Planetary gear set

2.0 Planetary Gear Set
• In a manual transmission, different gear ratios are
obtained by sliding the gears into mesh.
• Torque flow must be momentarily interrupted before
the gears are shifted (use clutch).
• With an automatic transmission there is no driver-
operated clutch, so gear shifts are not made by sliding
gears into mesh.
• Automatic transmissions use a planetary gear set
system that does not require an interruption of
torque flow to change gear ratios.

• Nearly all automatic transmissions rely on planetary
gear sets to transfer power and multiply engine
torque to the drive axle.
• A simple planetary gear set consists of three parts:
a) sun gear,
b) Planet carrier
c) ring gear (a.k.a annulus)
• All gears in a planetary gear set are in constant mesh.
The torque flow through a planetary gear set, both
input and output, occurs along a single axis.

How planetary gear works?
• Depending on which member is the driver, which is
held, and which is driven, either a torque increase
(underdrive) or a speed increase (overdrive) is
produced by the planetary gear set.
• 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 reaction, held, or
stationary member.

How planetary gear works?
• The third member then becomes the driven or output
member.
• Output direction can also be reversed through various
combinations.

2.1 Compound Planetary Gear Sets
• A limited number of gear ratios are available from a
single planetary gear set.
• Gear sets can be combined to increase the number of
available gear ratios.
• A typical automatic transmission has at least two
planetary gear sets connected together to provide the
various gear ratios needed to efficiently move a
vehicle.

• There are three designs of compound gear sets:
1. Simpson gear set, in which two planet carriers,
two ring gear and share a common sun gear.
Providing three forward gears, as well as neutral
and reverse.
2. Ravingeaux gear set (pronounced RAVENO),
which has two sun gears, two of planet carriers,
and a common ring gear. Provides four forward
gears, as well as neutral and reverse.

3. Lepelletier System: The Lepelletier system
connects a simple planetary gear set to a
Ravigneaux gear set. Provide six to eight
forward gear.

TYPE SUN GEAR PLANET CARRIER RING GEAR
SIMPSON SHARE 2 2
RAVINGEAUX 2 2 SHARE
LEPELLETIER Planetary gear set + Ravingeaux gear set

SIMPSON GEAR SET
• A Simpson geartrain is the most commonly used
compound planetary gear set.
• One half of the compound set or one planetary unit is
referred to as the front planetary and the other
planetary unit is the rear planetary.
• The two planetary units do not need to be the same
size or have the same number of teeth on their gears.
The size and number of gear teeth determine the
actual gear ratios obtained

SIMPSON 1ST GEAR
• Input shaft (engine torque) is locked to the FRONT
ring gear.
• Front ring gear drives the front planet gears.
• Front planet gears drive the sun gear .
• Rear planet carrier is held.
• Sun gear spins the rear planet gears.
• Planet gears drive the REAR ring gear which is locked
to the output shaft.

SIMPSON 2ND GEAR
• Input shaft (engine torque) is locked to the FRONT
ring gear.
• Front ring gear drives the front planet gears.
• Sun gear is held.
• Front planet gears walk around the sun gear.
• Front planet carrier is forced to turn by this ‘walk’.
• This turns the output shaft since the planet carrier is
locked to the output shaft.

SIMPSON 3RD GEAR (Direct Drive)
• Input is received by the front ring gear, as in the
other forward positions.
• However, the input is also received by the sun gear.
• Since the sun and ring gear are rotating at the same
speed and in the same direction, the front planet
carrier is locked between the two and is forced to
move with them.
• Since the front carrier is locked to the output shaft,
direct drive results.

SIMPSON REVERSE GEAR
• The input is received by the sun gear, as in the third
gear position, and rotates in a clockwise direction.
• sun gear then drives the rear planet gears in
counterclockwise direction.
• The rear planet carrier is held; the planet gears drive
the rear ring gear in a counterclockwise direction.
• Ring gear is locked to the output shaft that turns at
the same speed and direction as the rear ring gear.
The result is a reverse gear.

RAVIGNEAUX GEARTRAIN
• The Ravigneaux offers some advantages over a
Simpson geartrain.
• It is very compact. It can carry large amounts of
torque because of the great amount of tooth contact.
• It can also have three different output members.
• However, it has a disadvantage because it is more
complex.

• The Ravigneaux geartrain is designed to use two sun
gears: one small (forward) and one large (reverse).
• They also have two sets of planetary pinion gears:
three long pinions and three short pinions.
• The planetary pinion gears rotate on their own shafts
that are fastened to a common planetary carrier.
• A single ring gear surrounds the complete assembly.

• The small sun gear is meshed with the short planetary
pinion gears.
• These short pinions act as idler gears to drive the long
planetary pinion gears.
• The long planetary pinion gears mesh with the large
sun gear and the ring gear.

RAVIGNEAUX 1ST GEAR
• Engine torque drives the small sun gear.
• Planet carrier is held.
• Sun gear drives short planet gears.
• Short planet gears drive the long planet gears.
• Long planet gears drive ring gear and output shaft in a
lower speed than the input.

RAVIGNEAUX 2ND GEAR
• Engine drive the small sun gear.
• Small sun gear rotate the short planet gears.
• Short planet gears drive the long planet gears.
• Long planet gears walk around large sun gear.
• Large sun gear is held.
• This ‘walking’ drives the ring gear and output shaft in
at a torque reduction.

RAVIGNEAUX 3RD GEAR (Direct Drive)
• Engine torque drives the small sun gear.
• Input is also received by the planetary gear carrier.
• Because two members of the geartrain are being
driven at the same time, the planetary gear carrier
and the small sun gear rotate as a unit.
• The long planet gears transfer the torque in through
the gear set to the ring gear and output shaft.
• This results in direct drive.

RAVIGNEAUX 4TH GEAR
• Input is received at the planetary carrier.
• The long planet gears walk around the big sun gear.
• Big sun gear is held stationary.
• This ‘walk’ drive the ring gear and output shaft.
• This results in an overdrive condition.

RAVIGNEAUX REVERSE GEAR
• Input is received at the small sun gear.
• The planetary gear carrier is held.
• The clockwise rotation of the reverse sun gear drives
the long planet gears in a counterclockwise direction.
• The long planets then drive the ring gear and output
shaft in a counterclockwise direction with a speed
reduction.

Planetary gear set

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