On the basis of the force analysis of two mobile mechanisms containing a mobile closed contour the theoretical preconditions of creation of self-adjusted mechanisms is developed. The load design of mechanism is executed on the basis of the proved theorem of a derivative on a time from the force moment. Brand new property of self-adjustment is found. Mechanism self-adjustment allows it to be independently conforming to variable external loading by change of speed. Property of self-adjustment allows creating the mechanisms with variable transfer ratio depending on loading. Self-adjusted mechanisms are simple on the device and are working without a control system. Self-adjusted mechanisms (for example, self-adjusted gear boxes) are determining a new highly effective generation of drives of machines with variable technological resistance.