Normally, in the tightening process of Full Threaded Stud, the torque actually converted into bolt clamping force accounts for only 10%, the remaining 50% is used to overcome the friction under the bolt head, and 40% is used to overcome the friction in the thread pair Force, this is the "541" rule, which mainly reflects the relationship between clamping force and friction. However, if certain improvement measures are applied (such as lubricating oil) or there are defects in the thread pair (such as impurities, bumps, etc.), the proportional relationship will be affected and changed in different ways.
Changes in volume during bolt tightening:
When the bolt is tightened, the overall force is that the bolt is pulled and the connecting piece is compressed. But in the process of force, the magnitude of force is different. It is roughly divided into the following stages:
1. At the beginning of tightening, the pressing force F is zero because the bolt is not seated. However, due to the friction, the torque T is kept at a small value.
2. When the seat is back (point Z), the real tightening starts, and the pressing force F and torque T rise rapidly with the increase of the angle A.
3. When the yield point is reached, the bolt begins to plastically deform, the rotation angle increases greatly, but the compression force and torque increase little or even unchanged.
4. If the tightening is continued, the torque T and the pressing force F will drop, and even the bolt will break.
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. Preload accuracy is ±25%.
2. Torque angle control method: firstly screw the bolt to a small torque, and then from this point, screw a prescribed angle of control method. The accuracy of the 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 friction on the axial preload of the bolt no longer exists.
3. Yield point control method: The yield point is determined by 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 indicates that the yield point has been reached, and immediately stop tightening. The accuracy of the preload is within ±8%.
The disadvantage of the yield point tightening method of Stud Manufacturer is that it is easy to cause thread breakage. The reason is: a. The yield point tightening method, the tightening stress will be close to the guaranteed stress. b Using a hardened mandrel to test the nut with a higher breaking strength than the corresponding grade of bolts.