Evaluation of Mechanical Behavior of Taper Pipe Threads in the Tightening Process by Finite Element Analysis and Elementary Theory of Solid Mechanics

There are two types of pipe threads, i.e., parallel and tapered ones. The former is used for mechanically connecting hollow cylinder-shaped structures, and the latter is usually employed for connecting thin pipes and tubes. The primary function required for taper pipe threads is to prevent the leakage of contained fluids. In order to ensure the sealing performance, target taper pipe threads need to be tightened with proper conditions. However, it seems that a standard tightening guideline with sufficient mechanical background has not been established. In this paper, using helical thread models, the relationship between assembly torque and rotation angle of threaded pipe is studied by FEA. The relationship between rotation angle and radial contact force between male and female threads, which is regarded as an index of the sealing performance, is also evaluated in like manner. In the numerical calculations, finite element analyses are performed as elastic and elastic-plastic problems, in which nominal dimeter of threads, pipe wall thickness and coefficient of friction on the thread contact surface are changed systematically, aiming at the establishment of a practical tightening guideline. Additionally, a simple method is proposed to evaluate the contact force between male and female threads, using elementary theory of solid mechanics. It is shown that the simple method can predict the contact force with sufficient accuracy, comparing to the calculation results by FEA.