Abstract: The utility model relates to internal cycloidal gear mechanisms and can be used in various branches of mechanical engineering as working members of hydraulic machines (pumps and engines), compressors, internal combustion engines, as well as in planetary gearboxes, in particular in technical systems for drilling and repairing oil and gas wells. The problems to be solved by the utility model include the improvement of the quality of the process of designing working members having a cycloidal tooth profile, as well as the substantiation of the conditions for modifying cycloidal face profiles (by choosing the required combination of dimensionless gearing coefficients) in order to achieve the maximum or minimum open area of the gerotor mechanism having a different kinematic ratio.