Fatigue Of Threaded Sucker-Rod Couplings

Abstract Accelerated laboratory tests on the fatigue characteristics of sucker-rod couplings were carried out on a closed-loop hydraulic testing system. The purpose was to evaluate a number of different thread designs currently being used in the manufacture of sucker-rod couplings. The thread forms tested included the simple cut, a keyhole, cut and burnished, cut and cold worked and fully cold worked. The results show that the fatigue life increases as the degree of cold-working increases. It is also seen that the effects of accelerating the testing are insignificant. Introduction The failure of sucker-rod joints is of considerable interest to those involved in petroleum production. To eliminate or minimize such failures many attempts have been made both, to producing new rod and coupling designs, and to eliminating the coupling altogether. The literature shows a number of papers dealing with the subject of joint failures, specifically related to improving the pin connection. There is apparently very little written on the coupling itself yet there is a multitude of coupling designs commercially available for use in joining sucker rods. The question arises as to which of these designs is the best. Thus, to aid in the design and selection of a coupling, the purpose of this paper is to evaluate the fatigue life of a number of purpose of this paper is to evaluate the fatigue life of a number of different thread designs which are being used in the manufacture of sucker- rod couplings. Coupling design has, for the most part, been consistent with the pin design to which it must mate. Thus, coupling design has evolved from the simple cut thread, to a keyhole, burnished, cut and cold-worked and recently, to the fully cold-worked or rolled thread. To the consumer the coupling represents but a part of the cost of a rod. However, to the manufacturer, not all couplings can be made with the same ease and hence, same cost.