The loading and unloading of the bolt can be performed by simultaneous stretching, and all the bolts can be loaded and stretched, and the stepwise and stepwise stretching can also be adopted. When the bolt is loaded and stretched into two stages, the load distribution is roughly: the first stage loading is 60% to 70% of the total load; the second stage loading is 100% of the total load. When the bolt is loaded and stretched 3 At the stage, the load distribution is roughly: the first stage loading is about 60% of the total load; the second stage loading is 80% of the total load; the third stage loading is 100% of the total load.
When stepwise stretching is adopted, the order of bolt stretching should be arranged symmetrically and reasonably, and the influence of the rear tension bolt on the first stretching bolt should be reduced, and the operation should be simple and convenient.
The bolts shall be measured before and after the bolt is loaded and stretched. The difference between the measured values ​​before and after the stretching is the net elongation of the bolt. Before the measurement, the measuring rod must be placed into the center hole of the bolt a few hours earlier, otherwise the thermal expansion and contraction of the measuring rod will affect the accuracy of the measurement results. And when the bolt is measured twice before and after the bolt is stretched, the dial gauge should be placed in the same orientation of the bolt. When the elongation is out of tolerance, the bolt with excessive elongation should be subjected to another loading and stretching operation until it is qualified.
The large-diameter bolts are stretched and pre-tensioned by hydraulic stretching device, which not only ensures sufficient pre-tightening force, but also precisely controls the pre-tightening force, so that the pre-tightening of the bolts is safe and reliable. Practice has shown that the pre-tightening of the bolt by the hydraulic stretching method can ensure the quality of the bolt connection and the reliability of the container seal, and there is no frictional strain on the contact surface. The hydraulic tension pre-tightening method has been widely used in production practice, and its scientificity and feasibility have been tested in practice.
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