Classical dynamics: Spinning eggs — a paradox resolved

An internal-gear fluid-displacement machine, such as a fluid-displacing pump or a fluid-displaced motor includes an internal gear and a spur gear which meshes with the internal gear at a meshing region. A separating body fills a separating region between the gears which narrows toward the meshing region and separates the same from the remainder of the interspace between the gears. An actuating lever is mounted on the housing for pivoting and extends through the interspace, and a piston subjected to the pressure prevailing at the meshing region abuts against a free end of the actuating lever, pressing the same into abutment with that end of the separating body which faces into the remainder of the interspace. A strap connects the separating body to the actuating lever. The axial sides of the separating body have depressions therein which are bounded by narrow sealing projections which converge toward the meshing region.