Screw installation and anti-loosening technology
Author: admin Origin: This website Date: 2017-7-5 9:24:41 Popularity:Label:
Technical exchange of screw installation and anti-loosening technology bookmark0 Zhao Yunyi and Li Jinping, 3rd Research Institute of the Aerospace Science and Industry Group, 3rd Research Institute, plays a large part in the process. During the installation process, threads often appear due to the tightening torque being too large or too small.fastenerIn order to improve the consistency and reliability of screw connections due to the failure of connecting parts, we first determined the actual tightening torque values of screws of different specifications through engineering method calculations, and summarized the anti-loosening measures. Secondly, we verified through vibration experiments whether the engineering calculation values of screw tightening torque are accurate and legal. Finally, the screw tightening torque selection criteria were deduced and summarized to facilitate process designers to make choices based on different usage conditions.
The key to the safety of screw connections is to maintain sufficient surplus pre-tightening force. When the inertial navigation system is vibrating and the working force on the screws is large enough to offset the pre-tightening force, the screws will become loose or broken. In order to prevent screw connections from failing and ensure the maintainability and disassembly of the product, thread fastening glue and elastic pads are often used to prevent loosening. In addition, thread fastening glue is used to prevent loosening. The solid glue method prevents loosening. The role of the adhesive is high, and the disassembly torque increases exponentially. The elastic and flat washer prevents loosening. The flat washer is mainly used to improve the contact state of the supporting surface to ensure that the friction coefficient on the supporting surface is stable, and plays a certain role in preventing loosening. The spring washer uses its elasticity to generate shaft back force to compress the back of the threaded countershaft and generate friction to prevent loosening, but it is usually used for threaded pair connections that are not tight.
2. Engineering calculation method of thread tightening torque. In the engineering plan, the tightening torque of threaded connecting parts is usually calculated according to the formula. In addition, the tightening torque coefficient K is determined by the thread size, thread rise angle, thread equivalent friction angle, and thread equivalent friction. Coefficient, the screw head is related to the friction coefficient of the supporting surface of the connected part. It can be measured according to the test method specified in GB715.15 for specific working conditions. When the surface of the connected part is oxidized and has no lubrication, take 0.24. F is the shaft back force, that is, the pretightening force determined during design, K tightening torque coefficient; d nominal thread diameter, unit mm; for carbon steel screws, the pretightening force is: For alloy steel screws, the pretightening force is: As thread nominal stress section area, d2 external thread outer diameter; d3 calculated diameter of the thread "3" H is the original triangular height of the thread; di external thread diameter.
3 The engineering calculation values of thread tightening torque are 9.8, 10.9, and 12.9. The number before the decimal place represents 1\/100 of the tensile strength (") of the material, and the number after the decimal place represents 10 times the ratio of the material's yield limit (approximately, .2) to its tensile strength ("). For example, a screw with grade 4.8 has a tensile strength of 4x100 =400MPa, the yield limit is the tensile strength and yield limit of common screw performance grades, see Table 1, and the tightening torque of common threaded fasteners, see Table 2. System special adjustment technology.
Table 1 Tensile strength and yield limit of common performance grades of screws Table 2 Engineering calculation results of common thread fastening 1N tightening torque Screw standards National standards Aviation industry standards Performance grades\/material trademarks Thread specificationsboltTightening torque\/Nm Bolt tightening torque\/Nm 4 How to choose thread fastening glue To prevent screw fasteners from loosening, it is more common to use threaded fastening glue to prevent loosening. In addition, epoxy glue has a strong adhesive force and generally cannot be disassembled after use in screw fastening. Moreover, epoxy glue is usually two-component and is complicated to use. Anaerobic glue tools It has excellent vibration resistance, adhesion and sealing properties, and has strong anti-loosening properties. Anaerobic adhesives are single-component adhesives that are liquid in the air. After blocking the air, a polymerization reaction occurs and solidifies. Anaerobic adhesives can be used for repeated disassembly of threaded fasteners, so anaerobic adhesives are more commonly used.
Anaerobic adhesives are usually a series of adhesives, which are divided into different brands according to the bonding strength. Glues with different properties can be selected according to different needs such as thread fastening and sealing. Loctite 200 series is thread locking sealant, which is widely used. Its performance is shown in the table 3. Table 3 Loctite 200 series product performance Terry M12 strength low strength external strength high strength ultra-high strength ultra-high strength large viscosity external strength low viscosity serial number thread fastening glue loosening torque increase situation 1 Loctite 22213 Percent 2 Loctite 24325 Percent 3 Loctite 26240 Percent Table 5 Vibration test conditions Test direction frequency\/Hz Power spectrum density value\/g2Hz-1 Total mean square value\/g time\/min Experimental verification of nail fastener tightening torque 5.1 Experimental conditions In order to verify the accuracy of the screw tightening torque selection criteria, a screw fastener vibration experiment was conducted. The vibration experiment conditions were determined, and the experimental conditions are shown in Table 5. Since the X-back and Z-back vibration experiments have the same stress direction on the screw, only the X-back direction and the Y-direction are vibrated.
5.2 Experimental preparation The material of the test piece is 2A12T4. At the same time, in order to ensure the installation strength of the screws, threads above M4 are inlaid with wire thread sleeves. The threaded fasteners used in the experiment mainly choose HB and GB screws, in addition: GB screw performance products Grades are 4.8, 5.8. The selected material for HB screws is ML25 stud screws). 20 screws of each specification are used to conduct vibration tests on threaded fasteners without anti-loosening measures, elastic flat pads to prevent loosening, threaded fastening glue to prevent loosening, elastic flat pads and rubber anti-loosening.
5.3 Test process: Threaded fasteners of different specifications are reinforced according to different test conditions. After the glue solidifies, the marking paint is applied, and then the vibration test is carried out according to the vibration test conditions. After the vibration test, check whether the thread marking paint is damaged, and use a torque screwdriver or torque wrench to check whether the threaded fasteners are loose.
The test piece of the vibration test instrument does not have vibration damping measures, so the vibration test conditions are more severe. Two control surfaces and a monitoring surface are installed on the vibration tooling, as shown.
5.4 Experimental conclusion: Through the vibration test, the X-back vibration test magnification is 0.999, and the Y-back vibration test magnification is 2.26. It shows that during the vibration test, the force experienced by some screws is greater than the test conditions. After the vibration test, the inspection marks are all intact, and a torque screwdriver is used to check according to the prescribed torque value. No screws are loose. It proves that the tightening torque value of threaded fasteners stipulated in this article is in compliance with the requirements.
Based on many years of production experience, the selection criteria for the tightening torque of screw fasteners are as follows: the tightening torque value of screw fasteners should not exceed the engineering calculation value in Table 2. Otherwise, the screw connection tightening torque will be too large, causing the thread teeth to be sheared and tripped or the screw fasteners to be twisted off.
For the connection of important parts with high strength requirements, you can choose screw fasteners with high quality grade or navigation mark 30CrMnSiA material. At the same time, you can choose high-strength Loctite locking sealant to prevent loosening, but the product is not easy to disassemble.
- Generally speaking, in order to meet the maintainability requirements of the product, Loctite locking sealant 222 is used for screw fasteners with a nominal thread diameter of <2.5, and="" loctite="" locking="" sealant="" 243="" is="" used="" for="" screw="" fasteners="" with="" a="" nominal="" thread="" diameter="" greater="" than="" 2.5="" to="" prevent="">
If the external material to be compressed is a hard material such as plastic material, the tightening torque value of the screw fastener needs to be reduced, otherwise the plastic parts will be deformed due to the large tightening torque. For screws, performance grade 4.8 screws or navigation mark ML25 screws can be selected. The tightening torque value of screw fasteners can be based on the engineering calculation values calculated in Table 2 1\/2. If it is necessary to increase the tightening torque value of screw fasteners, it must be based on not crushing the plastic parts. At the same time, the maximum torque value of the screw fasteners for tightening the plastic parts should not exceed the maximum torque value given in Table 2. At the same time, in order to ensure that the threads are not loose, it is recommended to use Loctite locking sealant 243 and elastic flat pads to prevent looseness at the same time.
If thread fastening glue is used for reinforcement during screw reinforcement, the screw loosening torque will increase to varying degrees due to the use of different thread fastening glue. Therefore, the screw tightening torque can be appropriately reduced to meet the strength requirements.
The method of using elastic and flat pads to prevent loosening is generally used for threaded pair connections that are not critical. Therefore, it is recommended to use elastic and flat pads to prevent loosening of threaded connections. The tightening torque is the value in Table 2.
7 Conclusion In order to avoid the frequent failure of screws during installation and improve the consistency and reliability of screw connections, the tightening torque of screws needs to be determined.
The screw tightening torque values provided in this article are engineering calculation values and are derived based on actual production experience. In production, the tightening torque of threaded connections needs to be determined in detail based on the different working conditions used by the screws.
6 Criteria for selecting tightening torque of screw fasteners
