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, that 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 gear set.

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 gear sets,

watch our video

on automatic transmission.

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