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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

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that it 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,

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that the 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

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has to be the same

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otherwise,

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the gear teeth will penetrate

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each other 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

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to predict how

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speed variation 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 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 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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