Planetary gearboxes are characterized by a high power density and a compact design with coaxial in- and output. Consequently, planetary gear units are frequently used in many practical applications, e.g. in wind energy and industrial transmission systems as well as in automotive applications such as automatic transmissions. As a result of the scaling-up of the required system power, the risk of flank damages increased during the last decade. Certain damage types, such as pitting and micropitting, occur predominantly in contact areas of negative specific sliding on the tooth flank surface. Unlike external gears, internal gears offer the possibility of positioning the pitch point C outside of the active profile. Depending on the gear design, this allows a complete avoidance of negative specific sliding either on the planet or on the ring gear. In contrast, the corresponding gear is subjected to negative specific sliding only. It can be assumed that the lifetime regarding flank damages of each meshing partner can be affected by different positions of the pitch point and the resulting differences in the specific sliding, which allows an optimization of the full gear system, taking into account e.g. the load carrying capacity reserves for different material pairings. It is to be noted that such gear designs are already part of some practical applications.However, the existing analytical approaches within standardized load carrying capacity calculation methods are typically based on investigations considering external gears with balanced sliding conditions. Systematic results on internal gear designs with pitch point positioned outside of the active profile are not available.Within this paper, extended theoretical studies on the tribological characteristics of different internal gear designs are presented. Therefore, a reference geometry with balanced sliding conditions is compared to internal gears with pitch point below respectively above the active profile. The focus of the studies is the analysis of the lubricant film thickness and the contact temperature in the meshing contact. Referring to this, a high-order TEHL contact simulation model was used. The results of the numerical analysis with the FZG calculation software TriboMesh are compared to different analytical approaches in the context of the calculation of load carrying capacity to derive recommendations for the design of internal gears.
