1. The concept of bolt tightening:
The purpose of using bolts to connect parts is to make the two connected parts closely fit, and in order to withstand a certain dynamic load, there is also a need for sufficient compression force between the two connected parts to ensure reliable connection and normal operation of the connected parts. . In this way, sufficient axial preload (that is, axial tensile stress) is required after tightening.
Second, the amount of change in the process of bolt tightening:
When the bolts are tightened, the overall force situation is that the bolts are in tension and the joints are in compression. But in the process of stress, the magnitude of the force is different. It is roughly divided into the following stages:
1. At the beginning of tightening, because the bolts are not seated, the pressing force F is zero. However, due to the existence of friction, the torque T remains at a small value.
2. When the seat is seated (point Z), the real tightening begins, and the pressing force F and torque T rise rapidly with the increase of the rotation angle A.
3. When the yield point is reached, the bolt begins to deform plastically, the rotation angle increases greatly, but the compression force and torque increase little or even remain unchanged.
4. Continue to tighten, the torque T and the pressing force F will drop, and even the bolt will break.
3. Bolt tightening method:
1. Torque control method: Torque control method, which is a control method that stops tightening immediately when the tightening torque reaches a certain set control value Tc. Accuracy of preload is ±25%.
2. Torque rotation angle control method: firstly screw the bolt to a small torque, and then from this point, screw a specified rotation angle control method. Accuracy of preload is ±15%. The advantage is that the difference in frictional resistance only affects the starting point of the measured rotation angle. That is, the influence of frictional force on the axial pretightening force of the bolt no longer exists.
3. Yield point control method: the yield point is determined through continuous calculation and judgment of the slope of the tightening torque/rotation angle curve. When the slope drops to a certain value (generally defined, when the slope drops to one-half of the maximum value), it means that the yield point has been reached, and the tightening is stopped immediately. The accuracy of preload is within ±8%.
The disadvantage of the yield point tightening method is that it is easy to cause the thread to come off. The reason is: a. The yield point tightening method, the tightening stress will be close to the guaranteed stress. b The pull-out strength obtained by testing the nut with a hardened mandrel is higher than that of the corresponding grade of bolt.
