first entrainer element 8c at the lower end of slide rod 8a and a second entrainer element 8d is located at an upper portion of rod 8 as will appear from the following description the mutual axial spacing between both entrainer elements 8c and 8d should be approximately equal to or a little smaller than the stroke of the piston 6 thus in the position shown in fig1 slide member 8 has been displaced upwards to an upper position in control valve 9 due to the fact that abutment element 6c in bore 6a has engaged entrainer element 8d during the final portion of the upward movement of piston 6 control valve 9 which has various inlets and outlets 9a 9f in cooperation with the movement of slide member 8 serves to control drilling fluid flows for the purpose of the repositioning or movement of the valve means as discussed above fig4 shows an actuator cylinder 19 having a transverse orientation with respect to the central longitudinal axis corresponding to bore 6a in piston 6 in fig1 it should be noted that for simplicity fig1 does not show the actuator cylinder 19 and fig4 only shows a portion of control valve 9 fig4 shows a transmission mechanism for converting the linear movements of a through running slide rod 19t in the actuator cylinder 19 to the rotary movement of valve bodies 4a and 4b discussed above this transmission mechanism comprises a chain element 21 engaging two gear wheels 14a and 14b which are keyed to the top of valve body 4a and 4b respectively see fig1 the two ends of chain element 21 are attached each to one end of the actuator slide rod 19t upon establishing a pressure difference.