Abstract: The subject of the invention is a device with a reciprocating motion mechanism enabling the conversion of its moment of inertia into rotational speed or rotational speed into moment of inertia, characterised in that on the rotating shaft (11) there is a releasable mechanism (2) of reciprocating motion in two perpendicular directions, including two circular discs (3 and 4), tiled in parallel, with profiled notches (7 and 8) on their surfaces, whereby both discs are connected with each other by bolts (10) and have a releasable connection with the rotating shaft (11) and between each pair of profiled notches (7 and 8) of both discs there are upper connectors (14) of the upper ends of each pair of opposite moving arms (15), having an articulated connection with each other, of which the other ends also have an articulated connection with the two ring connectors (17) of two hydraulic actuators (18), which have a releasable connection with this shaft, whereby all upper connectors are equipped with functional compone