Stress process and tightening method of high strength bolt

The stress process and tightening method of high strength bolt the purpose of bolt connection of parts is to make the two connectors fit closely. In order to bear a certain dynamic load, enough pressing force should be provided between the two connectors to ensure the reliable connection and normal operation of the connector. In this way, there must be enough axial preload (i.e. axial tensile stress) after tightening.

Quantity change during bolt tightening:

When tightening the bolt, the total stress condition is that the bolt is in the tensile state and the connector is in the compression state. But in the process of stress, the force is different. It can be roughly divided into the following stages:

1 at the beginning of tightening, the tightening force F is zero because the bolts are not seated. However, the torque T remains small due to friction.

2 when it is close to the seat (Z point), the true tightening begins. With the increase of angle a, the pressure F and torque t increase rapidly.

When the yield point is reached, the bolt begins to plastic deformation and the angle increases greatly, but the compression force and torque almost do not increase or even change.

4. After further tightening, the torque T and pressure f decrease, even the bolt breaks.

Bolt fastening method

1 torque control method: torque control method, when the tightening torque reaches the set control value TC, immediately stop tightening. The preloading accuracy is ± 25%.

2 torque angle control method: first tighten the high strength bolt to small torque, and then from this point on, use the specified angle control method. The preloading accuracy is ± 15%. Its advantage is that the difference of friction resistance only affects the starting point of the measured angle. That is to say, the influence of friction on the axial preload of bolts no longer exists.

3. Yield point control method: the yield point is determined by continuously calculating and judging the slope of tightening torque / angle curve. When the slope drops to a certain value (usually defined as half of the maximum), the yield point is reached and the tightening stops immediately. The preloading accuracy is within ± 8%.

The disadvantage of yield point tightening method is that the thread is easy to fall off. The reason is: using the yield point tightening method, the tightening stress will be close to the guarantee stress. B. The results show that the debonding strength measured by hardened mandrel is higher than that measured by corresponding grade bolts.

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