Length of Contacting Generating Lines Interpreted in the Regular Rectangular Contact Zone of Helical Gears and Zone Dependence of Their Change

External helical gears play an important role in the development of drive chains. Powertrain gears are expected to show a minimum level of vibration and noise level. The most significant source of vibration excitation and noise is the contact zone. Effects of errors caused by the manufacture, assembly and load are also displayed in the zone of contact. In this article, we will focus our investigation only on error-free gears (free from manufacturing, assembly and deformation errors). The continuous change in the contact zone is the change in the length of the contacting generating lines of the meshing. The contact zone can change its shape from a regular rectangle to a completely general one as a result of modifying the top land surface. We will examine the effect of the parameters and the shape of the contact zone on the total length of the contacting generating lines. In this article, the first step is to study the regular rectangular contact zone of external-external gear pairs.