According to geometric relationships, the intersection of the normal plane and the indexing cylindrical surface at a certain point on the tooth line of a helical cylindrical gear is an ellipse. The maximum curvature radius of this ellipse is used as the indexing circle radius of a hypothetical straight cylindrical gear, and the normal modulus and normal tooth shape angle of the helical cylindrical gear are used as the modulus and tooth shape angle of the hypothetical straight cylindrical gear. The hypothetical straight cylindrical gear is called the equivalent gear of the helical cylindrical gear. The equivalent number of teeth of a helical cylindrical gear refers to the equivalent number of teeth of the helical cylindrical gear.