Principles and methods of fastener selection

As we all know, only through fasteners can various parts be connected and fastened to form a host. However, in the host design process, some designs often focus on the structure, function, life, or operating rate of the mechanical equipment, and only consider the selection of connections and fasteners until the end. This may be a thing of the past, but this kind of thing does not need to exist. This selection method has gradually attracted people's attention after many mechanical and personal accidents have occurred.

　　 In the actual application selection, it is very important to select the appropriate fastener types, specifications, accuracy and level. This is the experience that the predecessors paid the price of life and blood.

　　 When selecting fasteners, first choose from the national standard varieties and specifications, and then select in the industry, and if necessary, design according to the special needs of the host. Among the various requirements, preventing loosening and preventing breakage are the two main considerations.

　　 In the 1980s, Michigan, the US, a local newspaper published an article saying that there were about 3 to 4 accidents caused by out-of-control car driving in the state every week. It is suspected that these drivers were asleep or drunk. . Later, after they had driven, the mechanical parts of the car were inspected and found that 80% of the fasteners were loose, including: the bolts and nuts on the front suspension, rear suspension and shock absorber, and these devices control the car. Critical. The report also pointed out that there are also loose bolts and nuts on mechanical parts such as the transmission, differential system, engine rocker cover, catalytic converter, and body metal components. In 1986, the rear section of a passenger plane fell in flight and caused 532 deaths in an air crash. Afterwards, investigations were carried out on the rear of the same aircraft in various countries, and it was found that there were loose nuts and broken bolts. This situation may occur first. The strain at the tail of the aircraft exceeded the tightening force that the fasteners could withstand, and secondly, internal displacement between the flanks of the meshing thread was found. (Guide: Cold heading and processing of threaded fasteners)

　　 The loosening of fasteners is a common problem. A pair of scissors has only one fastener. If it is loosened, it can only be used if it is tightened. However, a large aircraft may have 300,000 fasteners, and if something goes wrong, it will be serious.

　　Industrial people in developed countries have realized that there is no problem that troubles the industry more than fasteners, and no matter which problem is more important than solving the problem of fasteners, this is their personal experience. Therefore, there is a choice of fasteners that are safe, applicable and economical.

　　 1. Choose according to different characteristics of fasteners

　　 Fasteners are used in various fields of the national economy. According to their respective uses and hosts, the application forms are also different. The following table summarizes the types and characteristics of general fasteners.

　There are many types of fasteners with different characteristics, and there are many specialized production enterprises. First of all, we must understand the production and quality of domestic fasteners, such as the types and specifications of fasteners produced and the special uses of various products. As well as the excellent products of the Ministry and the country, the famous brand new products or new, excellent and special products of key enterprises, these products should be selected first, and the famous, excellent new and special fastener products should be given priority.

　　 2. Model selection should be based on the applicable economy principle

　　 should be selected according to the characteristics of the fastener. Fasteners are small, but they are diverse. According to the major categories, there are twelve categories and more than 400 varieties. Only bolts and screws have 32 subcategories and nearly 100 varieties. There are also end types of rods. 10 categories. It is impossible for such a large family of fasteners to tell all of them. The selection should be based on the principle of saving. Now take the head type as an example.

　　1. Hexagonal head. This type of product also has a shape with a table, a hole, a boss, and a flange. This type of shape does not have a higher mechanical performance, which is convenient for tightening the wrench and can be applied more Large tightening torque, wide application, bearing surface with convex or flange surface, has the function of gasket, enlarges the bearing surface, has good anti-loosening performance, and can be used for the connection of steel structure and important main machine structure.

　　2, square head, large head area, easy to tighten the wrench, can apply large tightening torque, not easy to be bald, mostly used for thicker, frequently disassembled structures, but not suitable for moving parts .

　　3. Pan head, head size, head has a groove, etc., which is convenient for tool screwing. It is mostly used for products with low strength and low torque. It is generally used for home appliances and other small-sized host products. It is mostly used below MIO, and more used below M6.

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