Evaluation of Various Methods to Inspect Bolted Joint Integrity by Torque Measurement

Abstract

The integrity of bolted joints is often checked after a certain period of running time. These checks are designed to examine whether the target bolt has still properly been tightened with prescribed bolt force staying within acceptable ranges. Three types of inspection methods are commonly used for verification, i.e., the release torque method, the re-tightening torque method and the marking method. Depending on the type of the method, inspections are carried out at the site by either measuring loosening torque, re-tightening torque or tightening torque up to the marked position. In this study, aiming at establishing general guidelines for the inspection methods, characteristics of these methods are examined using rigorous equations that represent the relationship between applied torque and generated bolt force. Torque ratios anticipated to be used as a guideline for each inspection method are comprehensively calculated by varying friction coefficients, nominal diameter and thread pitch. Coefficient of friction values, which affect inspection accuracy, are measured by means of tightening equipment which has been developed to measure relevant values, such as torque and bolt force, with high accuracy.