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The Working Principle And Main Features Of Self-extruded Screws

Apr 17, 2018

1, working principle

      In the process of rotating the self-tapping locking screw into the prefabricated hole (drilling, punching or die-casting light hole) of the connected part, the internal thread is formed in the prefabricated hole by squeezing the screw thread of the screw to achieve the connection and tightness. Solid purpose.

      The structure of self-extruded screws differs from that of machine screws, self-cutting screws and ordinary self-tapping screws, and its working principle is not the same. The machine screw can only be screwed into the hole with internal thread. Generally speaking, there is a gap between the internal thread and the external thread, mainly relying on the pre-tightening force to ensure the anti-loosening performance between the mating thread and the side and the bearing surface; The tapping screw is used to screw the screw into a prefabricated hole without threads. Generally, there is interference between the inner and outer threads, mainly relying on the pretightening force and the interference amount to ensure that the friction between the side surfaces of the mating threads and the bearing surface is prevented. performance. The principle of anti-loosening of the self-cutting screws lies between the machine screws and ordinary self-tapping screws. In addition to the above-mentioned anti-loosening performance, the self-extrusion screws have their own unique features. The screw of the self-extrusion screw has an arc-shaped triangular cross-section thread. For ease of analysis, we treat its thread as a ring-shaped tooth.

      Due to the presence of elastic deformation, the internal thread diameters outside the three regions of the vertices A, B and C on the annular teeth are smaller than the diameter of the circumscribed circle of the annular teeth, that is, the three regions A, B and c of the annular teeth are embedded in the screw holes. Inside the metal, according to the minimum resistance law and the volume invariable law of plastic deformation in the principle of metal pressure processing, the metal flows toward both sides of the apex, when the ring teeth rotate, in front of the movement direction of the apex of the ring teeth, on the screw hole Three internal iliac crests are formed and move forward with the rotation of the screw, creating a certain resistance. Therefore, regardless of the direction in which the ring teeth turn, this resistance must be overcome. Self-extrusion screws are based on this principle to improve the anti-loosening performance.

      2, the main features

      (1) save the processing of internal thread

Self-extinguishing screws, like ordinary self-tapping screws and self-cutting screws, can save the machining effort of the internal thread. For this advantage alone, it constitutes an important basis for today's worldwide development of self-tapping screws. The tapping procedure can be eliminated and it seems that it is not valued by people and generally does not consider its economic value. However, in some industrial advanced countries, such "small things" have gradually been attached importance. For example, Japan issued an article in 1977 that explicitly stated: In the screw connection, the cost of screws is only 15% of the total cost, and the remaining 85% is used for Costs of equipment, tools, and labor costs for drilling, tapping, and securing devices (washers). At the same time, the United States, West Germany and other patents and papers on the self-tapping locking screws are indispensable to demonstrate the economic benefits of eliminating the tapping process. With China's opening up to the outside world, especially the introduction of new technologies, this issue has also begun to be taken seriously. The self-extrusion screw can save the internal thread processing, has a low screwing torque, high locking performance, can be screwed into the body of many materials and high thread strength and superior performance, has been design, use, production distribution and other parties have reached consensus. The adoption of the international standard system has been adopted and implemented more widely.

      (2) Low screwing torque, high locking performance

The self-extruded screw has an arc-shaped triangular section thread, that is, a unique thread shape, which not only makes it superior to ordinary self-tapping screws in the installation of the screwing torque, but also greatly exceeds the reliability of the conventional self-tapping screws and lock washers. Machine screws. In this regard, in recent years, Japan and West Germany have published monographs on experimental results. China now has test conditions.

      (3) Many body materials that can be screwed in

      Self-extrusion screws are surface-hardened and can be screwed into prefabricated holes of black or non-ferrous metal material and extruded to form threads. Prefabricated holes can be made by drilling, die cutting or die casting. It can be twisted into all the extensible materials, ie it can be screwed into a material with a certain plastic deformation capacity. The material's elongation δ can be used as the basis, generally δ5 ≥ 8% ~ 10% of the material is most suitable, but even for the δ5 <5% of the die-casting alloy is also applicable, but for such materials should be determined through the process verification The best prefabricated hole size and lubrication conditions. Therefore, the materials that can be screwed in include unalloyed, low-alloy, aluminum and aluminum alloys (commonly used die-cast parts), copper and copper alloys (having a content of copper greater than 63%) and zinc and zinc alloys having tensile strength σb ≤ 60 O N/mm2. (Common die casting) etc.

       For materials with poor plastic deformability or work hardening, such as stainless steel, gray cast iron and magnesium alloy, the screw can be used under certain conditions of use.

      Inapplicable materials or conditions of use are inappropriate, when the screws are screwed in, there may be swarf, seizure (or cold welding), and the roughness of tapped screw holes may be low.

      (4) High thread strength

      According to the standard of the self-extruded screw, the minimum tensile strength of the screw is 800 N/mm2, which is the same as the index of the 8.8 high-strength bolt or screw. In addition, screwing into a prefabricated hole and forming an internal thread can increase the connection strength of the internal thread by more than 30%. Therefore, the strength of the self-resisting screw connection pair far exceeds that of the machine screw connection. Under the same use conditions, the smaller size of the self-extruding screw can be used in place of the machine screw, which may also reduce the size of the host machine. .


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