A rotary mechanical transmission, comprising: - a containment structure; - a first rotary element, connected or connectable to a drive unit to define a mechanical power input unit and rotatable about an axis; - a fixed guide, in particular forming part of said containment structure; - a second rotary element rotatable about said axis and defining a power output unit; - a connecting element extending along said axis of rotation and coupled to said first rotary element by a first threaded connection (3, 8), in particular a lead nut and screw or recirculating ball or roller connection, wherein said connecting element is also coupled with one of either said fixed guide or said second rotary element by means of a second threaded connection (4, 14), in particular a lead nut and screw or recirculating ball or roller connection, and with the other of either said fixed guide or said second rotary element by means of a linear guide parallel to aid axis; and wherein said first threaded connection (3, 8) and second threaded connection (4, 14) have different pitches in such a way as to vary the angular speed between the connecting element and the first rotary element; wherein: - said connecting element is made in the form of a roto-translational element extending along an axis of rotation and configured for translating along said axis and simultaneously rotating about said axis; - said second threaded connection (4, 14) is positioned between the connecting element and said fixed guide; and - said linear guide is positioned between the connecting element and the second rotary element in such a way that the second rotary element is rotatably connected to the connecting element for rotating at the same angular speed as the connecting element, characterized in that said linear guide is defined by a recirculating ball connection. - a containment structure; - a first rotary element, connected or connectable to a drive unit to define a mechanical power input unit and rotatable about an axis; - a fixed guide, in particular forming part of said containment structure; - a second rotary element rotatable about said axis and defining a power output unit; - a connecting element extending along said axis of rotation and coupled to said first rotary element by a first threaded connection (3, 8), in particular a lead nut and screw or recirculating ball or roller connection, wherein said connecting element is also coupled with one of either said fixed guide or said second rotary element by means of a second threaded connection (4, 14), in particular a lead nut and screw or recirculating ball or roller connection, and with the other of either said fixed guide or said second rotary element by means of a linear guide parallel to aid axis; and wherein said first threaded connection (3, 8) and second threaded connection (4, 14) have different pitches in such a way as to vary the angular speed between the connecting element and the first rotary element; wherein: - said connecting element is made in the form of a roto-translational element extending along an axis of rotation and configured for translating along said axis and simultaneously rotating about said axis; - said second threaded connection (4, 14) is positioned between the connecting element and said fixed guide; and - said linear guide is positioned between the connecting element and the second rotary element in such a way that the second rotary element is rotatably connected to the connecting element for rotating at the same angular speed as the connecting element, - said connecting element is made in the form of a roto-translational element extending along an axis of rotation and configured for translating along said axis and simultaneously rotating about said axis; - said second threaded connection (4, 14) is positioned between the connecting element and said fixed guide; and - said linear guide is positioned between the connecting element and the second rotary element in such a way that the second rotary element is rotatably connected to the connecting element for rotating at the same angular speed as the connecting element, characterized in that said linear guide is defined by a recirculating ball connection.