Analysis and Research on the Relaxation Curve of Thread Fasteners

The purpose of installing the threaded fastener is to make the connection secure and secure if the fastener is not slack. To solve the problem of anti-looseness of threaded fasteners, it is necessary to obtain a graph of the locking performance by testing the threaded fasteners, systematically analyze and deeply study the slack phenomenon and the regularity of the threaded fasteners.

A threaded fastener relaxation index and its curve

1. Measure the index of loose fasteners

The slack in the threaded fastener occurs with the tightening of the threaded fastener. Since the fastening method is tightening, the tightening process must have a tendency to relax or slack. The tighter the threaded fastener that is actually installed, the stronger the tendency to slack. Tightening and slack are two contradictory aspects of the threaded fastener locking system during the locking process. The axial pull force that measures the tightening of the threaded fastener can be used to measure the degree of slack. The fasteners are of course expected to maintain a good locking state for a long time, and the fastener locking system is less slack for a long time, indicating that the locking system has good locking performance, and vice versa. Therefore, another variable that reflects the slack in the fastener locking system is time.

2. Test to determine the relaxation curve

Any system under stress has a tendency to release stress, and threaded fastener locking systems are no exception. The tightening process of the fastener is actually a process of applying stress to the system. After the external action of applying the tightening torque is stopped, the fastener locking system is always in the process of constant relaxation. The Junkers vibration testing machine of the former Ministry of Machinery Standardization adopted the internationally accepted test method for testing the locking performance of threaded fasteners. It uses the above principle to highlight the slack of threaded fasteners. Characteristics, exerting a certain kind of stable external action, making it loosely characterized, so as to facilitate research. In 1996, the “Junke” vibration testing machine of 708 was used to test the locking performance of different types and specifications of “bolt-nut” fastener locking system. In a vibration environment with an amplitude of ±0.50 mm and a frequency of 30 Hz, almost all bolt-nut axial tension changes were measured as shown in Fig. 1.

Fig.1 Vibration relaxation curve of threaded fastener locking system

In Figure 1, the “axial tension-vibration time” curve (referred to as the “F-T” curve) reflects the phenomenon that the axial tensile force F of the fastener locking system becomes smaller with the prolongation of the vibration time T under vibration conditions. , reflects the relaxation of the locking system under vibration conditions. Therefore, the “F — T ” curve is the vibration relaxation curve of the thread locking system with an amplitude of ±0.5 mm and a frequency of 30 Hz.

Analysis of two relaxation curves and influencing factors

As shown in Figure 1, the slack curve of the threaded fastener locking system has two distinct features. One is the rapid relaxation section, the "I" part of the curve. This curve shows that part of the axial tension F of the locking system is released quickly in a short time; the second is the relatively stable section, ie the "II" part of the curve. This curve shows that the axial tension force F of the locking system has a relatively long period of relative stability after releasing part of the axial force.

In the above test, the balancing torque of the tightening torque applied by the operator to the fastener can be decomposed into a "bolt-nut" torque generated by the friction between the thread pair Mf, between the nut and the bearing surface of the connected member. The moment Mu generated by the friction force and the moment Me generated by the elastic deformation of the fastener. That is, the tightening torque:

M = Mf +Mr +Me

All three moments are positively related to the axial pull force F produced by the fastener, and any change in torque will cause a change in F and vice versa. Under the action of a certain F, Mf and Mr depend on the surface state of the thread contact pair and the connected member, and Me depends on the stiffness and torsional deformation of the locking system. Among the three influencing factors, the surface state of the thread contact pair and the connected member is relatively stable when the test piece is not replaced and is not reinstalled, and only the torsional elastic deformation of the locking system is in external conditions such as impact or vibration. The bottom will quickly become smaller, making F smaller and the system slack. At the same time, the locking system also creeps under the action of the torque, which is also the reason for making F smaller.

Therefore, the smaller the torsional deformation of the locking system and the creep of the locking system under the external force are the main causes of the rapid decrease of F in the "I" segment of the locking system and the severe slack in the initial stage after locking. .

The practical significance of the study of three relaxation curves

The study of fastener slack or relaxation curves is as important as the fastener locking study.

1. The significance of the study of the "I" segment relaxation curve

After the fasteners are installed, the amount of elastic deformation will be reduced and creep rapidly under the influence of the external environment, which will reduce the axial tensile force of the fastener and reduce the locking effect of the fastener. However, the tendency to reduce the amount of torsional deformation and the creep under external force are inherent physical properties of the locking system, and as long as the conditions are met, the amount of elastic deformation becomes small and creep occurs. As a conditional stress balance system, the thread locking system has a strong elastic deformation amount or recovery and creep tendency, and it is inevitable to relax rapidly under the influence of vibration or other environmental conditions. This seems to have little to do with everyday life, but the impact on defense and high-tech industrial products such as aerospace can be fatal. Imagine that if the spacecraft is lifted off, the connection of its gyro platform bolts may affect the elastic deformation stress and creep stress of the platform reference. If it is lifted off, the bolt will occur under the action of strong impact vibration or space environment. Elastic deformation recovery and creep, resulting in reduced axial tensile force; since the stress relief of the connecting bolts cannot be synchronized, the asynchronous stress changes of different connecting bolts will amplify the stress distribution and deformation of the gyro platform, which will result in The platform reference skews, which have serious consequences for the life of the spacecraft.

Similarly, the slackening effect of the fastener locking system will also have an adverse effect on the important joints of the launch vehicle and the like.

2. The significance of the study of the relaxation curve of "II" segment

With the selection and installation of fasteners, we all want to have good locking performance and maintain a good locking condition for a long time. The “II” segment relaxation curve image accurately reflects the locking characteristics of the fastener. Through the test, according to the length of the "II" segment relaxation curve, we can choose fasteners with longer locking time and better locking performance; we can also test and select better fastener materials and surface treatment methods and installation. Process.

Four conclusions

In summary, the analysis of the slack curve of the thread locking system, the relaxation phenomenon of the thread locking system, and the relaxation law are necessary. We should actively develop and use new and advanced fasteners, and develop new standards. At the same time, we should pay attention to the research of fastener installation process, the pre-treatment of fasteners, the quantitative control of process parameters during installation, and the installation. Process methods, etc., systematically study and formulate corresponding process specifications.

In view of the fact that the slack of the threaded fastener locking system may have a fatal impact on key critical parts of the product, it is recommended that the relevant unit take effective measures to eliminate the slack in the fastener locking system before the rocket is launched, before the satellite is launched; Eliminate the slack in the fastener locking system before key parts, commissioning and calibration of important mechanisms to ensure that the model is safe and reliable.

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