WEBVTT 00:00:00.308 --> 00:00:01.748 The planetary gear set, 00:00:01.748 --> 00:00:02.718 also known as, 00:00:02.718 --> 00:00:04.256 the epicyclic gear train 00:00:04.256 --> 00:00:05.260 is one of the most 00:00:05.260 --> 00:00:06.470 important and interesting 00:00:06.470 --> 00:00:08.350 inventions in engineering. 00:00:08.350 --> 00:00:09.490 They are great speed 00:00:09.490 --> 00:00:10.811 variation mechanisms 00:00:10.811 --> 00:00:11.882 and are often used in 00:00:11.882 --> 00:00:13.813 automobiles as a vital part of 00:00:13.813 --> 00:00:15.233 automatic transmissions. 00:00:18.585 --> 00:00:19.967 Let's explore the secrets 00:00:19.967 --> 00:00:21.234 of the planetary gear set 00:00:21.234 --> 00:00:22.834 in this video! 00:00:24.664 --> 00:00:26.129 A planetary gear set 00:00:26.129 --> 00:00:28.029 has four main parts: 00:00:28.479 --> 00:00:30.038 the sun, 00:00:30.368 --> 00:00:31.897 planet gears, 00:00:32.867 --> 00:00:34.537 ring gear, 00:00:35.537 --> 00:00:37.140 and carrier. 00:00:37.405 --> 00:00:38.408 You can see that it 00:00:38.408 --> 00:00:40.528 sometimes rotates quickly, 00:00:41.708 --> 00:00:43.882 sometimes slowly, 00:00:46.023 --> 00:00:47.283 and sometimes 00:00:47.283 --> 00:00:48.873 even in reverse. 00:00:50.203 --> 00:00:52.494 But, how does this happen? 00:00:52.494 --> 00:00:53.474 You will be able 00:00:53.474 --> 00:00:54.514 to predict the motion 00:00:54.514 --> 00:00:55.896 of this gear set completely, 00:00:55.896 --> 00:00:56.836 if you understand 00:00:56.836 --> 00:00:58.473 one simple fact! 00:00:59.123 --> 00:01:00.273 When two gears 00:01:00.273 --> 00:01:01.522 are moving as shown, 00:01:01.522 --> 00:01:02.362 they should have 00:01:02.362 --> 00:01:03.362 the same speed 00:01:03.362 --> 00:01:04.943 at the interface. 00:01:04.943 --> 00:01:06.073 This means, that the 00:01:06.073 --> 00:01:07.503 speed of gear A, 00:01:07.503 --> 00:01:08.494 should be the same 00:01:08.494 --> 00:01:09.493 as gear B, 00:01:09.493 --> 00:01:11.393 at their mating point. 00:01:17.139 --> 00:01:18.409 The speed has to be 00:01:18.409 --> 00:01:19.322 the same, 00:01:19.322 --> 00:01:20.302 otherwise, 00:01:20.302 --> 00:01:21.321 the gear teeth will 00:01:21.321 --> 00:01:22.340 penetrate each other 00:01:22.340 --> 00:01:23.550 as shown. 00:01:23.550 --> 00:01:25.806 That is an impossible condition. 00:01:26.818 --> 00:01:28.108 Just apply this fact 00:01:28.108 --> 00:01:29.540 to planetary gear sets, 00:01:29.540 --> 00:01:31.024 and you will be able to predict 00:01:31.024 --> 00:01:32.434 how speed variation 00:01:32.434 --> 00:01:34.124 is achieved. 00:01:35.864 --> 00:01:37.254 Assume that the ring gear 00:01:37.254 --> 00:01:38.374 is held stationary 00:01:38.374 --> 00:01:40.280 and we rotate the sun gear. 00:01:43.930 --> 00:01:45.168 Think of what happens 00:01:45.168 --> 00:01:46.858 to the planet gears! 00:01:47.968 --> 00:01:48.968 At point A, 00:01:48.968 --> 00:01:49.968 the planet gear 00:01:49.968 --> 00:01:51.746 should have a certain speed 00:01:51.746 --> 00:01:53.123 and at point B, 00:01:53.123 --> 00:01:54.343 the speed should be zero, 00:01:54.343 --> 00:01:55.343 as the ring gear 00:01:55.343 --> 00:01:56.963 is stationary. 00:01:56.963 --> 00:01:57.741 However, 00:01:57.741 --> 00:01:58.531 how are both 00:01:58.531 --> 00:01:59.511 of these conditions 00:01:59.511 --> 00:02:01.751 possible at the same time? 00:02:01.751 --> 00:02:03.661 There is only one way, 00:02:03.661 --> 00:02:04.801 the planet gear 00:02:04.801 --> 00:02:07.556 should spin as well as turn! 00:02:07.556 --> 00:02:08.843 The spinning will produce 00:02:08.843 --> 00:02:10.803 velocities in opposite [directions] 00:02:10.803 --> 00:02:12.273 at the top and bottom points, 00:02:12.273 --> 00:02:13.723 as shown, 00:02:13.723 --> 00:02:14.993 whereas, the turning 00:02:14.993 --> 00:02:17.523 produces unidirectional velocities. 00:02:18.853 --> 00:02:19.872 At the top, 00:02:19.872 --> 00:02:21.602 the spinning and turning velocities 00:02:21.602 --> 00:02:23.451 are in opposite directions, 00:02:23.451 --> 00:02:24.451 so the velocity of 00:02:24.451 --> 00:02:25.930 point B is zero. 00:02:26.450 --> 00:02:27.313 At the bottom, 00:02:27.313 --> 00:02:29.053 they get added up. 00:02:29.783 --> 00:02:30.811 In short, 00:02:30.811 --> 00:02:31.941 the planet gears 00:02:31.941 --> 00:02:33.192 are forced to turn 00:02:33.192 --> 00:02:34.372 in order to satisfy 00:02:34.372 --> 00:02:36.024 the condition of velocity. 00:02:36.024 --> 00:02:37.506 As the carrier is attached 00:02:37.506 --> 00:02:38.966 to the planet gear 00:02:38.966 --> 00:02:40.235 it will turn along 00:02:40.235 --> 00:02:42.074 with the planet gears. 00:02:42.674 --> 00:02:44.056 Now, let's see what happens, 00:02:44.056 --> 00:02:45.177 when the sun gear 00:02:45.177 --> 00:02:46.310 is held stationary 00:02:46.310 --> 00:02:48.473 and the ring gear is rotated. 00:02:50.243 --> 00:02:51.743 This is the exact opposite 00:02:51.743 --> 00:02:53.016 to the previous case. 00:02:53.016 --> 00:02:54.266 At the inner point 00:02:54.266 --> 00:02:55.490 of the planet gear, 00:02:55.490 --> 00:02:57.710 the velocity should be zero 00:02:59.770 --> 00:03:02.078 and the outer points 00:03:02.078 --> 00:03:03.078 should have the speed 00:03:03.078 --> 00:03:04.387 of the ring gear. 00:03:04.387 --> 00:03:05.995 In this case, 00:03:05.995 --> 00:03:06.995 the planetary spin 00:03:06.995 --> 00:03:08.575 will reverse 00:03:08.575 --> 00:03:09.628 in order to satisfy 00:03:09.628 --> 00:03:10.681 the speed conditions. 00:03:10.681 --> 00:03:12.969 However, 00:03:12.969 --> 00:03:13.969 this case, 00:03:13.969 --> 00:03:15.257 has one more difference. 00:03:15.257 --> 00:03:17.546 The speed of point B. Will be higher than [the] speed of point A in the previous case 00:03:18.214 --> 00:03:21.771 This is obvious as the ring gear radius is higher 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 00:03:36.570 --> 00:03:40.366 Let's now explore this reverse mechanism of Planet Gears 00:03:42.580 --> 00:03:46.690 For this what you have to do is just arrest the motion of the carrier 00:03:47.997 --> 00:03:53.146 This means the Planet gears are not allowed to turn and can only spin 00:03:55.019 --> 00:03:58.441 This Spin will be opposite to the rotation of the sun gear 00:03:59.633 --> 00:04:04.282 This spinning Planet Gear will make the ring gear rotate in the same Direction 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 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 00:04:22.628 --> 00:04:25.699 This is practically difficult in an actual mechanism 00:04:26.527 --> 00:04:29.080 in an automatic transmission [to] achieve this 00:04:29.080 --> 00:04:35.209 Three planetary gear sets are connected in series as shown with coaxial Shafts 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 00:04:41.818 --> 00:04:44.663 Watch our video on automatic transmission 00:04:45.608 --> 00:04:49.608 Your support on patreon.com is greatly appreciated to make our educational service sustainable. 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