Abstract: A rotary mechanical seal assembly for a shaft comprises a freely floating ring of flexible material which has inner and outer axially directed lips to bear against the surfaces of the shaft and housing, at least one of these lips being normally out of contact with its associated surface so that no wear takes place. The seal is brought into action by the application of pressure either applied from an outside source or created by leakage past an adjacent upstream face-type seal for which the seal of the invention forms an emergency back-up. There may be a detector to sense the leakage. There could be two such lip seals facing one another.