Method and Device for Controlling a Hydraulically Operated Power Wrench

A method for controlling a hydraulically operated power wrench comprising a pivotable lever pivoted by a piston-cylinder unit, comprising a ratchet arrangement for a stepwise rotation of ratchet bushing by advancing strokes and return strokes, and a drive unit with a motor-driven pump for tightening a screw connection;

• the method comprising:
• a) entering a nominal torque or a nominal pre-load or a screw size, material quality, pitch of a thread, and clamping length into an electronic input unit;
• b) determining with an electronic evaluation unit a nominal pressure to be generated by the pump, which nominal pressure corresponds to the nominal torque or the nominal pre-load or the screw size, material quality, thread pitch, and clamping length;
• c) pressure loading of the piston-cylinder unit for performing a pre-stroke until the nominal pressure is reached and immediate reversing to a return stroke upon reaching the nominal pressure by pressure relief and spring pressure or by controlling pressure loading of the piston-cylinder unit to perform a return stroke;
• d) measuring a time interval from beginning of the pressure loading step until the nominal pressure is reached or until an initial pressure after pressure relief is reached;
• e) repeating the steps c) and d);
• f) comparing the measured time intervals and switching off the pressure loading cycles according to the step c) when at least the last measured time interval is shorter than at least the penultimate measured time interval.