Petroleum: The Pumping Unit (OG-09)
If sufficient subsurface pressure exists, oil (mixed with gas and sometimes water) will flow from a well unassisted. But in time, as pressure declines with production, oil will cease to flow and must be lifted artificially by a pumping device. The pumping unit commonly consists of a power-driven walking beam, which is connected to a pump at the bottom of the well by a string of steel rods running through the tubing. The up-and-down motion of the walking beam activates the subsurface pump piston and lifts the oil up in the tubing to the surface. The size of the pumping unit is proportionate to the depth of the well and the amount of liquid that must be lifted.

Our Hoosier State Beneath Us: Oil and Gas