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Analysis And Research On Relaxation Curves Of Screw Fasteners

May 23, 2018

The purpose of installing the threaded fastener is to make the connection firm, the premise is that the fastener is not slack. To solve the problem of preventing the loosening of threaded fasteners, it is necessary to obtain the locking performance curve by testing the threaded fasteners, systematically analyzing and deeply studying the phenomenon of relaxation and the law of the threaded fasteners. The

A slackness indicator for a threaded fastener and its curve measure the slack indicator for a threaded fastener. The slackening phenomenon of a threaded fastener occurs with the tightening of a threaded fastener. Because the fastening method is tightening, the tightening process is There must be a tendency to relax or relax. The tighter the tightening of the actually installed threaded fasteners, the stronger the tendency to relax. Tightening and loosening are two contradictory aspects of the tightening process of the threaded fasteners. The axial pull to measure the degree of tightening of the threaded fastener can be used to measure the degree of slack. The

The installation of fasteners, of course, requires the fasteners to maintain a good locking state over a long period of time. The fastener locking system takes less relaxation during a long period of time, which indicates that the locking system has good locking performance, and vice versa. Therefore, another variable reflecting the relaxation phenomenon of the fastener locking system is time. The

Any system under stress has a tendency to release stress, and the threaded fastener locking system is 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 stops, the fastener locking system has been in a process of constant relaxation. Junko, former Ministry of Machinery Industry Standardization Institute

The (unkers) vibration tester meter is a test method commonly used in the world for testing the locking performance of threaded fasteners. It uses the above principle to exert a stable external environment by highlighting the slack characteristics of threaded fasteners. The role of relaxation characteristics so that it is easy to study. The

On the inspection machine, the locking performance comparison tests of different types and specifications of "bolt-nut" fastener locking system were carried out. The

In the vibration environment with an amplitude of 0.50 mm and a frequency of 30 Hz, almost all changes in the axial tension of the bolt-nut are measured following the curve shown. The

In the vibration relaxation curve of the locking system of threaded fasteners, the “axial tension-vibration time” curve (abbreviated as “curve”) reflects the extension of the axial tension F of the fastener locking system with the vibration time T under vibration conditions. The smaller phenomenon reflects the relaxation of the locking system under vibration conditions. Therefore, the illustrated curve is the vibration relaxation curve of the thread locking system under the conditions of an amplitude of 0.5 mm and a frequency of 30 Hz. The

Analysis of the Relaxation Curve and Influencing Factors As shown, the relaxation curve of the locking system of the threaded fastener has two distinct characteristic segments. One is the rapid relaxation segment, ie, the "I-speed release of the curve; the second is the relatively stable segment, ie, the "curve" portion. This curve shows that the axial force of the locking system is released after releasing part of the axial force. F has a longer relative stability period.

In the above test, the balance moment of the tightening torque applied to the fastener by the operator can be decomposed into the moment Mf generated by the friction between the “bolt-nut” thread pair and the bearing surface between the nut and the connected part. Torque generated by the friction force Mr and torque due to torsional elastic deformation of the fastener Me. Tightening torque: These three moments are positively related to the axial tension F produced by the fastener. Any moment change will cause F Change, and vice versa. In the state of action of 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 the amount of torsional deformation of the locking system. Among the three groups of influencing factors, the surface state of the thread contact pair and the connected part is relatively stable when the test piece is not replaced or re-installed and tightened, and only the torsional elastic deformation amount of the locking system is subjected to external conditions such as impact or vibration. It will quickly become smaller, so that F will become smaller and the system will relax. The

At the same time, the locking system also creeps under the effect of moments, which is also the reason for F to become smaller. The

Therefore, the reduction in the amount of torsional elastic deformation of the locking system and the creep of the locking system under external forces are the main causes for the rapid decrease of the locking system in the “I” section and the rapid relaxation in the early stages after the locking. the reason. The

The practical significance of the study of three relaxation curves The study of fastener relaxation phenomena or relaxation curves is of special significance as is the fastener locking study. The

After installation of the fastener is completed, under the influence of the external environment, the amount of elastic deformation will rapidly decrease and creep, so that the axial tension of the fastener will be reduced, and the locking effect of the fastener will be reduced. However, the smaller tendency of torsional elastic deformation and the creep under external force are the inherent physical properties of the locking system. As long as the conditions are satisfied, the reduction in the amount of elastic deformation and creep will occur. As a conditional stress balance system, the thread locking system has a strong reduction in elastic deformation or recovery and creep tendency. It is also inevitable that the thread locking system relaxes rapidly under the influence of vibration or other environmental conditions. This does not seem to matter in everyday life, but the impact on aerospace and other defense high-tech industrial products may be fatal. Imagine that if the spacecraft is lifted, connecting the bolts of its gyro platform may affect the elastic deformation stress and creep stress of the platform's reference. The elastic deformation stress and the creep stress are not fully released. When the spacecraft is lifted off, the bolt will be generated under the strong impact vibration or the space environment. Elastic deformation recovers and creeps, resulting in a reduction of the axial tension; due to 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 Deflection of the platform reference affects the serious consequences of the service life of the spacecraft. The

Similarly, the loosening effect of the fastener locking system will also have an adverse effect on important attachment locations such as launch vehicles. The

With the selection and installation of fasteners, we all hope that its locking performance will be good and it will maintain a good locking state for a long period of time. "" The segmental slack curve image accurately reflects the locking characteristics of the fastener. Through experiments, according to the length of the "" relaxation curve, we can choose fasteners with longer locking time and better locking performance; we can also test and select better fastener materials, surface treatment methods and installation techniques.

Manager: Allen Liang

Tel: +86-573-80705030


E-mail: allen@dyfastener.net

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