Concept of bolt tightening
The purpose of bolted connection of parts is to make the two connected bodies closely fit, and in order to bear a certain dynamic load, it is also necessary to have sufficient pressing force between the two connected bodies to ensure the reliable connection and normal operation of the connected parts. This requires sufficient axial preload (i.e. axial tensile stress) after tightening.
Changes in the amount during bolt tightening:
When the bolt is tightened, the overall stress condition is that the bolt is tensioned and the connector is pressurized. But in the process of stress, the magnitude of stress is different. It can be roughly divided into the following stages:
1. At the beginning of tightening, since the bolt is not against the seat, the pressing force F is zero. However, due to friction, torque T remains at a small value.
2. After the seat (point z), the real tightening starts, and the pressing force F and torque T rise rapidly with the increase of angle A.
3. When the yield point is reached, the bolt begins to plastic deformation, the angle increases greatly, but the compression force and torque increase little or even unchanged.
4. Continue to tighten again, the torque T and pressing force F decrease, and even the bolt breaks.
Bolt tightening method
1. Torque control method: torque control method, which is a control method to stop tightening immediately when the tightening torque reaches a set control value TC. The accuracy of preload is ± 25%.
2. Torque angle control method: first screw the bolt to a small torque, and then screw a specified angle from this point. The accuracy of preload is ± 15%. The advantage is that the difference of friction resistance only affects the starting point of the measured angle. That is, the influence of friction on the axial preload of bolts no longer exists.
3. Yield point control method: the yield point is determined through continuous calculation and judgment of the slope of tightening torque / angle curve. When the slope drops to a certain value (generally defined, when the slope drops to half of the maximum value), it indicates that the yield point has been reached, and the tightening shall be stopped immediately. The accuracy of preload is within ± 8%.
The disadvantage of the yield point tightening method is that it is easy to cause thread shedding. The reason is that: a the tightening stress of the yield point tightening method will be close to the guarantee stress. B the peel strength obtained by using hardened mandrel is higher than that obtained by testing nuts with bolts of corresponding grade.
