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The planetary gear set,

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also known as,

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the epicyclic gear train

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is one of the most

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important and interesting

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inventions in engineering.

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They are great speed 


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variation mechanisms

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and are often used in

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automobiles as a vital part of

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automatic transmissions.

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Let's explore the secrets

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of the planetary gear set

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in this video!

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A planetary gear set

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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

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sometimes rotates quickly,

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sometimes slowly,

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and sometimes

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even in reverse.

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But, how does this happen?

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You will be able

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to predict the motion


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of this gear set completely,

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if you understand

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one simple fact!

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When two gears

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are moving as shown,

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they should have

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the same speed

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at the interface.

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This means, that the

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speed of gear A,

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should be the same 


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as gear B,

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at their mating point.

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The speed has to be

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the same,

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otherwise,

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the gear teeth will

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penetrate each other

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as shown.

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That is an impossible condition.

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Just apply this fact

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to planetary gear sets,

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and you will be able to predict

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how speed variation

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is achieved.

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Assume that the ring gear

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is held stationary

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and we rotate the sun gear.

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Think of what happens

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to the planet gears!

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At point A,

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the planet gear

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should have a certain speed

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and at point B,

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the speed should be zero,


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as the ring gear

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is stationary.

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However,

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how are both

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of these conditions


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possible at the same time?

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There is only one way,

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the planet gear

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should spin as well as turn!

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The spinning will produce

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velocities in opposite [directions]

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at the top and bottom points,

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as shown,

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whereas, the turning

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produces unidirectional velocities.

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At the top,

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the spinning and turning velocities

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are in opposite directions,

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so the velocity of

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point B is zero.

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At the bottom,

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they get added up.

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In short,

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the planet gears

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are forced to turn

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in order to satisfy

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the condition of velocity.

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As the carrier is attached

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to the planet gear

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it will turn along

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with the planet gears.

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Now, let's see what happens,

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when the sun gear

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is held stationary

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and the ring gear

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is rotated.

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This is the exact opposite

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to the previous case.

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At the inner point

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of the planet gear,

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the velocity should be zero

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and the outer points

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should have the speed

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of the ring gear.

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In this case,

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the planetary spin

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will reverse

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in order to satisfy

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the speed conditions.

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However,

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this case,

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has one more difference.

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The speed of point B,

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will be higher

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than [the] speed of point A

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in the previous case.

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This is obvious,

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as the ring gear radius

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is higher.

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This will make

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the planet gear spin

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and turn at a higher speed.

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Thus, the carrier will turn

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at a higher speed.

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Let's now explore

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this reverse mechanism

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of planet gears.

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For this,

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what you have to do

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is just arrest

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the motion of the carrier.

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This means, that the

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planet gears

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are not allowed to turn

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and can only spin.

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This Spin will be opposite to

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the rotation of the sun gear.

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This spinning planet gear

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will make the ring gear

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rotate in the same direction.

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In short,

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the direction of rotation

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of the ring gear

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will be the opposite

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to the sun gear

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thus we will get

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the reverse gear.

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Here you can note,

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that in order to achieve

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different speeds, the input must be given to different parts of the planetary gearset

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the input must be given

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to different parts

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of the planetary gear set.

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This is practically difficult

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in an actual mechanism.

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In an automatic transmission,

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to achieve this,

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three planetary gear sets

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are connected in series

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as shown with coaxial shafts.

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To understand

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how this arrangement

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effectively transfers

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the input rotation

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to different parts

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of the planetary gear set,

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Watch our video on automatic transmission

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on automatic transmission.

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Your support on

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is greatly appreciated to make

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our educational service sustainable.

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Thank you.