In a cased borehole 1, an extraction pump 2 is driven from the surface by a pump unit 3 by means of a rod set for pumping 4 (according to the Pattent No. 51580, authors Nicolae Drăgotescu, Ioan Paul Popescu, Dan Bodnărescu). The pump rod set 4 is composed of a number of sections a, b, ... n, each section having different diameters.