Abstract: A double-acting hydraulic cylinder unit imparts rocking movement to the walking beam of well pumping apparatus and has either (1) a stationary piston and a generally vertically reciprocating cylinder or (2) a stationary cylinder and a generally vertically reciprocating piston. Upper and lower piston rods of equal size extend from opposite sides of the piston and project through the upper and lower ends, respectively, of the cylinder. In one arrangement the lower end of the lower piston rod is pivotally anchored so as to pivotally absorb both compression and tension forces transmitted through the lower piston rod while the upper piston rod is not anchored and the primary function of the upper piston rod is to provide for equal displacement of fluid on opposite sides of the piston during vertical reciprocation of the piston.

Abstract: An improved hydraulic drive system for a doubleacting hydraulic cylinder which drives or rocks the walking beam of an oil well pump. A closed circuit hydrostatic drive system is used to operate the double-acting hydraulic power cylinder unit and includes a variable displacement reversing swashplate pump driven at a predetermined constant speed by a prime mover such as an electric motor. Suitable automatic control means are provided for synchronizing the reversing movement of the swashplate with the rocking movement of the walking beam.