There are many kinds gear applications,. According to the tooth profile curve can be divided into the involute gear, cycloidal gear, arc gear, etc. According to the shape can be divided into cylindrical gear, bevel gear, the non-circular gear, rack, worm, worm gear, etc.; According to the tooth's surface can be divided into external gear and internal gear; According to the tooth line shape can be divided into straight gear, helical gears, herringbone gear, gear curve, etc. According to the manufacturing method can be divided into casting rolling gear, gear, gear cutting, sintered gears, etc. Able to transfer between any two axis movement and power, transmission smooth, reliable, high efficiency, long service life, compact structure, wide transmission speed and power. But need special equipment manufacturing, machining precision and installation accuracy is higher, and not suitable for long distance transmission. The role of the gear for transmission torque power, dividing and positioning.