Influence of Unbalanced Sliding Conditions on the Pitting Load Carrying Capacity of Internal Gears

Certain gear failure mechanisms are strongly affected by tribological effects on the flank surface. Here, the contact sliding conditions can play a decisive role due to i.e., crack direction and near-surface TEHL induced stresses. Macropitting and micropitting for example occur predominantly in contact areas of negative specific sliding. Unlike for external gears, a positioning of pitch point C outside of the active profile is possible for internal gear designs. This allows to completely avoid negative sliding either on the planet or the ring gear. Conversely, the corresponding gear is exposed to negative sliding only. It is assumed that the flank load carrying capacity of each meshing partner can be affected by different positions of the pitch point and the resulting differences in the specific sliding, allowing a lifetime optimization of the full gear stage by taking into account e.g. the load capacity reserves for different material pairings. Systematic results on internal gears with pitch point positioned outside of the active profile are not available. Existing standardized load carrying capacity calculation methods are typically based on investigations considering external gears with balanced sliding conditions. Systematic theoretical and experimental investigations on the pitting load capacity of internal gears in the pairing of a through-hardened respectively nitrided ring gear with case-hardened planets were performed. Therefore, a reference geometry with balanced sliding conditions was compared to gear designs with pitch point below and above the active profile. The results confirm a distinct influence of unbalanced sliding conditions, showing a significant increase of the pitting load capacity by avoiding contact areas of negative sliding on the ring gear, while an exposure to negative sliding only leads to a reduction of the pitting resistance. This influence was quantified and recommendations for the design of internal gears were derived.