Screw assembly and anti-loosening technique
Adopted frequently appear in the process of assembly for tightening torque is too big or too small threaded fasteners or fittings failure problems, in order to improve the consistency and reliability of the screw connection, first by engineering method to calculate and determine the different specifications of the screw theory to express tightening torque values, and the locking methods are summarized, secondly by vibration test values adopted bolt tightening torque engineering calculation is correct and reasonable.Finally, the selection principle of screw tightening torque is summarized so that the process designers can choose according to different working environment.
The key to the safety of 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 preloading force, the risk of screw loosening or breaking will occur.In order to prevent the failure of screw connection and ensure the maintainability and disassembly of the product, the commonly used thread fastening adhesive and elastic flat pad are used to prevent looseness.The method of thread fastening adhesive prevents looseness, and under the action of adhesive, the removing torque increases exponentially.The elastic flat cushion method is used to prevent looseness. The flat washer is mainly used to improve the contact state of the supporting surface and ensure the stability of the friction factor of the supporting surface.Spring washer makes use of its elasticity to generate axial force to make screw pair axial compression and produce friction resistance, but it is generally used for connection of unimportant screw pair.
2 screw fastening torque of engineering calculation method in engineering design, threaded fittings tightening torque generally calculated on type, the tightening torque coefficient K and the thread size, Angle and friction Angle, thread equivalent friction coefficient, screw head with the fittings supporting surface friction coefficient, but specific conditions stipulated in the GB715.15 test method to measure, when the connection is oxidation on the surface of the lubrication state, take 0.24 F axial force, namely the design of pre-tightening force, torque coefficient K;D thread nominal diameter, unit mm;For carbon steel screws, the preloading force is: for alloy steel screws, the preloading force is: As thread nominal stress cross-sectional area, d2 middle diameter of external thread;The calculated diameter of d3 thread "3 H is the original triangular height of the thread;Di external thread path.
The engineering calculation values of the 3 thread fastening moment are 9.8, 10.9 and 12.9. The decimal number represents 1/100 of the tensile strength (") of the material, and the decimal number represents the yield limit (or,) of the material.2) 10 times its tensile strength (") ratio.For example, the tensile strength of 4.8 screws is 4x100 =400MPa; the yield limit is the tensile strength and yield limit of the common performance grades of screws, as shown in table 1; the tightening torque of common thread fasteners is shown in table 2.
Table 1 screw common performance level of tensile strength and yield limit table 2 common 1 cattle tightening torque engineering calculation results threaded fastening screw standard national aviation industry standard performance level/materials brand threaded bolt tightening torque specifications/N - m bolt tightening torque/Nm 4 glue the ways of the selection of screw fastening screw fasteners, locking the commonly used ones are for the use of threaded fastening adhesive for locking.Among them, epoxy adhesive relay is larger, generally can not be removed after use in screw fastening, and epoxy adhesive is generally two-component, complex operation.Anaerobic adhesive has good vibration resistance, adhesion and sealing, and strong anti-loosening function. Anaerobic adhesive is a single-component adhesive, which is liquid in the air. When isolated from air, polymerization reaction solidifies.
Anaerobic adhesive is generally a series of adhesive, divided into different grades according to the bonding strength, according to thread fastening, sealing and other different needs to choose different performance of the adhesive solution.Loctite 200 series as the thread locking sealant, widely used, its performance is shown in table 3. The table 3 loctite 200 series product performance characteristics M12 strength low intensity medium intensity high strength ultra high strength ultra high strength large viscosity strength low viscosity number threaded fastening adhesive loosen the torque increases 1 loctite 22213% 26240% 2 24325% loctite 3 loctite direction of vibration test condition test frequency/table 5 Hz power spectral density value/g2Hz - 1 the mean square value/g time/minX, Y to 20 after using loctite locking glue, can make the screw loosening torque increase,Table 4 shows the increase of loosening torque after using glue.The vibration test conditions are determined, which are shown in table 5.Since the stress direction of the screw is the same in the X and Z directions of vibration test, it only vibrates in the X and Y directions.
5.2 test preparation test article material is 2A12T4. At the same time, in order to ensure the mounting strength of the screws, steel wire bolts are inserted into the threads above M4.HB and GB screws were mainly selected for screw fasteners used in the test, among which, the performance grades of GB screws were 4.8 and 5.8.There are 20 screws of each specification, and the vibration test of elastic plain pad, thread fastener, elastic plain pad and rubber is carried out.
5.3 during the test, the threaded fasteners of different specifications shall be strengthened according to different test conditions. After the glue solution is cured, the marking paint shall be marked. Then, the vibration test shall be conducted according to the vibration test conditions.After vibration test, check whether the thread marking paint is damaged, and use torque screwdriver or torque wrench to check whether the thread fastener is loose.
There are no measures to reduce the vibration of the test pieces in the vibration test, so the vibration test conditions are more severe.Two control points and a monitoring point are arranged on the vibration equipment.As is shown in.
5.4 test conclusion through the vibration test, the magnification factor of X direction vibration test is 0.999, and the magnification factor of Y direction vibration test is 2.26. This indicates that the force borne by some screws during the vibration test is greater than the test conditions.After the vibration test, check that the marking paint is intact, and use torque screwdriver to check that no screws loosen according to the specified torque value.It is proved that the tightening torque of screw fastener specified in this paper is in line with the requirements.
According to years of production experience, the tightening torque of screw fasteners should be selected according to the following principles: the tightening torque value of screw fasteners should not exceed the engineering calculation value in table 2, otherwise, the tightening torque of screw fasteners will be too large, which will cause the thread teeth to be cut off and trip or cause the screw fastener to be broken.
For the connection of important parts with high strength requirement, you can choose the screw fastener with high quality grade or mariner 30CrMnSiA material.At the same time, you can choose high strength letai lock sealant to prevent loose, but the product is not easy to remove.
-- in general, in order to meet the requirements of product maintainability, letai locking sealant 222 is used for screw fasteners with screw nominal diameter <2.5, and letai locking sealant 243 is used for screw fasteners with screw nominal diameter > 2.5 to prevent loosening.
For the intermediate materials that are compressed are soft materials such as plastic materials, it is necessary to reduce the tightening torque value of screw fasteners, otherwise the plastic parts will be deformed due to the large tightening torque.Screw choice performance class 4.8 screw or beacon ML25 screw.The tightening torque value of screw fastener can be 1/2 of the engineering calculation value calculated in table 2. If it is necessary to increase the tightening torque value of screw fastener, the plastic parts should not be crushed, and the maximum torque value of screw fastener tightening plastic parts should not exceed the maximum torque value given in table 2.At the same time, in order to ensure the thread is not loose, it is recommended to use letai locking sealant 243 and elastic pad at the same time to prevent loose.
If thread fastening adhesive is used in screw strengthening process, the loosening torque of screw will increase to different degrees due to different thread fastening adhesive. Therefore, the tightening torque of screw can be appropriately reduced when meeting the requirement of strength.
Use the spring pad method to prevent loosing, generally used for minor thread pairs.Therefore, it is recommended to use elastic flat pad to prevent loose thread connection torque, as shown in table 2.
In order to avoid the frequent failure of screws in the assembly process and improve the consistency and reliability of screw connection, it is necessary to determine the tightening torque of screws.
The torque value of screw tightening provided in this paper is calculated by engineering and based on practical experience.In production, the tightening moment of screw connection should be determined according to different working conditions.