Abstract: A rotating machine such as a submersible pump which comprises a casing (1) and a rotor (2) which is rotatable relative to the casing (1). The casing defines an aperature adjacent to which the rotor is positioned such that an annular gap is formed around the aperture between a first surface defined by the rotor (2) and a second surface (9) defined by the casing (1). One or both of the first and second surfaces is provided with a surface structure which is operative to prevent the accumulation of material within the gap. The irregular surface may be defined by for example a screw thread.