1
00:00:00,408 --> 00:00:01,728
The planetary gear set,

2
00:00:01,728 --> 00:00:02,718
also known as,

3
00:00:02,718 --> 00:00:04,256
the epicyclic gear train

4
00:00:04,256 --> 00:00:05,350
is one of the most

5
00:00:05,350 --> 00:00:06,410
important and interesting

6
00:00:06,410 --> 00:00:07,950
inventions in engineering.

7
00:00:08,490 --> 00:00:10,633
They are great speed variation mechanisms


8
00:00:10,633 --> 00:00:12,373
and are often used in automobiles

9
00:00:12,373 --> 00:00:15,171
as a vital part of automatic transmissions.

10
00:00:18,585 --> 00:00:20,167
Let's explore the secrets

11
00:00:20,167 --> 00:00:23,407
of the planetary gear set in this video.

12
00:00:25,144 --> 00:00:27,647
A planetary gear set has four main parts:

13
00:00:29,039 --> 00:00:29,868
the sun,

14
00:00:30,927 --> 00:00:32,087
planet gears,

15
00:00:33,244 --> 00:00:34,370
ring gear,

16
00:00:35,897 --> 00:00:36,940
and carrier.

17
00:00:37,815 --> 00:00:40,441
You can see that it sometimes rotates quickly,

18
00:00:42,068 --> 00:00:43,262
sometimes slowly,

19
00:00:46,503 --> 00:00:49,014
and sometimes even in reverse.

20
00:00:50,483 --> 00:00:52,394
But how does this happen?

21
00:00:52,924 --> 00:00:55,746
You will be able to predict the motion
of this gear set completely

22
00:00:55,746 --> 00:00:58,122
if you understand one simple fact.

23
00:00:59,523 --> 00:01:01,732
When two gears are moving as shown

24
00:01:01,732 --> 00:01:04,765
they should have the same
speed at the interface.

25
00:01:05,223 --> 00:01:07,585
This means that the speed of gear A

26
00:01:07,585 --> 00:01:10,603
should be the same as gear B
at their mating point.

27
00:01:17,589 --> 00:01:19,612
The speed has to be the same

28
00:01:19,612 --> 00:01:23,270
otherwise the gear teeth
will penetrate each other as shown

29
00:01:23,270 --> 00:01:25,806
that is an impossible condition.

30
00:01:27,118 --> 00:01:29,630
Just apply this fact to 
planetary gear sets,

31
00:01:29,630 --> 00:01:33,574
and you will be able to predict how
speed variation is achieved.

32
00:01:36,174 --> 00:01:40,174
Assume that the ring gear is held
stationary and we rotate the sun gear.

33
00:01:44,130 --> 00:01:46,742
Think of what happens to the planet gears:

34
00:01:48,438 --> 00:01:51,746
At point A the planet gear should
have a certain speed

35
00:01:51,746 --> 00:01:56,560
and at point B the speed should be 
zero as the ring gear is stationary.

36
00:01:57,163 --> 00:02:01,163
However, how are both of these conditions
possible at the same time?

37
00:02:01,961 --> 00:02:07,161
There is only one way the planet gear should spin as well as turn

38
00:02:07,766 --> 00:02:13,595
The spinning will produce velocities in opposite [directions] at the top and bottom points as shown

39
00:02:13,943 --> 00:02:17,695
Whereas the turning Produces unidirectional velocities

40
00:02:19,073 --> 00:02:26,143
at the top the spinning and turning velocities are in opposite directions, so the velocity [of] point B is zero.

41
00:02:26,822 --> 00:02:29,230
at The bottom they get added up

42
00:02:30,083 --> 00:02:36,382
in short the Planet gears are forced to turn in order to satisfy the condition of velocity.

43
00:02:36,541 --> 00:02:41,806
as The carrier is attached to the planet gear it will turn along with the planet Gears

44
00:02:42,816 --> 00:02:48,373
Now let's see what happens when the sun gear is held stationary and the ring gear is rotated

45
00:02:50,397 --> 00:02:55,636
This is the exact opposite to the previous case at the inner point of the planet gear

46
00:02:55,636 --> 00:03:01,044
the Velocity should be zero and the outer points should have the speed of the ring gear.

47
00:03:01,044 --> 00:03:06,696
in This case the planetary Spin will reverse in order to satisfy the speed conditions

48
00:03:08,995 --> 00:03:11,735
However this case has one more difference

49
00:03:12,288 --> 00:03:17,546
The speed of point B. Will be higher than [the] speed of point A in the previous case

50
00:03:18,214 --> 00:03:21,771
This is obvious as the ring gear radius is higher

51
00:03:26,156 --> 00:03:33,763
This will make the planet gear spin and turn at a higher speed thus the carrier will turn at a higher speed

52
00:03:36,570 --> 00:03:40,366
Let's now explore this reverse mechanism of Planet Gears

53
00:03:42,580 --> 00:03:46,690
For this what you have to do is just arrest the motion of the carrier

54
00:03:47,997 --> 00:03:53,146
This means the Planet gears are not allowed to turn and can only spin

55
00:03:55,019 --> 00:03:58,441
This Spin will be opposite to the rotation of the sun gear

56
00:03:59,633 --> 00:04:04,282
This spinning Planet Gear will make the ring gear rotate in the same Direction

57
00:04:04,621 --> 00:04:12,017
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

58
00:04:15,373 --> 00:04:22,628
Here you can note that in order to achieve different speeds the input must be given to different parts of the planetary gearset

59
00:04:22,628 --> 00:04:25,699
This is practically difficult in an actual mechanism

60
00:04:26,527 --> 00:04:29,080
in an automatic transmission [to] achieve this

61
00:04:29,080 --> 00:04:35,209
Three planetary gear sets are connected in series as shown with coaxial Shafts

62
00:04:35,700 --> 00:04:41,818
To understand how this arrangement effectively transfers the input rotation to different parts of the planetary gearset

63
00:04:41,818 --> 00:04:44,663
Watch our video on automatic transmission

64
00:04:45,608 --> 00:04:49,608
Your support on patreon.com is greatly appreciated to make our educational service sustainable. Thank you