The topic of this paper is the design of reducer for two modes of operation. The function of this machine is primarily related to power tranmission. It's main elemets are cylindrical gears, which is why the gearbox is also considered as a mechanical power transmission. The introductory part of the paper describes the historical development of gearboxes. There is a talk in general about power transmission, fabrication materials and fabrication technologies. The following chapter describes main parts of the system: electric motor, elastic coupling and reducer with variable gear ratio. The third chapter presents a conceptual solution to the problem and its working principle. The accounting includes the calculation and optimization of gears, shafts and bearings. Selection of standard system components like electric motor and elastic coupling was made according to the manufacturer's catalogs. The conceptual solution to the core of this paper is based on the use of a claw coupling to change the mode of operation. The technical documentation contains a 3D model and assembly plan of the output part of the system, as well as workshop drawings of the output shaft, gears and claw coupling.