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The planetary gear set,

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also known as the epicyclic gear train,

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is one of the most important and
interesting inventions in engineering.

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They are great speed variation mechanisms

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and are often used in automobiles as 
a vital part of automatic transmissions.

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Let's explore the secrets of
the planetary gear set in this video!

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A planetary gear set has four main parts:

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the sun,

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planet gears,

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ring gear,

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and carrier.

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You can see that it sometimes
rotates quickly,

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sometimes slowly,

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and sometimes even in reverse.

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But, how does this happen?

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You will be able to predict the motion
of this gear set completely,

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if you understand one simple fact!

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When two gears are moving as shown,

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they should have the same 
speed at the interface.

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This means, that the speed of 
gear A should be the same

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as gear B at their mating point.

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The speed has to be the same,

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otherwise the gear teeth will
penetrate each other as shown.

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That is an impossible condition!

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Just apply this fact to planetary gear sets

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and you will be able to predict how
speed variation is achieved.

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Assume that the ring gear is held 
stationary and we rotate the sun gear.

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Think of what happens to the planet gears!

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At point A, the planet gear
should have a certain speed.

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And at point B, the speed should be zero,

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as the ring gear is stationary.

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However, how are both of these conditions
possible at the same time?

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There is only one way!

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The planet gear should 
spin as well as turn!

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The spinning will produce velocities 
in opposite directions

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at the top and bottom points, as shown.

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Whereas the turning produces 
unidirectional velocities.

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At the top, the spinning and turning
velocities are in opposite directions,

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so the velocity of point B is zero.

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At the bottom, they get added up.

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In short, the planet gears 
are forced to turn

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in order to satisfy 
the condition of velocity.

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As the carrier is attached 
to the planet gear

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it will turn along with the planet gears.

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Now, let's see what happens when
the sun gear is held stationary

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and the ring gear is rotated.

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This is the exact opposite 
to the previous case.

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At the inner point of the planet gear,

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the velocity should be zero

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and the outer points should have
the speed of the ring gear.

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In this case,

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the planetary spin will reverse in order
to satisfy the speed conditions.

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However, this case has
one more difference.

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The speed of point B, will be higher

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than the speed of point A 
in the previous case.

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This is obvious, as the ring
gear radius is higher.

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This will make the planet gear spin
and turn at a higher speed.

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Thus, the carrier will turn
at a higher speed.

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Let's now explore this reverse 
mechanism of planet gears.

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For this,

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what you have to do is just 
arrest the motion of the carrier.

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This means, that the planet gears
are not allowed to turn

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and can only spin.

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This spin will be opposite to
the rotation of the sun gear.

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This spinning planet gear will make 
the ring gear rotate in the same direction.

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In short,

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the direction of rotation of the ring gear
will be the opposite to the sun gear.

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Thus, we will get the reverse gear.

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Here, you can note, that in order
to achieve different speeds,

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the input must be given to different
parts of the planetary gear set.

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This is practically difficult 
in an actual mechanism.

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In an automatic transmission,
to achieve this,

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three planetary gear sets
are connected in series

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as shown with coaxial shafts.

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To understand, how this arrangement
effectively transfers the input rotation

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to different parts of 
the planetary gear set,

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watch our video on automatic transmission.

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