Qualiseal Hydrodynamic Lift Off Seals are non-contacting film-riding seals that have been successfully used in industrial pump and compressor applications for decades and are now increasingly finding their way into demanding aerospace applications where the hydrodynamic features can be even more beneficial. We design hydrodynamic features into the face of the mating ring. As the mating ring rotates, these features force air into a converging gap in the sealing surface, causing a pressure increase between the primary sealing elements. Once the pressure becomes great enough, the faces separate and the part operates in a hydrodynamic or non-contacting mode. The hydrodynamic features use air from the inside diameter to generate lift. A static sealing dam prevents leakage when the part is not rotating. In many aerospace applications, the rapid thermal transition, constantly changing pressures and high altitude conditions demand solutions where seal flatness must be maintained reliably over the long haul. Using a non-contacting seal enables the face seal and mating ring to operate virtually leak-free and experience almost no wear.