Mechanical automatic-box is developed on the basis of discovery «Effect of force adaptation in mechanics». Automatic- box represents the gear differential with two degrees of freedom which has a cogwheels closed contour. Closed contour imposes additional constraint on motion of links and provides definability of motion. Automatic-box has constant engagement of cogwheels. Transfer ratio of the box is independently brought into accord to output loading. Laws of transfer parameters interaction in geometrical and analytical form are presented in the work.