The purpose of installing the threaded fasteners is to make the connection secure and secure if the fasteners are not loose. To solve the problem of anti-looseness of threaded fasteners, it is necessary to obtain the locking performance curve by testing the threaded fasteners, systematically analyze and deeply study the slack phenomenon and the regularity of the threaded fasteners.
A threaded fastener slack indicator and its curve measure the looseness of the threaded fastener. The slack of the threaded fastener occurs with the tightening of the threaded fastener. Since the tightening method is tightening, the tightening process There must be a tendency to relax or relax. The tighter the threaded fasteners that are actually installed, the stronger the tendency to relax. Tightening and slackening are two contradictory aspects of the threaded fastener locking system during the locking process. The axial tension that measures the tightening of the threaded fastener can be used to measure the degree of slack.
It is of course desirable to install the fasteners to maintain a good locking state for a long time. The fastener locking system is less relaxed for a long time, indicating that the locking system has good locking performance, and vice versa. Therefore, another variable that reflects the relaxation of the fastener locking system is time.
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 continuous relaxation. "Junke" of the former Ministry of Machinery Standardization Institute
(unkers) vibration tester meters are used in the international test method for testing the locking performance of threaded fasteners. It uses the above principle to emphasize the slack characteristics of threaded fasteners and exert a stable external environment. The effect is to make the slack characteristics obvious for research.
On the inspection machine, different types and specifications of "bolt-nut" fastener locking system were tested for locking performance.
In a vibration environment with an amplitude of 0.50 mm and a frequency of 30 Hz, the axial tensile force change of almost all bolts and nuts was measured to follow the curve shown.
In the vibration relaxation curve of the threaded fastener locking system, the "axial tension-vibration time" curve (referred to as the "" curve) reflects the axial tensile force F of the fastener locking system with the vibration time T extended under vibration conditions. The smaller phenomenon reflects the relaxation of the locking system under vibration conditions. Therefore, the graphical 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 the two relaxation curves and the influencing factors As shown, the slack curve of the threaded fastener locking system has two distinct features. One is the rapid relaxation segment, which is the "I-speed release of the curve; the second is the relatively stable segment, that is, the curve" portion. This curve shows that the axial tension of the locking system after releasing part of the axial force. F has a longer relative stability period.
In the above test, the balancing torque of the tightening torque applied by the operator to the fastener can be decomposed into a torque Mf generated by the friction between the "bolt-nut" thread pair, and between the nut and the supported surface of the connected member. The torque generated by the friction force Mr and the moment generated by the elastic deformation of the fastener Me. That is, the tightening torque: all three moments are positively related to the axial tensile force F generated by the fastener, and any change in the torque will cause F Change and vice versa. Under a certain F action state, 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 next 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 reduction of the torsional elastic 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 the F in the "I" section of the locking system and the rapid relaxation in the initial stage after the locking. the reason.
The practical significance of the study of the three relaxation curves is that the study of the fastener relaxation phenomenon or the relaxation curve has the same special significance as the fastener locking research.
After the fastener is installed, the amount of elastic deformation will be reduced and creep rapidly under the influence of the external environment, so that the axial tensile force of the fastener is reduced, and the locking effect of the fastener is reduced. However, the tendency to reduce the amount of torsional deformation and the creep under external force are the inherent physical properties of the locking system. 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 quickly relax 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 a reduction in axial tensile force; since the stress relief of the connecting bolts cannot be synchronized, the asynchronous stress changes of the different connecting bolts will amplify the stress distribution and deformation of the gyro platform, which will result in Deviation of the platform reference affects the serious consequences of the life of the spacecraft.
Similarly, the loosening of the fastener locking system will also have an adverse effect on the important joints of the launch vehicle and other models.
With the selection and installation of fasteners, we all hope that the locking performance is good and can maintain a good locking state for a long time. "" Segmental curve image accurately reflects the locking characteristics of the fastener. Through experiments, according to the length of the "" slack 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 processes.
In summary, the analysis of the slack curve of the thread locking system, the relaxation phenomenon of the thread locking system, and the mastery of its slack law are very necessary. We should actively develop and use new and advanced fasteners and formulate new standards. At the same time, we must pay attention to the problems of fastener installation process. Several problems need to be solved in the epoxy adhesive cement. Reliability of epoxy limbs, for the problems existing in the adhesion of epoxy limbs, based on the original formula, a large number of tests were carried out on the epoxy limb configuration, a new formula was proposed, and the reliability was verified by experiments. The cementing process is incorporated into the process specification, effectively solving the problems in cementing. At the same time, in response to the problems related to the new formula, it is proposed that measures should be taken to ensure the effectiveness of the new formula.
The use of epoxy adhesive-adhered electronic components in electrical equipment is a process in which some unit models are developed and produced. Due to problems such as breakage of the epoxy-bonded joints in the type test, in order to prevent such problems from recurring, The Aerospace Corporation issued a notice on the related issues of the banned process in Circular 034, requesting the unit of the epoxy adhesive cementing process to further improve and perfect the cementing process, and incorporate the method and location of the product to fix the epoxy glue into the process. Standardize to eliminate quality hazards. According to the basic requirements of the company's epoxy adhesive cementing process, the original shell and the production practice of our factory, to solve the epoxy adhesive, must deal with the following problems.
It is required to ensure that the adhesive does not flow to the lead of the component and the surface of the epoxy adhesive on the solder joint is cured for 24 hours, and the epoxy adhesive has a large fluidity after being configured. How to start from the material configuration and process, to ensure that the epoxy glue does not flow to the component leads and solder joints after bonding, is a central problem that must be solved.
Improve the strength of the epoxy glue Before the 034 file is issued, the bonding of the components is to be bonded together with the leads, and the amount of glue is large, and the bonding strength is large. According to the requirements of Document No. 034, the components cannot be fixed to the leads, so the position of the fixing is very limited, the components have a certain mounting height, and the amount of glue is reduced, but the bonding strength is required to be constant. In order to avoid the epoxy glue flowing to the component leads or solder joints, increasing the adhesive strength of the epoxy glue is another problem that must be solved.
Standardization of the bonding process Under the changing environmental stress, the epoxy adhesive has its own bonding stress. What method is used to cement the component, and where the epoxy adhesive is applied to the component, the bonding strength can be ensured. Damage to components, this is the third problem that must be solved.
Although there are such conditions as the pot life, curing temperature, curing time, etc., it must be solved on the basis of handling the above three problems.
From the analysis of the above problems, it mainly involves material research, systematic research on pre-installation of fasteners, quantitative control of process parameters during installation, and post-installation treatment methods, 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 units take effective measures to eliminate the loose hidden danger of the fastener locking system before the rocket launches. Before; the loosening of the fastener locking system is eliminated in critical parts, debugging and calibration of important mechanisms to ensure that the model is safe and reliable.
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