Research on new method for measuring the pre-tension force of bolts

Abstract

The cable clamp is the key node to connect the main cable and the suspender cable on the suspension bridge. The cable clamp which is divided into left and right halves or upper and lower halves is made of cast steel. In order to make sure the main cable can be confined by the cable clamp effectively, the bolt should retain appropriate pre-tension force when the bridges are under operational conditions. In construction period, several twists are done to make the pre-tension satisfy the design require. And it is inevitable to do additional twist during the operation period according to the statistics. According to relevant statistical data, variation of main cable section caused by creep of zinc coating and becoming small of the thinness of the main cable after pulling are the main causes leading to the decrease of the pre-tension of the bolts. When the additional twist is done with conventional method, manual pump matching with the jack is used. Pull the bolt to the required value, twist the nut using rotation bar with hand, and then unload the pump. After analysis the reason why the error of additional twist force is so big is that oil pump table, rotation circle number and retraction force can’t be controlled properly. Ultrasonic testing is a new testing method in recent years. The ultrasonic sensor placed at one end of the bolt can transmit and receive signals at the same time. According to the reflection theory, when the acoustic wave transmits from the low impedance to the high impedance, reflection will occur. Using this principle, the transmit time can be obtained. Based on the ultrasonic testing technology, a new pre-tension test method is introduced. In the process of the pull off there are two stages. Stage one, before the pressure on the jack reaches to the pre-tension of the bolt, the tension of the bolt is essentially unchanged, and the acoustic transmit time is unchanged theoretically. In stage two, when the pressure on the jack reaches to the pre-tension of the bolt, the bolt will stretch, and the stress in the bolt increases which makes the sound velocity reduces. The joint action of the two aspects increases the sound transmit time in the bolt, and the increase is linear. The core technology in this method is the test technology of pre-tension of bolts. On the basis of conventional additional twists, using high accurate ultrasonic test equipment, the ultrasonic transmit time from one side to the other side on the bolt is recorded. Relationship curve between tension and sound transmit time is recorded when the jack is tensioned hierarchically. Before and after the pressure of jack reaches the pretension of the bolt, the gradient of the relationship curve changes obviously, and then the pre-tension of the bolt can be obtained. Then a new additional twist method is introduced. The method flow is, test, then twist, and then test. According to the current bolt tension value, the bolt is tightened again or unloaded, ensuring the pre-tension reaches the target value. Finally the validity of the new method is verified by a practical engineering example of additional twists on a suspension bridge. In order to verify the validity of the operation process, the target tension and the measured tension are compared in the process of the additional twist of the bolts in a long span suspension bridge. Ten compared data are recorded. There are 2 bolts on which the error of the target tension and the measured tension is less than 1 ton. 4 bolts error are less than 2 tons., etc. Considering the 5 tons error of pressure on the jack, differences between the target force and the measured tension are in the range of 5 tons, so the additional twist procedure is valid.