Abstract: An improvement in the combination of a cylindrical outer frame which is coaxially rotatably mounted on the outside of a cylindrical inner frame as disclosed. The improvement which maintains a sealed condition between the two frames includes a first circumferential groove on the outside of the inner frame, this groove having sloped sidewalls. A second circumferential groove is provided on the inside surface of the outer frame opposite the first groove, the second groove having parallel sidewalls. A sealing ring is positioned within the space defined by the first and second grooves and contacts opposing sidewalls of the two grooves. A biasing means is provided for axially biasing the inner and outer frame with respect to each other so as to assure the sealing condition established by the sealing ring.