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What are the Factors for the Relaxation of the Preload of Threaded Fasteners?

Aug. 25, 2020

As an Anchor Bolt Manufacturer, share with you. In the connection of threaded fasteners, the main possible factors that lead to the relaxation of the pre-tightening force are: contact surface subsidence, mutual influence of bolt groups, seal ring deformation, high temperature creep and torsion relaxation.

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1. Subsidence of contact surface

When the threaded fastener is tightened for the first time, the protruding parts in the microstructure of the internal and external threads, the bearing surface of the bolt and the nut, and the contact surface of the connected part first touch. The material in these local protrusions will be overloaded, first exceeding its yield point, and then the material: creep deformation, until there are more and enough contact surfaces to withstand the load. The sinking of the contact surface is the main cause of the relaxation of the pre-tightening force; the main reason is also the most common situation encountered in the threaded connection.

These relatively short-term stress relaxation effects are called "sinking." Studies have shown that the relaxation rate just after tightening is the largest and decreases exponentially, usually after an initial period of time, that is, it decreases to a very low creep rate. Usually, the relaxation of the pre-tightening force due to subsidence is roughly 5% to 10% of the initial pre-tightening force.

2. The bolt groups affect each other

The mutual influence of bolt groups mostly occurs when multiple bolts are connected in a plane such as bolt flange connection. When the first bolt is tightened, the bolt is elongated and the vicinity of the connected part is compressed. When the nearby bolts are tightened, the connected parts near the first bolt will be further compressed, causing the pre-tightening force of the first bolt to relax. The mutual influence mainly depends on the rigidity of the connected parts, the spacing between the bolts, whether the sealing ring is used or not. Tests have shown that the pre-tightening force can be lost by 50% or even 100%. Take a certain type of flange bolt group connection as an example, the 8 bolts on it are tightened in diagonal order, and the mutual influence between each bolt.

3. Deformation of the sealing ring

Sealing ring deformation is another form of short-term relaxation. Structural connections with sealing rings will cause greater slack than normal subsidence. In fact, the plastic deformation of the sealing ring is used to prevent the leakage of liquid or gas in the pipeline during the structural connection. Similar to other plastic materials, when the sealing ring is subjected to high surface stress for the first time, the material will deform to a certain extent. But because the deformation is related to many factors, it is difficult to accurately predict its magnitude. When a sealing ring is included in the structural connection, its load offset characteristic is non-linear, and the flexibility of the sealing ring is much higher than other parts of the connected parts.

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