The Reduction of Stress Concentration by Tapering Threads

Abstract

For bolted joints failures usually happen at the root of the first bolt thread where the maximum stress concentration occurs. Previously several methods were proposed to reduce the stress concentration through improving bolts and nuts profile. Among those methods, tapering threads have been widely used in mechanical structures. Those types of threads are called CD bolts, which were proposed by Nishida as an effective method for stress reduction. Several experimental studies indicated that CD bolts have higher fatigue strength although little FEM analyses are available. In this study, therefore, the bolted joints with tapered threads are analyzed with the finite element method, and stress reduction effect of CD bolts is discussed with varying geometrical conditions. The reduction of the stress concentration is realized by tapering bolt threads. Then, it is found that the stress reduction is notable when the height of bolt threads is reduced significantly near the bolt head and the nut is closer to the bolt head. According to those results, optimum conditions for stress reduction are discussed. Then, it is shown that the maximum stress can be decreased by 20% compared with the cases of standard bolts and nuts.