US Patent 8162322 Hydrodynamic seal with circumferentially varying lift force

A seal assembly which operates efficiently at high altitudes and low surface speeds includes a first seal ring, a second seal ring, and hydropads. The first seal ring is of a generally annular shape and defining radial and circumferential directions. The second seal ring is positioned in facing relation to the first seal ring and rotatably mounted relative to the first seal ring about an axis of rotation. A plurality of hydropads formed on one of the first seal ring and second seal ring provides a lift force that varies about the circumference of the mating ring.