In the research of modern gear reduction motors, arc gears will replace cylindrical gears and become the focus of development. In terms of stability, torque and service life, arc gears have more advantages than cylindrical gears. It has great advantages, and its development trend is also very good. The tooth profile of the arc gear is non-conjugate tooth profile. In order to achieve continuous and stable transmission, it must have the following motion characteristics:

1. The working surface of the two gears (the contact line in theory or the contact belt in actual operation) must be a helical line or a helical surface.

2. For a pair of meshing gears, the helix angles on the two pitch cylinders must be equal in size and opposite in direction. That is, the axial pitch (pitch) of the two gears must be equal. This is consistent with involute helical gears.

3. For a pair of meshing gears, the axial coincidence degree must be greater than 1.

4. The modules of a pair of meshing gears must be equal.

5. For a pair of meshing gears, the moving direction of the meshing point (regional plane) is parallel to the axis direction of the two gears.

6. For a pair of meshing gears, there shall be no tooth interference between the tooth surfaces.