The Stress Process And Tightening Method Of Bolt

The purpose of the bolt connection of parts is to make the two connected bodies closely fit, and bear a certain dynamic load but also needs to have enough compression force between the two connected bodies, to ensure the reliable connection and normal work of the connected parts. In this way, sufficient axial preload (axial tensile stress) is required after tightening the bolt.

Force change during bolt tightening

When the bolt is tightened, the overall force situation is that the bolt is under tension and the connector is under pressure. But in the process of force, the force is different. It can be divided into the following stages.

Force change during bolt tightening

At the beginning of tightening, because the bolt is not seated, the compression force F is zero. But because of friction, the torque T is kept at a small value.

When the bolt is reseated (Z point), the real tightening begins. The compression force F and torque T rise rapidly with the increase of Angle A.

When the yield point is reached, the bolt begins to deform plastic, the Angle increases greatly while the compression force and torque increase little or even unchanged.

After further tightening, the torque T and the compression force F decrease, and even the bolt breaks.

Method of tightening bolts

Torque control method: It is a control method to stop tightening immediately when the tightening torque reaches a certain set control value Tc. The accuracy of preload is ±25%.

Torque Angle control method: First screw the bolt to a small torque, and then screw a specified Angle control method from this point. Preload accuracy ±15%. The advantage is that the difference in friction resistance only affects the starting point of the Angle measurement. That is, the influence of friction force on the axial preload of the bolt no longer exists.

Yield point control method: it is through the continuous calculation and judgment of tightening torque/Angle curve slope to determine the yield point. When the slope drops to a certain value (generally defined as when the slope drops to half of the maximum value), it indicates that the yield point has been reached and tightening is stopped immediately. Preload accuracy within ±8%.The disadvantage of the yield point tightening method is that it is easy to cause thread blurting. The reasons are as follows: On the one hand, in the yield point tightening method, the tightening stress will be close to the guaranteed stress. On the other hand, a nut tested with a hardened mandrel has higher blurt strength than a nut tested with a bolt of the corresponding grade.

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