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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