The paper presents an assessment of the functional qualities of self-tightening nuts developed in «Agropromimpeks-2000» Trade House Ltd. The goal of the article is in studying the influence of the design parameters of a nut on its functional properties. The main functional property of this nut is self-retardation in the absence of axial forces. This property is provided because the nut together with the screw bolt represents a previously strained couple in which permanent preliminary loading is provided with guaranteed tightness. The second functional property of the nut is its ability to self-tighten under loading in case this loading decreases. The self-tightening conditions are revealed, which are force and deformation; suggestions for improving the quality of self-tightening, and also connection durability are given (for bolts and self-tightening nuts). The studied self-tightening nut is recognized as promising both for mainline and industrial tracks. We have identified the future directions of studying the self-tightening nuts. The nut could be improved by reducing its weight and increasing its strength and durability.