Fastener selection principle and method

As we all know, only throughFastenersIn order to connect and fasten various parts and components to form a host. However, in the host design process, some designs often focus on the structure, function, life or starting rate of mechanical equipment, and finally consider the selection of connections and fasteners. This may be a thing of the past, but such a thing does not need to exist. This selection method, but also in the occurrence of a lot of mechanical and personal accidents, it gradually attracted people's attention.

In the practical application selection, it is very important to choose the appropriate fastener variety, specification, precision and level, which is the experience gained at the cost of life and blood.

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

In the 1980 s, in the United States, a local newspaper published a message saying that there were about 3 to 4 cases of car driving out of control every week in the state, causing casualties. People 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 bolts and nuts on the front suspension, rear suspension and shock absorber, and these devices were very important to the control of the car. The report also pointed out that bolts and nuts are also loose on mechanical parts such as transmissions, differential systems, engine rocker covers, catalytic converters, and body metal components. In 1986, an air crash occurred in which 532 people were killed when the rear section of an airliner fell in flight. Later, an investigation was conducted on the same aircraft rear sections in various countries. It was found that nuts were loose and bolts were broken. In this case, firstly, the strain on the tail of the airliner exceeded the fastening force that the fastener could bear. Secondly, it was also found that the internal displacement between the tooth sides of the meshing thread was found.

Loosening of fasteners is a general problem. A pair of scissors has only one fastener. If it is loose, it can only be used if it is tightened. However, a large aircraft may have 300000 fasteners, and if something goes wrong, it will be extremely difficult.

Industrialists in developed countries have realized that no problem is more troubling to the industry than fasteners, and no matter which problem is more important than solving the fastener problem, this is their own experience. So there is a "how to choose" that is safe, applicable and economical fasteners.

1. according to the different characteristics of fasteners
Fasteners are used in all fields of the national economy. According to their respective uses and different 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. There are many specialized production enterprises. First of all, we should understand the production and quality of domestic fasteners, such as the varieties and specifications of fasteners produced and the special uses of various products, as well as the famous brand new products or new, excellent and special products of key enterprises. While these products should be given priority, we should give priority to famous, excellent and special fastener products.
2. shall be selected according to the applicable principle of economy
Should be selected according to the characteristics of fasteners. Fasteners are small, but they are diverse. According to the major categories, there are 12 categories and more than 400 varieties. Only bolts and screws have 32 small categories and nearly 100 varieties. There are also 10 types of end types of rods. Such a large fastener family members, it is impossible to be one by one, should apply the principle of saving selection, now take the head type as an example for selection.
1. Hexagonal head. This kind of products are also equipped with tables, holes, bosses and flanges. This kind of products do not have high mechanical properties, are convenient for wrench tightening, can apply large tightening torque, and are widely used. The supporting surface has convex or flange surface, has gasket function, increases pressure bearing surface, has good anti-loosening performance, and can be used for connection of steel structure parts and important host structures.
2. Square head, large head area, easy to tighten and rotate the wrench, can apply a large tightening torque, but not easy to screw bald, mostly used for relatively thick, often disassembled structure, but not suitable for moving parts.
3. Pan head, head size, head slot, etc., are convenient for tool screwing. They are mostly used for products with low strength and low screwing torque. They are generally used for host products with small size such as household appliances. The specifications are mostly below MIO and more below M6.