In general there are three methods available to the operator to determine the net torque loadings on a pumping unit's gearbox. Two dynamic methods determine the instantaneous torque throughout the pumping cycle: method 1) Combines the measured surface dynamometer card and calculated torque factors together with measured or calculated counterbalance moments from the crank and weights and method 2) uses measured motor power with motor and drive efficiencies and the pumping unit speed to determine gearbox torque. Performing a counter balance effect, CBE, test is a direct method of determining net gearbox torque at a specific crank position to estimate the counterbalance moment; this static test is where the cranks and counter weights are held level until no upward or downward movement is noticed when the break is released. Field case studies of applying all three methods to determining gearbox torque are presented in this paper. The pros and cons of using each method are discussed.


The oldest and most common method of artificial lift used for producing oil wells is sucker rod pumping. In the United States and Canada sucker rod lift is used in over 85% of artificial lift wells. The sucker rod lift system is made up of four components 1) prime mover, 2) pumping unit, 3) counterbalance to the rod loading, and 4) sucker rods and associated downhole equipment. The function of the pumping unit is to change the rotating motion of the prime mover into the vertical up and down linear pumping motion at the polished rod. The role of the prime mover is to furnish the necessary power to drive the system.

In a sucker rod pumping system, the polished rod work needed to lift the fluid column is required only during the upstroke. If the sucker rod load on the surface pumping equipment were not counterbalanced, then the total work required from the prime mover would be performed during the upstroke lifting the buoyed sucker rod load and fluid load. During the downstroke the prime mover would not be doing any work, while the force of gravity pulls the rods and pump back down to the bottom of the stroke. Operating in this inefficient manner would require an extremely powerful prime mover and gearbox. For this inefficient system, the uncounterbalanced sucker rod load would determine the torque on the gearbox. To improve the efficiency and to reduce the size of the prime mover and gearbox, plus to load the gearbox more uniformly, the sucker rod pumping system is furnished with some type of counterbalance system, where the counterbalance load effect at the polished rod is approximately equal to the buoyant weight of the rods plus half the weight of the fluid load on the plunger. The effect of the "correct" counterbalance required to equally balance the peak upstroke and down stroke torque loads on the gearbox, when measured as a load at the polished rod, is approximately equal to the weight of the rods floating in fluid plus 1/2 the fluid load.

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