What are the three functions of gears?

What are the three functions of gears? There are the following related contents:

Transfer power and change the speed and direction of movement. Gear refers to a mechanical element with gears on the rim, and the gears are constantly meshed to transmit motion and power. Gears have long been used for transmission. At the end of 19, the principle of generating teeth and cutting hair began to appear, and then under the research of people, the theory of using special machine tools and tools to cut teeth appeared one after another. With the development of production, the stability of gear operation is concerned. The working principle of gear is to transmit the direction of motion and power through the meshing between gear teeth and links. The advantages of gears are high transmission efficiency, large transmission ratio, large transmission torque and long service life. However, it also has some disadvantages, such as heavy weight, high noise, short transmission distance and high friction, which requires frequent lubrication. Gears and gear products are important basic parts of mechanical equipment, which play a very important role in the whole power system. It can be said that most of the main transmission parts of complete mechanical equipment are gear transmission at present.