Pump jack device

A pump jack comprising a double acting equal displacement, through rod type piston and cylinder motor;

• means for connecting a piston rod of said motor to a polished rod in a well head;
• said motor being arranged to be mounted above said well head and coaxial with said polished rod;
• a variable displacement type hydraulic pump having a pair of output ports;
• a pair of hydraulic conduits each placing one of said output ports in communication with an opposite end of said motor and forming a closed hydraulic loop with said motor and pump;
• drive means for said pump;
• a pump control means for controlling the direction and volume of flow in said loop to thereby determine the length and position of the stroke and the velocity of said piston rod;
• a compressible fluid counterbalance means for accumulation of energy during a down stroke of said piston rod and returning said energy to said piston rod during an up stroke of said piston rod;
• said counterbalance means including a cylinder having a piston with means connecting the piston thereof to the piston of said motor for drive therewith, said cylinder and piston forming a chamber containing a gaseous charge disposed to be compressed by the piston of said counterbalance means during the downward stroke of said polished rod, said counterbalance means including an additional closed chamber means in communication with said chamber formed by said cylinder and piston, and the cylinder of said counterbalance means being coaxially mounted with said motor, said piston of said counterbalance means being directly connected to a second piston rod formed by the through rod of said motor.