39 includes double planet wheels first gear component 11 39 is a satellite carrier and second gear component 12 39 is an annular wheel third gear component 13 is a sun wheel as in fig3 the operation of the embodiment in fig4 corresponds to the operation of the embodiment in fig3 referring now to fig5 a control system is shown for retarder 15 of the embodiment of the invention shown in fig3 individual parts of the gear system in fig5 are designated by the same reference numerals as in fig3 working fluid for retarder 15 is connected through a supply line 32 and a return line 33 which are both connected to a switching valve 39 a cooler 35 is connected to switching valve 39 by an inlet line 34 and an outlet line 36 a fluid container 40 is divided by a partition 41 into an upper collecting portion 42 and a lower supply container 43 switching valve 39 is shown in its rest position in which it connects retarder supply line 32 to a venting line 52 and return line 33 to a draining line switching valve 39 also connects cooler inlet line 34 to draining line 54 and shuts off cooler outlet line 36 lines 52 53 and 54 are connected to upper collecting portion.