Experiment on detection of preload force in bolt clamp by pulling method
In order to reveal the causes of errors in the detection of screw preload force by the pull-out method and to improve measurement accuracy,the cable clamps of the Longmen Bridge were taken as the example.This study combined theoretical analysis with model experiments to study the mechanism of the pull-out method.A mechanical model of the effective preload force of the screw during two-stage by pull-out method was constructed.The M27 screw model was made indoors,and pressure sensors were used to detect the preload force of the nuts at both ends of the screw,the change law of the preload force during the tensioning process was analyzed,which verified the theoretical model.The MJ45 and MJ56 screw of the cable clamp of the Longmen Bridge was used,and the pull-out ultrasonic method for preload force was studied,the relationship between the ultrasonic time difference,the effective preload force,elongation and tension force of the screw was analyzed under different preload force conditions.The results show that the preload force of the screw is not constant but increases gradually with the increase of the tension force.When the nut is loosening,the tension force is usually 10％to 20％higher than the initial effective preload force,and the pull-out ultrasonic method can accurately capture the critical moment of nut loosening.This study proposes a detection reduction factor of 1.1 to 1.2 to address the errors that may occur during the detection process.The research results can provide a basis for the detection of screw preload force in actual engineering.1 tab,13 figs,21 refs.
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