the piston 7 divides the cylinder 8 into working chambers 10 and 11 the working chamber 10 is provided with a port a 1 which is connected with the port a of a control valve 13 through conduit 12 the working chamber 11 of the servo motor 9 is connected with port b of the control valve 13 by port b 1 and a conduit 14 the control valve 13 is provided with two additional ports p and t port p is connected with a fluid pressure source 15 not shown in detail an outlet conduit is connected with the port t the control valve has a casing 16 not shown in detail which is fixedly connected to the power output member 3 in a bore 17 of the casing 16 is a reciprocatable control piston 18 which is integrally connected to the power input member 1 by means of operating rod 19 the control piston 18 blocks in a centered position the port p or connects the ports a and b each with the port t in a one switch position the connection from port p to the port b is opened while the connection between the ports a and t are further opened finally in another switch position the port p is connected in a reverse manner with port a while the connection between the ports b and t is further opened the control piston 18 is provided with opposed reaction areas as indicated at 34 a 34 reaction chambers are formed in bore 17 adjacent the reaction areas 34 a 34 and are indicated at 20 and 21 the reaction chamber 20 is connected to conduit 12 through a control line 22 and reaction.