Abstract: Constraining rings are provided in a fluid displacement machine of the kind in which an eccentrically mounted rotor revolves within a hollow chamber and is equipped with radially displaceable vanes to contact the wall of the chamber and separate working spaces of the machine. The constraining rings surround lugs in the vanes and limit outward centrifugal displacement of the vanes to prevent excessive contact pressure between the vanes and the wall of the chamber. Recesses for the rings may be provided in the ends of the vanes or the rotor may have end pieces sealingly engaging the ends of the hollow chamber and incorporating cavities to accommodate the rings and lugs. To center the rings they may be formed as the inner rings of bearings whose outer rings are fast with the ends of the chamber. Stress may be distributed by providing additional rings intermediately along the axial length of the vanes. Runners may be interposed between the lugs and the rings.