What are the correct meshing conditions for gears?

[DiLeo]

Here are the relevant introductions to the correct meshing conditions for gears: 1. Spur gears: The modules of the two gears must be equal, and the pressure angles on the pitch circles of the two gears must be equal. 2. Helical gears: The normal modules of the two gears must be equal, the pressure angles must be equal, the helix angles must be equal, and the helix directions must be opposite. 3. Straight bevel gears: (1) The modules and pressure angles at the large ends of the two gears must be equal respectively. (2) The modules of the two gears must be equal, and the pressure angles on the pitch circles of the two gears must be equal. The normal modules of the two gears must be equal, the pressure angles must be equal, the helix angles must be equal, and the helix directions must be opposite. The modules and pressure angles at the large ends of the two gears must be equal respectively. (3) Because gear cutting tools are standardized, the meshing condition for gears is that the modules and pressure angles must be equal respectively. The continuous meshing condition for gears is that the contact ratio must be greater than 1.