rotation at the output. When an external
gear tooth is in mesh with an internal gear, the output rotation for both gears
is the same.
MAXIMUM FORWARD REDUCTION With the ring gear stationary and the sun gear
turning clockwise, the sun gear rotates the planetary pinions counterclockwise
on their shafts. The small sun gear (driving) rotates several times, driving the
mid-size planetary carrier one complete revolution, resulting in the most gear
reduction or the maximum torque multiplication that can be achieved in one
planetary gearset. Input speed is high, but output speed is low.
MAXIMUM FORWARD REDUCTION In this combination the sun gear is stationary and
the ring gear rotates clockwise. The ring gear drives the planetary pinions
clockwise and walks around the stationary sun gear. The planetary pinions drive
the planetary carrier in the same direction as the ring gear - forward. This
results in more than one turn of the input as compared to one complete
revolution of the output. The result is torque multiplication. The planetary
gearset is operating in a forward reduction with the large ring gear driving the
small planetary carrier. Therefore, the combination produces minimum forward
reduction.
MAXIMUM OVERDRIVE With the ring gear stationary and the planetary carrier
rotating clockwise, the three planetary pinion shafts push against the inside
diameter of the planetary pinions. The pinions are forced to walk around the
inside of the ring gear, driving the sun gear clockwise. In this combination,
the mid-size planetary carrier is rotating less than one turn and driving the
smaller sun gear at a speed greater than the input speed. The result is
overdrive with maximum speed increase.
SLOW OVERDRIVE In this combination, the sun gear is stationary and the
carrier rotates clockwise. As the carrier rotates, the pinion shafts push
against the inside diameter of the pinions and they are forced to walk around
the held sun gear. This drives the ring gear faster and the speed increases. The
carrier turning less than one turn causes the pinions to drive the ring gear one
complete revolution in the same direction as the planetary carrier and a slow
overdrive occurs.
SLOW REVERSE Here the sun gear is driving the ring gear with the planetary
carrier held stationary. The planetary pinions, driven by the sun gear, rotate
counterclockwise on their shafts. While the sun gear is driving , the planetary
pinions are used as idler gears to drive the ring gear counterclockwise. This
means the input and output shafts are operating in the opposite or reverse
direction to provide a reverse power flow. Since the driving sun gear is small
and the driven ring gear is large, the result is slow reverse.
FAST REVERSE For fast reverse, the carrier is held, but the sun gear and
ring gear reverse roles, with the ring gear now being the driving member and the
sun gear driven. As the ring gear rotates counterclockwise, the pinions rotate
counterclockwise as well, while the sun gear turns clockwise. In this
combination, the input ring gear uses the planetary pinions to drive the output
sun gear. The sun gear rotates in reverse to the input ring gear, providing fast
reverse.
DIRECT DRIVE In the direct drive combination, both the ring gear and the sun
gear are input members. They turn clockwise at the same speed. The internal
teeth of the clockwise turning ring gear try to rotate the planetary pinions
clockwise as well. But the sun gear, which rotates clockwise, tries to drive the
planetary pinions counterclockwise. The opposing forces lock the planetary
pinions against rotation so the entire planetary gearset rotates as one complete
unit. This ties together the input and output members and provides a direct
drive.
NEUTRAL OPERATION When no member is held stationary or locked, there is
input into the gearset, but no output. The result is a neutral condition.