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Understanding PLANETARY GEAR set ! - YouTube
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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
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of 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 is stationary.
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However, 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 has
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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, the direction of
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rotation 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,
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the input must be given to
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different parts of the
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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
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on automatic transmission.
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Your support on PATREON.COM
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is greatly appreciated to make our
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educational service sustainable.
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Thank you.
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