Estimation method for axial tension of various bolted joints using hydraulic tensioner

Abstract

In this paper, a new method for estimating the axial tension of each bolt after the hydraulic tightening, and an appropriate structure for the hydraulic tightening have been proposed. A hydraulic tensioner is frequently used to tighten the pressure vessels by the bolted joints. The hydraulic tensioner can apply higher value and lower variation of the axial tension than the torque tightening. However, the axial tension remaining in the bolt after the hydraulic tightening is less than the initial tension by the hydraulic load, and the ratio of the residual tension to the initial tension, the effective tensile coefficient, is the most critical factor to tighten the pressure vessel securely. Some estimation methods of the ratio considering the stiffness of tightening parts have been proposed. However, there are no previous studies that verified the theory, the characteristics and the appropriate structure for the hydraulic tightening using various bolts, such as a stud bolt, a through bolt and a two-nuts bolt. In this study, we developed some new equations for estimating the effective tensile coefficient of each bolt by considering the compliance at each part of the bolted joint. Then the analysis method of the hydraulic tightening for each bolt was developed by using a finite element model simulated the joint process with the hydraulic tensioner. In addition, we evaluated the effective tensile coefficient of each bolt by the analysis and considered improvement measures of that. As a result, it was clarified that the analysis method and the equations could estimate accurately the effective tensile coefficient and the coefficient of the two-nuts bolt was the highest of the three bolt types. It was confirmed that the coefficient could be improved by locating the foot close to the bolt and reducing the diameter of the bolt-hole.