during the suction stroke the piston 8 is moved backward by the pressure in the cushioning circuit and it is now subjected to a work w p u00d7 v contrary to the case during the compression stroke since the volume of the accumulator 15 is selected very great as compared with that of the cylinder chamber 16 the difference in pressure p between the beginning and end of the compression stroke as well as between the beginning and end of the suction stroke is very small fig2 shows an arrangement in which two sets of the pump system shown in fig1 are arranged in parallel and 180 u00b0 apart in phase with respect to each other and the respective cylinder and piston units 12 have their head sides communicated with the hydraulic pressure generator 14 and the accumulator 15 through the flexible pipe 13 this duplex pump arrangement is advantageous in that due to the pump operation mentioned previously the work performed by one of the pump systems on the compression stroke will be compensated by the work to which the other pump system is subjected on the suction stroke with the result that as long as there is no delivery the pump systems will be operated under no load condition substantially only with a power consumption required by the mechanical loss next the operation of the delivery circuit 22 for fixed delivery purposes will be described on the compression stroke the piston 8 attached to the crosshead 4 is moved forward by the rotation.