Stress process and tightening method of high-strength bolts

Stress process and tightening method of high-strength bolts

The concept of high-strength bolt tightening

The purpose of bolted connection of parts is to ensure a tight fit between the two connecting bodies. In order to withstand a certain dynamic load, sufficient compression force should be provided between two connecting bodies to ensure the reliable connection and normal operation of the connecting parts. In this way, there must be sufficient axial preload (i.e. axial tensile stress) after tightening.

Quantity changes during bolt tightening process:

When tightening the bolts, the overall force situation is that the bolts are in tension and the connectors are in compression. But in the process of being subjected to force, the force is different. It can be roughly divided into the following stages:

At the beginning of tightening, due to the bolt not seating properly, the tightening force F was zero. However, due to friction, the torque T remains at a small value.

When it approaches the valve seat (point z), the actual tightening begins. As the angle A increases, the pressure F and torque t rapidly increase.

When the yield point is reached, the bolt begins plastic deformation and the angle greatly increases, but the compression force and torque hardly increase or even remain unchanged.

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

Bolt tightening method

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

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

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

The disadvantage of the yield point tightening method is that it can easily cause thread detachment. The reason is that using the yield point tightening method, the tightening stress will be close to the guaranteed stress. B. The results indicate that the debonding strength measured with hardened core rods is higher than the peel strength of nuts tested with corresponding grade bolts.