The Absolute Evaluation and Loosening Life Prediction Method on Self-Loosening of Bolted Joints During Actual Machine Operation

Abstract

This paper presents a method for estimating the absolute lock effect in bolted joints during actual machine operation. The absolute evaluation on self-loosening is indispensable for the prediction of the life of locking devices (parts)/method at actual machine operation. The loosening phenomenon of bolted joint can be viewed as the decrease tendency of the initial axial tension or residual axial tension. According to the author’s previous report, there is a good linear relation on logarithmic coordinates (log-log paper) between the decrease of axial tension (measured tension/initial axial tension) and number of operations (or working time or mileage) since last tightening. Based on this relation, and observation of loosening in the early stages of product development, we estimate the rate of axial tension decrease (degree of self-loosening) accurately after prolonged operation since tightening by measuring the initial axial tension behavior by using the proposed regression formula. The trial calculation of the residual axial tension after prolonged operation is made using the industrial vehicle (stacking capacity 16-ton large-sized forklift-truck) at customer’s site by loosening measurement. According to the estimation using only a few day’s data, it is presumed that initial axial tension is kept 55% or more in case of anaerobic adhesive tightening and almost 30% in case of lubrication tightening after ten years. In the working load type loosening, authors propose the L-N diagram (Loosening Life - Numbers of Cycles to Loosening N Diagram) for loosening life estimation like the S-N diagram for fatigue life estimation. Using the loosening frequency diagram and L-N diagram, the degree of loosening damage for every axial tension’s residual ratio is figured out to analyze the loosening life of the locking device/method by Miner’s rule and cumulative loosening damage models.