The output shaft is attached to the blue ring gear, and the planet carrier is held stationary - this gives the same 6:1 gear ratio. Because there are three red gears instead of one, this gear train is extremely rugged. All three are attached to a plate (the planet carrier), and they engage the inside of the blue gear (the ring) instead of the outside. In this gear system, the yellow gear (the sun) engages all three red gears (the planets) simultaneously. In that case, you can use a planetary gear system, as shown here: A common place where this same-axis capability is needed is in an electric screwdriver. However, imagine that you want the axis of the output gear to be the same as that of the input gear. The size of the red gear is not important because it is just there to reverse the direction of rotation so that the blue and yellow gears turn the same way. In this train, the blue gear has six times the diameter of the yellow gear (giving a 6:1 ratio). One way to create that ratio is with the following three-gear train: Let's say you want a gear ratio of 6:1 with the input turning in the same direction as the output. Planetary gears solve the following problem.
One specialized gear train is called a planetary gear train.
Because the dials are directly connected to one another, they spin in opposite directions (you will see that the numbers are reversed on dials next to one another). If you can see inside your power meter and it's of the older style with five mechanical dials, you will see that the five dials are connected to one another through a gear train like this, with the gears having a ratio of 10:1. In the same way, you could attach a 2,500-rpm motor to the red gear to get 100 rpm on the purple gear. That means that if you connect the purple gear to a motor spinning at 100 revolutions per minute (rpm), the green gear will turn at a rate of 500 rpm and the red gear will turn at a rate of 2,500 rpm. In this train, the smaller gears are one-fifth the size of the larger gears. The gear train shown below has a higher gear ratio: The red gear turns at twice the rate as the green gear. The green gear turns at twice the rate of the purple gear. In the case above, the purple gear turns at a rate twice that of the blue gear.
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