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How to improve the strength of bolt connection?

In most cases, the strength of the tension bolt connection depends on the strength of the bolt. There are many factors that affect Bolt strength, including material, structure, size parameters, manufacturing and assembly process, etc.
The following introduces some measures to improve bolt strength.

01. Improve the load distribution between thread teeth.

When ordinary nut is adopted, the axial load distributes unevenly among all laps of spinning and closing thread. As shown in figure 1(a), counting from the supporting surface of nut, the load on the first lap will be the largest and on the later laps will decrease.
Theoretical analysis and experiments have proved that the more rotating coil, the more significant the degree of uneven load distribution. After the 8th to 10th coil, the thread almost does not receive load.
Therefore, using a thick nut with many turns cannot improve the connection strength. If using the mounting (pull) nut as shown in Figure 1 (B), the taper mounting section of the nut and the bolt rod are both in tension deformation, which is helpful to reduce the difference of the moment between the nut and the stud rod, so as to make the load distribution relatively uniform. Fig. 1(c) is a ring groove nut, which has similar functions to a suspension nut.
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Figure 1

02. Avoid or reduce additional stress.

Due to negligence in design, manufacture or installation, the bolt may be subject to additional bending stress (Figure 2), which has a great impact on the fatigue strength of the bolt and should be avoided.
For example, when installing bolts on unprocessed surfaces such as castings and forgings, a convex head or countersunk head seat is often used, and a flat supporting surface can be obtained after cutting (Fig. 3).
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Figure 2
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Figure 3

03. Reducing stress concentration.

The thread of the tooth root, the head of the bolt and the joint of the bolt rod all have stress concentration, which is a dangerous part of fracture. Among them, the stress concentration of threaded tooth root has a great influence on the fatigue strength of bolts.
Measures such as increasing the fillet radius of the threaded tooth root, enlarging the fillet on the transitional part of the bolt head (Figure 4a) or cutting unloading groove (Figure 4b, 4c) can be adopted to reduce the stress concentration.
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Figure 4

04. Decrease stress amplitude.

When the maximum stress of the bolt is fixed, the smaller the stress amplitude is, the higher the fatigue strength is. Under the condition that the working load and residual pre-tightening force remain unchanged, reducing the bolt stiffness or increasing the stiffness of connected parts can achieve the purpose of reducing the stress range (see figure 5), but the pre-tightening force should be increased.
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Figure 5
The measures of reducing the rigidity of the bolt are as follows: increase the length of the bolt appropriately; Partly reduce the diameter of the bolt or make the hollow structure as flexible bolt. The installation of elastic elements (Fig. 6) under the nut can also achieve the effect of flexible bolts. The flexible bolt has a large amount of deformation and strong energy absorption effect, and is also suitable for bearing impact and vibration.
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Figure 6
In order to increase the stiffness of the connected system, it is not suitable to use gaskets with small stiffness. The sealing connection shown in figure 7 is preferred with a sealing ring.
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Figure 7

05. Improve manufacturing process.

Manufacturing process has a great influence on the fatigue strength of bolts. For high-strength steel bolts, it is more significant. When using thread rolling, due to the effect of cold work hardening, the surface has residual pressure stress, the metal flow is reasonable, and the fatigue strength of bolts is higher than that of turning. Carbonitriding, nitriding and shot blasting can improve bolt fatigue strength.

Source: Network Arrangement – China Bolts Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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