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 gearset
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
on automatic transmission.
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