What Are The Factors Affecting The Preload Of Fasteners?
Factors of preload slack in threaded fasteners
In the threaded fastener connection, the possible factors leading to preload slack are: contact surface subsidence, bolt group interaction, seal deformation, and high temperature creep and torsional slack.
1.Contact surface subsidence
When the threaded fastener is tightened for the first time, the internal and external threads, the bolt and nut support surface, the contact surface of the connected parts of the microstructure of the raised place first contact. The material in these locally raised areas will be overloaded, first exceeding its yield point, and then the material will creep and deform until there are more and enough contact surfaces to carry the load. Contact surface subsidence is the primary cause of preload relaxation and is one of the most common conditions encountered in threaded joints.
These relatively short stress relaxation effects are known as "sinkage". Studies have shown that the rate of relaxation is greatest just after tightening and decreases exponentially, usually to a very low creep rate after an initial period of time. Usually the preload relaxation due to subsidence is roughly 5%~10% of the initial preload force.
2.Bolt group interaction
Bolt group interaction mostly occurs in the bolt flange connection and other flat multiple bolt connection occasions. When the first bolt is tightened, the bolt is elongated, while the nearby connected parts are compressed. When the nearby bolts are tightened, the connected parts near the first bolt will be further compressed, resulting in a slackening of the preload of the first bolt. The mutual influence depends on the stiffness of the connected parts, the spacing between the bolts, and whether seals are used. Some tests show that the preload can be lost by 50% or even 100%.
3.Deformation of the seal
Seal deformation is another form of short-term relaxation. A structural connection with a seal will cause more slack than normal sink. In fact, in the structural connection through the plastic deformation of the seal to prevent the leakage of liquid or gas in the pipeline. Similar to other plastic materials, when a seal is subjected to high surface stresses for the first time, the material will undergo some deformation. However, since the deformation is related to many factors, it is difficult to predict the exact amount. When a structural joint contains a seal, the load deflection characteristics are non-linear and the seal is much more flexible than the rest of the jointed part. Therefore, the elastic deformation accounts for most of the total deformation, and has a hysteresis effect due to its partial plasticity. If subjected to dynamic load, the hysteresis effect will disappear after a period of time. Therefore, the analysis conducted by static load and dynamic load is different.
4.High temperature creep
High temperature generally reduces the strength of the material, which in turn will lead to a certain degree of relaxation. As a result of long-term work at high temperatures, the bolt will undergo slow creep deformation, and stress relaxation will also occur. When creep occurs, the length of the bolt will increase, resulting in an increase in the distance between the nuts on both sides of the bolt, the flange compression degree is reduced, the sealing performance will be reduced; when stress relaxation occurs, the elastic deformation will be transformed into plastic deformation, the axial force of the bolt is reduced due to the reduction of elastic strain, the compression degree of the flange is also reduced, the sealing performance will also be reduced.
5.Torsional relaxation
In the fastening process, in addition to the axial stress, the corresponding torsional stress is generated in the fastener. Torsional stress is susceptible to different degrees of relaxation, and it is usually considered to be the most susceptible stress when relaxation occurs. In most cases, when the wrench is removed, the torsional stress disappears immediately. It has been shown that the relaxation of the torsional stress can produce additional tensile stresses. This "self-tightening" phenomenon is always masked by a greater axial sink relaxation. The torsional stress is mainly determined by the amount of thread friction torque and relaxation, and whether the relaxation is generated by head rotation or self-tightening depends mainly on the comparison between the under-head friction conditions and the thread friction conditions.
These are the main factors affecting the preload slackness of threaded fasteners, and you can take precautions according to these points summarized by Qisheng Metal. Our company is a professional fastener manufacturing company with professional technicians and equipment, if you want to buy fasteners, you can pay attention to Qisheng Metal Co.
