接触剛性によるボルト締結モデルと透過超音波パルスを用いた定量評価

Evaluation of bolt tightening plays an important role to keep a performance of a mechanical structure. Lack of the fastening force induces slack and falling of the tightening bolt. Excessive fastening force induces a plastic deformation of screw thread and causes the decline of the fastening force. Therefore, guarantee of the bolt tightening is an important subject in order to secure the mechanical structure against unforeseen accidents. However, most of preceding studies are restricted within qualitative evaluation of the slackness. In the field of industrial manufacturing, quantitative evaluation of the bolt tightening is strongly required. In this paper, the authors propose a technique for quantitative evaluation of bolt tightening, and in this technique, the flank of the screw thread is assumed to contact through a contact stiffness. The transmissivity and reflectivity of an ultrasonic pulse wave through the flank are derived as a function of contact stiffness, and the contact stiffness is capable of being identified from experimentally acquired amplitude of ultrasonic pulse wave. The contact stiffness is quantitatively identified and the surface pressure on the screw thread is estimated by the identified contact stiffness through the results of the preceding study. The proposed technique treats an ultrasonic pulse perpendicularly transmits a fastening bolt, and is applied to an experimentally acquired ultrasonic pulse which enters a side surface of nut and transmits a bolt-nut fastening element. The identified contact stiffness is shown to be adequate for the given fastening force to the bolt-nut fastening element, and validity of the proposed technique is demonstrated.