Device wherein the co-operation of a rotatable shaft and the displacement of a hydraulic medium forms a variable transmission unit or a pump, said device comprising an ingoing (driving) shaft having a widened part/rotor and an outgoing (driven) shaft co-axial with the ingoing shaft, the widened part of the ingoing shaft being provided with a number of cylindrical bores, substantially parallel to its center line, each bore containing a plunger which can reciprocate in said bore, said outgoing shaft being provided with a plate (mirror plate) obliquely arranged under an angle .alpha. with respect to the axis of the shaft for co-operating with protruding end(s) of the plunger(s) such that upon rotation of the rotor or the outgoing shaft the plunger(s) reciprocate in its (their) bores, each bore presenting in its wall a radially directed channel to establish a connection between the bore(s) and a circuit containing a hydraulic medium.