0:00:00.308,0:00:01.748
The planetary gear set,

0:00:01.748,0:00:02.718
also known as,

0:00:02.718,0:00:04.256
the epicyclic gear train

0:00:04.256,0:00:05.260
is one of the most

0:00:05.260,0:00:06.470
important and interesting

0:00:06.470,0:00:08.350
inventions in engineering.

0:00:08.350,0:00:09.490
They are great speed [br]

0:00:09.490,0:00:10.811
variation mechanisms

0:00:10.811,0:00:11.882
and are often used in

0:00:11.882,0:00:13.813
automobiles as a vital part of

0:00:13.813,0:00:15.233
automatic transmissions.

0:00:18.585,0:00:19.967
Let's explore the secrets

0:00:19.967,0:00:21.284
of the planetary gear set

0:00:21.284,0:00:22.804
in this video!

0:00:24.694,0:00:26.129
A planetary gear set

0:00:26.129,0:00:28.029
has four main parts:

0:00:28.479,0:00:29.938
the sun,

0:00:30.368,0:00:31.877
planet gears,

0:00:32.957,0:00:34.537
ring gear,

0:00:35.537,0:00:37.140
and carrier.

0:00:37.405,0:00:38.408
You can see that it

0:00:38.408,0:00:40.528
sometimes rotates quickly,

0:00:41.708,0:00:43.882
sometimes slowly,

0:00:46.023,0:00:47.283
and sometimes

0:00:47.283,0:00:48.873
even in reverse.

0:00:50.203,0:00:52.494
But, how does this happen?

0:00:52.494,0:00:53.474
You will be able

0:00:53.474,0:00:54.514
to predict the motion[br]

0:00:54.514,0:00:55.896
of this gear set completely,

0:00:55.896,0:00:56.836
if you understand

0:00:56.836,0:00:58.473
one simple fact!

0:00:59.103,0:01:00.323
When two gears

0:01:00.323,0:01:01.522
are moving as shown,

0:01:01.522,0:01:02.362
they should have

0:01:02.362,0:01:03.362
the same speed

0:01:03.362,0:01:05.013
at the interface.

0:01:05.013,0:01:06.003
This means, that the

0:01:06.003,0:01:07.503
speed of gear A,

0:01:07.503,0:01:08.494
should be the same [br]

0:01:08.494,0:01:09.493
as gear B,

0:01:09.493,0:01:11.523
at their mating point.

0:01:17.149,0:01:18.049
The speed

0:01:18.049,0:01:19.612
has to be the same

0:01:19.612,0:01:20.612
otherwise,

0:01:20.612,0:01:21.612
the gear teeth will penetrate

0:01:21.612,0:01:22.990
each other as shown.

0:01:22.990,0:01:25.806
That is an impossible condition.

0:01:26.868,0:01:28.118
Just apply this fact

0:01:28.118,0:01:29.630
to planetary gear sets,

0:01:29.630,0:01:30.754
and you will be able

0:01:30.754,0:01:31.759
to predict how

0:01:31.759,0:01:33.844
speed variation is achieved.

0:01:35.884,0:01:37.274
Assume that the ring gear

0:01:37.274,0:01:38.444
is held stationary

0:01:38.444,0:01:40.280
and we rotate the sun gear.

0:01:43.930,0:01:45.168
Think of what happens

0:01:45.168,0:01:46.828
to the planet gears!

0:01:47.968,0:01:48.968
At point A,

0:01:48.968,0:01:49.968
the planet gear

0:01:49.968,0:01:51.746
should have a certain speed

0:01:51.746,0:01:53.123
and at point B,

0:01:53.123,0:01:54.343
the speed should be zero,[br]

0:01:54.343,0:01:55.343
as the ring gear

0:01:55.343,0:01:56.963
is stationary.

0:01:56.963,0:01:57.741
However,

0:01:57.741,0:01:58.531
how are both

0:01:58.531,0:01:59.511
of these conditions[br]

0:01:59.511,0:02:01.411
possible at the same time?

0:02:01.741,0:02:03.751
There is only one way,

0:02:03.751,0:02:04.831
the planet gear

0:02:04.831,0:02:07.556
should spin as well as turn!

0:02:07.556,0:02:08.843
The spinning will produce

0:02:08.843,0:02:10.803
velocities in opposite [directions]

0:02:10.803,0:02:12.283
at the top and bottom points,

0:02:12.283,0:02:13.793
as shown,

0:02:13.793,0:02:15.033
whereas, the turning

0:02:15.033,0:02:17.393
produces unidirectional velocities.

0:02:18.853,0:02:19.872
At the top,

0:02:19.872,0:02:21.602
the spinning and turning velocities

0:02:21.602,0:02:23.451
are in opposite directions,

0:02:23.451,0:02:24.451
so the velocity of

0:02:24.451,0:02:25.850
point B is zero.

0:02:25.850,0:02:27.083
At the bottom,

0:02:27.083,0:02:29.733
they get added up.

0:02:29.733,0:02:30.871
In short,

0:02:30.871,0:02:31.981
the Planet gears

0:02:31.981,0:02:33.192
are forced to turn

0:02:33.192,0:02:34.372
in order to satisfy

0:02:34.372,0:02:35.734
the condition of velocity.

0:02:35.734,0:02:37.506
As the carrier is attached

0:02:37.506,0:02:38.966
to the planet gear

0:02:38.966,0:02:40.235
it will turn along

0:02:40.235,0:02:42.394
with the planet gears.

0:02:45.854,0:02:48.373
Now let's see what happens when the sun gear is held stationary and the ring gear is rotated

0:02:50.397,0:02:55.636
This is the exact opposite to the previous case at the inner point of the planet gear

0:02:55.636,0:03:01.044
the Velocity should be zero and the outer points should have the speed of the ring gear.

0:03:01.044,0:03:06.696
in This case the planetary Spin will reverse in order to satisfy the speed conditions

0:03:08.995,0:03:11.735
However this case has one more difference

0:03:12.288,0:03:17.546
The speed of point B. Will be higher than [the] speed of point A in the previous case

0:03:18.214,0:03:21.771
This is obvious as the ring gear radius is higher

0:03:26.156,0:03:33.763
This will make the planet gear spin and turn at a higher speed thus the carrier will turn at a higher speed

0:03:36.570,0:03:40.366
Let's now explore this reverse mechanism of Planet Gears

0:03:42.580,0:03:46.690
For this what you have to do is just arrest the motion of the carrier

0:03:47.997,0:03:53.146
This means the Planet gears are not allowed to turn and can only spin

0:03:55.019,0:03:58.441
This Spin will be opposite to the rotation of the sun gear

0:03:59.633,0:04:04.282
This spinning Planet Gear will make the ring gear rotate in the same Direction

0:04:04.621,0:04:12.017
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

0:04:15.373,0:04:22.628
Here you can note that in order to achieve different speeds the input must be given to different parts of the planetary gearset

0:04:22.628,0:04:25.699
This is practically difficult in an actual mechanism

0:04:26.527,0:04:29.080
in an automatic transmission [to] achieve this

0:04:29.080,0:04:35.209
Three planetary gear sets are connected in series as shown with coaxial Shafts

0:04:35.700,0:04:41.818
To understand how this arrangement effectively transfers the input rotation to different parts of the planetary gearset

0:04:41.818,0:04:44.663
Watch our video on automatic transmission

0:04:45.608,0:04:49.608
Your support on patreon.com is greatly appreciated to make our educational service sustainable. Thank you