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What are the measures to improve the strength of the bolt connection?

Update:15-11-2021
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Measures to improve the strength of bolt connections In […]

Measures to improve the strength of bolt connections In most cases, the compressive strength of tension bolt connections is determined by the bolt's compressive strength. There are many factors that affect the compressive strength of bolts, including materials, structure, dimensional parameters, manufacturing and assembly processes, and so on. Here are some common measures to improve the compressive strength of bolts.

1. Improve the load distribution between the threads
When using ordinary nuts, the axial load is not uniformly distributed among the turns of the screw thread. As shown in the figure below, starting from the nut bearing surface, the load on the first turn is relatively large, and the subsequent turns decrease. Theoretical analysis and experiments have proved that the more the number of screwing turns, the more significant the unevenness of the load distribution. After the 8th to 10th turns, the thread is hardly loaded. Therefore, the use of thick nuts with a large number of turns cannot improve the compression strength of the connection. The picture below shows the ring nut, which functions similarly to the suspension nut.

2. Avoid or reduce additional stress
Due to negligence in design, manufacturing or installation, the bolts may be subjected to additional bending stress, which has a great influence on the fatigue strength of the bolts and should be avoided. For example, when installing bolts on unprocessed surfaces such as castings or forgings, structures such as bosses or countersinks are often used, and a flat bearing surface can be obtained after cutting.

3. Reduce stress concentration
The root of the thread, the head of the bolt and the joint of the bolt have stress concentration, which is a dangerous part of fracture. Among them, the stress concentration of 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 or cutting the unloading groove can be taken to reduce the stress concentration.

4. Reduce the stress amplitude
When the stress of the bolt is larger, the smaller the stress amplitude, the higher the fatigue strength. Under the condition that the working load and the remaining pre-tightening force remain unchanged, reducing the bolt stiffness or increasing the stiffness of the connected parts can achieve the purpose of reducing the stress amplitude, but the pre-tightening force should be increased.

Measures to reduce the stiffness of the bolt include: appropriately increasing the length of the bolt; partially reducing the diameter of the bolt or making it into a hollow structure, that is, a flexible bolt. Installing an elastic element under the nut can also function as a flexible bolt. The flexible bolt has a large amount of deformation when subjected to a force, has a strong effect on absorbing energy, and is also suitable for withstanding shock and vibration.
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