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
00:00:08,510 --> 00:00:09,490
They are great speed 


8
00:00:09,490 --> 00:00:10,811
variation mechanisms

9
00:00:10,811 --> 00:00:11,782
and are often used in

10
00:00:11,782 --> 00:00:13,813
automobiles as a vital part of

11
00:00:13,813 --> 00:00:15,233
automatic transmissions.

12
00:00:18,585 --> 00:00:19,967
Let's explore the secrets

13
00:00:19,967 --> 00:00:21,254
of the planetary gear set

14
00:00:21,254 --> 00:00:22,714
in this video.

15
00:00:24,694 --> 00:00:26,039
A planetary gear set

16
00:00:26,039 --> 00:00:27,639
has four main parts:

17
00:00:28,299 --> 00:00:29,868
the sun,

18
00:00:30,597 --> 00:00:32,087
planet gears,

19
00:00:33,034 --> 00:00:34,370
ring gear,

20
00:00:35,747 --> 00:00:36,790
and carrier.

21
00:00:37,415 --> 00:00:38,458
You can see

22
00:00:38,458 --> 00:00:41,188
that it sometimes rotates quickly,

23
00:00:41,868 --> 00:00:43,262
sometimes slowly,

24
00:00:46,213 --> 00:00:47,283
and sometimes

25
00:00:47,283 --> 00:00:48,873
even in reverse.

26
00:00:50,203 --> 00:00:52,564
But, how does this happen?

27
00:00:52,564 --> 00:00:53,474
You will be able

28
00:00:53,474 --> 00:00:54,464
to predict the motion


29
00:00:54,464 --> 00:00:55,726
of this gear set completely,

30
00:00:55,726 --> 00:00:56,746
if you understand

31
00:00:56,746 --> 00:00:58,543
one simple fact!

32
00:00:59,373 --> 00:01:01,522
When two gears are moving as shown

33
00:01:01,522 --> 00:01:02,362
they should have

34
00:01:02,362 --> 00:01:05,193
the same speed at the interface.

35
00:01:05,193 --> 00:01:07,445
This means that the speed of gear A

36
00:01:07,445 --> 00:01:09,323
should be the same as gear B


37
00:01:09,323 --> 00:01:11,523
at their mating point.

38
00:01:17,179 --> 00:01:19,612
The speed has to be the same

39
00:01:19,612 --> 00:01:20,612
otherwise...

40
00:01:20,612 --> 00:01:21,612
the gear teeth will penetrate

41
00:01:21,612 --> 00:01:22,990
each other as shown.

42
00:01:22,990 --> 00:01:25,806
That is an impossible condition.

43
00:01:27,018 --> 00:01:28,118
Just apply this fact

44
00:01:28,118 --> 00:01:29,630
to planetary gear sets,

45
00:01:29,630 --> 00:01:31,274
and you will be able to predict

46
00:01:31,274 --> 00:01:33,454
how speed variation is achieved.

47
00:01:36,034 --> 00:01:37,494
Assume that the ring gear

48
00:01:37,494 --> 00:01:41,044
is held stationary

49
00:01:41,174 --> 00:01:44,130
and we rotate the sun gear.

50
00:01:44,130 --> 00:01:46,742
Think of what happens to the planet gears:

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

52
00:01:51,746 --> 00:01:56,560
and at point B the speed should be 
zero as the ring gear is stationary.

53
00:01:57,163 --> 00:02:01,163
However, how are both of these conditions
possible at the same time?

54
00:02:01,961 --> 00:02:07,161
There is only one way the planet gear should spin as well as turn

55
00:02:07,766 --> 00:02:13,595
The spinning will produce velocities in opposite [directions] at the top and bottom points as shown

56
00:02:13,943 --> 00:02:17,695
Whereas the turning Produces unidirectional velocities

57
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.

58
00:02:26,822 --> 00:02:29,230
at The bottom they get added up

59
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.

60
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

61
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

62
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

63
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.

64
00:03:01,044 --> 00:03:06,696
in This case the planetary Spin will reverse in order to satisfy the speed conditions

65
00:03:08,995 --> 00:03:11,735
However this case has one more difference

66
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

67
00:03:18,214 --> 00:03:21,771
This is obvious as the ring gear radius is higher

68
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

69
00:03:36,570 --> 00:03:40,366
Let's now explore this reverse mechanism of Planet Gears

70
00:03:42,580 --> 00:03:46,690
For this what you have to do is just arrest the motion of the carrier

71
00:03:47,997 --> 00:03:53,146
This means the Planet gears are not allowed to turn and can only spin

72
00:03:55,019 --> 00:03:58,441
This Spin will be opposite to the rotation of the sun gear

73
00:03:59,633 --> 00:04:04,282
This spinning Planet Gear will make the ring gear rotate in the same Direction

74
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

75
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

76
00:04:22,628 --> 00:04:25,699
This is practically difficult in an actual mechanism

77
00:04:26,527 --> 00:04:29,080
in an automatic transmission [to] achieve this

78
00:04:29,080 --> 00:04:35,209
Three planetary gear sets are connected in series as shown with coaxial Shafts

79
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

80
00:04:41,818 --> 00:04:44,663
Watch our video on automatic transmission

81
00:04:45,608 --> 00:04:49,608
Your support on patreon.com is greatly appreciated to make our educational service sustainable. Thank you