40 with which the cam rollers 30 are to interact during the rotational driving of the drum 10 is made up of a number of curved cam curve segments 41 which are fixed to the drum 10 by means of screws 54 the joint between the cam segments 41 can be made as shown in fig4 where a wedge shaped end portion 56 engages in a corresponding wedge shaped cavity 58 in the opposite end portion of the adjacent cam segment which provides a smooth transition as regards surface pressure for the cam rollers 30 when passing the joint between two adjacent cam segments 41 the wave shaped cam profile 38 of each cam segment 41 has in the embodiment shown three lobes and the joint between the curve segments 41 is placed in a valley portion between the cam segments each cam segment can also consist of individual lobe units which are welded together into a ring the operating principle of the hydraulic drive system according to the invention corresponds to the operating principle for a known so called 34 four piston machine 34 which means that the number of pistons is evenly divisible by four while the number of cam tops is evenly divisible by three in the embodiment shown in fig1 the cam ring 40 is composed of nine cam curve segments 41 each with three cam tops i e a total of twentyseven while the number of piston units 16 is four times four i e sixteen the piston units 16 are arranged in a manner known per se to cooperate with the cam curve 38 of the cam ring 40 so that at synchronized phased strokes of the pistons 20 and the associated cam rollers 30 in the different groups of piston units 14 there.