The modulus of the two gears must be equal, and the tooth angles on the dividing circle of the two gears must be equal. The normal module of the two gears is equal, the tooth profile angle is equal, the helix angle is equal, and the helix direction is opposite. The big end modulus and tooth profile angle of the two gears are equal respectively.

Due to the standardization of gear processing tools, the meshing condition of gears is that the modulus and pressure angle must be equal respectively. The condition of continuous meshing of gears is that the coincidence degree must be greater than 1.

Gear is a mechanical element with teeth on the rim that can continuously mesh to transmit motion and power, and it is a kind of toothed mechanical part that can mesh with each other. The diameter of the big gear is twice that of the small gear.