What are the conditions for proper gear meshing?

[Reid]

The conditions for proper gear meshing are equal modulus, equal pressure angle, and equal base circle tooth pitch. The following are the specific conditions for proper gear meshing: Equal modulus and equal pressure angle: The modulus of the two gears must be equal, and the tooth profile angle on the pitch circle of the two gears must be equal. The meshing conditions for helical cylindrical gears: the normal modulus of the two gears must be equal, the tooth profile angle must be equal, the helix angle must be equal, and the helix direction must be opposite. The meshing conditions for straight bevel gears: the large end modulus and tooth profile angle of the two gears must be equal respectively. Equal base circle tooth pitch: The base circle tooth pitch of a pair of gears must be equal. Because the processing tools for involute gears are standardized, the gear meshing conditions are that the pressure angles on the pitch circles of the two gears are equal, the moduli of the two gears are equal, and equal to the national standard values, etc. The condition for continuous gear meshing is that the contact ratio must be greater than 1.