Many “gears” are used for automobiles, but they are also used for many other machines. The most typical one is the “transmission” that conveys the power of engine to tires. There are broadly two roles the transmission of an automobile plays : one is to decelerate the high rotation speed emitted by the engine to transmit to tires; the other is to change the reduction ratio in accordance with the acceleration / deceleration or driving speed of an automobile.
The rotation speed of an automobile’s engine in the general state of driving amounts to 1,000 – 4,000 rotations per minute (17 – 67 per second). Since it is impossible to rotate tires with the same rotation speed to run, it is necessary to lower the rotation speed utilizing the ratio of the number of gear teeth. Such a role is called deceleration; the ratio of the rotation speed of engine and that of tires is called the reduction ratio.
Then, why is it necessary to change the reduction ratio in accordance with the acceleration / deceleration or driving speed ? This is because substances need a large force to start moving however they do not require such a large force to keep moving once they have started to move. Automobile can be cited as a good example. An engine, however, by its nature can’t so finely change its output. Therefore, one adjusts its output by changing the reduction ratio utilizing a transmission.
The transmission of motive power through gears very much resembles the principle of leverage (a lever). The ratio of the number of teeth of gears meshing with each other can be deemed as the ratio of the length of levers’ arms. That is, if the reduction ratio is large and the rotation speed as output is low in comparison to that as input, the power output by transmission (torque) will be large; if the rotation speed as output is not so low in comparison to that as input, on the other hand, the power output by transmission (torque) will be small. Thus, to change the reduction ratio utilizing transmission is much akin to the principle of moving things.
Then, how does a transmission change the reduction ratio ? The answer lies in the mechanism called a planetary gear mechanism.
A planetary gear mechanism is a gear mechanism consisting of 4 components, namely, sun gear A, several planet gears B, internal gear C and carrier D that connects planet gears as seen in the graph below. It has a very complex structure rendering its design or production most difficult; it can realize the high reduction ratio through gears, however, it is a mechanism suited to a reduction mechanism that requires both small size and high performance such as transmission for automobiles.