a mud pump is provided that includes a pump shaft having substantially circular eccentric lobes . each lobe is rotatably mounted in a connecting rod that , in turn , moves a slide in a horizontal and linearly manner . a piston rod is operatively couples a pump module to one or both sides of each slide frame and a pump module . as the pump shaft turns , each lobe causes the slide to move side to side . as the slide moves side to side , each piston rod operates a pump module whose outputs can be coupled to a common manifold .