Evaluation for Residual Lifetime and Remaining Axial Tension to Loosening Failure on Bolted Joints Used in Long-Life Machine and Plant Equipment

The bolted joints used in mechanical structures loosen owing to, for example, a decrease in the initial axial tension (clamping force) and repeated external forces in the direction perpendicular to the axis. A Junker-type loosening test used to evaluate the performance of various types of locking devices in a short period of time has been established. In such a test, it is possible to compare the loosening performance characteristics of various bolted joints and locking devices. However, it has not been possible to absolutely determine the period of time that they can maintain their initial locking performance. This paper presents a method of estimating the absolute locking performance of bolted joints. The absolute evaluation of self-loosening is indispensable for the prediction of the lifetime of locking devices (parts and methods) in actual machines. From the results of our experimental study, we found a strong linear correlation on logarithmic coordinates between the decreases in axial tension (measured/initial axial tension) and the number of operations (or working time or mileage) since the last tightening. The loosening phenomenon of bolted joints can be observed as the tendency of the initial or residual axial tension to decrease. By utilizing the regression formula at logarithmic coordinates, we show an estimation method for residual axial tension to answer various questions such as “How much of the axial tension remains after tens of thousands of hours (operations)?” Therefore, it is possible to determine to what extent can the bolted joints of long-life machines and plant equipment in medium-to-long-term use be considered to loosening failure. Finally, we describe the judgement criterion for loosening. We examine evaluation criteria such as “Is the residual axial tension sufficient for the prevention of loosening failure?” The limit surface pressure is examined as the upper limit axial tension. As the lower limit axial tension, the allowable value of the external transverse force is examined.