The apparatus, which can be used in aspirating face seals or thrust bearings, spaces two relatively rotatable facing surfaces in use by entraining gas between the surfaces. The apparatus includes a first portion (1) defining a generally frusto-conical surface (3); a second portion (2) defining at least one flat surface disposed adjacent the first portion so that the two surfaces face each other and define at least one point of closest engagement between the surfaces, with diverging gaps extending between the surfaces on either side of a plane which contains the point or points of closest engagement and which extends generally orthogonal to the direction of relative rotation, and a device (4) for biasing the flat surface towards the frusto-conical surface to maintain the gaps within a predetermined dimensional range.