The characteristics of bolted circular flange joints tightened with bolt number N are analyzed using FEM such as the interface stress distributions in initial clamping state, those in operation and the load factor under tensile loadings. Then the effects of the bolt pitch circle diameter D and the bolt number N on the load factor and the interface stress distributions are examined. The FEM results of the load factor and a load when the interfaces starts to separate are fairly coincided with the experimental results. The value of the load factor is the smaller and it is less than 0.1. In addition, it decreases as the value of D increases. Another objective is to demonstrate a new design method and two typical cases are described. One is a problem how to determine the bolt nominal diameter when the bolt strength grade, bolt number N, the interface stress for keeping the joint function and external load are provided and the other is how to determine the bolt strength when the bolt nominal diameter, interface stress and external load are provided. The measured tightening coefficient Q is introduced in the design and the bolt strength is evaluated using the newly introduced equivalent stress taking account the shear stress due to torque in bolt initial tightening. Using the obtained nominal diameter, the bolt maximum stress, the bolt stress amplitude and the critical stress at the bearing surfaces are checked. The availability of the method is shown with the flowchart for design.

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