into the hollow space 232 with variable operating volume located between the piston 228 of the hydraulic cylinder 208 and its bottom 230 a second branch line 226 2 is interrupted by a safety relief valve 234 and terminates at the constant displacement pump 220 a safety pressure limiting valve 236 is located in parallel with the series connected constant displacement pump 220 and safety relief valve 234 and thus connects the branch line 226 2 to the tank 186 through a line terminating above the liquid level of said tank a bypass check valve 238 interrupts a bypass line 240 which branches off the supply line 226 between the hydraulic cylinder 208 and the pressure regulating valve 224 on the one hand and between the latter and the fork 242 in the supply line 226 thereby bypassing the pressure regulating valve 224 as shown symbolically in fig3 the feeler roller 216 works against a variable restoring force acting upon the pressure limiting valve 218 the pressure regulating valve 224 undergoes pressure relief through the pressure limiting valve 218 acting as a preliminary control valve the principle of operation of the embodiment described is as follows at the start of the forward stroke of the two upsetting dies 30 the two first knee levers 40 are flexed so that their two legs 38 and 42 assume the position indicated in fig1 on the right by dash dotted lines for actuation of the knee joints 49 the positive cam mechanism 58 now causes the very rapid extension.