Pumpjack

Pumpjack : the pivoting beam atop an oil well that translates the rotation of a motor or engine into the up-and-down movement needed to pump oil

Oil wells, topped with pumpjacks, in Signal Hill, Calif.

Well pumps today are most often powered by electrical motors, one motor for each well. In the earlier years of the 20th century, oilfields might share one large engine between several wells, transferring energy with driveshafts or systems of ropes and pulleys, still connected to pumpjacks.

In the aging Southern California oilfields, wellheads often carry signs that warn passers-by that pumps may start suddenly and without warning.

Schematic drawing of power mechanical transmission in an oilfield, before widespread electrification, from HABS/HAER/HALS.