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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