Abstract: A ring-shaped sealing member is configured to effect a seal between a shaft arranged to rotate in one sense within a housing at a relatively high rate--typically in excess of 10,000 r.p.m. The sealing member fits closely around the shaft but is split to allow circumferential expansion thereof. At least one ramp surface facing the shaft is defined by the sealing member, the clearance between the shaft and ramp surface decreasing in the direction of normal rotation of the shaft.On rotation of the shaft at a high speed, air is carried round by the shaft within the gap of decreasing clearance between the shaft and ramp surface of the sealing member. The sealing member is expanded circumferentially thereby, and when the shaft rotates sufficiently quickly the air film carried round by the shaft lifts the sealing member completely clear of the shaft, the air film maintaining the seal between the shaft and the sealing member.