Do you know the theory of operation?
Well, The prime mover rotates the pinion shaft via drive belt.
The pinion shaft has mesh gear with crankshaft which transfer motion between them.
The rotation of crankshaft causes angular movement of the three connecting rods in harmonic consecutive way.
The connecting rods are pinned to something called crosshead.
Crosshead convert angular movement from connecting rods into linear movement, and transfer this motion to piston assembly (Pony rod and piston Rod)
Piston assembly move linearly inside a liner to pushes the drilling fluid to continue their path into circulation system.