The working principle and main characteristics of self-extruding screws

1. Working principle

In the process of screwing the self-tapping locking screw into the prefabricated hole of the connected part (drilling, punching or die-casting smooth hole), the thread of the screw is extruded to form an internal thread in the prefabricated hole to achieve The purpose of connection and fastening.

The structure of self-extruding screws is different from machine screws, self-cutting screws and ordinary self-tapping screws, and their working principles are also different. Machine screws can only be screwed into holes with internal threads. Generally speaking, there is a gap between the internal and external threads. The pre-tightening force is mainly used to ensure the anti-loosening performance between the matching threads and the side surfaces and the bearing surface. Tapping screw is to screw the screw into the prefabricated hole without thread. Usually, there is interference between the internal and external threads. The pre-tightening force and the interference between the matching thread sides and the friction between the supporting surface ensure anti-loosening performance. The anti-loosening principle of self-cutting screws is between machine screws and ordinary self-tapping screws. In addition to the above-mentioned anti-loosening performance, self-extruding screws have their unique features. The screw of the self-extruding screw has a thread with an arc-shaped triangular section. For the convenience of analysis, we regard its thread as a ring thread.

Curved triangular cross-section thread

Due to the existence of elastic deformation, the diameter of the internal thread outside the 3 areas of the apex A, B, and C on the ring tooth is smaller than the diameter of the circumscribed circle of the ring tooth, that is to say, the 3 areas of A, B, and c on the ring tooth In the metal embedded in the screw hole, according to the law of least resistance and the law of volume invariance of plastic deformation in the principle of metal pressure processing, the metal flows to both sides of the apex. When the ring tooth rotates, it is in front of the direction of movement of the apex of the ring tooth. Three inner tenons are formed on the screw hole, and they move forward with the rotation of the screw, generating a certain resistance. Therefore, no matter which direction the ring tooth rotates, it needs to overcome this resistance. Self-extrusion screws are based on this principle to improve the anti-loosening performance. (Guide: Commonly used hardware locks have so many classifications, see how much do you know?)

2. Main features

(1) Save internal thread processing

Self-extrusion screws, like ordinary self-tapping screws and self-cutting screws, can save the processing workload of internal threads. Only for this advantage, it has constituted an important basis for the development of self-tapping screw products worldwide. . The tapping process can be eliminated, which seems to be ignored by people, and its economic value is generally not considered. However, some advanced industrial countries have gradually paid attention to such trivial matters.For example, as early as 1977, Japan published a related article that clearly stated: In screw connections, the cost of screws only accounts for 15% of the total cost, while the remaining 85% is used for punching. , Tapping, anti-loosening device (washer) and other aspects of equipment, tool costs and labor costs. At the same time, patents and papers related to self-tapping locking screws in the United States and West Germany have indispensably demonstrated the economic benefits of eliminating the tapping process. With my country's opening to the outside world, especially the introduction of new technologies, people have begun to pay attention to this issue. For self-extruding screws, it can save internal thread processing. It has superior performance such as low screwing torque, high locking performance, more body materials that can be screwed in, and high thread connection strength. It has been designed, used, produced and distributed by all parties. The national standards formulated and revised by international standards have been adopted and implemented more widely.

(2) Low screwing torque, high locking performance

The self-extruding screw has a thread with an arc-shaped triangular section, that is, a unique thread shape, which not only makes it superior to ordinary self-tapping screws in terms of installation and screwing torque, but also greatly exceeds the reliability of locking Machine screws for locking washers. In this regard, in recent years, both Japan and West Germany have published relevant test results in monographs, and our country now has the test conditions.

(3) There are many body materials that can be screwed in

Self-extruding screws are hardened on the surface and can be screwed into prefabricated holes of black or non-ferrous metal materials to be extruded to form threads. The prefabricated holes can be made by drilling, punching or die casting. It can be screwed into all malleable materials, that is, it can be screwed into materials with a certain plastic deformation ability. The elongation δ of the material can be used as the basis. Generally, it is most suitable for materials with δ5≥8%~10%, but it is also suitable for die-casting alloys with δ5<5%, but for such materials, it should be determined by process verification. Optimal prefabricated hole size and lubrication conditions. Therefore, the materials that can be screwed in include non-alloyed, low-alloy, aluminum and aluminum alloys (commonly used die castings) with tensile strength σb≤60O N/mm2, copper and copper alloys (copper content greater than 63%), and zinc and zinc alloys (Commonly used die castings) and so on.

For materials that have poor plastic deformation ability or are prone to work hardening, such as stainless steel, gray cast iron and magnesium alloy, this type of screw can also be used under certain conditions of use.

For unsuitable materials or improper use conditions, chipping, seizure (or cold welding) and low roughness of the tapped screw hole may occur when screwing in the screw.

(4) High thread connection strength

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

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