Screw assembly and anti-loose technology

Technical exchange screw assembly and anti-loose technology bookmark0 Aerospace Science and Industry Group Third Institute Research Institute Zhao Yunqi Li Jinping Chengzhong has a large proportion. During the assembly process, the problem of threaded fasteners or connectors failing due to excessive or too small tightening torque is encountered. In order to improve the consistency and reliability of the screw connection, the theoretical tightening of the screws of different specifications is first determined by engineering methods. The torque value is summarized and the anti-loose method is summarized. Secondly, the vibration test is used to verify whether the calculated value of the screw tightening torque is correct and reasonable. Finally, the principles of screw tightening torque selection are summarized and summarized so that the process designers can choose according to different environments.

The key to the safety of the screw connection is to maintain sufficient residual preload. When the inertial navigation system is vibrating, the working force of the screw is large enough to offset the preload, there is a danger of the screw loosening or breaking. In order to prevent the failure of the screw connection and ensure the maintainability and detachability of the product, the commonly used thread fastening rubber anti-loose and elastic flat pad is anti-loose. Among them, the thread fastening method is anti-loose, and the disassembly torque is multiplied under the action of the adhesive. The flat washer method is anti-loose. The flat washer is mainly used to improve the contact state of the bearing surface, to ensure the friction coefficient of the bearing surface is stable, and has a certain effect on the anti-loose. The spring washer utilizes its elasticity to generate an axial force to cause the thread pair to be axially compressed to generate friction to prevent loosening, but is generally used for unimportant threaded secondary connections.

2 Engineering calculation method of thread tightening torque In the engineering design, the tightening torque of the threaded joint is generally calculated according to the formula, wherein the tightening torque coefficient K and the thread size, the thread lift angle, the thread equivalent friction angle, the thread equivalent friction coefficient, the screw head and It is related to the friction coefficient of the supporting surface of the connecting piece. 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 unlubricated, take 0.24. F-one axial force. That is, the pre-tightening force determined during design, K tightening torque coefficient; d thread nominal diameter, unit mm; for carbon steel screws, the pre-tightening force is: for alloy steel screws, the pre-tightening force is: As thread nominal stress cross-sectional area, D2 external diameter; d3 thread calculation diameter "3 H is the original triangle height of the thread; di external thread diameter.

3 The calculated value of the thread tightening torque is 9.8, 10.9, 12.9. The number before the decimal point represents 1/100 of the tensile strength of the material ("), and the number after the decimal point represents the yield limit of the material (or, 2) and its tensile strength. 10 times the intensity (") ratio. For example, a screw with a rating of 4.8 has a tensile strength of 4x100 = 400 MPa. The yield limit is the tensile strength and yield limit of the common performance grade of the screw. See Table 1. The tightening torque of common threaded fasteners is shown in Table 2. System-specific technology .

Table 1 Tensile Strength and Yield Limits of Common Performance Grades of Screws Table 2 Common Thread Fastening 1 Cat Tightening Torque Engineering Calculation Results Screw Standard National Standard Aviation Industry Standard Performance Grade / Material Grade Thread Size Bolt Tightening Torque / Nm Bolt Tightening Torque / Nm 4 thread fastening glue selection method Screw fasteners are more commonly used to prevent loosening with thread fastening glue. Among them, the epoxy adhesive has a large adhesive force, and is generally not removable after being used in screw fastening, and the epoxy adhesive is generally two-component, and the operation is complicated. Anaerobic adhesive has good vibration resistance, adhesion and sealing, and has strong anti-loose function. Anaerobic adhesive is a single-component adhesive. It is liquid in air. After air is isolated, polymerization cures and anaerobic adhesive is used. Anaerobic adhesives are more commonly used for repeated disassembly of threaded fasteners.

Anaerobic adhesives are generally series of rubbers. They are divided into different grades according to the bonding strength. Different properties of glue can be selected according to different requirements such as thread fastening and sealing. Loctite 200 series is a thread locking sealant, which is widely used. Its performance is shown in Table 3. Table 3 Loctite 200 series product performance characteristics M12 strength low strength medium strength high strength ultra high strength super high strength large viscosity medium strength low viscosity serial number Thread fastening rubber loosening torque increase situation 1 Loctite 22213% 2 Loctite 24325% 3 Loctite 26240% Table 5 Vibration test conditions Test direction frequency / Hz power spectral density value / g2Hz-1 total mean square value / g time /minX, Y to 20 after using Loctite lock glue, the thread release torque can be increased, and the increase of the release torque after the glue is shown in Table 4. Table 4 uses different thread fastening rubber thread release torque increase Case 5 Test Verification of Tightening Torque of Screw Fasteners 5.1 Test Conditions In order to verify the correctness of the selection principle of the tightening torque of the screw, the vibration test of the screw fastener was carried out. The vibration test conditions were determined, and the test conditions are shown in Table 5. Since the X-direction and Z-direction vibration tests have the same force direction to the screw, only the X-direction and the Y-direction are vibrated.

5.2 Test preparation The test piece material is 2A12T4. At the same time, in order to ensure the installation strength of the screw, the thread above the M4 thread is inlaid with the wire screw sleeve. The test threaded fasteners mainly select HB and GB screws, of which: GB screw performance grade is 4.8, 5.8.HB screw selection material is ML25 stud screw). There are 20 screws for each type of screw, no anti-loosening measures for threaded fasteners, anti-loose flat washers, anti-loose rubber fastening, flat washers and anti-loose vibration test.

5.3 Test procedure Thread fasteners of different specifications shall be reinforced according to different test conditions. After the glue is solidified, mark the paint and then carry out the vibration test according to the vibration test conditions. Check the thread mark paint for damage after the vibration test, and use a torque screwdriver or a torque wrench to check whether the threaded fasteners are loose.

The test piece in the vibration test has no vibration damping measures, so the vibration test condition is even worse. Set two control points and one monitoring point on the vibration tooling. As shown.

5.4 Test conclusion Through the vibration test, the X-direction vibration test magnification is 0.999, and the Y-direction vibration test magnification is 2.26. It is indicated that during the vibration test, the partial screw is subjected to a force greater than the test condition. After the vibration test, check that the marking paint is intact, and use a torque screwdriver to check that no screw is loosened according to the specified torque value. It is proved that the tightening torque value of the threaded fastener specified in this paper is in compliance with the requirements.

According to years of production experience, the tightening torque of the screw fasteners is selected as follows: The tightening torque value of the screw fasteners should not exceed the engineering calculation value of Table 2. Otherwise, the tightening torque due to the screw connection is too large, resulting in the thread teeth being Tripping or tripping or causing the screw fastener to break.

For the connection of important parts with high strength requirements, screw fasteners with high quality grade or beacon 30CrMnSiA material can be selected. At the same time, high-strength Loctite lock sealant can be selected for anti-loose, but the product is not easy to disassemble.

Under normal circumstances, in order to meet the product maintainability requirements, Loctite Sealant 222 is used for screw fasteners with a nominal diameter of <2.5, and Loctite Sealing is used for screw fasteners with a nominal diameter of more than 2.5. The glue 243 is secured against loosening.

For the softer material such as plastic material to be pressed, it is necessary to reduce the tightening torque value of the screw fastener, otherwise the plastic parts will be deformed due to the large tightening torque. The screw can be selected with a performance rating of 4.8 screws or a navigation aid ML25 screw. The tightening torque value of the screw fastener can be 1/2 of the calculated value calculated in Table 2. If it is necessary to increase the tightening torque value of the screw fastener, the screw of the plastic part should be tightened without crushing the plastic part. The maximum torque value of the fastener shall not exceed the maximum torque value given in Table 2. At the same time, in order to ensure that the thread is not loose, it is recommended to use Loctite Sealing Sealant 243 and the flat washer to prevent loosening.

If the screw fastening process is used for reinforcement during the screw reinforcement process, the screw release torque is increased to different degrees due to the use of different thread fastening rubbers, so the screw tightening torque can be appropriately reduced under the requirement of strength.

Use the flat pad to prevent loosening, generally used for unimportant threaded sub-connections. Therefore, it is recommended to use the flat washer to prevent loosening of the tightening torque, which can be the value of Table 2.

7 Conclusion In order to avoid the problem of repeated failure of the screw in the assembly process, to improve the consistency and reliability of the screw connection, it is necessary to determine the tightening torque of the screw.

The screw tightening torque values ​​provided in this paper are engineering calculations and are based on actual production experience. In the production, the tightening torque of the threaded connection is also determined according to the different working conditions of the screw.

6 screw fastener tightening torque selection principle

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