A fully rotating rotary actuator of a felling head, comprising: • an axle to be suspended from a boom, • a frame supported co-axially by means of bearings (5, 15) from the axle • a first, driven gear arranged in the axle, • a second, driver gear and a motor that rotates it installed in the frame for driving said first gear and for rotating the frame with respect to the axle, • rotatable hydraulic oil distribution means between the axle and the frame and first couplings (21', 22', 23') connected thereto on the side of the axle and second couplings (211, 221, 231) on the side of the frame, wherein the rotary actuator comprises at least one additional conduit in the axle for an additive fluid line or a tube for additive fluid arranged in the additional conduit and a rotatable connector having a first part and a second part, and wherein the lower end of the additional fluid line conduit or tube is connected to the first part of the rotatable connector, and the second rotating part of which is connected to a second tube fitted in the frame for conducting the additive fluid for application, and wherein the axle comprises one conduit for conducting an electrical cable through the axle in an axial direction and wherein, at the bottom end of the axle between the axle and the frame, there is a rotatable electricity distributor connected to a co-axially arranged electrical cable, the electricity distributor comprising parts that rotate with respect to one another, namely an outer ring and an inner ring, wherein one part is fastened to the axle and the other part to the frame while leaving a gap in the middle of the inner ring, and in which the additive fluid line tube (201, 2013) is conducted out of the axle via the gap in the inner ring of said electricity distributor. • an axle to be suspended from a boom, • a frame supported co-axially by means of bearings (5, 15) from the axle • a first, driven gear arranged in the axle, • a second, driver gear and a motor that rotates it installed in the frame for driving said first gear and for rotating the frame with respect to the axle, • rotatable hydraulic oil distribution means between the axle and the frame and first couplings (21', 22', 23') connected thereto on the side of the axle and second couplings (211, 221, 231) on the side of the frame, wherein the rotary actuator comprises at least one additional conduit in the axle for an additive fluid line or a tube for additive fluid arranged in the additional conduit and a rotatable connector having a first part and a second part, and wherein the lower end of the additional fluid line conduit or tube is connected to the first part of the rotatable connector, and the second rotating part of which is connected to a second tube fitted in the frame for conducting the additive fluid for application, and wherein the axle comprises one conduit for conducting an electrical cable through the axle in an axial direction and wherein, at the bottom end of the axle between the axle and the frame, there is a rotatable electricity distributor connected to a co-axially arranged electrical cable, the electricity distributor comprising parts that rotate with respect to one another, namely an outer ring and an inner ring, wherein one part is fastened to the axle and the other part to the frame while leaving a gap in the middle of the inner ring, and in which the additive fluid line tube (201, 2013) is conducted out of the axle via the gap in the inner ring of said electricity distributor. wherein the rotary actuator comprises at least one additional conduit in the axle for an additive fluid line or a tube for additive fluid arranged in the additional conduit and a rotatable connector having a first part and a second part, and wherein the lower end of the additional fluid line conduit or tube is connected to the first part of the rotatable connector, and the second rotating part of which is connected to a second tube fitted in the frame for conducting the additive fluid for application, and wherein the axle comprises one conduit for conducting an electrical cable through the axle in an axial direction and wherein, at the bottom end of the axle between the axle and the frame, there is a rotatable electricity distributor connected to a co-axially arranged electrical cable, the electricity distributor comprising parts that rotate with respect to one another, namely an outer ring and an inner ring, wherein one part is fastened to the axle and the other part to the frame while leaving a gap in the middle of the inner ring, and in which the additive fluid line tube (201, 2013) is conducted out of the axle via the gap in the inner ring of said electricity distributor.