Stresses in Bolt and Nut : Effects of Contact Conditions at the First Ridge

Effects of the contact condition at the first ridge on mechanical behaviors of threaded portions were analysed using an axi-symmetric FEM which is appropriate for the three-body elastic contact problem including the effects of friction on two contact surfaces between threads of bolt and nut, and between fastened plate and nut. The cross sectional shape of nut at the first ridge changes circumferentially due to the lead angle of thread, which causes non-symmetric mechanical behaviors concerning the ratio of flank loads and the stress concentration at the root of bolt. Four types of models were used to estimate these non-symmetric behaviors approximately. Each of them represents the cross section rotated one fourth revolution around axis. The analytical members are bolt, nut and fastened plate, and only the shape of nut is changed. The analysis is done by the axi-symmetric FEM without considering the effects of lead angle. The ratio of flank loads and the stress concentration at each cross section show some remarkable effects of the non-symmetric behaviors.