The interface stress distributions between the unsintered PTFE (Polytetrafluoroethlene) tape and threads of the fittings under an internal pressure are analyzed by using the finite element method (FEM). From the numerical calculations, the following results were obtained: (1) the stress distributes uniformly at the interface except near the edges, (2) the stress becomes singular at the edges of the interfaces and (3) the stress increases as the Young’s modulus of the PTFE tape thickness increases. The experiment of the leakage test of the connections consist of the socket and the plug manufactured of cast iron or stainless steel, which was also carried out by applying an internal oil pressure to the threaded connections. From the experiments, the following results are obtained: (1) the sealing performance in pipe fittings with PTFE tape increased as the number of turns in the PTFE tape increased, and it was not proportional to the numbers of turns, and (2) also as the number of turns in the screw plug increased, and it became a quadratic form, (3) the flow rate of leakage oil from the fittings under the internal pressure decreased as the number of turns in the screw plug increased, and (4) it could be estimated by using the Darcy-Weisbach and the Hagen-Poiseuille equations in the laminar-flow.

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