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The planetary gear set, 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 mechanism 
and are often used in automobiles

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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

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should be the same 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 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 the planet gear should spin as well as turn

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The spinning will produce velocities in opposite [directions] 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, 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 in order to satisfy the condition of velocity.

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as The carrier is attached to the planet gear 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 and the ring gear is rotated

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This is the exact opposite to the previous case at the inner point of the planet gear

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the Velocity should be zero and the outer points should have the speed of the ring gear.

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in This case 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 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 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 what you have to do is just arrest the motion of the carrier

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This means the Planet gears are not allowed to turn 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 the direction of rotation of the ring Gear will be the opposite to the sun gear thus we will get the reverse gear

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Here you can note that in order to achieve different speeds the input must be given to different parts of the planetary gearset

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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 as shown with coaxial Shafts

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To understand how this arrangement effectively transfers the input rotation to different parts of the planetary gearset

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Watch our video on automatic transmission

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