WEBVTT 00:00:00.278 --> 00:00:01.748 The planetary gear set, 00:00:02.088 --> 00:00:04.514 also known as the epicyclic gear train, 00:00:04.578 --> 00:00:07.930 is one of the most important and interesting inventions in engineering. 00:00:08.790 --> 00:00:10.901 They are great speed variation mechanisms 00:00:11.102 --> 00:00:15.483 and are often used in automobiles as a vital part of automatic transmissions. 00:00:18.565 --> 00:00:22.117 Let's explore the secrets of the planetary gear set in this video! 00:00:25.149 --> 00:00:27.899 A planetary gear set has four main parts: 00:00:28.869 --> 00:00:29.893 the sun, 00:00:30.768 --> 00:00:32.027 planet gears, 00:00:32.867 --> 00:00:34.137 ring gear, 00:00:35.657 --> 00:00:36.960 and carrier. 00:00:37.643 --> 00:00:40.313 You can see that it sometimes rotates quickly, 00:00:41.818 --> 00:00:43.239 sometimes slowly, 00:00:46.383 --> 00:00:48.376 and sometimes even in reverse. 00:00:50.203 --> 00:00:52.313 But, how does this happen? 00:00:52.674 --> 00:00:55.814 You will be able to predict the motion of this gear set completely, 00:00:55.826 --> 00:00:58.036 if you understand one simple fact! 00:00:59.553 --> 00:01:01.522 When two gears are moving as shown, 00:01:01.772 --> 00:01:04.680 they should have the same speed at the interface. 00:01:05.283 --> 00:01:08.284 This means, that the speed of gear A should be the same 00:01:08.284 --> 00:01:10.536 as gear B at their mating point. 00:01:17.559 --> 00:01:19.415 The speed has to be the same, 00:01:19.573 --> 00:01:23.121 otherwise the gear teeth will penetrate each other as shown. 00:01:23.450 --> 00:01:25.623 That is an impossible condition! 00:01:27.148 --> 00:01:29.704 Just apply this fact to planetary gear sets 00:01:29.995 --> 00:01:33.600 and you will be able to predict how speed variation is achieved. 00:01:36.234 --> 00:01:39.944 Assume that the ring gear is held stationary and we rotate the sun gear. 00:01:43.930 --> 00:01:46.278 Think of what happens to the planet gears! 00:01:48.392 --> 00:01:51.646 At point A, the planet gear should have a certain speed. 00:01:51.836 --> 00:01:54.793 And at point B, the speed should be zero, 00:01:54.793 --> 00:01:56.731 as the ring gear is stationary. 00:01:57.088 --> 00:02:01.220 However, how are both of these conditions possible at the same time? 00:02:01.941 --> 00:02:03.661 There is only one way! 00:02:04.029 --> 00:02:06.653 The planet gear should spin as well as turn! 00:02:07.908 --> 00:02:10.783 The spinning will produce velocities in opposite directions 00:02:10.783 --> 00:02:13.327 at the top and bottom points, as shown. 00:02:13.975 --> 00:02:17.613 Whereas the turning produces unidirectional velocities. 00:02:19.142 --> 00:02:23.362 At the top, the spinning and turning velocities are in opposite directions, 00:02:23.451 --> 00:02:25.891 so the velocity of point B is zero. 00:02:26.843 --> 00:02:29.053 At the bottom, they get added up. 00:02:30.151 --> 00:02:33.081 In short, the planet gears are forced to turn 00:02:33.192 --> 00:02:36.002 in order to satisfy the condition of velocity. 00:02:36.326 --> 00:02:38.966 As the carrier is attached to the planet gear 00:02:38.966 --> 00:02:41.631 it will turn along with the planet gears. 00:02:43.003 --> 00:02:46.230 Now, let's see what happens when the sun gear is held stationary 00:02:46.230 --> 00:02:48.173 and the ring gear is rotated. 00:02:50.343 --> 00:02:53.273 This is the exact opposite to the previous case. 00:02:53.360 --> 00:02:55.506 At the inner point of the planet gear, 00:02:55.506 --> 00:02:57.504 the velocity should be zero 00:02:57.624 --> 00:03:00.699 and the outer points should have the speed of the ring gear. 00:03:00.978 --> 00:03:02.015 In this case, 00:03:02.165 --> 00:03:06.625 the planetary spin will reverse in order to satisfy the speed conditions. 00:03:09.058 --> 00:03:11.616 However, this case has one more difference. 00:03:12.352 --> 00:03:14.493 The speed of point B, will be higher 00:03:14.493 --> 00:03:17.488 than the speed of point A in the previous case. 00:03:18.274 --> 00:03:21.810 This is obvious, as the ring gear radius is higher. 00:03:26.232 --> 00:03:29.983 This will make the planet gear spin and turn at a higher speed. 00:03:30.282 --> 00:03:33.417 Thus, the carrier will turn at a higher speed. 00:03:36.604 --> 00:03:40.280 Let's now explore this reverse mechanism of planet gears. 00:03:42.647 --> 00:03:43.559 For this, 00:03:43.559 --> 00:03:46.681 what you have to do is just arrest the motion of the carrier. 00:03:48.032 --> 00:03:51.151 This means, that the planet gears are not allowed to turn 00:03:51.443 --> 00:03:52.817 and can only spin. 00:03:55.023 --> 00:03:58.207 This spin will be opposite to the rotation of the sun gear. 00:03:59.563 --> 00:04:03.993 This spinning planet gear will make the ring gear rotate in the same direction. 00:04:04.521 --> 00:04:05.329 In short, 00:04:05.329 --> 00:04:09.262 the direction of rotation of the ring gear will be the opposite to the sun gear. 00:04:09.481 --> 00:04:11.696 Thus, we will get the reverse gear. 00:04:15.368 --> 00:04:18.646 Here, you can note, that in order to achieve different speeds, 00:04:18.682 --> 00:04:22.482 the input must be given to different parts of the planetary gear set. 00:04:22.589 --> 00:04:25.563 This is practically difficult in an actual mechanism. 00:04:26.370 --> 00:04:28.920 In an automatic transmission, to achieve this, 00:04:28.920 --> 00:04:32.353 three planetary gear sets are connected in series 00:04:32.353 --> 00:04:34.697 as shown with coaxial shafts. 00:04:35.714 --> 00:04:39.465 To understand, how this arrangement effectively transfers the input rotation 00:04:39.465 --> 00:04:41.913 to different parts of the planetary gear set, 00:04:41.913 --> 00:04:44.126 watch our video on automatic transmission. 00:04:45.571 --> 00:04:47.301 Your support on patreon.com 00:04:47.301 --> 00:04:50.781 is greatly appreciated to make our educational service sustainable. 00:04:50.781 --> 00:04:51.731 Thank you.