In this study, a coefficient of impulsive friction μi is measured on the contact surfaces, which have the same surface finishes as screw thread surfaces, and the results are applied to clarify the loosening mechanism of screw threads by repeated impacts. Since μi decreases with the increase of normal force, the following facts are newly found concerning the loosening by repeated impacts. (1) The relationship between impact force and the critical axial force of loosening (called critical diagram for loosening) shows a closed curve. (2) When the friction on the bearing surface of nut is not lost, screw thread is the hardest to loosen, and when the friction of the bearing surfaces are lost, screw threads are harder to loosen as its spring constant is smaller. (3) For the forecast of loosening of screw threads by repeated impacts, it is necessary to use μi measured taking account of the hysteresis of screw thread surfaces. (4) It can be said that the fact impact shearing stress has the maximum value in the case of impulsive friction is verified by this study.
