Introduction

The sucker-rod string is the most vital part of the pumping system, since it provides the link between the surface pumping unit and the subsurface pump. It is a peculiar piece of mechanical equipment and has almost no analogies in man-made structures, being several thousand feet long and having a maximum diameter of slightly more than one inch. The behavior of this perfect "slender bar" can have a fundamental impact on the efficiency of fluid lifting and its eventual failure leads to a total loss of production. Therefore, a properly designed rod string not only assures good operating conditions but can considerably reduce total production costs as well.

The rod string is composed of individual sucker rods that are connected to each other until the required pumping depth is reached. Early pumping installations used wooden poles, usually made of hickory, which had steel end fittings to facilitate joining of the rods. As average well depths increased and greater rod strength was required, the all-steel sucker rods appeared around the turn of the last century. These are solid steel bars with forged upset ends to accommodate male or female threads a design that has not changed since then. The most important improvements in sucker-rod manufacturing methods through the years were the application of heat treating to improve corrosion resistance, better pin constructions, and the use of rolling instead of cutting for making the necessary threads. Steel rods different from the solid type were also made available, such as...