Development of Testing Equipment for Tightening Characteristics Evaluation

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Tightening experiments for evaluating frictional characteristics between bearing surfaces and mating threads are, the only way to maintain the reliability of bolted joints. However, the conventional testing machines have some problems in terms of the accuracy, applicability and rigidity.This study aims to develop the new equipment having the possibility to resolve such problems. The piezo electric force sensors to be used for the equipment have two advantages that the rigidity of them is about 7 times larger than conventional load cell, and that the accuracy and the linearity are very high and are independent of the range to be measured. On the other hand, the characteristics of the sensor eccentrically placed and preloaded should be clarified. Experiments and analytical investigations using FEM have shown that the pressure distribution on the sensor surface affects the sensitivity value but that the linearity remains its ideal value, and it is concluded that the axial force sensor can be used as the load cell for this equipment.As for the evaluation for the torque measurement, the investigations were done for the torque cell construction with four shear force sensors. As a result, it was clarified that the cross talk of the axial force due to the errors generated in both manufacturing and installing stages affects the virtual sensitivity of shear force sensors. However, such a cross talk can be controlled to be proportional to the axial force, and can be compensated by calculation.Finally, the possibility to perform the tightening experiment with sufficient accuracy by using the newly designed load cell unit with 12 sensors was confirmed.