| Classification | Meshing mode | Basic feature |
| Spur gear | external meshing, internal meshing, gear and rack | The contact line of tooth profile is a straight line parallel to the axis. A pair of tooth profiles enter or exit the meshing along the tooth width at the same time, which is easy to cause impact and noise and poor transmission stability. |
| Straight bevel gears | — | Pitch cone, indexing cone, tooth top cone, tooth root cone and base cone. For the transmission between intersecting two shafts, the gear teeth are distributed along the conical surface, and the size of the lifting gear teeth is gradually reduced towards the top of the cone. |
In order to make the gear rotate in both directions, the tooth profiles on both sides of the tooth are composed of involute curved surfaces with the same shape and opposite direction. The names and symbols of each part are: tooth top circle, tooth root circle, tooth groove, tooth thickness, tooth pitch, module m, indexing circle d, tooth top and tooth root, and top clearance.