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Pumpjacks do have some interesting design features. Everyone knows about the counterweight lever action, but it also has a neat check valve system in the well itself. Think of a toilet reservoir stopper but with another in the tank with a reciprocating piston up and down the height of the tank and a side pipe. The downward action causes the bottom plunger to close by the hydraulic force of the fluid and prevents flow back into the well or through the side channel.

Likewise on the upstroke, the suction action opens the top and bottom valve and sucks the fluid upwards like in a drinking straw until it reaches the top of the stroke, then pushes the fluid through the side channel on the next downstroke which then connects farther up the pipe, which then goes up again on the upstroke. This is all done by hydraulic pressure in the well and has no moving machinery (other than the motor aboveground) and only needs periodic O-ring replacement.

Thus, it is acting as a positive displacement pump (like a heart beating in a sense but less complicated).