Understanding PLANETARY GEAR system !

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 set,

watch our video on automatic transmission.

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