The implementation of the new national standard mechanical performance level, creating reasonable and convenient conditions for the selection and manufacture of threaded fasteners. Threaded fasteners are a wide range of connectors. In weapon equipment, cutting-edge technology and common electromechanical products, a large number of threaded fasteners are used for structural fixing and connection. Threaded fasteners such as screws, bolts and nuts are simple in structure, but if they are not used properly, it may cause accidents such as thread tripping or screw breakage during use, and even lead to failure of the entire mechanism. We must pay attention to it. Threaded fasteners are available in two applications, one without preload and the other with preload. Most threaded connections are preloaded during assembly. The pre-tightening force can enhance the rigidity, tightness, anti-vibration capability of the connecting structure and prevent the sliding of the threaded connecting member during lateral interception. Usually, the nut is tightened by a force wrench, and the tightening torque is used to overcome the screw resistance torque of the thread pair and the end surface friction torque between the nut and the bearing surface of the connected member (or the washer), thereby causing the connection to generate a preload. The magnitude of the preload is determined by the force of the threaded assembly and the requirements of the connection. Before selecting the threaded fastener, firstly determine the thread assembly pre-tightening force according to the load condition and the use requirement, and then calculate the required fastening according to the determined pre-tightening force, the allowable stress of the threaded fastener and the safety factor. The nominal diameter of the thread. The fastener allows the stress crs (aa2) to be selected mainly based on the inner valley marked in the design material column. The following review and comparison of the new and old national standard of threaded fasteners, the problems reflected in the selection and manufacture of threaded fasteners due to the different forms marked on the drawings. 1 Threaded fasteners The development of national standards for new and old national standard markings on the drawings has experienced three major changes in content, which can be called the national standard for fasteners in the 60s, 70s and 80s. A review of the labeling form of the national standard for threaded fasteners in the three years on the drawing material column is now available. In the 1960s national standard, for the standard fastener coded fasteners, only the shape and size specifications are given. The material is selected by the designer, and the steel code is marked in the drawing material column. In the 1970s national standard, there were five mechanical performance grades for bolts of 3.6, 6, 8, 10 and 12. In the standard, it is also stipulated that when the mechanical performance level cannot meet the requirements of use, the relevant materials can be selected according to the regulations. In the 1970s, the mechanical performance rating of screws, studs and gaskets was not specified in the national standard. In the 1970s, the national standard made great progress in the content of the national standard in the 1960s. However, due to the unfamiliar level of mechanical properties of fasteners and the fact that materials were allowed in the standard, people still liked to use old habits when designing, that is, Most units still indicate the steel code in the drawing material column. In the 1980s, the national standard was in line with international standards and its technical content was advanced. In the 1980s national standard, there were ten mechanical performance grades of 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9 for bolts, screws and studs. The nut is specified as 04, 05, 4, 5, 6, 8, 9, 10 and 12, and a total of nine performance levels ab nominal abminaS nominal mechanical performance grade. There are four mechanical performance grades for 14X, 22H, 33H and 45H for the set screws. The number and type of mechanical performance grades of threaded fasteners are relatively complete, and the combination is convenient, easy to use and feasible. The new national standard stipulates that the threaded fastener indicates the mechanical performance grade code of the fastener in the material column of the design drawing. The two new and old national standard forms will have different effects on the following two aspects: First, the calculation method of pre-tightening force will result in complicated calculation and accuracy due to different value methods of ~("..2). Different; second, there are different effects on the manufacturing process of threaded fasteners. (a.2) Value method The old national standard indicates the fastener material, and the heart (2) value of the material is generally obtained by checking the data. This has the following problems: there are many material manuals, and the data of each manual is different; It is to check the material standard, but the (0.2) value of the material is related to the heat treatment specification of the material, the test piece, the heat, the batch, etc., and the heat treatment of the material of the actual wide product may not be exactly the same. Many situations are difficult to grasp, so the accuracy is poorer and more complicated. The new national standard marks the mechanical performance level of fasteners. For example, a bolt (screw or stud) pattern material column marked 5.8, indicating that the nominal tensile strength of the bolt (screw or stud) is (N/mm2), the nominal yield point as (or nominal yield strength 2) and the nominal resistance The ratio of the tensile strength (ie, the yield ratio) is 0.8. The product of the two parts of the front and back of the "" in the performance level code is one tenth of the nominal yield point ~ (or the nominal yield strength aa2), so the bolt ( The nominal yield point of the screw or stud is 400 MPa (N/mm2). The L(.2) value can be directly converted, which is more intuitive and accurate. There are two other points that I think should also be considered: First, the manufacturing process of threaded fastener products (especially grinding) causes the material to undergo a once-strength (cold work hardening) treatment. In addition, the standard not only gives crb, (7S ((7.2) nominal value, but also gives the minimum of each performance level b, minimum (7S ((7.2) value, see Table 1. For example Table 1 shows that bmm, (.) is equal to or greater than the respective nominal value. It is safe to use the nominal value specified in the national standard for calculation. For example: 35 steel after quenching and tempering, can produce bolts with performance class 8.8, as can be seen from Table 1, MS 8.8 bolts, the nominal tensile strength and minimum tensile strength are 800MPa, nominal yielding The strength and minimum yield strength are both 640 MPa. It can be found in the standard of high-quality carbon structural steel that the nominal tensile strength of the 35 steel after quenching and tempering is 530 MPa, and the nominal yield strength is 310 MPa. Compare the above values: bolt products The nominal tensile strength is twice that of the material and the nominal yield strength is 1.5 times that of the material. The diameter of the ribs or the number of threaded fasteners is reduced to reduce the size of the structure. The new national standard only specifies the mechanical performance level of the product, and does not stipulate the material for manufacturing the fastener, thus giving the process a greater degree of freedom, and also conforms to the principle of maximum freedom of writing standard principles. Because different steels pass different heat treatment specifications, similar tensile strength and yield strength can be obtained. Different steel materials can also pass different heat treatment specifications to obtain several different tensile strengths and yield strengths. Therefore, threaded fastener products of the same mechanical performance grade can be manufactured with different kinds of steel materials, and threaded fastener products of different mechanical performance grades can be manufactured by the same steel. Manufacturers can combine the process equipment, process level and inventory steel types of the factory to compile the corresponding process methods to manufacture thread fastener products with various mechanical performance grades. Expanding the degree of freedom of the process will help improve the process level, reduce costs and increase competitiveness. Stevia Rebaudioside D,which is a glycoside. Stevia Rd 95%,Pure Bulk Powder,Bulk Powder Sweetener,0 Calorie Pure Powder JINING USP INTERNATIONAL CO.,LTD. , https://www.uspintl.com