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Think about the relationship between the sun gear and the pinion used in the first gear. Try to compare the radius from the center of the sun gear to its serrations and the radius from its center to the center of the planet carrier. It will be found that when the radius of the pinion (sun gear) is 1, the radius of the big gear (from the center of the sun gear to the center of the planet carrier) is 2, that is, the radius ratio of the gears is 2:1. At this point, you may think that the spur gear used in the manual transmission has a gear ratio of 4:1 in the first gear, and it is strange that the radius ratio of planetary gears is 2:1 in the first gear. In fact, automatic transmissions generally have two planetary gears. After passing through two 2:1 planetary gears, the gear ratio will become 4:1, which is the same as the first gear of the manual transmission. The state of using two planetary gears at the same time is very complex. Let's just learn about the difference in the radius of each gear in a planetary gear when the gear is above gear 2. Compare the radius from the center of the sun gear to the planet carrier and the radius from the center of the sun gear to the serration of the internal gear ring in gear 2. In the first gear, the radius ratio of the pinion gear to the sun gear is 2:1, and in the second gear, the radius ratio of the internal gear ring to the pinion gear is 3:2. Compared with the gear combination in the first gear, the radius difference of the gear combination in the second gear is smaller. In the third gear, all three kinds of gears are rotating. Please imagine what would happen if there were no planetary gears. If the three kinds of gears are connected in a straight line by one shaft, the rotational force of the engine can be transmitted to the tire without deceleration and loss. When the sun gear is fixed and the rotating force of the engine is transmitted to the internal gear ring, the relationship between the radius of the internal gear ring and the radius from the center of the sun gear to the center of the planet carrier is completely opposite to that in the first and second gears. When the internal gear ring is fixed and the rotating force of the engine is transmitted to the pinion to drive the sun gear to rotate, the relationship between the pinion and the pinion is also opposite to that in the first and second gears. As mentioned above, planetary gears are quite complex, but like spur gears, the combination of large and small gears should also be selected. Also, it is easy to understand the principle of gear ratio by paying attention to the radius of the rotating gear.