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How to improve the strength of bolted connections?


In most cases, the strength of a tension bolted connection is determined by the strength of the bolt. There are many factors that affect the strength of bolts, including materials, structures, dimensional parameters, manufacturing and assembly processes, etc.
So what are the measures to improve the strength of bolts?

① Improve load distribution between thread teeth

When using an ordinary nut, the axial load is distributed unevenly among the turns of the screw thread, as shown in Figure 1(a). Counting from the nut bearing surface, the load is greatest in the first turn, and decreases in subsequent turns.

Theoretical analysis and experiments have proven that the more turns the screw has, the more significant the uneven load distribution becomes. After the 8th to 10th turns, the thread is almost unloaded.

Therefore, using a thick nut with many turns cannot improve the connection strength. If the suspended (tension) nut in Figure 1(b) is used, the conical suspension section of the nut and the bolt rod are both stretched and deformed, which helps to reduce the difference in the change of the screw moment between the nut and the bolt rod, thereby making the load distribution Relatively even. Figure 1(c) shows an annular groove nut, which functions similarly to a suspension nut.

② Avoid or reduce additional stress

Due to negligence in design, manufacturing or installation, the bolt may be subjected to additional bending stress (Fig. 2), which has a great impact on the fatigue strength of the bolt and should be avoided.

For example, when installing bolts on unmachined surfaces such as castings or forgings, structures such as bosses or countersunk seats are often used, and a flat supporting surface can be obtained after cutting (Figure 3).

③ Reduce stress concentration

The thread root, the intersection of the bolt head and the bolt rod all have stress concentration and are dangerous parts for fracture. Among them, the stress concentration at the thread root has a great influence on the fatigue strength of the bolt.

Measures such as increasing the fillet radius of the thread root, increasing the fillet at the transition part of the bolt head (Figure 4a), or cutting an unloading groove (Figure 4b, 4c) can be taken to reduce stress concentration.

④ Reduce stress amplitude

When the maximum stress of the bolt is constant, the smaller the stress amplitude, the higher the fatigue strength. When the working load and residual pretightening force remain unchanged, reducing the bolt stiffness or increasing the stiffness of the connected parts can reduce the stress amplitude (see Figure 5), but the pretightening force should be increased.

Measures to reduce the stiffness of bolts include: appropriately increasing the length of the bolt; partially reducing the diameter of the bolt rod or making a hollow structure, that is, a flexible bolt. Installing an elastic element under the nut (Figure 6) can also act as a flexible bolt. Flexible bolts have large deformation when stressed, have strong energy absorption effect, and are also suitable for withstanding shock and vibration.

⑤ Improve manufacturing process

The manufacturing process has a great influence on the fatigue strength of bolts. For high-strength steel bolts, it is even more significant. When rolling threads are used, due to the effect of cold work hardening, there is residual compressive stress on the surface, the metal streamlines are reasonable, and the fatigue strength of the bolt is higher than that of turning. Carbonitriding, nitriding, and shot peening can all improve bolt fatigue strength.