the initial stage of the tightening process the torque resistance from the screw joint is very low and the rotation speed as well as the air flow through the motor increase rapidly this means that the back pressure from the motor and the feed pressure in the inlet passage 14 is relatively low this relatively low inlet pressure during the initial operation sequence results in a relatively low pressure is being transferred to the actuator chamber 46 via the tube 38 the bore 36 the needle valve 40 41 and the opening 42 this relatively low activating pressure produces a force on activating surface 45 of the valve element 28 that is effective only to displace the valve head 43 just a small distance relative to the seat 26 thereby letting through a restricted air flow only the pneumatic activating force acting on the activating surface 45 is partly balanced by the bias force of the spring 49 however the pressure drop across the valve head 43 generates a force on the activation surface 33 of the valve head 43 which acts in the same direction as the spring 49 this means that during the starting phase of the motor the valve element 28 is displaced a very short distance from its lower end position this results in the valve head 43 in cooperation with the seat 26 forming a small area opening and allows a restricted outlet flow only through the neck portion 25 this initial limited opening area of the exhaust valve results in a substantial reduction of the motor speed during the screw joint running down phase the initial screw joint running down speed would otherwise have been very.