Larry Angelo, J.M. Huber Corporation: Flow Control Division
Most polished rods break at the bottom of the polished rod clamp. Almost all of these breaks are fatigue failures. A study of polished rod fatigue failures entitled How to Minimize Polished Rod Breaks (Ref. 1) was presented at the 41st Annual Southwestern Petroleum Short Course in April, 1994. The study concluded that bending stresses amplified by stress concentrations from polished rod clamps are the most significant contributors to polished rod fatigue failures. The study also pointed out a noticeable absence of information on stress concentrations caused by various polished rod clamps. This paper picks up where the above study left off. Recent experimental results are reported on the effects of stress concentrations generated by polished rod clamps on the fatigue strength of polished rods. The major objective of the investigation was to determine and to compare polished rod endurance limits resulting from indention and friction style clamps. This objective was achieved by conducting fatigue tests on polished rods using both clamp designs. Test results are presented in the form of stress-cycle curves which were used to determine the endurance limits for polished rods equipped with indention and friction style clamps.