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 mechanism
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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