positive displacement pumps displace a specific volume of fluid on each stroke or rotation. positive displacement pumps can be classified as rotary or reciprocating. rotary pumps displace fluids by means of rotating screws, gears, vanes, or lobes. reciprocating pumps move fluids by drawing them into a chamber on the intake stroke and displacing them by means of a piston, diaphragm, or plunger on the discharge stroke.