Effects of taper variation on conical threaded connections load distribution

Threaded connections are used in a lot of mechanical and civil engineering applications and, nowadays, are perhaps the most developed and economical way to join two elements made of any kind of material. It is well known that when a bolt is screwed into a threaded hole, the first threads engaged bear more than half of the axial load induced by the make-up torque. This overload in correspondence with the first threads engaged, together with the steep stress gradient induced by the notch effect at the thread root, is the cause of dramatic fatigue failures. Moreover, it is on the first threads engaged, because of the presence of high contact loads on the flanks of the threads, that galling can arise and promote surface damage if the threaded connection has to be screwed and unscrewed many times. The object of this paper is to propose a numerical finite element procedure, confirmed by means of full-scale experimental tests, which makes it possible to quantify the effects induced by varying the taper of rotary shouldered connections (RSCs), in terms of stress state, loads carried by the threads and pressure on the thread flanks. RSCs are conical threaded connections used in the oil industry and the aim of the procedure is to provide the designer with a useful tool able to minimize the dangerous effects of galling and overload.