An arm structure intended for controlling the impulse valve (5) of the oscillating cylinder which performs a back and forth movement whereupon the oscillating cylinder comprises a cylinder body (14), a cylinder chamber (1) formed into the mentioned body, a piston (2) which can be moved in the cylinder chamber with the help of a pressure media, feeding channels (15, 16, 17) of the pressure media to the cylinder chamber (1) and control devices in order to direct the pressure media to the cylinder chamber to various sides of the piston and away from the cylinder chamber in order to achieve a back and forth movement for the piston and whereupon the impulse valves (5) belong to the control devices the operation of which at least one arm (10); (12), which explores mechanically the position of the piston arm (7) the movement of which arm is adjusted to open the pressure relieving channel of the impulse valve (5). The contact element which can be moved in relation to the arm, is adjusted to the arm which contact element opens the mentioned pressure relieving channel.