The planetary gear set,

also known as,

the epicyclic gear train

is one of the most

important and interesting

inventions in engineering.

They are great speed

variation mechanisms

and are often used in

automobiles as a vital part of

automatic transmissions.

Let's explore the secrets

of the planetary gear set

in this video.

A planetary gear set

has four main parts:

the sun,

planet gears,

ring gear,

and carrier.

You can see

that it sometimes

rotates quickly,

sometimes slowly,

and sometimes

even in reverse.

But, how does this happen?

You will be able

to predict the motion

of this gear set completely,

if you understand

one simple fact!

When two gears

are moving as shown,

they should have

the same speed

at the interface.

This means,

that the speed of gear A,

should be the same

as gear B,

at their mating point.

The speed

has to be the same

otherwise,

the gear teeth will penetrate

each other as shown.

That is an impossible condition.

Just apply this fact

to planetary gear sets,

and you will be able

to predict how

speed variation is achieved.

Assume that the ring gear

is held stationary

and we rotate the sun gear.

Think of what happens

to the planet gears!

At point A,

the planet gear

should have a certain speed

and at point B,

the speed should be zero,

as the ring gear

is stationary.

However,

how are both

of these conditions

possible at the same time?

There is only one way,

the planet gear

should spin as well as turn!

The spinning will produce

velocities in opposite [directions]

at the top and bottom points,

as shown,

whereas, the turning

produces unidirectional velocities.

At the top,

the spinning and turning velocities

are in opposite directions,

so the velocity of

point B is zero.

At the bottom,

they get added up.

In short,

the Planet gears

are forced to turn

in order to satisfy

the condition of velocity.

As the carrier is attached

to the planet gear

it will turn along

with the planet gears.

Now let's see what happens when the sun gear is held stationary and the ring gear is rotated

This is the exact opposite to the previous case at the inner point of the planet gear

the Velocity should be zero and the outer points should have the speed of the ring gear.

in This case the planetary Spin will reverse in order to satisfy the speed conditions

However this case has one more difference

The speed of point B. Will be higher than [the] speed of point A in the previous case

This is obvious as the ring gear radius is higher

This will make the planet gear spin and turn at a higher speed thus the carrier will turn at a higher speed

Let's now explore this reverse mechanism of Planet Gears

For this what you have to do is just arrest the motion of the carrier

This means the Planet gears are not allowed to turn and can only spin

This Spin will be opposite to the rotation of the sun gear

This spinning Planet Gear will make the ring gear rotate in the same Direction

In short the direction of rotation of the ring Gear will be the opposite to the sun gear thus we will get the reverse gear

Here you can note that in order to achieve different speeds the input must be given to different parts of the planetary gearset

This is practically difficult in an actual mechanism

in an automatic transmission [to] achieve this

Three planetary gear sets are connected in series as shown with coaxial Shafts

To understand how this arrangement effectively transfers the input rotation to different parts of the planetary gearset

Watch our video on automatic transmission

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